Updating event handlers to support an incoming runtime upgrade

Each event handler has to support all versions of a given event. If there is an incoming runtime upgrade and some existing event signature is going to change (for example, a new field will be added to Members.MemberRemarked event), you will need to:

• Update the joystream.jsonl file,
• Generate new types (src/types/events.ts) using make typegen,
• Adjust the event handler by adding support for the new version of the event.

Different versions of the same event are identified by the Joystream runtime spec version, which changes with each runtime upgrade.

Existing example: Members.MemberRemarked event

Let's use the existing Members.MemberRemarked event as an example:

export class MembersMemberRemarkedEvent {
    /* ... */

    get isV1000(): boolean {
        return this._chain.getEventHash('Members.MemberRemarked') === '455000da2c8f650044c433ea0fc69e39c5cb2db11e7a81e15e0fcba6f0757e16'
    }

    get asV1000(): [bigint, Uint8Array] {
        assert(this.isV1000)
        return this._chain.decodeEvent(this.event)
    }

    get isV2001(): boolean {
        return this._chain.getEventHash('Members.MemberRemarked') === '800e11437fa752c6c57a4245f54183c0c5c445b438324a6d5c2f2272b4bd0e2a'
    }

    get asV2001(): [bigint, Uint8Array, ([Uint8Array, bigint] | undefined)] {
        assert(this.isV2001)
        return this._chain.decodeEvent(this.event)
    }
}

As you can see, this particular event is currently supported in two versions: V1000 and V2001. You can use isV1000 and isV2001 getters to check the version of the event inside the event handler. After you check the version, you can use asV1000 and asV2001 to decode the event data in a type-safe way.

Important: You should always check the version of the event before decoding it! If you try to decode an event using asV1000 when the event is actually V2001, an error will be thrown!

• The V1000 is version of the event at runtime spec version 1000, which was the first spec version of the Joystream mainnet runtime. At the time, the event consisted of two fields: memberId (bigint) and remark (Uint8Array). Important: Note that since 1000 there was also a runtime upgrade to 1001, but since the event signature didn't change at 1001, V1000 represents the event at both 1000 and 1001 spec versions.
• The V2001 is version of the event at runtime spec version 2001, which was the Ephesus runtime upgrade. At the time, a new field was introduced in the Members.MemberRemarked event: Option<(AccountId, Balance)> (decoded to ([Uint8Array, bigint] | undefined)).

If you take a look at the current implementation of Members.MemberRemarked event handler, you will see that it supports both V1000 and V2001 versions of the event:

export async function processMemberRemarkedEvent({
  /* ... */
  event,
}: EventHandlerContext<'Members.MemberRemarked'>) {
  const [memberId, message, payment] = event.isV2001 ? event.asV2001 : event.asV1000
  /* ... */
}

In this case, array destructuring works, because payment is an optional field and will always be undefined when the event version is V1000. However, in some cases you may need to separate the handler into multiple if statements with distinct logic for each version of the event.

Possible future runtime upgrade scenario

Now let's imagine a scenario where the runtime is about to be upgraded to a spec version 3000 and the Members.MemberRemarked event signature is going to change again. To make things more complex, we won't be introducing a new field, but instead change the type of the existing payment field, so that the event signature will now look like this:

MemberRemarked(MemberId, Vec<u8>, Vec<(AccountId, Balance)>)

Where Vec<(AccountId, Balance)> will be a vector of payments, instead of a single optional payment.

In order to support this change in Orion, you will need to:

1. Update joystream.jsonl file. You can do that by running version 3000 of the Joystream runtime locally and then executing:

   npx squid-substrate-metadata-explorer --chain ws://localhost:9944 --output tmp.jsonl
   cat tmp.jsonl >> joystream.jsonl
   rm tmp.jsonl
   

   This will append the metadata associated with the new runtime spec version to the joystream.jsonl file.
2. Regenerate types using make typegen. This will update the src/types/events.ts file with the new class for Members.MemberRemarked event, including the new isV3000 and asV3000 getters.
3. Update processMemberRemarkedEvent event handler in src/mappings/membership/index.ts:

   export async function processMemberRemarkedEvent({
       /* ... */
       event,
   }: EventHandlerContext<'Members.MemberRemarked'>) {
       if (event.isV1000 || event.isV2001) {
           const [memberId, message, payment] = event.isV2001 ? event.asV2001 : event.asV1000
           /* ... */
       }
       if (event.isV3000) {
           const [memberId, message, payments] = event.asV3000
           /* ... */
       }
   }
   

The processor should now be able to handle all three versions of the event. Important: In case any other existing events were to change during a runtime upgrade to spec version 3000, you would need to update their corresponding handlers in a similar way!