A Protobuf `Msg` service processes [messages](./messages-and-queries.md#messages). Protobuf `Msg` services are specific to the module in which they are defined, and only process messages defined within the said module. They are called from `BaseApp` during [`DeliverTx`](../core/baseapp.md#delivertx). {synopsis}
Each module should define a Protobuf `Msg` service, which will be responsible for processing requests (implementing `sdk.Msg`) and returning responses.
As further described in [ADR 031](../architecture/adr-031-msg-service.md), this approach has the advantage of clearly specifying return types and generating server and client code.
Protobuf generates a `MsgServer` interface based on a definition of `Msg` service. It is the role of the module developer to implement this interface, by implementing the state transition logic that should happen upon receival of each `sdk.Msg`. As an example, here is the generated `MsgServer` interface for `x/bank`, which exposes two `sdk.Msg`s:
When possible, the existing module's [`Keeper`](keeper.md) should implement `MsgServer`, otherwise a `msgServer` struct that embeds the `Keeper` can be created, typically in `./keeper/msg_server.go`:
Before a `msgServer` method is executed, the message's [`ValidateBasic()`](../basics/tx-lifecycle.md#ValidateBasic) method has already been called. Since `msg.ValidateBasic()` performs only the most basic checks, this stage must perform all other validation (both *stateful* and *stateless*) to make sure the `message` is valid. Checks performed in the `msgServer` method can be more expensive and the signer is charged gas for these operations.
For example, a `msgServer` method for a `transfer` message might check that the sending account has enough funds to actually perform the transfer.
It is recommended to implement all validation checks in a separate function that passes state values as arguments. This implementation simplifies testing. As expected, expensive validation functions charge additional gas. Example:
```go
ValidateMsgA(msg MsgA, now Time, gm GasMeter) error {
After the validation is successful, the `msgServer` method uses the [`keeper`](./keeper.md) functions to access the state and perform a state transition.
### Events
Before returning, `msgServer` methods generally emit one or more [events](../core/events.md) by using the `EventManager` held in the `ctx`. Use the new `EmitTypedEvent` function that uses protobuf-based event types:
These events are relayed back to the underlying consensus engine and can be used by service providers to implement services around the application. Click [here](../core/events.md) to learn more about events.
The invoked `msgServer` method returns a `proto.Message` response and an `error`. These return values are then wrapped into an `*sdk.Result` or an `error` using `sdk.WrapServiceResult(ctx sdk.Context, res proto.Message, err error)`:
* The [`LegacyMsg`](./messages-and-queries.md#messages) is the actual object being processed.
* The [`Context`](../core/context.md) contains all the necessary information needed to process the `msg`, as well as a branch of the latest state. If the `msg` is successfully processed, the branched version of the state contained in the `ctx` will be written to the main state (branch).
* The `*Result` returned to `BaseApp` contains (among other things) information on the execution of the `handler` and [events](../core/events.md).
Module `handler`s are typically implemented in a `./handler.go` file inside the module's folder. The [module manager](./module-manager.md) is used to add the module's `handler`s to the
[application's `router`](../core/baseapp.md#message-routing) via the `Route()` method. Typically,
the manager's `Route()` method simply constructs a Route that calls a `NewHandler()` method defined in `handler.go`.
In this regard, `handler`s functions need to be implemented for each module `LegacyMsg`. This will also involve manual handler registration of `LegacyMsg` types.
