cosmos-sdk/docs/building-modules/structure.md

Recommended Folder Structure

Structure

A typical Cosmos SDK module can be structured as follows:

x/{module}
├── client
│   ├── cli
│   │   ├── query.go
│   │   └── tx.go
│   └── rest
│       ├── query.go
│       └── tx.go
├── exported
│   └── exported.go
├── internal
│   ├── keeper
│   │   ├── invariants.go
│   │   ├── keeper.go
│   │   ├── ...
│   │   └── querier.go
│   └── types
│       ├── codec.go
│       ├── errors.go
│       ├── events.go
│       ├── expected_keepers.go
│       ├── genesis.go
│       ├── keys.go
│       ├── msgs.go
│       ├── params.go
│       ├── ...
│       └── querier.go
├── simulation
│   ├── decoder.go
│   ├── genesis.go
│   ├── operations.go
│   ├── params.go
│   └── proposals.go
├── abci.go
├── alias.go
├── genesis.go
├── handler.go
├── ...
└── module.go

• abci.go: The module's BeginBlocker and EndBlocker implementations (if any).
• alias.go: The module's exported types, constants, and variables. These are mainly to improve developer ergonomics by only needing to import a single package. Note, there is nothing preventing developers from importing other packages from the module (excludinginternal/) but it is recommended that alias.go have everything exposed that other modules may need. The majority of the exported values here will typically come from internal/ (see below).
• client/: The module's CLI and REST client functionality implementation and testing.
• exported/: The module's exported types -- typically type interfaces. If a module relies on other module keepers, it is expected to receive them as interface contracts through the expected_keepers.go (which are detailed below) design to avoid having a direct dependency on the implementing module. However, these contracts can define methods that operate on and/or return types that are specific to the contract's implementing module and this is where exported/ comes into play. Types defined here allow for expected_keepers.go in other modules to define contracts that use single canonical types. This pattern allows for code to remain DRY and also alleviates import cycle chaos.
• genesis.go: The module's genesis related business logic (e.g. InitGenesis). Note, genesis types are defined in internal/types.
• handler.go: The module's message handlers.
• internal/: The module's internal types and implementations. The purpose of this package is mainly two fold. First, it signals that this package is not intended to be used or imported anywhere outside the defining module. Secondly, it goes hand-in-hand with alias.go in that it allows public types and functions to be used internally while not being exposed outside to the outside world. This allows for greater freedom of development while maintaining API stability.
  • internal/types: The module's type definitions such as messages, KVStore keys, parameter types, and expected_keepers.go contracts.
  • internal/keeper: The module's keeper implementation along with any auxiliary implementations such as the querier and invariants.
• module.go: The module's implementation of the AppModule and AppModuleBasic interfaces.
• simulation/: The module's simulation package defines all the required functions used on the blockchain simulator: randomized genesis state, parameters, weigthed operations, proposal contents and types decoders.

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