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We've got plans to update this README, but in the mean time, check out the updated documentation here.

Project setup

Finagle is built using sbt. We've included a bootstrap script to ensure the correct version of sbt is used. To build:

$ ./sbt test

Finagle and its dependencies are published to maven central with crosspath versions. It is published with the most current Scala 2.9.x version. Use with sbt is simple:

libraryDependencies += "com.twitter" %% "finagle-core" % "6.0.5"

---

Finagle Developer Guide (December 15, 2011 Draft)

• Quick Start
  • Simple HTTP Server
  • Simple HTTP Client
  • HTTP Client To A Standard Web Server
  • Simple Client and Server for Thrift
• Finagle Overview
  • Client Features
  • Server Features
  • Supported Protocols
• Architecture
  • Future Objects
  • Service Objects
  • Filter Objects
  • Codec Objects
  • Servers
  • Clients
  • Threading Model
  • Starting and Stopping Servers
  • Exception Handling
• Finagle Projects and Packages
• Using Future Objects
  • Future Callbacks
  • Future Timeouts
  • Future Exceptions
  • Promises
  • Using Future map and flatMap Operations
  • Using Future in Scatter/Gather Patterns
  • Using Future Pools
• Creating a Service
• Creating Filters
• Building a Robust Server
• Building a Robust Client
• Creating Filters to Transform Requests and Responses
• Configuring Finagle Servers and Clients
• Using ServerSet Objects
• Java Design Patterns for Finagle
  • Using Future Objects With Java
  • Imperative Java Style
  • Functional Java Style
  • Building a Server in Java
  • Building a Client in Java
  • Implementing a Pool for Blocking Operations in Java
• Additional Samples
• API Reference Documentation
• Example Maven Project

Quick Start

Finagle is an asynchronous network stack for the JVM that you can use to build asynchronous Remote Procedure Call (RPC) clients and servers in Java, Scala, or any JVM-hosted language. Finagle provides a rich set of tools that are protocol independent.

The following Quick Start sections show how to implement simple RPC servers and clients in Scala and Java. The first example shows the creation a simple HTTP server and corresponding client. The second example shows the creation of a Thrift server and client. You can use these examples to get started quickly and have something that works in just a few lines of code. For a more detailed description of Finagle and its features, start with Finagle Overview and come back to Quick Start later.

Note: The examples in this section include both Scala and Java implementations. Other sections show only Scala examples. For more information about Java, see Java Design Patterns for Finagle.

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Simple HTTP Server

Consider a very simple implementation of an HTTP server and client in which clients make HTTP GET requests and the server responds to each one with an HTTP 200 OK response.

The following server, which is shown in both Scala and Java, responds to a client's HTTP request with an HTTP 200 OK response:

Scala HTTP Server Implementation

val service: Service[HttpRequest, HttpResponse] = new Service[HttpRequest, HttpResponse] { // 1
  def apply(request: HttpRequest) = Future(new DefaultHttpResponse(HTTP_1_1, OK))          // 2
}

val address: SocketAddress = new InetSocketAddress(10000)                                  // 3

val server: Server = ServerBuilder()                                                       // 4
  .codec(Http())
  .bindTo(address)
  .name("HttpServer")
  .build(service)

Java HTTP Server Implementation

Service<HttpRequest, HttpResponse> service = new Service<HttpRequest, HttpResponse>() {    // 1
  public Future<HttpResponse> apply(HttpRequest request) {
    return Future.value(                                                                   // 2
        new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK));
  }
};

ServerBuilder.safeBuild(service, ServerBuilder.get()                                       // 4
  .codec(Http.get())
  .name("HttpServer")
  .bindTo(new InetSocketAddress("localhost", 10000)));                                     // 3

HTTP Server Code Annotations

1. Create a new Service that handles HTTP requests and responses.
2. For each request, respond asynchronously with an HTTP 200 OK response. A Future instance represents an asynchronous operation that may be performed later.
3. Specify the socket addresses on which your server responds; in this case, on port 10000 of localhost.
4. Build a server that responds to HTTP requests on the socket and associate it with your service. In this case, the Server builder specifies

• an HTTP codec, which ensures that only valid HTTP requests are received by the server
• the host socket that listens for requests
• the association between the server and the service, which is specified by .build in Scala and the first argument to safeBuild in Java
• the name of the service

Note: For more information about the Java implementation, see Java Design Patterns for Finagle.

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Simple HTTP Client

The client, which is shown in both Scala and Java, connects to the server, and issues a simple HTTP GET request:

Scala HTTP Client Implementation

val client: Service[HttpRequest, HttpResponse] = ClientBuilder()                           // 1
  .codec(Http())
  .hosts(address)
  .hostConnectionLimit(1)
  .build()

// Issue a request, get a response:
val request: HttpRequest = new DefaultHttpRequest(HTTP_1_1, GET, "/")                      // 2
val responseFuture: Future[HttpResponse] = client(request)                                 // 3
  onSuccess { response => println("Received response: " + response)                        // 4
  }

Java HTTP Client Implementation

Service<HttpRequest, HttpResponse> client = ClientBuilder.safeBuild(ClientBuilder.get()    // 1
  .codec(Http.get())
  .hosts("localhost:10000")
  .hostConnectionLimit(1));

// Issue a request, get a response:
HttpRequest request = new DefaultHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.GET, "/");   // 2
client.apply(request).addEventListener(new FutureEventListener<HttpResponse>() {           // 3
  public void onSuccess(HttpResponse response) {                                           // 4
    System.out.println("received response: " + response);
  }
  public void onFailure(Throwable cause) {
    System.out.println("failed with cause: " + cause);
  }
});

HTTP Client Code Annotations

1. Build a client that sends an HTTP request to the host identified by its socket address. In this case, the Client builder specifies

• an HTTP request filter, which ensures that only valid HTTP requests are sent to the server
• a list of the server's hosts that can process requests
• maximum number of connections from the client to the host
• to build this client service

2. Create an HTTP GET request.
3. Make the request to the host identified in your client.
4. Specify a callback, onSuccess, that Finagle executes when the response arrives.

Note: Although the example shows building the client and execution of the built client on the same thread, you should build your clients only once and execute them separately. There is no requirement to maintain a 1:1 relationship between building a client and executing a client.

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HTTP Client To A Standard Web Server

If you create a request using HttpVersion.HTTP_1_1, a validating server will require a Hosts header. We could fix this by setting the Host header as in

httpRequest.setHeader("Host", "someHostName")

More concisely, you can use the RequestBuilder object, shown below. In the following example, we use a Jetty server "someJettyServer:80" which happens to come with a small test file "/d.txt":

import org.jboss.netty.handler.codec.http.HttpRequest
import org.jboss.netty.handler.codec.http.HttpResponse

import com.twitter.finagle.Service
import com.twitter.finagle.builder.ClientBuilder
import com.twitter.finagle.http.Http
import com.twitter.finagle.http.RequestBuilder
import com.twitter.util.Future

object ClientToValidatingServer {
  def main(args: Array[String]) {
    val hostNamePort = "someJettyServer:80"
    val client: Service[HttpRequest, HttpResponse] = ClientBuilder()
      .codec(Http())
      .hosts(hostNamePort)
      .hostConnectionLimit(1)
      .build()

    val httpRequest = RequestBuilder().url("http://" + hostNamePort + "/d.txt").buildGet
    val responseFuture: Future[HttpResponse] = client(httpRequest)
    responseFuture onSuccess { response => println("Received response: " + response) }
  }
}

Simple Client and Server for Thrift

Apache Thrift is a binary communication protocol that defines available methods using an interface definition language (IDL). Consider the following Thrift IDL definition for a Hello service that defines only one method, hi:

service Hello {
  string hi();
}

To create a Finagle Thrift service, you must implement the FutureIface Interface that Scrooge (a custom Thrift compiler) generates for your service. Scrooge wraps your service method return values with asynchronous Future objects to be compatible with Finagle.

• If you are using sbt to build your project, the sbt-scrooge plugin automatically compiles your Thrift IDL. Note: The latest release version of this plugin is only compatible with sbt 0.11.2.
• If you are using maven to manage your project, maven-finagle-thrift-plugin can also compile Thrift IDL for Finagle.

Simple Thrift Server

In this Finagle example, the ThriftServer object implements the Hello service defined using the Thrift IDL.

Scala Thrift Server Implementation

object ThriftServer {
  def main(args: Array[String]) {
    // Implement the Thrift Interface
    val processor = new Hello.ServiceIface {                                 // 1
    def hi() = Future.value("hi")                                            // 2
  }

  val service = new Hello.Service(processor, new TBinaryProtocol.Factory())  // 3

  val server: Server = ServerBuilder()                                       // 4
    .name("HelloService")
    .bindTo(new InetSocketAddress(8080))
    .codec(ThriftServerFramedCodec())
    .build(service)
  }
}

Java Thrift Server Implementation

Hello.FutureIface processor = new Hello.FutureIface() {                      // 1
  public Future<String> hi() {                                               // 2
    return Future.value("hi");
  }
};

ServerBuilder.safeBuild(                                                     // 4
  new Hello.FinagledService(processor, new TBinaryProtocol.Factory()),       // 3
  ServerBuilder.get()
    .name("HelloService")
    .codec(ThriftServerFramedCodec.get())
 // .codec(ThriftServerFramedCodecFactory$.MODULE$) previously
    .bindTo(new InetSocketAddress(8080)));

Thrift Server Code Annotations

1. Create a Thrift processor that implements the Thrift service interface, which is Hello in this example.
2. Implement the service interface. In this case, the only method in the interface is hi, which only returns the string "hi". The returned value must be a Future to conform the signature of a Finagle Service. (In a more robust example, the Thrift service might perform asynchronous communication.)
3. Create an adapter from the Thrift processor to a Finagle service. In this case, the Hello Thrift service uses TBinaryProtocol as the Thrift protocol.
4. Build a server that responds to Thrift requests on the socket and associate it with your service. In this case, the Server builder specifies

• the name of the service
• the host addresses that can receive requests
• the Finagle-provided ThriftServerFramedCodec codec, which ensures that only valid Thrift requests are received by the server
• the association between the server and the service

Simple Thrift Client

In this Finagle example, the ThriftClient object creates a Finagle client that executes the methods defined in the Hello Thrift service.

Scala Thrift Client Implementation

object ThriftClient {
  def main(args: Array[String]) {
    // Create a raw Thrift client service. This implements the
    // ThriftClientRequest => Future[Array[Byte]] interface.
    val service: Service[ThriftClientRequest, Array[Byte]] = ClientBuilder()               // 1
      .hosts(new InetSocketAddress(8080))
      .codec(ThriftClientFramedCodec())
      .hostConnectionLimit(1)
      .build()

    // Wrap the raw Thrift service in a Client decorator. The client provides
    // a convenient procedural interface for accessing the Thrift server.
    val client = new Hello.ServiceToClient(service, new TBinaryProtocol.Factory())         // 2

    client.hi() onSuccess { response =>                                                    // 3
      println("Received response: " + response)
    } ensure {
      service.release()                                                                    // 4
    }
  }
}

Java Thrift Client Implementation

Service<ThriftClientRequest, byte[]> service = ClientBuilder.safeBuild(ClientBuilder.get() // 1
  .hosts(new InetSocketAddress(8080))
  .codec(ThriftClientFramedCodec.get())
  .hostConnectionLimit(1));

Hello.FinagledClient client = new Hello.FinagledClient(                                    // 2
  service,
  new TBinaryProtocol.Factory(),
  "HelloService",
  new InMemoryStatsReceiver());

client.hi().addEventListener(new FutureEventListener<String>() {
  public void onSuccess(String s) {                                                        // 3
    System.out.println(s);
  }

  public void onFailure(Throwable t) {
    System.out.println("Exception! " + t.toString());
  }
});

Thrift Client Code Annotation

1. Build a client that sends a Thrift protocol-based request to the host identified by its socket address. In this case, the Client builder specifies
   • the host addresses that can receive requests
   • the Finagle-provided ThriftServerFramedCodec codec, which ensures that only valid Thrift requests are received by the server
   • to build this client service
2. Make a remote procedure call to the Hello Thrift service's Hi method. This returns a Future that represents the eventual arrival of a response.
3. When the response arrives, the onSuccess callback executes to print the result.
4. Release resources acquired by the client.

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Finagle Overview

Use the Finagle library to implement asynchronous Remote Procedure Call (RPC) clients and servers. Finagle is flexible enough to support a variety of RPC styles, including request-response, streaming, and pipelining; for example, HTTP pipelining and Redis pipelining. It also makes it easy to work with stateful RPC styles; for example, RPCs that require authentication and those that support transactions.

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Client Features

• Connection Pooling
• Load Balancing
• Failure Detection
• Failover/Retry
• Distributed Tracing (à la Dapper)
• Service Discovery (e.g., via Zookeeper)
• Rich Statistics
• Native OpenSSL Bindings

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Server Features

• Backpressure (to defend against abusive clients)
• Service Registration (e.g., via Zookeeper)
• Distributed Tracing
• Native OpenSSL bindings

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Supported Protocols

• HTTP
• HTTP streaming (Comet)
• Thrift
• Memcached/Kestrel
• More to come!

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Architecture

Finagle extends the stream-oriented Netty model to provide asynchronous requests and responses for remote procedure calls (RPC). Internally, Finagle manages a service stack to track outstanding requests, responses, and the events related to them. Finagle uses a Netty pipeline to manage connections between the streams underlying request and response messages. The following diagram shows the relationship between your RPC client or server, Finagle, Netty, and Java libraries:

Finagle manages a Netty pipeline for servers built on Finagle RPC services. Netty itself is built on the Java NIO library, which supports asynchronous IO. While an understanding of Netty or NIO might be useful, you can use Finagle without this background information.

Finagle objects are the building blocks of RPC clients and servers:

• Future objects enable asynchronous operations required by a service
• Service objects perform the work associated with a remote procedure call
• Filter objects enable you to transform data or act on messages before or after the data or messages are processed by a service
• Codec objects decode messages in a specific protocol before they are handled by a service and encode messages before they are transported to a client or server.

You combine these objects to create:

• Servers
• Clients

Finagle provides a ServerBuilder and a ClientBuilder object, which enable you to configure servers and clients, respectively.

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Future Objects

In Finagle, Future objects are the unifying abstraction for all asynchronous computation. A Future represents a computation that has not yet completed, which can either succeed or fail. The two most basic ways to use a Future are to

• block and wait for the computation to return
• register a callback to be invoked when the computation eventually succeeds or fails

For more information about Future objects, see Using Future Objects.

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Service Objects

A Service is simply a function that receives a request and returns a Future object as a response. You extend the abstract Service class to implement your service; specifically, you must define an apply method that transforms the request into the future response.

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Filter Objects

It is useful to isolate distinct phases of your application into a pipeline. For example, you may need to handle exceptions, authorization, and other phases before your service responds to a request. A Filter provides an easy way to decouple the protocol handling code from the implementation of the business rules. A Filter wraps a Service and, potentially, converts the input and output types of the service to other types. For an example of a filter, see Creating Filters to Transform Requests and Responses.

A SimpleFilter is a kind of Filter that does not convert the request and response types. For an example of a simple filter, see Creating Filters.

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Codec Objects

A Codec object encodes and decodes wire protocols, such as HTTP. You can use Finagle-provided Codec objects for encoding and decoding the Thrift, HTTP, memcache, Kestrel, HTTP chunked streaming (ala Twitter Streaming) protocols. You can also extend the CodecFactory class to implement encoding and decoding of other protocols.

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Servers

In Finagle, RPC servers are built out of a Service and zero or more Filter objects. You apply filters to the service request after which you execute the service itself:

Typically, you use a ServerBuilder to create your server. A ServerBuilder enables you to specify the following general attributes:

Attribute	Description	Default Value
codec	Object to handle encoding and decoding of the service's request/response protocol	None
statsReceiver	Statistics receiver object, which enables logging of important events and statistics	None
name	Name of the service	None
bindTo	The IP host:port pairs on which to listen for requests; localhost is assumed if the host is not specified	None
logger	Logger object	None

You can specify the following attributes to handle fault tolerance and manage clients:

Attribute	Description	Default Value
maxConcurrentRequests	Maximum number of requests that can be handled concurrently by the server	None
hostConnectionMaxIdleTime	Maximum time that this server can be idle before the connection is closed	None
hostConnectionMaxLifeTime	Maximum time that this server can be connected before the connection is closed	None
requestTimeout	Maximum time to complete a request	None
readTimeout	Maximum time to wait for the first byte to be read	None
writeCompletionTimeout	Maximum time to wait for notification of write completion from a client	None

You can specify the following attributes to manage TCP connections:

Attribute	Description	Default Value
sendBufferSize	Requested TCP buffer size for responses	None
recvBufferSize	Actual TCP buffer size for requests	None

You can also specify these attributes:

Attribute	Description	Default Value
tls	The kind of transport layer security	None
channelFactory	Channel service factory object	None
traceReceiver	Trace receiver object	new NullTraceReceiver object

Once you have defined your Service, it can be bound to an IP socket address, thus becoming an RPC server.

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Clients

Finagle makes it easy to build RPC clients with connection pooling, load balancing, logging, and statistics reporting. The balancing strategy is to pick the endpoint with the least number of outstanding requests, which is similar to least connections in other load balancers. The load-balancer deliberately introduces jitter to avoid synchronicity (and thundering herds) in a distributed system.

Your code should separate building the client from invocation of the client. A client, once built, can be used with lazy binding, saving the resources required to build a client. Note: The examples, which show the creation of the client and its first execution together, represent the first-execution scenario. Typically, subsequent execution of the client does not require rebuilding.

Finagle will retry the request in the event of an error, up to the number of times specified; however, Finagle does not assume your RPC service is Idempotent. Retries occur only when the request is known to be idempotent, such as in the event of TCP-related WriteException errors, for which the RPC has not been transmitted to the remote server.

A robust way to use RPC clients is to have an upper-bound on how long to wait for a response to arrive. With Future objects, you can

• block, waiting for a response to arrive and throw an exception if it does not arrive in time.
• register a callback to handle the result if it arrives in time, and register another callback to invoke if the result does not arrive in time

A client is a Service and can be wrapped by Filter objects. Typically, you call ClientBuilder to create your client service. ClientBuilder enables you to specify the following general attributes:

Attribute	Description	Default Value
name	Name of the service	None
codec	Object to handle encoding and decoding of the service's request/response protocol	None
statsReceiver	Statistics receiver object, which enables logging of important events and statistics	None
loadStatistics	How often to load statistics from the server	(60, 10.seconds)
logger	A Logger object with which to log Finagle messages	None
retries	Number of tries (not retries) per request (only applies to recoverable errors)	None

You can specify the following attributes to manage the host connection:

Attribute	Description	Default Value
connectionTimeout	Time allowed to establish a connection	10.milliseconds
requestTimeout	Request timeout	None, meaning it waits forever
hostConnectionLimit	Number of connections allowed from this client to the host	None
hostConnectionCoresize	Host connection's cache allocation	None
hostConnectionIdleTime		None
hostConnectionMaxWaiters	The maximum number of queued requests awaiting a connection	None
hostConnectionMaxIdleTime	Maximum time that the client can be idle until the connection is closed	None
hostConnectionMaxLifeTime	Maximum time that client can be connected before the connection is closed	None

You can specify the following attributes to manage TCP connections:

Attribute	Description	Default Value
sendBufferSize	Requested TCP buffer size for responses	None
recvBufferSize	Actual TCP buffer size for requests	None

You can also specify these attributes:

Attribute	Description	Default Value
cluster	The cluster connections associated with the client	None
channelFactory	Channel factory associated with this client	None
tls	The kind of transport layer security	None

If you are using stateful protocols, such as those used for transaction processing or authentication, you should call buildFactory, which creates a ServiceFactory to support stateful connections.

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Threading Model

The Finagle threading model requires that you avoid blocking operations in the Finagle event loop. Finagle-provided methods do not block; however, you could inadvertently implement a client, service or a Future callback that blocks.

Blocking events include but are not limited to

• network calls
• system calls
• database calls

Note: You do not need to be concerned with long-running or CPU intensive operations if they do not block. Examples of these operations include image processing operations, public key cryptography, or anything that might take a non-trivial amount of clock time to perform. Only operations that block in Finagle are of concern. Because Finagle and its event loop use a relatively low number of threads, blocked threads can cause performance issues.

Consider the following diagram, which shows how a client uses the Finagle event loop:

Your threads, which are shown on the left, are allowed to block. When you call a Finagle method or Finagle calls a method for you, it dispatches execution of these methods to its internal threads. Thus, the Finagle event loop and its threads cannot block without degrading the performance of other clients and servers that use the same Finagle instance.

In complex RPC operations, it may be necessary to perform blocking operations. In these cases, you must set up your own thread pool and use Future or FuturePool objects to execute the blocking operation on your own thread. Consider the following diagram:

In this example, you can use a FuturePool object to provide threads for blocking operations outside of Finagle. Finagle can then dispatch the blocking operation to your thread. For more information about FuturePool objects, see Using Future Pools.

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Starting and Stopping Servers

A server automatically starts when you call build on the server after assigning the IP address on which it runs. To stop a server, call its close method. The server will immediately stop accepting requests; however, the server will continue to process outstanding requests until all have been handled or until a specific duration has elapsed. You specify the duration when you call close. In this way, the server is allowed to drain out outstanding requests but will not run indefinitely. You are responsible for releasing all resources when the server is no longer needed.

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Finagle Projects and Packages

The Core project contains the execution framework, Finagle classes, and supporting classes, whose objects are only of use within Finagle. The Core project includes the following packages:

• builder - contains ClientBuilder, ServerBuilder
• channel
• http
• loadbalancer
• pool
• service
• stats
• tracing
• util

It also contains packages to support remote procedure calls over Kestrel, Thrift, streams, clusters, and provides statistics collection (Ostrich).

The Util project contains classes, such as Future, which are both generally useful and specifically useful to Finagle.

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Using Future Objects

In the simplest case, you can use Future to block for a request to complete. Consider an example that blocks for an HTTP GET request:

// Issue a request, get a response:
val request: HttpRequest = new DefaultHttpRequest(HTTP_1_1, GET, "/")
val responseFuture: Future[HttpResponse] = client(request)

In this example, a client issuing the request will wait forever for a response unless you specified a value for the requestTimeout attribute when you built the client.

Consider another example:

val responseFuture: Future[String] = executor.schedule(job)

In this example, the value of responseFuture is not available until after the scheduled job has finished executing and the caller will block until responseFuture has a value.

Note: For examples of using Finagle Future objects in Java, see Using Future Objects With Java.

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Future Callbacks

In cases where you want to continue execution immediately, you can specify a callback. The callback is identified by the onSuccess keyword:

val request: HttpRequest = new DefaultHttpRequest(HTTP_1_1, GET, "/")
val responseFuture: Future[HttpResponse] = client(request)
responseFuture onSuccess { responseFuture =>
  println(responseFuture)
}

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Future Timeouts

In cases where you want to continue execution after some amount of elapsed time, you can specify the length of time to wait in the Future object. The following example waits 1 second before displaying the value of the response:

val request: HttpRequest = new DefaultHttpRequest(HTTP_1_1, GET, "/")
val responseFuture: Future[HttpResponse] = client(request)
println(responseFuture(1.second))

In the above example, you do not know whether the response timed out before the request was satisfied. To determine what kind of response you actually received, you can provide two callbacks, one to handle onSuccess conditions and one for onFailure conditions. You use the within method of Future to specify how long to wait for the response. Finagle also creates a Timer thread on which to wait until one of the conditions are satisfied. Consider the following example:

import com.twitter.finagle.util.Timer._
...
val request: HttpRequest = new DefaultHttpRequest(HTTP_1_1, GET, "/")
val responseFuture: Future[HttpResponse] = client(request)
responseFuture.within(1.second) onSuccess { response =>
  println("responseFuture)
} onFailure {
  case e: TimeoutException => ...
}

If a timeout occurs, Finagle takes the onFailure path. You can use a TimeoutException object to display a message or take other actions.

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Future Exceptions

To set up an exception, specify the action in a try block and handle failures in a catch block. Consider an example that handles Future timeouts as an exception:

val request: HttpRequest = new DefaultHttpRequest(HTTP_1_1, GET, "/")
val responseFuture: Future[HttpResponse] = client(request)
try {
  println(responseFuture(1.second))
} catch {
  case e: TimeoutException => ...
}

In this example, after 1 second, either the HTTP response is displayed or the TimeoutException is thrown.

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Promises

Promise is a subclass of Future. Although a Future can only be read, a Promise can be both read and written. Usually a producer makes a Promise and casts it to a Future before giving it to the consumer. The following example shows how this might be useful in the case where you intend to make a Future service but need to anticipate errors:

def make() = {
...
val promise = new Promise[Service[Req, Rep]]
... {
  case Ok(myObject) =>
    ...
    promise() = myConfiguredObject
  case Error(cause) =>
    promise() = Throw(new ...Exception(cause))
  case Cancelled =>
    promise() = Throw(new WriteException(new ...Exception))
  }
  promise
}

You are discouraged from creating your own Promises. Instead, where possible, use Future combinators to compose actions (discussed next).

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Using Future map and flatMap Operations

In addition to waiting for results to return, Future can be transformed in interesting ways. For instance, it is possible to convert a Future[String] to a Future[Int] by using map:

val stringFuture: Future[String] = Future("1")
val intFuture: Future[Int] = stringFuture map (_.toInt)

Similar to map, you can use flatMap to easily pipeline a sequence of Futures:

val authenticateUser: Future[User] = User.authenticate(email, password)
val lookupTweets: Future[Seq[Tweet]] = authenticateUser flatMap { user =>
  Tweet.findAllByUser(user)
}

In this example, Tweet.findAllByUser(user) is a function of type User => Future[Seq[Tweet]].

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Using Future in Scatter/Gather Patterns

For scatter/gather patterns, the challenge is to issue a series of requests in parallel and wait for all of them to arrive. To wait for a sequence of Future objects to return, you can define a sequence to hold the objects and use the Future.collect method to wait for them, as follows:

val myFutures: Seq[Future[Int]] = ...
val waitTillAllComplete: Future[Seq[Int]] = Future.collect(myFutures)

A more complex variation of scatter/gather pattern is to perform a sequence of asynchronous operations and harvest only those that return within a certain time, ignoring those that don't return within the specified time. For example, you might want to issue a set of parallel requests to N partitions of a search index; those that don't return in time are assumed to be empty. The following example allows 1 second for the query to return:

import com.twitter.finagle.util.Timer._

val timedResults: Seq[Future[Result]] = partitions.map { partition =>
  partition.get(query).within(1.second) handle {
    case _: TimeoutException => EmptyResult
  }
}
val allResults: Future[Seq[Result]] = Future.collect(timedResults)

allResults onSuccess { results =>
  println(results)
}

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Using Future Pools

A FuturePool object enables you to place a blocking operation on its own thread. In the following example, a service's apply method, which executes in the Finagle event loop, creates the FuturePool object and places the blocking operation on a thread associated with the FuturePool object. The apply method returns immediately without blocking.

class ThriftFileReader extends Service[String, Array[Byte]] {
  val diskIoFuturePool = FuturePool(Executors.newFixedThreadPool(4))

  def apply(path: String) = {
    def blockingOperation = {
      scala.Source.fromFile(path) // potential to block
    }
    // give this blockingOperation to the future pool to execute
    diskIoFuturePool(blockingOperation)
    // returns immediately while the future pool executes the operation on a different thread
  }
}

Note: For an example implementation of a thread pool in Java, see Implementing a Pool for Blocking Operations in Java.

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Creating a Service

The following example extends the Service class to respond to an HTTP request:

class Respond extends Service[HttpRequest, HttpResponse] {
  def apply(request: HttpRequest) = {
    val response = new DefaultHttpResponse(HTTP_1_1, OK)
    response.setContent(copiedBuffer(myContent, UTF_8))
    Future.value(response)
  }
}

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Creating Simple Filters

The following example extends the SimpleFilter class to throw an exception if the HTTP authorization header contains a different value than the specified string:

class Authorize extends SimpleFilter[HttpRequest, HttpResponse] {
  def apply(request: HttpRequest, continue: Service[HttpRequest, HttpResponse]) = {
    if ("shared secret" == request.getHeader("Authorization")) {
      continue(request)
    } else {
      Future.exception(new IllegalArgumentException("You don't know the secret"))
    }
  }
}

The following example extends the SimpleFilterclass to set the HTTP response code if an error occurs and return the error and stack trace in the response:

class HandleExceptions extends SimpleFilter[HttpRequest, HttpResponse] {
  def apply(request: HttpRequest, service: Service[HttpRequest, HttpResponse]) = {
    service(request) handle { case error =>
      val statusCode = error match {
        case _: IllegalArgumentException =>
          FORBIDDEN
        case _ =>
          INTERNAL_SERVER_ERROR
        }

      val errorResponse = new DefaultHttpResponse(HTTP_1_1, statusCode)
      errorResponse.setContent(copiedBuffer(error.getStackTraceString, UTF_8))

      errorResponse
    }
  }
}

For an example implementation using a Filter object, see Creating Filters to Transform Requests and Responses.

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Building a Robust Server

The following example encapsulates the filters and service in the previous examples and defines the execution order of the filters, followed by the service. The ServerBuilder object specifies the service that indicates the execution order along with the codec and IP address on which to bind the service:

object HttpServer {
  class HandleExceptions extends SimpleFilter[HttpRequest, HttpResponse] {...}
  class Authorize extends SimpleFilter[HttpRequest, HttpResponse] {...}
  class Respond extends Service[HttpRequest, HttpResponse] {... }


  def main(args: Array[String]) {
    val handleExceptions = new HandleExceptions
    val authorize = new Authorize
    val respond = new Respond

  val myService: Service[HttpRequest, HttpResponse]
    = handleExceptions andThen authorize andThen respond

  val server: Server = ServerBuilder()
    .name("myService")
    .codec(Http())
    .bindTo(new InetSocketAddress(8080))
    .build(myService)
  }
}

In this example, the HandleExceptions filter is executed before the authorize filter. All filters are executed before the service. The server is robust not because of its complexity; rather, it is robust because it uses filters to remove issues before the service executes.

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Building a Robust Client

A robust client has little to do with the lines of code (SLOC) that goes into it; rather, the robustness depends on how you configure the client and the testing you put into it. Consider the following HTTP client:

val client = ClientBuilder()
  .codec(Http())
  .hosts("localhost:10000,localhost:10001,localhost:10003")
  .hostConnectionLimit(1)             // max number of connections at a time to a host
  .connectionTimeout(1.second)        // max time to spend establishing a TCP connection
  .retries(2)                         // (1) per-request retries
  .reportTo(new OstrichStatsReceiver) // export host-level load data to ostrich
  .logger(Logger.getLogger("http"))
  .build()

The ClientBuilder object creates and configures a load-balanced HTTP client that balances requests among 3 (local) endpoints. The Finagle balancing strategy is to pick the endpoint with the least number of outstanding requests, which is similar to a least connections strategy in other load balancers. The Finagle load balancer deliberately introduces jitter to avoid synchronicity (and thundering herds) in a distributed system. It also supports failover.

The following examples show how to invoke this client from Scala and Java, respectively:

Scala Client Invocation

val request: HttpRequest = new DefaultHttpRequest(HTTP_1_1, Get, "/")
val futureResponse: Future[HttpResponse] = client(request)

Java Client Invocation

HttpRequest request = new DefaultHttpRequest(HTTP_1_1, Get, "/")
Future<HttpResponse> futureResponse = client.apply(request)

For information about using Future objects with Java, see Using Future Objects With Java.

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Creating Filters to Transform Requests and Responses

The following example extends the Filter class to authenticate requests. The request is transformed into an HTTP response before being handled by the AuthResult service. In this case, the RequireAuthentication filter does not transform the resulting HTTP response:

class RequireAuthentication(val p: ...)
  extends Filter[Request, HttpResponse, AuthenticatedRequest, HttpResponse]
  {
    def apply(request: Request, service: Service[AuthenticatedRequest, HttpResponse]) = {
      p.authenticate(request) flatMap {
        case AuthResult(AuthResultCode.OK, Some(passport: OAuthPassport), _, _) =>
          service(AuthenticatedRequest(request, passport))
        case AuthResult(AuthResultCode.OK, Some(passport: SessionPassport), _, _) =>
          service(AuthenticatedRequest(request, passport))
        case ar: AuthResult =>
          Trace.record("Authentication failed with " + ar)
          Future.exception(new RequestUnauthenticated(ar.resultCode))
    }
  }
}

In this example, the flatMap object enables pipelining of the requests.

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Using ServerSet Objects

finagle-serversets is an implementation of the Finagle Cluster interface using com.twitter.com.zookeeper ServerSets.

You can instantiate a ServerSet object as follows:

val serverSet = new ServerSetImpl(zookeeperClient, "/twitter/services/...")
val cluster = new ZookeeperServerSetCluster(serverSet)

Servers join a cluster, as in the following example:

val serviceAddress = new InetSocketAddress(...)
val server = ServerBuilder()
  .bindTo(serviceAddress)
  .build()

cluster.join(serviceAddress)

A client can access a cluster, as follows:

val client = ClientBuilder()
  .cluster(cluster)
  .hostConnectionLimit(1)
  .codec(new StringCodec)
  .build()

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Configuring Finagle Servers and Clients

Finagle offers a wealth of options for configuring servers and clients. For many if not most Finagle users, the defaults are both sensible and sufficient, and this section is unnecessary. Both servers and clients require a small number of configuration parameters (below: clients, servers).

• Using the ClientBuilder and ServerBuilder
• ClientBuilder Required Parameters
• ServerBuilder Required Parameters
• Clusters
• Idle Times
• Timeouts
• <a href="#Configuring Connections>Configuring Connections
• <a href="#maxConcurrentRequests>maxConcurrentRequests
• Retries
• Debugging

Using the ClientBuilder and ServerBuilder

A client is specified as follows:

val client: Service[Req, Resp] = ClientBuilder()
  .configParam1(val1)
  .configParam2(val2)
  ...
  .build()

Each configParam is initialized with a value (val); the params are initialized in order, top to bottom. Conceptually, the builder acts like an immutable map. If configParamX and configParamY are mutually exclusive, the later one overrides the earlier one. At the end, build, called with no arguments, actually constructs the client; this is the only operation with any side-effects.

A server looks similar:

val server: Server = ServerBuilder()
  .configParam1(val1)
  .configParam2(val2)
  ...
  .build(myService)

Unlike in the ClientBuilder, the ServerBuilder's build call takes a single argument, the service that will be visible to connected clients.

These builders are immutable/persistent; this has correctness advantages and also allows constructing "template" builders that might encapsulate a certain set of parameters. This is a useful and common design pattern.

ClientBuilder Required Parameters

The ClientBuilder has two main abstractions. The first is the trio of client, hosts, and connections. A client can connect to one or more hosts and specify a policy that distributes requests to those hosts. And each host may allow one or more individual connections to it, exposing concurrency among requests from its connected clients and allowing parallel execution.

The second main abstraction is the codec, which is responsible for turning a discrete request or response into a stream of bytes to send across the network, and vice versa.

These concepts are so important that they are required when specifying any client:

The ClientBuilder requires the definition of cluster or hosts, codec, and hostConnectionLimit. In Scala, these are statically type checked, and in Java the lack of any of the above causes a runtime error.

Alternatively, a Java ClientBuilder is statically type checked by using ClientBuilder.safeBuild().

• hosts must contain a list of hosts or cluster an explicitly specified cluster.
• The codec implements the network protocol used by the client, and consequently determines the types of request and reply.
• hostConnectionLimit specifies the maximum number of connections that are established to each host.

If you don't specify those, and you're using Scala, you'll see an error message that indicates the Builder is not fully configured with details on the requested (incomplete) configuration.

ServerBuilder Required Parameters

ServerBuilder has three required parameters: a codec, a service name (a string) (called as name), and an address (typically InetSocketAddress(serverPort)) (called as bindTo). Just as with the client, in Scala these are statically type checked; Java can be statically type checked by using ServerBuilder.safeBuild(); otherwise in Java the lack of any of the above causes a runtime error.

Clusters

The purpose of a Cluster is to abstract a group of identical servers, where requests to the cluster can be routed to any server in that cluster. Note that clusters have dynamic membership. Recall that:

The Finagle balancing strategy is to pick the endpoint with the least number of outstanding requests, which is similar to a least connections strategy in other load balancers. The Finagle load balancer deliberately introduces jitter to avoid synchronicity (and thundering herds) in a distributed system. It also supports failover.

Idle Times

hostConnectionIdleTime vs. hostConnectionMaxIdleTime: with respect to the ClientBuilder, what's the difference?

hostConnectionIdleTime applies to the caching pool: "the amount of time a connection is allowed to linger (when it otherwise would have been closed by the pool) before being closed". More precisely, it is applied to any connection between the low and high watermarks. hostConnectionMaxIdleTime applies to the physical connection: "the maximum time a connection is allowed to linger unused".

Timeouts

Clients have several timeout parameters during the connection process, which consists of the following steps:

1. Make a request.
2. Ask the load balancer where to connect to (the load balancer chooses the least connected endpoint).
3. Ask that endpoint's pool for a connection. Most of the time, there is an existing unused connection in the pool and that connection is returned. Otherwise, a connection is acquired or, depending on pool policies, the request may be queued. A queued request may be satisfied by giving a connection back to the pool, or establishing one when possible according to pooling policies.
4. When a connection is established, we create a socket and connect.
5. Dispatch the request on the given connection.
6. Wait until you receive a reply, then satisfy the Future.
7. Give the connection back to the pool.

The configurable timeout parameters are:

• connectTimeout - total time to acquire a connection regardless of whether it's an actual connect attempt or waiting in the queue for one to free up. (1–3)
• tcpConnectTimeout - TCP-level connect timeout, equivalent to Netty's connectTimeoutMillis and the second parameter to Java's java.net.Socket.connect(SocketAddress, int). It is specifically the maximum time to wait between a socket connection attempt and its success. By default, this is set to 10 milliseconds, which may be insufficient for distant servers. (4)
• requestTimeout - per-request timeout, meaning for each retry, the attempt may take this long. This timer begins counting only when the connection is established. (5–6)
• timeout - top-level timeout applied from the issuance of a request (through service(request)) until the satisfaction of its reply future. No request will take longer than this. (1–6)

Configuring Connections

Finagle manages a connection pool for clients. Connections to a server are expensive to build, so when Finagle establishes a connection for a particular request, it maintains that connection even after the request is complete. Then another request can reuse the same connection. The management of this pool is handled by Finagle, but you can configure some of the pool parameters.

How do connections work in the presence of the connection pool? The following points explain the relevant behavior in terms of the parameters that you can set.

• When the client is built, no connections are established eagerly.
• When you send the first request, it will establish a connection and give it to that request.
• When that request is complete, the request will release the connection to the watermark pool. hostConnectionCoresize sets the size of this pool; the watermark pool maintains this number of connections (per host).
• If there are more than hostConnectionCoresize outstanding requests, new connections will be established on demand up to hostConnectionLimit. Once those requests complete, they will be released to the cachingPool, which will keep them around for hostConnectionIdleTime. if a new connection is requested within hostConnectionIdleTime, it will reuse that connection.
• Any connection-level errors (write exceptions or timeouts) will make the connection unavailable and will be discarded immediately.

Setting hostConnectionLimit specifies the maximum number of connections that are allowed per host; Finagle guarantees it will never have more active connections than this limit. hostConnectionCoresize sets a minimum number of connections; unless they time out from idleness, the pool never has fewer connections than this limit.

If you set these two parameters to be the same, the consequence is that Finagle won't establish more than that number of connections per host, and it won't relinquish healthy connections. However, this doesn't mean that you will always see the same number of connections. To get these connections, you need requests. Finagle does not proactively establish connections (when the client is built, no connections are established eagerly). When a new request is dispatched, the following happens:

if num(active connections) == max, enqueue it
otherwise establish a new connection and dispatch it

When a request is complete, the connection is re-added to the pool only if it is healthy (is alive and hasn't been closed by the server).

However, there are some other parameters at play, too: if the client specifies idle timeouts, the connection is jettisoned if idle (for the parameterized amount of time). Unless the connection count is maximized to every host, requests will never queue.

Another useful parameter is limiting the number of waiters by setting hostConnectionMaxWaiters; requests that arrive when the number of waiters exceeds this number immediately return a Future.exception(TooManyWaitersException).

Reducing high lock contention via a fast, lock-free buffer is the goal of the experimental expHostConnectionBufferSize(n) parameter in the pool, where n should be set to the number of expected outstanding requests at a time (overestimating is better than underestimating, and power-of-two sizes of n may be faster). This parameter is currently experimental and will eventually be integrated into the mainline code (as hostConnectionBufferSize).

Note that finagle also exports a number of useful stats that allow you to inspect the state of the pool(s), load balancers, and queues. These are usually illustrative in explaining exactly why there are N connections to a given host.

maxConcurrentRequests

maxConcurrentRequests is the maximum number of requests you are telling Finagle that your server implementation can handle concurrently at any time. If exceeded, Finagle will insert new requests in an unbounded queue waiting for their turn. Note that setting maxConcurrentRequests will not result in explicitly rejecting requests, due to the unbounded queue. However, it effectively can, due to timeouts upstream and consequent cancellations.

Retries

The ClientBuilder allows specifying a retryPolicy or a number of retries. These are mutually exclusive and each can override the other; if you specify both in the ClientBuilder, the later one will override the earlier one.

Note that "retries" actually means "tries"; retry=1 means 1 try with no retries; retry=2 means 1 try with 1 retry, and so on. We apologize for this historical vestige.

Debugging

One good trick for debugging is to add a logger:

ServerBuilder() // or ClientBuilder()
  ...
  .logger(java.util.logging.Logger.getLogger("debug"))
  ...

While logging server behavior, it may be useful to have access to information about the client for any particular request. The ClientId companion object provides a .current() method that returns the id of the client from which the current request originated. Its docs (finagle/finagle-thrift/src/main/scala/com/twitter/finagle/thrift/authentication.scala) note that

[ClientID] is set at the beginning of the request and is available throughout the life-cycle of the request. It is [available] iff the client has an upgraded finagle connection and has chosen to specify the client ID in its codec.

Loggers should only be used for debugging; logs are very verbose.

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Java Design Patterns for Finagle

The implementations of RPC servers and clients in Java are similar to Scala implementations. You can write your Java services in the imperative style, which is the traditional Java programming style, or you can write in the functional style, which uses functions as objects and is the basis for Scala. Most differences between Java and Scala implementations are related to exception handling.

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Using Future Objects With Java

A Future object in Java is defined as Future<Type>, as in the following example:

Future<String> future = executor.schedule(job);

Note: The Future class is defined in com.twitter.util.Future and is not the same as the Java Future class.

You can explicitly call the Future object's get method to retrieve the contents of a Future object:

// Wait indefinitely for result
String result = future.get();

Calling get is the more common pattern because you can more easily perform exception handling. See Handling Synchronous Responses With Exception Handling for more information.

You can alternatively call the Future object's apply method. Arguments to the apply method are passed as functions:

// Wait up to 1 second for result
String result = future.apply(Duration.apply(1, SECOND));

This technique is most appropriate when exception handling is not an issue.

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Imperative Java Style

The following example shows the imperative style, which uses an event listener that responds to a change in the Future object and calls the appropriate method:

Future<String> future = executor.schedule(job);
future.addEventListener(
  new FutureEventListener<String>() {
    public void onSuccess(String value) {
      println(value);
    }
    public void onFailure(Throwable t) ...
  }
)

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Functional Java Style

The following example shows the functional style, which is similar to the way in which you write Scala code:

import scala.runtime.BoxedUnit;
Future<String> future = executor.schedule(job);
  future.onSuccess( new Function<String, BoxedUnit>() {
    public BoxedUnit apply(String value) {
      System.out.println(value);
      return BoxedUnit.UNIT;
    }
  }).onFailure(...).ensure(...);

The following example shows the functional style for the map method:

Future<String> future = executor.schedule(job);
Future<Integer> result = future.map( new Function<String, Integer>() {
  public Integer apply(String value) { return Integer.valueOf(value); }
});

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Building a Server in Java

When you create a server in Java, you have several options. You can create a server that processes requests synchronously or asynchronously. You must also choose an appropriate level of exception handling. In all cases, either a Future or an exception is returned.This section shows several techniques that are relevant for servers written in Java:

• Server Imports
• Performing Synchronous Operations
• Chaining Asynchronous Operations
• Invoking the Server

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Server Imports

As you write a server in Java, you will become familiar with the following packages and classes. Some netty classes are specifically related to HTTP. Most of the classes you will use are defined in the com.twitter.finagle and com.twitter.util packages.

import java.net.InetSocketAddress;

import org.jboss.netty.buffer.ChannelBuffers;
import org.jboss.netty.handler.codec.http.DefaultHttpResponse;
import org.jboss.netty.handler.codec.http.HttpRequest;
import org.jboss.netty.handler.codec.http.HttpResponse;
import org.jboss.netty.handler.codec.http.HttpResponseStatus;
import org.jboss.netty.handler.codec.http.HttpVersion;

import com.twitter.finagle.Service;
import com.twitter.finagle.builder.ServerBuilder;
import com.twitter.finagle.http.Http;
import com.twitter.util.Future;
import com.twitter.util.FutureEventListener;
import com.twitter.util.FutureTransformer;

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Performing Synchronous Operations

If your server can respond synchronously, you can use the following pattern to implement your service:

public class HTTPServer extends Service<HttpRequest, HttpResponse> {
  public Future<HttpResponse> apply(HttpRequest request) {
  // If I can generate the response synchronously, then I just do this.
  try {
    HttpResponse response = processRequest(request);
    return Future.value(response);
  } catch (MyException e) {
    return Future.exception(e);
  }

In this example, the try catch block causes the server to either return a response or an exception.

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Chaining Asynchronous Operations

In Java, you can chain multiple asynchronous operations by calling a Future object's transformedBy method. This is done by supplying a FutureTransformer object to a Future object's transformedBy method. You typically implement FutureTransformer's map method to perform the actual conversion, and FutureTransformer's handle method which is called when an exception occurs. The following example shows this pattern: Note: If you need to perform blocking operations, see: Implementing a Pool for Blocking Operations in Java

public class HTTPServer extends Service<HttpRequest, HttpResponse> {

  private Future<String> getContentAsync(HttpRequest request) {
    // asynchronously gets content, possibly by submitting
    // a function to a FuturePool
    ...
  }

  public Future<HttpResponse> apply(HttpRequest request) {

    Future<String> contentFuture = getContentAsync(request);
    return contentFuture.transformedBy(new FutureTransformer<String, HttpResponse>() {
      @Override
      public HttpResponse map(String content) {
        HttpResponse httpResponse =
          new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK);
        httpResponse.setContent(ChannelBuffers.wrappedBuffer(content.getBytes()));
          return httpResponse;
      }

      @Override
      public HttpResponse handle(Throwable throwable) {
        HttpResponse httpResponse =
          new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.SERVICE_UNAVAILABLE);
        httpResponse.setContent(ChannelBuffers.wrappedBuffer(throwable.toString().getBytes()));
        return httpResponse;
      }
    }
  });
}

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Invoking the Server

The following example shows the instantiation and invocation of the server. Calling the ServerBuilder's safeBuild method statically checks arguments to ServerBuilder, which prevents a runtime error if a required argument is missing:

  public static void main(String[] args) {
    ServerBuilder.safeBuild(new HTTPServer(),
                            ServerBuilder.get()
                                         .codec(Http.get())
                                         .name("HTTPServer")
                                         .bindTo(new InetSocketAddress("localhost", 8080)));

  }
}

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Building a Client in Java

When you create a client in Java, you have several options. You can create a client that processes responses synchronously or asynchronously. You must also choose an appropriate level of exception handling. This section shows several techniques that are relevant for clients written in Java:

• Client Imports
• Creating the Client
• Handling Synchronous Responses
• Handling Synchronous Responses With Timeouts
• Handling Synchronous Responses With Exception Handling
• Handling Asynchronous Responses

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Client Imports

As you write a client in Java, you will become familiar with the following packages and classes. Some netty classes are specifically related to HTTP. Most of the classes you will use are defined in the com.twitter.finagle and com.twitter.util packages.

import java.net.InetSocketAddress;
import java.util.concurrent.TimeUnit;

import org.jboss.netty.handler.codec.http.DefaultHttpRequest;
import org.jboss.netty.handler.codec.http.HttpMethod;
import org.jboss.netty.handler.codec.http.HttpRequest;
import org.jboss.netty.handler.codec.http.HttpResponse;
import org.jboss.netty.handler.codec.http.HttpVersion;

import com.twitter.finagle.Service;
import com.twitter.finagle.builder.ClientBuilder;
import com.twitter.finagle.http.Http;
import com.twitter.util.Duration;
import com.twitter.util.FutureEventListener;
import com.twitter.util.Throw;
import com.twitter.util.Try;

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Creating the Client

The following example shows the instantiation and invocation of a client. Calling the ClientBuilder's safeBuild method statically checks arguments to ClientBuilder, which prevents a runtime error if a required argument is missing:

public class HTTPClient {
  public static void main(String[] args) {
    Service<HttpRequest, HttpResponse> httpClient =
      ClientBuilder.safeBuild(
        ClientBuilder.get()
                     .codec(Http.get())
                     .hosts(new InetSocketAddress(8080))
                     .hostConnectionLimit(1));

Note: Choosing a value of 1 for hostConnectionLimit eliminates contention for a host.

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Handling Synchronous Responses

In the simplest case, you can wait for a response, potentially forever. Typically, you should handle both a valid response and an exception:

    HttpRequest request = new DefaultHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.GET, "/");

    try {
      HttpResponse response1 = httpClient.apply(request).apply();
    } catch (Exception e) {
        ...
    }

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Handling Synchronous Responses With Timeouts

To avoid waiting forever for a response, you can specify a duration, which throws an exception if the duration expires. The following example sets a duration of 1 second:

    try {
      HttpResponse response2 = httpClient.apply(request).apply(
        new Duration(TimeUnit.SECONDS.toNanos(1)));
    } catch (Exception e) {
        ...
    }

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Handling Synchronous Responses With Exception Handling

Use the Try and Throw classes in com.twitter.util to implement a more general approach to exception handling for synchronous responses. In addition to specifying a timeout duration, which can throw an exception, other exceptions can also be thrown.

    Try<HttpResponse> responseTry = httpClient.apply(request).get(
      new Duration(TimeUnit.SECONDS.toNanos(1)));
    if (responseTry.isReturn()) {
      // Cool, I have a response! Get it and do something
      HttpResponse response3 = responseTry.get();
      ...
    } else {
        // Throw an exception
      Throwable throwable = ((Throw)responseTry).e();
      System.out.println("Exception thrown by client: " +  throwable);
    }

Note: You must call the request's get method instead of the apply method to retrieve the Try object.

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Handling Asynchronous Responses

To handle asynchronous responses, you add a FutureEventListener to listen for a response. Finagle invokes the onSuccess method when a response arrives or invokes onFailure for an exception:

    httpClient.apply(request).addEventListener(new FutureEventListener<HttpResponse>() {
      @Override
      public void onSuccess(HttpResponse response4) {
        // Cool, I have a response, do something with it!
        ...
      }

      @Override
      public void onFailure(Throwable throwable) {
        System.out.println("Exception thrown by client: " +  throwable);
      }
    });
  }
}

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Implementing a Thread Pool for Blocking Operations in Java

To prevent blocking operations from executing on the main Finagle thread, you must wrap the blocking operation in a Scala closure and execute the closure on the Java thread that you create. Typically, your Java thread is part of a thread pool. The following sections show how to wrap your blocking operation, set up a thread pool, and execute the blocking operation on a thread in your pool:

• Wrapping the Blocking Operation
• Setting Up Your Thread Pool
• Invoking the Blocking Operation

Note: Jakob Homan provides an example implementation of a thread pool that executes Scala closures on GitHub.

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Wrapping the Blocking Operation

The Util project contains a Function0 class that represents a Scala closure. You can override the apply method to wrap your blocking operation:

public static class BlockingOperation extends com.twitter.util.Function0<Integer> {
  public Integer apply() {
    // Implement your blocking operation here
    ...
  }
}

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Setting Up Your Thread Pool

The following example shows a Thrift server that places the blocking operation defined in the Future0 object's apply method in the Java thread pool, where it will eventually execute and return a result:

public static class HelloServer implements Hello.ServiceIface {
  ExecutorService pool = Executors.newFixedThreadPool(4);                     // Java thread pool
  ExecutorServiceFuturePool futurePool = new ExecutorServiceFuturePool(pool); // Java Future thread pool

  public Future<Integer> blockingOperation() {
      Function0<Integer> blockingWork = new BlockingOperation();
    return futurePool.apply(blockingWork);
  }

  public static void main(String[] args) {
    Hello.ServiceIface processor = new Hello.ServiceIface();

    ServerBuilder.safeBuild(
      new Hello.Service(processor, new TBinaryProtocol.Factory()),
      ServerBuilder.get()
                   .name("HelloService")
                   .codec(ThriftServerFramedCodec.get())
                   .bindTo(new InetSocketAddress(8080))
      );
    )
  )
)

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Invoking the Blocking Operation

To invoke the blocking operation, you call the method that wraps your blocking operation and add an event listener that waits for either success or failure:

	  Service<ThriftClientRequest, byte[]> client = ClientBuilder.safeBuild(ClientBuilder.get()
	    .hosts(new InetSocketAddress(8080))
	    .codec(new ThriftClientFramedCodecFactory())
	    .hostConnectionLimit(100)); // Must be more than 1 to enable parallel execution

	  Hello.ServiceIface client =
	    new Hello.ServiceToClient(client, new TBinaryProtocol.Factory());

	  client.blockingOperation().addEventListener(new FutureEventListener<Integer>() {
	    public void onSuccess(Integer i) {
	      System.out.println(i);
	    }

	  public void onFailure(Throwable t) {
	    System.out.println("Exception! ", t.toString());
	  });

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Additional Samples

• Echo - A simple echo client and server using a newline-delimited protocol. Illustrates the basics of asynchronous control-flow.
• Http - An advanced HTTP client and server that illustrates the use of Filters to compositionally organize your code. Filters are used here to isolate authentication and error handling concerns.
• Memcached Proxy - A simple proxy supporting the Memcached protocol.
• Stream - An illustration of Channels, the abstraction for Streaming protocols.
• Spritzer 2 Kestrel - An illustration of Channels, the abstraction for Streaming protocols. Here the Twitter Firehose is "piped" into a Kestrel message queue, illustrating some of the compositionality of Channels.
• Stress - A high-throughput HTTP client for driving stressful traffic to an HTTP server. Illustrates more advanced asynchronous control-flow.
• Thrift - A simple client and server for a Thrift protocol.
• Kestrel Client - A client for doing reliable reads from one or more Kestrel servers.

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API Reference Documentation

• Builders
• Service
• Future
• Complete Core Project Scaladoc
• Kestrel
• Memcached
• Ostrich 4
• ServerSets
• Stream
• Thrift
• Complete Util Project Scaladoc

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We use Semantic Versioning for published artifacts.

Example Maven Project

Wondering how to get started with a finagle project of your very own? Here's a good place to start:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.myorg</groupId>
  <artifactId>myapp</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>myapp</name>

  <!-- Tell maven where to find finagle -->
  <repositories>
    <repository>
      <id>twitter</id>
      <url>http://maven.twttr.com/</url>
    </repository>
  </repositories>

  <dependencies>
    <!-- At the very least you will need finagle-core, and probably
         some other sub modules as well (see below) -->
    <dependency>
      <groupId>com.twitter</groupId>
      <artifactId>finagle-core</artifactId>
      <type>pom</type>
      <version>5.3.1</version>
    </dependency>

    <!-- Be sure to depend on the various finagle sub modules that you need.
         For example, here's how you would depend on finagle-thrift -->
    <dependency>
      <groupId>com.twitter</groupId>
      <artifactId>finagle-thrift</artifactId>
      <type>pom</type>
      <version>5.3.1</version>
    </dependency>
  </dependencies>
</project>

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