A Protobuf Msg service processes 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. {synopsis}
- Module Manager {prereq}
- Messages and Queries {prereq}
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, 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.Msgs:
+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc3/x/bank/types/tx.pb.go#L285-L291
When possible, the existing module's Keeper should implement MsgServer, otherwise a msgServer struct that embeds the Keeper can be created, typically in ./keeper/msg_server.go:
+++ https://github.com/cosmos/cosmos-sdk/blob/v0.46.0-beta2/x/bank/keeper/msg_server.go#L14:L16
msgServer methods can retrieve the sdk.Context from the context.Context parameter method using the sdk.UnwrapSDKContext:
+++ https://github.com/cosmos/cosmos-sdk/blob/v0.46.0-beta2/x/bank/keeper/msg_server.go#L27-L28
sdk.Msg processing usually follows these 3 steps:
Before a msgServer method is executed, the message's 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:
ValidateMsgA(msg MsgA, now Time, gm GasMeter) error {
if now.Before(msg.Expire) {
return sdkerrrors.ErrInvalidRequest.Wrap("msg expired")
}
gm.ConsumeGas(1000, "signature verification")
return signatureVerificaton(msg.Prover, msg.Data)
}After the validation is successful, the msgServer method uses the keeper functions to access the state and perform a state transition.
Before returning, msgServer methods generally emit one or more events by using the EventManager held in the ctx. Use the new EmitTypedEvent function that uses protobuf-based event types:
ctx.EventManager().EmitTypedEvent(
&group.EventABC{Key1: Value1, Key2, Value2})or the older EmitEvent function:
ctx.EventManager().EmitEvent(
sdk.NewEvent(
eventType, // e.g. sdk.EventTypeMessage for a message, types.CustomEventType for a custom event defined in the module
sdk.NewAttribute(key1, value1),
sdk.NewAttribute(key2, value2),
),
)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 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):
+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc2/baseapp/msg_service_router.go#L104-L104
This method takes care of marshaling the res parameter to protobuf and attaching any events on the ctx.EventManager() to the sdk.Result.
This diagram shows a typical structure of a Protobuf Msg service, and how the message propagates through the module.
The handler type defined in the Cosmos SDK will be deprecated in favor of Msg Services.
Here is the typical structure of a handler function:
+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc2/types/handler.go#L4-L4
Let us break it down:
- The
LegacyMsgis the actual object being processed. - The
Contextcontains all the necessary information needed to process themsg, as well as a branch of the latest state. If themsgis successfully processed, the branched version of the state contained in thectxwill be written to the main state (branch). - The
*Resultreturned toBaseAppcontains (among other things) information on the execution of thehandlerand events.
Module handlers are typically implemented in a ./handler.go file inside the module's folder. The module manager is used to add the module's handlers to the
application's router via the Route() method. Typically,
the manager's Route() method simply constructs a Route that calls a NewHandler() method defined in handler.go.
NewHandler function dispatches a LegacyMsg to appropriate handler function, usually by using a switch statement:
First, NewHandler function sets a new EventManager to the context to isolate events per msg.
Then, a simple switch calls the appropriate handler based on the LegacyMsg type.
In this regard, handlers functions need to be implemented for each module LegacyMsg. This will also involve manual handler registration of LegacyMsg types.
handlers functions should return a *Result and an error.
New telemetry metrics can be created from msgServer methods when handling messages.
This is an example from the x/auth/vesting module:
+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc1/x/auth/vesting/msg_server.go#L73-L85
Learn about query services {hide}