PolicyRetryAndCircuitBreaker.md

Supporting Retry and Circuit Breaker

Brighter is a Command Processor and supports a pipeline of Handlers to handle orthogonal requests.

Amongst the valuable uses of orthogonal requests is patterns to support Quality of Service in a distributed environment: Timeout, Retry, and Circuit Breaker.

Even if you don't believe that you are writing a distributed system that needs this protection, consider that as soon as you have multiple processes, such as a database server, you are distributed.

Brighter uses Polly to support Retry and Circuit-Breaker. Through our Russian Doll Model we are able to run the target handler in the context of a Policy Handler, that catches exceptions, and applies a Policy on how to deal with them.

Using Brighter's UsePolicy Attribute

By adding the UsePolicy attribute, you instruct the Command Processor to insert a handler (filter) into the pipeline that runs all later steps using that Polly policy.

internal class MyQoSProtectedHandler : RequestHandler<MyCommand>
{
    static MyQoSProtectedHandler()
    {
        ReceivedCommand = false;
    }

    [UsePolicy(policy: "MyExceptionPolicy", step: 1)]
    public override MyCommand Handle(MyCommand command)
    {
        /*Do work that could throw error because of distributed computing reliability*/
    }
}

To configure the Polly policy you use the PolicyRegistry to register the Polly Policy with a name. At runtime we look up that Policy by name.

var policyRegistry = new PolicyRegistry();

var policy = Policy
    .Handle<Exception>()
    .WaitAndRetry(new[]
    {
        1.Seconds(),
        2.Seconds(),
        3.Seconds()
    }, (exception, timeSpan) =>
    {
        s_retryCount++;
    });

policyRegistry.Add("MyExceptionPolicy", policy);

You can use multiple policies with a handler, instead of passing in a single policy identifier, you can pass in an array of policy identifiers:

So if in addition to the above policy we have:

var circuitBreakerPolicy = Policy.Handle<Exception>().CircuitBreaker(
		1, TimeSpan.FromMilliseconds(500));

policyRegistry.Add("MyCircuitBreakerPolicy", policy);

then you can add them both to your handler as follows:

internal class MyQoSProtectedHandler : RequestHandler<MyCommand>
{
    static MyQoSProtectedHandler()
    {
        ReceivedCommand = false;
    }

    [UsePolicy(new [] {"MyCircuitBreakerPolicy", "MyExceptionPolicy"} , step: 1)]
    public override MyCommand Handle(MyCommand command)
    {
        /*Do work that could throw error because of distributed computing reliability*/
    }
}

Where we have multiple policies they are evaluated left to right, so in this case "MyCircuitBreakerPolicy" wraps "MyExceptionPolicy".

When creating policies, refer to the Polly documentation.

Whilst Polly does not support a Policy that is both Circuit Breaker and Retry i.e. retry n times with an interval between each retry, and then break circuit, to implement that simply put a Circuit Breaker UsePolicy attribute as an earlier step than the Retry UsePolicy attribute. If retries expire, the exception will bubble out to the Circuit Breaker.

Timeout

You should not allow a handler that calls out to another process (e.g. a call to a Database, queue, or an API) to run without a timeout. If the process has failed, you will consumer a resource in your application polling that resource. This can cause your application to fail because another process failed.

Usually the client library you are using will have a timeout value that you can set.

In some scenarios the client library does not provide a timeout, so you have no way to abort.

We provide the Timeout attribute for that circumstance. You can apply it to a Handler to force that Handler into a thread which we will timeout, if it does not complete within the required time period.

public class EditTaskCommandHandler : RequestHandler<EditTaskCommand>
{
    private readonly ITasksDAO _tasksDAO;

    public EditTaskCommandHandler(ITasksDAO tasksDAO)
    {
        _tasksDAO = tasksDAO;
    }

    [RequestLogging(step: 1, timing: HandlerTiming.Before)]
    [Validation(step: 2, timing: HandlerTiming.Before)]
    [TimeoutPolicy(step: 3, milliseconds: 300)]
    public override EditTaskCommand Handle(EditTaskCommand editTaskCommand)
    {
        using (var scope = _tasksDAO.BeginTransaction())
        {
            Task task = _tasksDAO.FindById(editTaskCommand.TaskId);

            task.TaskName = editTaskCommand.TaskName;
            task.TaskDescription = editTaskCommand.TaskDescription;
            task.DueDate = editTaskCommand.TaskDueDate;

            _tasksDAO.Update(task);
            scope.Commit();
        }

        return editTaskCommand;
    }
}