ModuleEvent event_index definition for 0,1,2?

[gftea]

Discord user ID

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Describe your question in detail.

When exploring graphQl api for module events, what is the definition/meaning of event_index 0,1,2, how they are decided?
I run the hello world example, and try to query the module event, only 0 and 1 returns data, but 2 return null. I cannot find the definition of these index.

What error, if any, are you getting?

No response

What have you tried or looked at? Or how can we reproduce the error?

yes, use api https://cloud.hasura.io/public/graphiql?endpoint=https://api.testnet.aptoslabs.com/v1/graphql
While I also see documents in https://aptos.dev/concepts/events/#module-events, which just mentioned index 0,1,2, but does not explain how the index is decided.

Which operating system are you using?

macOS

Which SDK or tool are you using? (if any)

REST API

Describe your environment or tooling in detail

No response

[Alexzander-Stone]

Event index

The indices indicate the order of emitted events during the TX. After an event is emitted, the event index is incremented.

Hello World example

For your hello world example, the MessageChange event was emitted first and the TX ended with the FeeStatement event. Only two events were emitted.

So, when you query against this TX, you'll see the first event (MessageChange) as being index 0, and the second/last event (FeeStatement) as being index 1.

Other examples

Check out the docs-supplied example for a showcase of a TX with 4 events.