[Discussion on migrate]

[99Kies]

Migrate is sparsely described in the documentation, and I'd like to know more details because I want my contract to support this method.

1. is migrate a compatible upgrade, a non-compatible upgrade, or can either mode be specified?
2. under what circumstances can an upgrade not be performed? Or are there any boundary conditions?
3. is there a better description and examples of migrate functionality?

links:
https://book.cosmwasm.com/actor-model/contract-as-actor.html?highlight=Migrate#migrations
https://docs.cosmwasm.com/core/entrypoints/migrate

[chipshort]

I don't know this distinction between compatible and non-compatible upgrade. Upgrading CosmWasm contracts works like this:
If you are the admin of the contract, you can change the underlying wasm file by sending a MsgMigrateContract to the chain. It does not need to have a migrate entrypoint. You can change all of the code of the contract, but the storage of the contract is untouched by this.
This can be a problem if you change the way the contract stores information (that's what the example in the docs page describes).
For that reason, the migrate entrypoint exists. When you send MsgMigrateContract, it calls that entrypoint of the new wasm file. Inside this you can run any arbitrary code, but most contracts load the state in the old format and save it in the new format again. You can think of it like a database migration.

About boundary conditions: When the migrate entrypoint exists on the new contract and returns an error, it stops the migration. Also all normal limits for entrypoints apply here, like block gas limit, recursion limits, etc.

One example is the burner contract. It just deletes all state entries and sends any funds to an address defined in the migrate message.

[99Kies]

Sorry I'm late. I thought cosmwasm's contract upgrade would have taken the old code approach into account. (Some ecosystem(like aptos) contract upgrades require that contract upgrades do not remove pre-existing contract methods and method entry parameters.) ), but as far as the docs and your reply and my practice are concerned, cosmwasm's contract upgrade is very simple, and provides a lot of room for developers to develop, so I think as long as it doesn't involve complex data migration, I shouldn't encounter any obstacle problems in contract upgrade. @chipshort Thanks!