This manual is for an old version of Hazelcast Jet, use the latest stable version.

Cooperative multithreading is one of the core features of Jet and can be roughly compared to green threads. It is purely a library-level feature and does not involve any low-level system or JVM tricks; the Processor API is simply designed in such a way that the processor can do a small amount of work each time it is invoked, then yield back to the Jet engine. The engine manages a thread pool of fixed size and on each thread, the processors take their turn in a round-robin fashion.

The point of cooperative multithreading is much lower context-switching cost and precise knowledge of the status of a processor's input and output buffers, which determines its ability to make progress.

Processor instances are cooperative by default. The processor can opt out of cooperative multithreading by overriding isCooperative() to return false. Jet will then start a dedicated thread for it.


To maintain an overall good throughput, a cooperative processor must take care not to hog the thread for too long (a rule of thumb is up to a millisecond at a time). Jet's design strongly favors cooperative processors and most processors can and should be implemented to fit these requirements. The major exception are sources and sinks because they often have no choice but calling into blocking I/O APIs.