Keyed State

Keyed state means each selected key has independent operator state.

stream.KeyBy(func(r types.Record) []byte {
    return []byte(customerID(r))
}).WithPartitions(8)

The selected key is both the routing key and the state key. Mailer writes it into Record.Key before stateful operators process the record.

Reduce state

Reduce uses ValueState("reduce").

For normal records, the state key is:

Record.Key

For windowed records, the state key is:

Record.Key/window_start/window_end

That makes the same customer get separate accumulators for different windows.

Window state

Window uses list-style state to buffer records per (key, window) until a watermark says the window can fire.

For large window buffers, Pebble state keeps those records on disk instead of growing Go heap usage.

Per-worker isolation

Every keyed worker gets cloned stateful operators. Each clone gets its own state backend owner id, such as worker-0, worker-1, and so on.

This matters for checkpoint restore: the checkpoint stores state per owner and restores it into the same worker clone before processing resumes.