Thread Synchronisation

Once you have become sufficiently advanced live coding with a number of functions and threads simultaneously, you’ve probably noticed that it’s pretty easy to make a mistake in one of the threads which kills it. That’s no big deal, because you can easily restart the thread by hitting Run. However, when you restart the thread it is now out of time with the original threads.

Inherited Time

As we discussed earlier, new threads created with in_thread inherit all of the settings from the parent thread. This includes the current time. This means that threads are always in time with each other when started simultaneously.

However, when you start a thread on its own it starts with its own time which is unlikely to be in sync with any of the other currently running threads.

Cue and Sync

Sonic Pi provides a solution to this problem with the functions cue and sync.

cue allows us to send out heartbeat messages to all other threads. By default the other threads aren’t interested and ignore these heartbeat messages. However, you can easily register interest with the sync function.

The important thing to be aware of is that sync is similar to sleep in that it stops the current thread from doing anything for a period of time. However, with sleep you specify how long you want to wait while with sync you don’t know how long you will wait - as sync waits for the next cue from another thread which may be soon or a long time away.

Let’s explore this in a little more detail:

in_thread do
  loop do
    cue :tick
    sleep 1
  end
end
in_thread do
  loop do
    sync :tick
    sample :drum_heavy_kick
  end
end

Here we have two threads - one acting like a metronome, not playing any sounds but sending out :tick heartbeat messages every beat. The second thread is synchronising on tick messages and when it receives one it inherits the time of the cue thread and continues running.

As a result, we will hear the :drum_heavy_kick sample exactly when the other thread sends the :tick message, even if the two threads didn’t start their execution at the same time:

in_thread do
  loop do
    cue :tick
    sleep 1
  end
end
sleep(0.3)
in_thread do
  loop do
    sync :tick
    sample :drum_heavy_kick
  end
end

That naughty sleep call would typically make the second thread out of phase with the first. However, as we’re using cue and sync, we automatically sync the threads bypassing any accidental timing offsets.

Cue Names

You are free to use whatever name you’d like for your cue messages - not just :tick. You just need to ensure that any other threads are syncing on the correct name - otherwise they’ll be waiting for ever (or at least until you press the Stop button).

Let’s play with a few cue names:

in_thread do
  loop do 
    cue [:foo, :bar, :baz].choose
    sleep 0.5
  end
end
in_thread do
  loop do 
    sync :foo 
    sample :elec_beep
  end
end
in_thread do
  loop do
    sync :bar
    sample :elec_flip
  end
end
in_thread do
  loop do
    sync :baz
    sample :elec_blup
  end
end

Here we have a main cue loop which is randomly sending one of the heartbeat names :foo, :bar or :baz. We then also have three loop threads syncing on each of those names independently and then playing a different sample. The net effect is that we hear a sound every 0.5 beats as each of the sync threads is randomly synced with the cue thread and plays its sample.

This of course also works if you order the threads in reverse as the sync threads will simply sit and wait for the next cue.