[swift-users] Recommendation for thread-safe dictionary

[Evan Maloney]

>> I'm writing some code where I'd like multiple threads to be writing to a common dictionary object.
>> Is there a recommended mechanism for doing this?
> 
> Wrap mutexes* around dictionary accesses. If you have a lot more reads than writes, a read/write mutex will be more efficient.


Speaking of which, the CleanroomConcurrency project for Swift provides a ReadWriteCoordinator that provides similar functionality using a Grand Central Dispatch feature:

https://github.com/emaloney/CleanroomConcurrency/tree/master/Code#readwritecoordinator <https://github.com/emaloney/CleanroomConcurrency/tree/master/Code#readwritecoordinator>

The flip side of the threadsafe dictionary is the thread-local dictionary associated with each NSThread which can be used in a Swifty fashion with:

https://github.com/emaloney/CleanroomConcurrency/tree/master/Code#threadlocalvalue <https://github.com/emaloney/CleanroomConcurrency/tree/master/Code#threadlocalvalue>

Disclosure: This open-source code is provided courtesy of my employer, Gilt Groupe, and ships in our "Gilt on TV" app for the new Apple TV.




> On Dec 10, 2015, at 12:28 PM, Jens Alfke via swift-users <swift-users at swift.org> wrote:
> 
> 
>> On Dec 10, 2015, at 9:18 AM, Lane Schwartz via swift-users <swift-users at swift.org> wrote:
>> 
>> I'm writing some code where I'd like multiple threads to be writing to a common dictionary object.
>> Is there a recommended mechanism for doing this?
> 
> Wrap mutexes* around dictionary accesses. If you have a lot more reads than writes, a read/write mutex will be more efficient.
> 
> Making a thread-safe dictionary class is usually not a good idea. (This is something Java learned in between JDK 1.0 and 1.2.) It adds unavoidable overhead to every single access, and doesn’t solve the higher-level synchronization problems of the code that’s using the dictionary. Instead, use synchronization primitives in your higher-level class at the appropriate points.
> 
> —Jens
> 
> * which I guess you’ll have to implement using C calls to pthreads, since the Swift concurrency library isn’t ready yet
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