Every store implementation follows the Store interface.
On startup, the server creates a unique instance of the Store. The constructor of a store implementation can do some initialization and configuration check, but must not access, or attempt to recover, the storage at this point. This is important because when the server runs on Fault Tolerance mode, the storage must be shared across many servers but only one server can be using it.
After instantiating the store, the server will then call Recover() in order to recover the persisted state. For implementations that do not support persistence, such as the provided MemoryStore, this call will simply return nil (without error) to indicate that no state was recovered.
The Store is used to add/delete clients, create/lookup channels, etc...
Creating/looking up a channel will return a ChannelStore, which points to two other interfaces, the SubStore and MsgStore. These stores, for a given channel, handle subscriptions and messages respectively.
If you wish to contribute to a new store type, your implementation must include all these interfaces. For stores that allow recovery (such as file store as opposed to memory store), there are additional structures that have been defined and should be returned by Recover().
The memory and the provided file store implementations both use a generic store implementation to avoid code duplication. When writing your own store implementation, you can do the same for APIs that don't need to do more than what the generic implementation provides. You can check MemStore and FileStore implementations for more details.