Thread model

The thread model assumes that a query received by Tarantool via network is processed with three operating system threads:

1. The network thread (or threads) on the server side receives the query, parses the statement, checks if it is correct, and then transforms it into a special structure – a message containing an executable statement and its options.

2. The network thread sends this message to the instance's transaction processor thread (TX thread) via a lock-free message bus. Lua programs are executed directly in the transaction processor thread, and do not need to be parsed and prepared.

   The TX thread either uses a space index to find and update the tuple, or executes a stored function that performs a data operation.

3. The execution of the operation results in a message to the write-ahead logging (WAL) thread used to commit the transaction and the fiber executing the transaction is suspended. When the transaction results in a COMMIT or ROLLBACK, the following actions are taken:

   • The WAL thread responds with a message to the TX thread.
   • The fiber executing the transaction is resumed to process the result of the transaction.
   • The result of the fiber execution is passed to the network thread, and the network thread returns the result to the client.

The TX thread can handle many fibers – a set of computer instructions that can contain "yield" signals. The TX thread executes all computer instructions up to a yield signal, and then switches to execute the instructions of another fiber.

Yields must happen, otherwise the TX thread would be permanently stuck on the same fiber.

There are also several supplementary threads that serve additional capabilities:

• For replication, Tarantool creates a separate thread for each connected replica. This thread reads a write-ahead log and sends it to the replica, following its position in the log. Separate threads are required because each replica can point to a different position in the log and can run at different speeds.

• There is a thread pool for ad hoc asynchronous tasks, such as a DNS resolver or fsync.

• There is a thread pool that can be used for parallel sorting (hence, to parallelize building indexes). To configure it, use the memtx.sort_threads configuration option. The option sets the number of threads used to sort keys of secondary indexes on loading a memtx database.