Messages and Queries

Synopsis

Msgs and Queries are the two primary objects handled by modules. Most of the core components defined in a module, like Msg services, keepers and Query services, exist to process messages and queries.

Pre-requisite Readings

• Introduction to Cosmos SDK Modules

Messages

Msgs are objects whose end-goal is to trigger state-transitions. They are wrapped in transactions, which may contain one or more of them.

When a transaction is relayed from the underlying consensus engine to the Cosmos SDK application, it is first decoded by BaseApp. Then, each message contained in the transaction is extracted and routed to the appropriate module via BaseApp's MsgServiceRouter so that it can be processed by the module's Msg service. For a more detailed explanation of the lifecycle of a transaction, click here.

Msg Services

Defining Protobuf Msg services is the recommended way to handle messages. A Protobuf Msg service should be created for each module, typically in tx.proto (see more info about conventions and naming). It must have an RPC service method defined for each message in the module.

Each Msg service method must have exactly one argument, which must implement the transaction.Msg interface, and a Protobuf response. The naming convention is to call the RPC argument Msg<service-rpc-name> and the RPC response Msg<service-rpc-name>Response. For example:

rpc Send(MsgSend) returns (MsgSendResponse);

See an example of a Msg service definition from x/bank module:

x/bank/proto/cosmos/bank/v1beta1/tx.proto

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See full example on GitHub

transaction.Msg Interface

transaction.Msg is an alias of proto.Message.

core/transaction/transaction.go

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Signers from the GetSigners() call is automated via a protobuf annotation. Read more about the signer field here.

x/bank/proto/cosmos/bank/v1beta1/tx.proto

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See full example on GitHub

If there is a need for custom signers then there is an alternative path which can be taken. A function which returns signing.CustomGetSigner for a specific message can be defined.

func ProvideBankSendTransactionGetSigners() signing.CustomGetSigner {
    // Extract the signer from the signature.
    signer, err := coretypes.LatestSigner(Tx).Sender(ethTx)
    if err != nil {
        return nil, err
    }

    // Return the signer in the required format.
    return [][]byte{signer.Bytes()}, nil
}

This can be provided to the application using depinject's Provide method in the application's app.go:

depinject.Provide(banktypes.ProvideBankSendTransactionGetSigners)

The Cosmos SDK uses Protobuf definitions to generate client and server code:

• MsgServer interface defines the server API for the Msg service and its implementation is described as part of the Msg services documentation.
• Structures are generated for all RPC request and response types.

A RegisterMsgServer method is also generated and should be used to register the module's MsgServer implementation in RegisterServices method from the AppModule interface.

In order for clients (CLI and gRPC-gateway) to have these URLs registered, the Cosmos SDK provides the function RegisterMsgServiceDesc(registry codectypes.InterfaceRegistry, sd *grpc.ServiceDesc) that should be called inside module's RegisterInterfaces method, using the proto-generated &_Msg_serviceDesc as *grpc.ServiceDesc argument.

Queries

A query is a request for information made by end-users of applications through an interface and processed by a full-node. A query is received by a full-node through its consensus engine and relayed to the application via the ABCI. It is then routed to the appropriate module via BaseApp's QueryRouter so that it can be processed by the module's query service (./04-query-services.md). For a deeper look at the lifecycle of a query, click here.

gRPC Queries

Queries should be defined using Protobuf services. A Query service should be created per module in query.proto. This service lists endpoints starting with rpc.

Here's an example of such a Query service definition:

proto/cosmos/auth/v1beta1/query.proto

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See full example on GitHub

As proto.Messages, generated Response types implement by default String() method of fmt.Stringer.

A RegisterQueryServer method is also generated and should be used to register the module's query server in the RegisterServices method from the AppModule interface.

Store Queries

Store queries query directly for store keys. They use clientCtx.QueryABCI(req abci.QueryRequest) to return the full abci.QueryResponse with inclusion Merkle proofs.

See following examples:

baseapp/abci.go

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See full example on GitHub