MutexOrChannel.md

# Use a sync.Mutex or a channel?

One of Go's mottos is _"Share memory by communicating, don't communicate by sharing memory."_

That said, Go does provide traditional locking mechanisms in the <a href='http://golang.org/pkg/sync/'>sync package</a>.  Most locking issues can be solved using either channels or traditional locks.

So which should you use?

Use whichever is most expressive and/or most simple.

A common Go newbie mistake is to over-use channels and goroutines just because it's possible, and/or because it's fun. Don't be afraid to use a <a href='http://golang.org/pkg/sync/#Mutex'><code>sync.Mutex</code></a> if that fits your problem best. Go is pragmatic in letting you use the tools that solve your problem best and not forcing you into one style of code.

As a general guide, though:

| **Channel** | **Mutex** |
|:------------|:----------|
| passing ownership of data,<br />distributing units of work,<br /> communicating async results | caches,<br />state |

If you ever find your sync.Mutex locking rules are getting too complex, ask yourself whether using channel(s) might be simpler.

### Wait Group

Another important synchronisation primitive is sync.WaitGroup. These allow co-operating goroutines to collectively wait for a threshold event before proceeding independently again. This is useful typically in two cases.

Firstly, when 'cleaning up', a sync.WaitGroup can be used to ensure that all goroutines - including the main one - wait before all terminating cleanly.

The second more general case is of a cyclic algorithm that involves a set of goroutines that all work independently for a while, then all wait on a barrier, before proceeding independently again. This pattern might be repeated many times. Data might be exchanged at the barrier event. This strategy is the basis of [Bulk Synchronous Parallelism](https://en.wikipedia.org/wiki/Bulk_synchronous_parallel) (BSP).

Channel communication, mutexes and wait-groups are complementary and can be combined.

### More Info

  * Channels in Effective Go: http://golang.org/doc/effective_go.html#channels
  * The sync package: http://golang.org/pkg/sync/