Nunc Stans

Nunc Stans is an event framework wrapper that provides a thread pool for event callback execution. It provides thread safety to event frameworks by isolating and protecting the thread safe parts from the non-thread safe parts, and allows multi-threaded applications to use event frameworks that are not thread safe. It has been primarily developed using libevent , but has also been tested with tevent. Nunc Stans uses lock free data structures where possible, to avoid mutex contention. The ‚Äčliblfds library is used.

There are two main components: the event loop thread and queue, and the worker threads and queues. The basic concept is the Thread Pool Pattern, where the primary source of tasks (job in nunc stans) for the task queue (the work queue in nunc stans) is provided by the event framework for I/O, timer, and signal events.


Nunc Stans is licensed under the GNU General Public License version 3 or later. Nunc Stans also provides an exception for the use of OpenSSL. See the files 'COPYING', 'COPYING.openssl', and 'COPYING.liblfds' for more information.

Event Loop Thread And Queue

The event queue is essentially the "main loop" of the application. It runs in its own thread. The event queue thread is the only thread that interfaces with the event framework - adding events, removing events, and issuing the callbacks when the event is triggered. This guarantees that all interactions with the event framework are performed in a thread safe manner. When a threaded application wants to be called back because of some event (I/O, timer, signal), it posts the event and callback data to the event queue. All interaction with the event queue is thread safe - multiple threads can post requests to the event queue at the same time. The event loop thread dequeues all of the event requests from the event queue, creates/removes events, then calls the event waiting function of the event framework. This assumes the underlying event framework has a function that allows waiting for a single event - something like event_base_loop() in libevent, or tevent_loop_wait() in tevent.

When the application wants events to be triggered as soon as possible, but the event framework is waiting for very long lived events, the event queue has a persistent I/O listener called the event_q_wakeup_pipe. When the application adds an event, nunc-stans will write to the pipe, which will cause the event framework to immediately wake up and add the pending events, then do a thread yield to allow the event framework thread to execute.

When an event is triggered by I/O, timer, or signal, the event callback is called. The callback can either be run in the event loop thread, or can be handed off to the work queue for execution in a worker thread. The application uses the flag NS_JOB_THREAD to specify that a job will be executed in a worker thread.

NOTE: Jobs executed in the event loop thread don't need locking if they don't use resources shared with other threads. This corresponds to a single threaded app where all jobs are run inside the main loop and no locking is required. However, just as in that case, jobs run in the event loop thread must be very careful to execute very quickly and not block on I/O or other resources. This can lead to event starvation.

Worker Threads and Queues

When a job is placed on the work queue, it will be executed in a worker thread. The number of worker threads is specified when nunc stans is initialized. Each worker thread sleeps on a condition variable (e.g. pthread_cond_wait()). When a job is placed on the work queue, nunc stans will notify the condition variable, waking up one of the worker threads. This worker thread will dequeue the job from the work queue and execute it. The work queue is thread safe - the event loop thread can enqueue jobs at the same time as the worker threads dequeue jobs. Note that the worker threads only execute jobs which have the NS_JOB_THREAD flag. Jobs without this flag will be executed in the event loop thread.


Nunc Stans Diagram

Diagram Explanation

The solid thick lines represent the flow of data, typically an ns_job_t object. The small dotted lines represent the flow of the program, or the flow of control. In the case of the signal and notification events, these represent the program sending a signal or notification, but not yielding control. The thick dashed lines represent the flow of data and program i.e. a function that takes an ns_job_t object and is the primary program path. The event queue and the work queue are thread safe FIFO/queue objects. The bottom of the stack of ellipses is the tail and the top is the head, labeled "head". The shaded box labeled "event framework" is the event framework (e.g. libevent). The boxes that are partially in and partially outside of the event framework are functions that take nunc stans objects and convert them into the format used by the event framework. Note that the "add/remove event in framework" function will pass ownership of the job into the event framework, so that the event framework will opaquely own that data in the case of add events. The shaded box labeled "event loop callback" is called by the event framework for each triggered event. The event loop callback will either execute the job immediately (for non-threaded jobs) or queue the job on the work queue for execution by a worker thread (for threaded jobs - the NS_JOB_THREADED job flag).

The event loop thread and the worker threads are represented by large boxes. Everything in the box happens inside that thread. The boxes that are partly inside and partly outside represent functions (e.g. the functions to add/delete an event job) and data structures (the event queue, the wakeup fd) that are thread safe or are otherwise protected and can be accessed both from within and outside of the thread. Although the diagram shows only 1 worker thread, there will usually be more than one, and they all share the same work queue, which is thread safe.

The usual starting point is the application represented by the APP icon on the left side. The application will typically create a new event job (e.g. a network socket listener). The job will be handed off to the event queue for processing by the event loop thread. If this is not happening inside the event loop thread, the event framework will be notified. This is necessary because the event framework could be waiting for a very long time if there are no I/O or signals happening, or if the timer jobs are very long lived. This will wakeup the event framework immediately so that it will loop back around to process the events in the event queue. The event loop will dequeue all of the jobs from the event queue and perform the appropriate add/remove job in the event framework. This ensures that only the single event loop thread, not multiple threads, will interact with the event framework. Then the event framework will wait for events. Once an event is triggered, the event framework will iterate through all of the triggered events and call the event loop callback for each one. This callback will either execute the job immediately or add the job to the work queue for a worker thread. This will also signal the worker threads (e.g. something like pthread_cond_wait) to notify them that there is a new job for processing. Once all of the events are processed, the event loop goes back to the top to see if there are more events to process. The worker thread signal will typically wake up 1 of the worker threads, which will dequeue the job and execute it.

Note that the job callback is called both with the data (the ns_job_t object) and the program flow. This callback is entry point into the application. It is the responsibility of the callback to manage the ns_job_t object, either by calling ns_job_done to dispose of it safely, or by calling ns_job_rearm to "re-arm" the event. If the job is not a threaded job, it is executed in the event loop thread, and can block all other events from being processed, so great care must be taken not to perform any long running task or otherwise block the thread.