http://www.opensips.org/html/docs/modules/devel/tm.html

TM module enables stateful processing of SIP transactions. The main use of stateful logic, which is costly in terms of memory and CPU, is some services inherently need state. For example, transaction-based accounting (module acc) needs to process transaction state as opposed to individual messages, and any kinds of forking must be implemented statefully. Other use of stateful processing is it trading CPU caused by retransmission processing for memory. That makes however only sense if CPUconsumption per request is huge. For example, if you want to avoid costly DNS resolution for every retransmission of a request to an unresolvable destination, use stateful mode. Then, only the initial message burdens server by DNS queries, subsequent retransmissions will be dropped and will not result in more processes blocked by DNS resolution. The price is more memory consumption and higher processing latency.

From user's perspective, the major function is t_relay(). It setup transaction state, absorb retransmissions from upstream, generate downstream retransmissions and correlate replies to requests.

In general, if TM is used, it copies clones of received SIP messages in shared memory. That costs the memory and also CPUtime (memcpys, lookups, shmem locks, etc.) Note that non-TM functions operate over the received message in private memory, that means that any core operations will have no effect on statefully processed messages after creating the transactional state. For example, calling record_route after t_relay is pretty useless, as the RR is added to privately held message whereas its TMclone is being forwarded.

TM is quite big and uneasy to program--lot of mutexes, shared memory access, malloc and free, timers--you really need to be careful when you do anything. To simplify TM programming, there is the instrument of callbacks. The callback mechanisms allow programmers to register their functions to specific event. See t_hooks.h for a list of possible events.

Other things programmers may want to know is UAC--it is a very simplistic code which allows you to generate your own transactions. Particularly useful for things like NOTIFYs or IM gateways. The UAC takes care of all the transaction machinery: retransmissions , FR timeouts, forking, etc. See t_uac prototype in uac.h for more details. Who wants to see the transaction result may register for a callback.