# 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 `sync`ing 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`.