Relayer Engine

The Relayer Engine is a package meant to provide the structure and a starting point for a custom relayer.

With the Relayer Engine, a developer can write specific logic for filtering to receive only the messages they care about.

Once a wormhole message is received, the developer may apply additional logic to parse custom payloads or submit the VAA to one or many destination chains.

To use the Relayer engine, a developer may specify how to relay wormhole messages for their app using an idiomatic express/koa middleware inspired api then let the library handle all the details!

Checkout the quick start example here, or for a more advanced relayer app, see the advanced example

Quick Start

The source for this example is available here

Install Package

First, install the relayer-engine package with your favorite package manager

``sh
npm i @wormhole-foundation/relayer-engine
`

Start Background Processes

> note: These processes _must_ be running in order for the relayer app below to work

Next, we must start a Spy to listen for available VAAs published on the guardian network as well as a persistence layer, in this case we're using Redis.

More details about the Spy are available in the docs

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In order for our Relayer app to receive messages, a local Spy must be running that watches the guardian network. Our relayer app will receive updates from this Spy.

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> Note: While we're using Redis here, the persistence layer can be swapped out for some other db by implementing the appropriate interface.

A Redis instance must also be available to persist job data for fetching VAAs from the Spy.

`bash
docker run --rm -p 6379:6379 --name redis-docker -d redis
`

Simple Relayer Code Example

In the following example, we'll:

1. Set up a StandardRelayerApp, passing configuration options for our Relayer
2. Add a filter to capture only those messages our app cares about with a callback to do _something_ with the VAA once we've gotten it
3. Start the Relayer app

`ts
import {
Environment,
StandardRelayerApp,
StandardRelayerContext,
} from "@wormhole-foundation/relayer-engine";
import { CHAIN_ID_SOLANA } from "@certusone/wormhole-sdk";

(async function main() {
// initialize relayer engine app, pass relevant config options
const app = new StandardRelayerApp(
Environment.TESTNET,
// other app specific config options can be set here for things
// like retries, logger, or redis connection settings.
{
name: "ExampleRelayer",
},
);

// add a filter with a callback that will be
// invoked on finding a VAA that matches the filter
app.chain(CHAIN_ID_SOLANA).address(
// emitter address on Solana
"DZnkkTmCiFWfYTfT41X3Rd1kDgozqzxWaHqsw6W4x2oe",
// callback function to invoke on new message
async (ctx, next) => {
const vaa = ctx.vaa;
const hash = ctx.sourceTxHash;
console.log(
Got a VAA with sequence: ${vaa.sequence} from with txhash: ${hash},
);
},
);

// add and configure any other middleware ..

// start app, blocks until unrecoverable error or process is stopped
await app.listen();
})();
`

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The first meaningful line instantiates the StandardRelayerApp, which is a subclass of the RelayerApp with common defaults.

`ts
export class StandardRelayerApp<
ContextT extends StandardRelayerContext = StandardRelayerContext,
> extends RelayerApp {
// ...
constructor(env: Environment, opts: StandardRelayerAppOpts) {
`

The only field we pass in the StandardRelayerAppOpts is the name to help with identifying log messages and reserve a namespace in Redis.

The next meaningful line in the example adds a filter middleware component. This middleware will cause the Relayer app to request a subscription from the Spy for any VAAs that match the criteria and invoke the callback with the VAA.

If you'd like your program to subscribe to multiple chains and addresses, the same method can be called several times or the multiple helper can be used.

`ts
app.multiple(
{
[CHAIN_ID_SOLANA]: "DZnkkTmCiFWfYTfT41X3Rd1kDgozqzxWaHqsw6W4x2oe"
[CHAIN_ID_ETH]: ["0xabc1230000000...","0xdef456000....."]
},
myCallback
);
`

The last line in the simple example runs await app.listen(), which will start the relayer engine. Once started, the relayer engine will issue subscription requests to the spy and begin any other workflows (e.g. tracking missed VAAs).

This will run until the process is killed or it encounters an unrecoverable error. If you'd like to shut down the relayer gracefully, call app.stop()`.

Advanced Example

For a more advanced example that details other middleware and more complex configuration and actions including a built in UI, see the Advanced Tutorial