Threading Model

Your application server has its own threads. Hazelcast does not use these - it manages its own threads.

I/O Threading

Hazelcast uses a pool of threads for I/O. A single thread does not do all the I/O: instead, multiple threads do the I/O. On each cluster member, the I/O threading is split up in 3 types of I/O threads:

  • I/O thread that takes care of accept requests,
  • I/O threads that take care of reading data from other members/clients,
  • I/O threads that take care of writing data to other members/clients.

You can configure the number of I/O threads using the system property. Its default value is 3 per member. This means that if 3 is used, in total there are 7 I/O threads; 1 accept I/O thread, 3 read I/O threads, and 3 write I/O threads. Each I/O thread has its own Selector instance and waits on if there is nothing to do.

Hazelcast periodically scans utilization of each I/O thread and can decide to migrate a connection to a new thread if the existing thread is servicing disproportionate number of I/O events. You can customize the scanning interval by configuring the system property. The default interval is 20 seconds. You can disable the balancing process by setting the property to a negative value.

In case of the read I/O thread, when sufficient bytes for a packet have been received, the Packet object is created. This Packet is then sent to the System where it is de-multiplexed. If the Packet header signals that it is an operation/response, the Packet is handed over to the operation service (please see the Operation Threading section). If the Packet is an event, it is handed over to the event service (please see the Event Threading section).