TNSDL

TNSDL stands for TeleNokia Specification and Description Language. TNSDL is based on the ITU-T SDL-88 language. It is used exclusively at Nokia Networks, primarily for developing applications for telephone exchanges. TNSDL is a general-purpose procedural programming language. It is especially well-suited for developing highly concurrent, distributed systems. It was originally designed for programming circuit switched exchanges. As the world shifted towards packet-switched and internet-based telecommunication, TNSDL turned out to be an excellent fit for developing internet servers, too. TNSDL is a very simple, easy-to-learn programming language. TNSDL is a strongly typed procedural programming language. Its basic capabilities are comparable to the C and Pascal languages. In TNSDL processes are created by the CREATE command. (It is somewhat similar to the POSIX fork or pthread_create commands.) The CREATE command creates either an operating system process or a cooperative task. The process model can be selected by configuration. The source code itself does not reflect which scheduling method is used. Still, to avoid certain race conditions, developers may need to be prepared for parallel execution. TNSDL explicitly supports critical sections to be marked in the code. In case of cooperative multitasking a program is scheduled as one operating system process. When a cooperative thread enters the state of waiting for asynchronous input, another thread of the program may run. The feature of TNSDL is the actor model. Processes are meant to be designed as event-driven finite state machines. Inter-process communication is done by asynchronous message passing. The OUTPUT command sends a message, while INPUT statements define the expected messages. Timers, from TNSDL perspective, are delayed messages. Just like ordinary messages, timer expiration is handled by the INPUT statement. The SET command starts and the RESET command cancels a timer. State machines can be optionally used, for example, to prevent accepting certain input messages at some stage of the processing.