Trident State

Trident includes high-level abstractions for managing persistent state in a topology. State management is fault tolerant: updates are idempotent when failures and retries occur. These properties can be combined to achieve exactly-once processing semantics. Implementing persistent state with the Storm core API would be more difficult.

Trident groups tuples into batches, each of which is given a unique transaction ID. When a batch is replayed, the batch is given the same transaction ID. State updates in Trident are ordered such that a state update for a particular batch will not take place until the state update for the previous batch is fully processed. This is reflected in Tridents State interface at the center of the state management API:

public interface State {
    void beginCommit(Long txid);
    void commit(Long txid);
}

When updating state, Trident informs the State implementation that a transaction is about to begin by calling beginCommit(), indicating that state updates can proceed. At that point the State implementation updates state as a batch operation. Finally, when the state update is complete, Trident calls the commit() method, indicating that the state update is ending. The inclusion of transaction ID in both methods allows the underlying implementation to manage any necessary rollbacks if a failure occurs.

Implementing Trident states against various data stores is beyond the scope of this document, but more information can be found in the Trident State documentation(https://storm.apache.org/releases/1.2.3/Trident-state.html).