Threads

The main purpose of liquidsoap is to create real time media streams. When streams are created, everything that is needed to compute them needs to happen very quickly so that we make sure that the stream can in fact be created in real time.

When a task is required that may take some time and whose result is not required for the stream generation, for instance when executing a on_stop or on_connect callback, it can be useful to execute this task in a thread.

Threads in liquidsoap are callback functions that are executed by an asynchronous queue. Here’s an example:

def connect_callback() =
  ignore(http.post("http://host/on_connect"))
end

thread.run(connect_callback)

By default, there are two type of queues available in liquidsoap:

• generic queues
• non_blocking queues

By convention, tasks that are known to be executing very fast should be sent to the non_blocking queues and all the other tasks should be sent to the generic queue.

You can decide which queue to send tasks to by using the queue parameter of the thread.run functions. Some other operators who also use threads can have a similar parameter such as thread_queue for request.dynamic and playlist.

You can also define your own named queue using the settings.scheduler.queues setting.

# Add 3 foo queue
settings.scheduler.queues.set([
   ...settings.scheduler.queues(),
   ("foo", 3)
])

def connect_callback() =
  ignore(http.post("http://host/on_connect"))
end

# Execute inside the foo queue
thread.run(queue="foo", connect_callback)

This is particularly useful for two applications:

• To control concurrent execution of specific tasks.
• To prevent deadlocks in case some tasks depends on other tasks.