Interactive Queries
Interactive Queries allow you to leverage the state of your application from outside your application. The Kafka Streams API enables your applications to be queryable.
The full state of your application is typically split across many distributed instances of your application, and across many state stores that are managed locally by these application instances.
There are local and remote components to interactively querying the state of your application.
Local state
An application instance can query the locally managed portion of the state and directly query its own local state stores. You can use the corresponding local data in other parts of your application code, as long as it doesn’t required calling the Kafka Streams API. Querying state stores is always read-only to guarantee that the underlying state stores will never be mutated out-of-band (e.g., you cannot add new entries). State stores should only be mutated by the corresponding processor topology and the input data it operates on. For more information, see Querying local state stores for an app instance.
Remote state
To query the full state of your application, you must connect the various fragments of the state, including:
- query local state stores
- discover all running instances of your application in the network and their state stores
- communicate with these instances over the network (e.g., an RPC layer)
Connecting these fragments enables communication between instances of the same app and communication from other applications for Interactive Queries. For more information, see Querying remote state stores for the entire app.
Kafka Streams natively provides all of the required functionality for interactively querying the state of your application, except if you want to expose the full state of your application via Interactive Queries. To allow application instances to communicate over the network, you must add a Remote Procedure Call (RPC) layer to your application (e.g., REST API).
This table shows the Kafka Streams native communication support for various procedures.
