Implements PUB-SUB for newHeads (eth_subscribe)

[colltoaction]

Now that we have WebSockets, we're able to do PUB-SUB. We focused on the newHeads notification because web3js:1.0 uses it when you call sendTransaction. With this feature, we're now able to run our integration test suite over WS.

• go-ethereum's spec: https://github.com/ethereum/go-ethereum/wiki/RPC-PUB-SUB
• parity's spec: https://wiki.parity.io/JSONRPC-Eth-Pub-Sub-Module.html

New RPC commands

• eth_subscribe, but only accepting newHeads parameter
• eth_unsubscribe

Implementation notes

jsonrpc4j dependency

This featured presented us with a hard scenario. The jsonrpc4j library wasn't extensible enough to easily support PUB-SUB. It abstracted away the network layer and only allowed a request-response interaction using reflection (Web3Impl classes)

new co.rsk.jsonrpc package

This package has many classes representing a generic JSON-RPC message. You will notice it doesn't have any dependencies on other co.rsk packages, it just models the specification: http://www.jsonrpc.org/specification

We use jackson to automatically decode incoming requests into objects of these classes, while performing strict validations on the input data according to the spec. We also use these classes to encode responses.

classes in co.rsk.rpc.modules.eth.subscribe package

These classes also model JSON-RPC commands (requests/responses/notifications), but specific to RSK. They are also used for encoding/decoding with strict validations.

RskJsonRpcHandler

This class is in the Netty pipeline just before the jsonrpc4j code. If it detects an eth_subscribe or eth_unsubscribe message, it intercepts and handles it. Otherwise, the preexisting handlers are executed.

Since jackson is able to decode to specific classes, we implement the Visitor pattern to easily dispatch specific commands. This could be extended to handle other commands and eventually replace jsonrpc4j and the Web3Impl classes if desired.

EthSubscriptionNotificationEmitter

Works along RskJsonRpcHandler. It basically stores subscriptions/handles unsubscriptions and listens to onBlock events to notify clients.

[diega]

This is a great move forward onto a working solution of eth_subscribe. I think we should work harder to abstract a lot of the Jackson magic occurring underneath, but I think this is fine for integrating into master as it's right now. I would start migrating other methods ASAP so we can have a real measure on how easy or not that task become.

[diega]

What about renaming it to JsonRpcIdentifiableMessage?

[diega]

Couldn't this boolean be resolved through polymorphism of the JsonRpcResultOrError hierarchy?

[diega]

I would think on renaming this, mostly to avoid consuming mind cycles when you search for the BlockHeader and most of the time you would mean org.ethereum.core.BlockHeader. What about BlockHeaderSubscription or BlockHeaderEvent?

[diega]

I would let JsonRpcSerializer to be an interface and the actual implementation to be JacksonBasedRpcSerializer. I'm not against injecting the ObjectMapper directly in the EthSubscriptionNotificationEmitter though

[aeidelman]

Approved

[rskops]

SonarQube analysis reported 2 issues

1. RskJsonRpcHandler.java#L92: Replace this "switch" statement by "if" statements to increase readability.
2. RskJsonRpcHandler.java#L64: Complete the task associated to this TODO comment.

[aeidelman]

Approved