Prepare Proposal

PrepareProposal handles construction of the block, meaning that when a proposer is preparing to propose a block, it requests the application to evaluate a RequestPrepareProposal, which contains a series of transactions from CometBFT's mempool. At this point, the application has complete control over the proposal. It can modify, delete, and inject transactions from its own app-side mempool into the proposal or even ignore all the transactions altogether. What the application does with the transactions provided to it by RequestPrepareProposal has no effect on CometBFT's mempool.

Note, that the application defines the semantics of the PrepareProposal and it MAY be non-deterministic and is only executed by the current block proposer.

Now, reading mempool twice in the previous sentence is confusing, lets break it down. CometBFT has a mempool that handles gossiping transactions to other nodes in the network. The order of these transactions is determined by CometBFT's mempool, using FIFO as the sole ordering mechanism. It's worth noting that the priority mempool in Comet was removed or deprecated. However, since the application is able to fully inspect all transactions, it can provide greater control over transaction ordering. Allowing the application to handle ordering enables the application to define how it would like the block constructed.

The Cosmos SDK defines the DefaultProposalHandler type, which provides applications with PrepareProposal and ProcessProposal handlers. If you decide to implement your own PrepareProposal handler, you must ensure that the transactions selected DO NOT exceed the maximum block gas (if set) and the maximum bytes provided by req.MaxBytes.

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.1/baseapp/abci_utils.go

This default implementation can be overridden by the application developer in favor of a custom implementation in app_di.go:

prepareOpt := func(app *baseapp.BaseApp) {
    abciPropHandler := baseapp.NewDefaultProposalHandler(mempool, app)
    app.SetPrepareProposal(abciPropHandler.PrepareProposalHandler())
}

baseAppOptions = append(baseAppOptions, prepareOpt)