Sorry I wasn't clear about that. The lock is not global, it is per cache. newCache creates a newMVar which serves as the thread safe lock for that cache.

Concurrently fetching from different caches can indeed run their separate fallbacks concurrently.

On Fri, Jun 13, 2014, 10:16 AM, Peter Simons <simons@cryp.to> wrote:

Hi Dan,

> 1. Create an empty cache with newCache
> 2. fetch repeatedly from the cache, each time providing a fallback in
> case it is empty.
>
> Fetching from a Cache is thread safe: only one "fallback" will execute
> at a time, and after the first successful fallback, the cached value
> will be set and no other fallbacks will be called.

I'm not sure whether I get this correctly. If multiple threads fetch
values from one cache, will only one of those threads run at a time? Or
will those that fetch pairwise different values still run concurrently?

In other words, does the cache have a global lock or is there a lock per
value?

Best regards,
Peter

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