cosmos-sdk/store/streaming/README.md

State Streaming Service

This package contains the constructors for the StreamingServices used to write state changes out from individual KVStores to a file or stream, as described in ADR-038 and defined in types/streaming.go. The child directories contain the implementations for specific output destinations.

Currently, a StreamingService implementation that writes state changes out to files is supported, in the future support for additional output destinations can be added.

The StreamingService is configured from within an App using the AppOptions loaded from the app.toml file:

# ...

[store]
# streaming is enabled if one or more streamers are defined
streamers = [
    # name of the streaming service, used by constructor
    "file"
]

[streamers]
[streamers.file]
    keys = ["list", "of", "store", "keys", "we", "want", "to", "expose", "for", "this", "streaming", "service"]
    write_dir = "path to the write directory"
    prefix = "optional prefix to prepend to the generated file names"

The store.streamers field contains a list of the names of the StreamingService implementations to employ which are used by ServiceTypeFromString to return the ServiceConstructor for that particular implementation:

listeners := cast.ToStringSlice(appOpts.Get("store.streamers"))
for _, listenerName := range listeners {
    constructor, err := ServiceTypeFromString(listenerName)
    if err != nil {
    	// handle error
    }
}

The streamers field contains a mapping of the specific StreamingService implementation name to the configuration parameters for that specific service.

The streamers.x.keys field contains the list of StoreKey names for the KVStores to expose using this service and is required by every type of StreamingService. In order to expose ALL KVStores, we can include * in this list. An empty list is equivalent to turning the service off.

Additional configuration parameters are optional and specific to the implementation. In the case of the file streaming service, the streamers.file.write_dir field contains the path to the directory to write the files to, and streamers.file.prefix contains an optional prefix to prepend to the output files to prevent potential collisions with other App StreamingService output files.

The ServiceConstructor accepts AppOptions, the store keys collected using streamers.x.keys, a BinaryMarshaller and returns a `StreamingService implementation.

The AppOptions are passed in to provide access to any implementation specific configuration options, e.g. in the case of the file streaming service the streamers.file.write_dir and streamers.file.prefix.

streamingService, err := constructor(appOpts, exposeStoreKeys, appCodec)
if err != nil {
    // handler error
}

The returned StreamingService is loaded into the BaseApp using the BaseApp's SetStreamingService method.

The Stream method is called on the service to begin the streaming process. Depending on the implementation this process may be synchronous or asynchronous with the message processing of the state machine.

bApp.SetStreamingService(streamingService)
wg := new(sync.WaitGroup)
quitChan := make(chan struct{})
streamingService.Stream(wg, quitChan)