The transaction processor is where all the logic of your Sawtooth blockchain lives. You can think of it as analogous to "smart contracts" on other platforms, but it is a much more powerful concept. You can write a transaction processor to be as simple just setting keys to values, or as complex as entire application environments. For example, the Sawtooth Seth transaction processor is an entire Ethereum Virtual Machine which can run Solidity smart contracts right on a Sawtooth blockchain.
As with the other components, npm is used to manage dependencies and extensive Mocha/Chai unit tests are provided to guide your efforts. To begin simply run:
cd processor/
npm install
npm testThere are a number of tests to run, but the test runner will stop on the first failure. There is no UI to build here, so your entire job is to build out the transaction processor until you get all of the tests to pass one by one.
In order to start up the full processor, run docker-compose up as detailed in
the Using Docker section of the part-two README.
Before you start on building this section make sure you have reviewed the Sawtooth Curriculum, in particular the sections on global state, and the transaction processor tutorial. Also be familiar with the Design of the Cryptomoji app. Although it applies to the client too, most of the actual implementation of this design will happen here, in the transaction processor.
In particular you should understand the apply method.
This is where the majority of the work of processing a transaction happens. It
takes two special objects, txn and context, which contain all of the data
and methods you will need, and are explained in detail below.
When you are done handling a transaction, the apply method should terminate in
one of two ways. If everything went well, it returns a Promise. Otherwise, it
throws an InvalidTransaction error. This works just like throwing a regular
Error, but the Sawtooth SDK will be watching for invalid transaction errors
in particular, and uses them as a trigger to properly handle and report
transactions that are invalid.
The txn object is essentially the transaction sent from your client, though
for convenience, the header has been deserialized for you. This gives you
access to some useful information that may come in handy when executing the
Cryptomoji design:
txn.payload- The payload sent from your client, the first thing you should do is decode ittxn.header- All of the header information sent with your transactiontxn.header.signerPublicKey- The public key used to sign the transaction, i.e. the identity of your usertxn.signature- Already validated by the validator, but maybe useful if you were looking for seemingly random seed for some reason . . .
The context object contains methods for accessing and modifying blockchain
state. Each returns a Promise which will resolve when the state data is fetched
or modified. Depending on the use case, you may call many of these methods in a
long Promise chain, and one common pattern is to return this chain directly out
of the apply method.
Now, really there are only two methods we need to complete this sprint:
getState, and setState. Their use should be fairly clear from the examples
below:
context.getState([ '5f4d7600...', '5f4d7601...' ]).then(state => {
console.log(state);
// {
// '5f4d7600...': <Buffer 00 01 02>,
// '5f4d7601...': <Buffer 04 05 06>
// }
});context.setState({
'5f4d7600...': <Buffer ff fe fd>,
'5f4d7601...': <Buffer fc fb fa>
}).then(setAddress => {
console.log(state);
// [ '5f4d7600...', '5f4d7601...' ]
});Note that the addresses here have been truncated for readability. You must use full and valid state addresses for both methods.
Modules:
The transaction processor makes use of many of the same helper functions that the client does. You will need to encode/decode data and calculate state addresses largely identical to how the client does it. Feel free to just copy/paste useful code between the two modules.
In addition to those repeats, the processor has a getPrng function. This
takes a seed, and returns a pseudo-random number generator function. That
function will return a seemingly random integers, but two PRNGs that start
with the same seed, will always generate the same set of numbers. This will be
important for creating pseudo-random effects (like generating new cryptomoji),
that will still be deterministic. Every blockchain node validating these
transactions will get the exact same results.
Oh, and we've already done getPrng for you. You're welcome.
Module: handler.js
All transaction processors include at least one transaction handler, and that
transaction handler has one very important method: apply. Apply is called
every time a transaction payload needs to be executed. It is the central router
for your processor, decoding the payload, figuring out what to do with it, and
sending it off to get done. These tests cover the basics of that interaction:
decoding the payload and rejecting unknown actions.
Spec: Create Collection
From here on out the processor will not test specific stub functions, but behavior. You can build out your MojiHandler however you like, but it must be able to properly handle the various payloads outlined in the specifications. It must correctly reject invalid payloads, and write good encoded entities to state with valid payloads. Use the design specs and the failing tests as your guide, and build your transaction processor.
This particular test will expect your handler to be able to create new
collection appropriately. It should throw an InvalidTransaction error if the
signer already has a collection, and populate a new collection with three
pseudo-random cryptomoji. Both the collection and the cryptomoji must be
properly constructed objects, with the properties and values outlined in the
design.
Spec: Select Sire
You must be able to handle SELECT_SIRE actions appropriately. Validate that
the signer has a collection, and that the selected sire is a part of that
collection. After validation, create a new sire listing for the signer,
replacing any existing listings.
Spec: Breed Moji
You must be able to handle BREED_MOJI actions appropriately. Validate that
the signer has a collection, that the sire is listed appropriately, and that
the breeder belongs to the signer. If so, create a new pseudo-random moji with
genes based on its parents, and add it to the signer's collection.
If you made it this far, congratulations! You've built a distributed
application on Hyperledger Sawtooth. You should have a decent enough grasp of
the fundamentals to start designing and building your own apps. Over the next
two days you will delve further and explore concepts like multi-party
agreements and deleting state. There are a full suite of additional tests based
around trading cryptomoji between collections. In order to run them you must
remove the .skip from the wrapping describe block in modules 06 - 09 in the
tests/ directory. Use these tests and the specs from the part-two
README to guide your implementation: