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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

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 sdk.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:

proto/cosmos/bank/v1beta1/tx.proto
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sdk.Msg Interface​

sdk.Msg is a alias of proto.Message.

To attach a ValidateBasic() method to a message then you must add methods to the type adhereing to the HasValidateBasic.

types/tx_msg.go
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In 0.50+ signers from the GetSigners() call is automated via a protobuf annotation.

Read more about the signer field here.

proto/cosmos/bank/v1beta1/tx.proto
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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
}

When using dependency injection (depinject) this can be provided to the application via the provide method.

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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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.

Legacy Queries​

Before the introduction of Protobuf and gRPC in the Cosmos SDK, there was usually no specific query object defined by module developers, contrary to messages. Instead, the Cosmos SDK took the simpler approach of using a simple path to define each query. The path contains the query type and all the arguments needed to process it. For most module queries, the path should look like the following:

queryCategory/queryRoute/queryType/arg1/arg2/...

where:

  • queryCategory is the category of the query, typically custom for module queries. It is used to differentiate between different kinds of queries within BaseApp's Query method.
  • queryRoute is used by BaseApp's queryRouter to map the query to its module. Usually, queryRoute should be the name of the module.
  • queryType is used by the module's querier to map the query to the appropriate querier function within the module.
  • args are the actual arguments needed to process the query. They are filled out by the end-user. Note that for bigger queries, you might prefer passing arguments in the Data field of the request req instead of the path.

The path for each query must be defined by the module developer in the module's command-line interface file.Overall, there are 3 mains components module developers need to implement in order to make the subset of the state defined by their module queryable:

  • A querier, to process the query once it has been routed to the module.
  • Query commands in the module's CLI file, where the path for each query is specified.
  • query return types. Typically defined in a file types/querier.go, they specify the result type of each of the module's queries. These custom types must implement the String() method of fmt.Stringer.

Store Queries​

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

See following examples:

baseapp/abci.go
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