cosmos-sdk/docs/building-modules/module-interfaces.md

Module Interfaces

This document details how to build CLI and REST interfaces for a module. Examples from various SDK modules are included. {synopsis}

Pre-requisite Readings

• Building Modules Intro {prereq}

CLI

One of the main interfaces for an application is the command-line interface. This entrypoint adds commands from the application's modules to let end-users create messages and queries. The CLI files are typically found in the module's ./client/cli folder.

Transaction Commands

Transactions are created by users to wrap messages that trigger state changes when they get included in a valid block. Transaction commands typically have their own tx.go file in the module's ./client/cli folder. The commands are specified in getter functions and include the name of the command.

Here is an example from the auth module:

+++ 64b6bb5270/x/auth/client/cli/tx_sign.go (L160-L194)

This getter function creates the command for the Sign transaction. It does the following:

• Construct the command: Read the Cobra Documentation for details on how to create commands.
  • Use: Specifies the format of a command-line entry users should type in order to invoke this command. In this case, the user uses buy-name as the name of the transaction command and provides the name the user wishes to buy and the amount the user is willing to pay.
  • Args: The number of arguments the user provides, in this case exactly two: name and amount.
  • Short and Long: A description for the function is provided here. A Short description is expected, and Long can be used to provide a more detailed description when a user uses the --help flag to ask for more information.
  • RunE: Defines a function that can return an error, called when the command is executed. Using Run would do the same thing, but would not allow for errors to be returned.
• RunE Function Body: The function should be specified as a RunE to allow for errors to be returned. This function encapsulates all of the logic to create a new transaction that is ready to be relayed to nodes.
  • The function should first get the clientCtx with client.GetClientContextFromCmd(cmd) and client.ReadTxCommandFlags(clientCtx, cmd.Flags()). This context contains all the information provided by the user and will be used to transfer this user-specific information between processes. To learn more about how contexts are used in a transaction, click here.
  • If applicable, the command's arguments are parsed.
  • If applicable, the Context is used to retrieve any parameters such as the transaction originator's address to be used in the transaction. Here, the from address is retrieved by calling clientCtx.GetFromAddress().
  • A message is created using all parameters parsed from the command arguments and Context. The constructor function of the specific message type is called directly. It is good practice to call ValidateBasic() on the newly created message to run a sanity check and check for invalid arguments.
  • Depending on what the user wants, the transaction is either generated offline or signed and broadcasted to the preconfigured node using GenerateOrBroadcastMsgs().
• Flags. Add any flags to the command. All transaction commands have flags to provide additional information from the user (e.g. amount of fees they are willing to pay). These persistent transaction flags can be added to a higher-level command so that they apply to all transaction commands.

Finally, the module needs to have a GetTxCmd(), which aggregates all of the transaction commands of the module. Often, each command getter function has its own file in the module's cli folder, and a separate tx.go file contains GetTxCmd(). Application developers wishing to include the module's transactions will call this function to add them as subcommands in their CLI. Here is the auth GetTxCmd() function, which adds the Sign, MultiSign, ValidateSignatures and SignBatch commands.

+++ 351192aa0b/x/auth/client/cli/tx.go (L10-L26)

An application using this module likely adds auth module commands to its root TxCmd command by calling txCmd.AddCommand(authModuleClient.GetTxCmd()).

Query Commands

Queries allow users to gather information about the application or network state; they are routed by the application and processed by the module in which they are defined. Query commands typically have their own query.go file in the module's ./client/cli folder. Like transaction commands, they are specified in getter functions. Here is an example of a query command from the auth module:

+++ d55c1a2665/x/auth/client/cli/query.go (L76-L108)

This query returns the account at a given address. The getter function does the following:

• Construct the command. Read the Cobra Documentation and the transaction command example above for more information. The user must type account and provide the address they are querying for as the only argument.
• RunE. The function should be specified as a RunE to allow for errors to be returned. This function encapsulates all of the logic to create a new query that is ready to be relayed to nodes.
  • The function should first initialize a new client Context as described in the previous section.
  • If applicable, the Context is used to retrieve any parameters (e.g. the query originator's address to be used in the query) and marshal them with the query parameter type, in preparation to be relayed to a node. There are no Context parameters in this case because the query does not involve any information about the user.
  • A new queryClient should be initialized using NewQueryClient(clientCtx), this method being generated from query.proto. Then it can be used to call the appropriate query.
  • The clientCtx.PrintProto method is used to format a proto.Message object and print it back to the user.
• Flags. Add any flags to the command.

Finally, the module also needs a GetQueryCmd, which aggregates all of the query commands of the module. Application developers wishing to include the module's queries will call this function to add them as subcommands in their CLI. Its structure is identical to the GetTxCmd command shown above.

Flags

Flags are entered by the user and allow for command customizations. Examples include the fees or gas prices users are willing to pay for their transactions.

The flags for a module are typically found in a flags.go file in the module's ./client/cli folder. Module developers can create a list of possible flags including the value type, default value, and a description displayed if the user uses a help command. In each transaction getter function, they can add flags to the commands and, optionally, mark flags as required so that an error is thrown if the user does not provide values for them.

For full details on flags, visit the Cobra Documentation.

For example, the SDK ./client/flags package includes a AddTxFlagsToCmd(cmd *cobra.Command) function that adds necessary flags to a transaction command, such as the from flag to indicate which address the transaction originates from.

+++ cfb5fc03e5/client/flags/flags.go (L85-L112)

Here is an example of how to add a flag using the from flag from this function.

cmd.Flags().String(FlagFrom, "", "Name or address of private key with which to sign")

The input provided for this flag - called FlagFrom is a string with the default value of "" if none is provided. If the user asks for a description of this flag, the description will be printed.

A flag can be marked as required so that an error is automatically thrown if the user does not provide a value:

cmd.MarkFlagRequired(FlagFrom)

Since AddTxFlagsToCmd(cmd *cobra.Command) includes all of the basic flags required for a transaction command, module developers may choose not to add any of their own (specifying arguments instead may often be more appropriate).

Similarly, there is a AddQueryFlagsToCmd(cmd *cobra.Command) to add common flags to a module query command.

gRPC

gRPC is the prefered way for external clients like wallets and exchanges to interact with a node.

In addition to providing an ABCI query pathway, modules custom queries can provide a GRPC proxy server that routes requests in the GRPC protocol to ABCI query requests under the hood.

In order to do that, modules should implement RegisterGRPCGatewayRoutes(clientCtx client.Context, mux *runtime.ServeMux) on AppModuleBasic to wire the client gRPC requests to the correct handler inside the module.

Here's an example from the auth module:

+++ 64b6bb5270/x/auth/module.go (L69-L72)

gRPC-gateway REST

Applications typically support web services that use HTTP requests (e.g. a web wallet like Lunie.io). Thus, application developers can also use REST Routes to route HTTP requests to the application's modules; these routes will be used by service providers.

grpc-gateway translates REST calls into gRPC calls, which might be useful for clients that do not use gRPC.

Modules that want to expose REST queries should add google.api.http annotations to their rpc methods, such as in the example below from the auth module:

// Query defines the gRPC querier service.
service Query{
    // Account returns account details based on address.
    rpc Account (QueryAccountRequest) returns (QueryAccountResponse) {
      option (google.api.http).get = "/cosmos/auth/v1beta1/accounts/{address}";
    }

    // Params queries all parameters.
    rpc Params (QueryParamsRequest) returns (QueryParamsResponse) {
      option (google.api.http).get = "/cosmos/auth/v1beta1/params";
    }
}

gRPC gateway is started in-process along with the application and Tendermint. It can be enabled or disabled by setting gRPC Configuration enable in app.toml.

The SDK provides a command for generating Swagger documentation (protoc-gen-swagger). Setting swagger in app.toml defines if swagger documentation should be automatically registered.

Legacy REST

Legacy REST endpoints will be deprecated. But developers may choose to keep using legacy REST endpoints for backward compatibility, although the recommended way is to use gRPC and gRPC-gateway.

With this implementation, module developers need to define the REST client by defining routes for all possible requests and handlers for each of them. It's up to the module developer how to organize the REST interface files; there is typically a rest.go file found in the module's ./client/rest folder.

To support HTTP requests, the module developer needs to define possible request types, how to handle them, and provide a way to register them with a provided router.

Request Types

Request types, which define structured interactions from users, must be defined for all transaction requests. Users using this method to interact with an application will send HTTP Requests with the required fields in order to trigger state changes in the application. Conventionally, each request is named with the suffix Req, e.g. SendReq for a Send transaction. Each struct should include a base request baseReq, the name of the transaction, and all the arguments the user must provide for the transaction.

Here is an example of a request to send coins from the bank module:

+++ 7f59723d88/x/bank/client/rest/tx.go (L15-L19)

The BaseReq includes basic information that every request needs to have, similar to required flags in a CLI. All of these values, including GasPrices and AccountNumber, will be provided in the request body. The user will also need to specify the argument Amount fields in the body.

BaseReq

BaseReq is a type defined in the SDK that encapsulates much of the transaction configurations similar to CLI command flags. Users must provide the information in the body of their requests.

• From indicates which account the transaction originates from. This account is used to sign the transaction.
• Memo sends a memo along with the transaction.
• ChainID specifies the unique identifier of the blockchain the transaction pertains to.
• AccountNumber is an identifier for the account.
• Sequenceis the value of a counter measuring how many transactions have been sent from the account. It is used to prevent replay attacks.
• TimeoutHeight allows a transaction to be rejected if it's committed at a height greater than the timeout.
• Gas refers to how much gas, which represents computational resources, a transaction consumes. Gas is dependent on the transaction and is not precisely calculated until execution, but can be estimated by providing auto as the value for Gas.
• GasAdjustment can be used to scale gas up in order to avoid underestimating. For example, users can specify their gas adjustment as 1.5 to use 1.5 times the estimated gas.
• GasPrices specifies how much the user is willing pay per unit of gas, which can be one or multiple denominations of tokens. For example, --gas-prices=0.025uatom, 0.025upho means the user is willing to pay 0.025uatom AND 0.025upho per unit of gas.
• Fees specifies how much in fees the user is willing to pay in total. Note that the user only needs to provide either gas-prices or fees, but not both, because they can be derived from each other.
• Simulate instructs the application to ignore gas and simulate the transaction running without broadcasting.

Request Handlers

Request handlers must be defined for both transaction and query requests. Handlers' arguments include a reference to the client Context.

Here is an example of a request handler for the bank module SendReq request (the same one shown above):

+++ 7f59723d88/x/bank/client/rest/tx.go (L21-L51)

The request handler can be broken down as follows:

• Parse Request: First, the request handler tries to parse the argument address into a AccountAddress. The request handler then attempts to parse the request, followed by running Sanitize and ValidateBasic on the underlying BaseReq to check the validity of the request. Finally, the request handler attempts to parse BaseReq.From to the type AccountAddress.
• Message: After parsing the request, a message of type MsgSend is created from the values, which is defined by the module developer to trigger the state changes for this transaction.
• Generate Transaction: Finally, the client Context, the HTTP ResponseWriter, the request's BaseReq, and the message are all passed to WriteGeneratedTxResponse to further process the request.

To read more about how a transaction is generated, visit the transactions documentation here.

Register Routes

The application CLI entrypoint will have a RegisterRoutes function in its main.go file, which calls the registerRoutes functions of each module utilized by the application. Module developers need to implement registerRoutes for their modules so that applications are able to route messages and queries to their corresponding handlers and queriers.

The router used by the SDK is Gorilla Mux. The router is initialized with the Gorilla Mux NewRouter() function. The router's HandleFunc function can then be used to route urls with the defined request handlers and the HTTP method (e.g. "POST", "GET") as a route matcher. It is recommended to prefix every route with the name of the module to avoid collisions with other modules that have the same query or transaction names.

Here is a registerRoutes function with one query route example from the nameservice tutorial:

func RegisterRoutes(cliCtx client.Context, r *mux.Router, cdc *codec.LegacyAmino, storeName string) {
  // ResolveName Query
  r.HandleFunc(fmt.Sprintf("/%s/names/{%s}", storeName, restName), resolveNameHandler(cdc, cliCtx, storeName)).Methods("GET")
}

A few things to note:

• The router r has already been initialized by the application and is passed in here as an argument - this function is able to add on the nameservice module's routes onto any application's router. The application must also provide a Context that the querier will need to process user requests and the application codec for encoding and decoding application-specific types.
• "/%s/names/{%s}", storeName, restName is the url for the HTTP request. storeName is the name of the module, restName is a variable provided by the user to specify what kind of query they are making.
• resolveNameHandler is the query request handler defined by the module developer. It also takes the application codec and Context passed in from the user side, as well as the storeName.
• "GET" is the HTTP Request method. As to be expected, queries are typically GET requests. Transactions are typically POST and PUT requests.

Next {hide}

Read about the recommended module structure {hide}