Clarify the Height and ChainId types (ref: "epochs")

[adizere]

Crate

ibc

Summary

The Height type and the ChainId types are closely related: the version_number field in a height of a chain denotes the revision of that chain. The revision used to be called "epoch," then renamed to "version", but it seems that the name of this field will stabilize to "revision".

There's two things to improve/clarify on our side:

1. Our ChainId implementation is a bit messy and uses both the names of "version" and "epoch".

https://github.com/informalsystems/ibc-rs/blob/45ae7b86844c906570f7b16792ef4806ed45ab54/modules/src/ics24_host/identifier.rs#L17-L25

2. The field of type Height might soon evolve so we probably need to rename version_number into revision_number.

https://github.com/informalsystems/ibc-rs/blob/b641f794cff8e5f21c308bd419fe4c29f028fda6/modules/src/ics02_client/height.rs#L9-L12

Dependencies:

• sdk: Rename IBC Height's "Version" to a unique name for IBC cosmos-sdk#7709
• spec: https://github.com/cosmos/ics/issues/481

Problem Definition

Proposal

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[thanethomson]

It'd make things a lot easier just to name this epoch everywhere. It has a pretty clear meaning and is quite distinct as a variable name.

Right now (from what I've managed to find so far) it's referred to as:

1. revision_number
2. version
3. counter

depending on where it's being referenced.

[adizere]

I think using "epoch" as a field name in the Height type is not really an option, it would be more appropriate to align with the specs and the .proto and have the Height type consist of fields revision_number and revision_height. We did this alignment in this PR I think.

I'm not convinced using epoch in the ChainId type is a good idea. Does the term epoch originate from tendermint-go?

I think this issue refers to the problem that we use the notion of ChainId::epoch side-by-side with the notion of version and also the notion of revision_number. It seems like we could entirely eliminate both epoch and version and stick to revision_number. What do we lose then?