flow-ethereum-event-watch

Flow Ethereum Event Watch

This CorDapp provides a simple example of how to watch (retrieve) Ethereum Events from Corda Flow.

A node starts event-watching by calling the StartEventWatchFlow. This creates a WatchState on the ledger. This WatchState has a scheduled activity to start the EventWatchFlow 10 seconds later.

When the EventWatchFlow runs, it consumes the existing WatchState. Then, the flow retrieves Ethereum Logs that contain Ethereum Events. If the Ethereum Event outputs the number which equals to searchId, the flow stops to produce WatchState; otherwise, the flow creates a new WatchState.

The new WatchState also has a scheduled activity to start the EventWatchFlow in 10 seconds.

After all, StartEventWatchFlow creates an endless chain of WatchStates that runs EventWatchFlow every 10 seconds as long as the flow cannot meet a certain condition.

Be aware that support of HTTP requests in flows is currently limited:

• The request must be executed in a BLOCKING way. Flows don't currently support suspending to await an HTTP call's response
• The request must be idempotent. If the flow fails and has to restart from a checkpoint, the request will also be replayed

Also, be aware that there is okhttp's dependency conflict between Corda Node v4 and web3j (later than 4.5.12).

Pre-requisites:

See https://docs.corda.net/getting-set-up.html.

Run ganache-cli

ganache-cli is a fast Ethereum RPC client for testing and development.

You can run ganache-cli and deploy sample Contract by following Minimal Ethereum Environment.

Create SmartContract Wrapper Class by web3j command

web3j truffle generate ../minimal-ethereum-env/build/contracts/SimpleStorage.json -o ./src/main/java -p jp.co.layerx.cordage.flowethereumeventwatch.ethWrapper

Usage

Running the nodes:

See https://docs.corda.net/tutorial-cordapp.html#running-the-example-cordapp.

Use the deployNodes task and ./build/nodes/runnodes script.

Interacting with the nodes:

Go to the CRaSH shell for ParticipantA, and run the StartEventWatchFlow with searchId:

flow start jp.co.layerx.cordage.flowethereumeventwatch.flow.StartEventWatchFlow searchId: 8

You can now start monitoring the node's flow activity...

flow watch

...you will see the EventWatch flow running every 10 seconds until you close the Flow Watch window using ctrl/cmd + c:

xxxxxxxx-xxxx-xxxx-xx Event Watch xxxxxxxxxxxxxxxxxxxx    Event Watched. (fromBlockNumber: x, toBlockNumber: xxxx)

...Or if aimed Ethereum Event was emitted on ethereum network, EventWatch flow will end with below log:

xxxxxxxx-xxxx-xxxx-xx Event Watch xxxxxxxxxxxxxxxxxxxx    Ethereum Event with id: xx watched and send TX Completed

You can send Ethereum tx to emit event using minimal ethereum env scripts.