messages-and-queries.md

Messages and Queries

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. {synopsis}

Pre-requisite Readings

• Introduction to SDK Modules {prereq}

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 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

Starting from v0.40, 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.

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

+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc1/proto/cosmos/bank/v1beta1/tx.proto#L10-L17

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);

sdk.Msg interface is a simplified version of the Amino LegacyMsg interface described below with only ValidateBasic() and GetSigners() methods. For backwards compatibility with Amino LegacyMsgs, existing LegacyMsg types should be used as the request parameter for service RPC definitions. Newer sdk.Msg types, which only support service definitions, should use canonical Msg... name.

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 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.

Legacy Amino LegacyMsgs

The following way of defining messages is deprecated and using Msg services is preferred.

Amino LegacyMsgs can be defined as protobuf messages. The messages definition usually includes a list of parameters needed to process the message that will be provided by end-users when they want to create a new transaction containing said message.

A LegacyMsg is typically accompanied by a standard constructor function, that is called from one of the module's interface. messages also need to implement the sdk.Msg interface:

+++ https://github.com/cosmos/cosmos-sdk/blob/4a1b2fba43b1052ca162b3a1e0b6db6db9c26656/types/tx_msg.go#L10-L33

It extends proto.Message and contains the following methods:

• Route() string: Name of the route for this message. Typically all messages in a module have the same route, which is most often the module's name.
• Type() string: Type of the message, used primarly in events. This should return a message-specific string, typically the denomination of the message itself.
• ValidateBasic() error: This method is called by BaseApp very early in the processing of the message (in both CheckTx and DeliverTx), in order to discard obviously invalid messages. ValidateBasic should only include stateless checks, i.e. checks that do not require access to the state. This usually consists in checking that the message's parameters are correctly formatted and valid (i.e. that the amount is strictly positive for a transfer).
• GetSignBytes() []byte: Return the canonical byte representation of the message. Used to generate a signature.
• GetSigners() []AccAddress: Return the list of signers. The SDK will make sure that each message contained in a transaction is signed by all the signers listed in the list returned by this method.

See an example implementation of a message from the gov module:

+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc1/x/gov/types/msgs.go#L77-L125

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 (./query-services.md). For a deeper look at the lifecycle of a query, click here.

gRPC Queries

Starting from v0.40, the prefered way to define queries is by 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:

+++ https://github.com/cosmos/cosmos-sdk/blob/d55c1a26657a0af937fa2273b38dcfa1bb3cff9f/proto/cosmos/auth/v1beta1/query.proto#L12-L23

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 SDK, there was usually no specific query object defined by module developers, contrary to messages. Instead, the 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 in order 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:

+++ https://github.com/cosmos/cosmos-sdk/blob/080fcf1df25ccdf97f3029b6b6f83caaf5a235e4/x/ibc/core/client/query.go#L36-L46

+++ https://github.com/cosmos/cosmos-sdk/blob/080fcf1df25ccdf97f3029b6b6f83caaf5a235e4/baseapp/abci.go#L722-L749

Next {hide}

Learn about Msg services {hide}