Question

You have two threads, a and b. Thread a is in a forever loop, listening on a blocking socket 1. Thread b is also in a forever loop, listening on blocking socket 2. Both socket 1 and socket 2 may return data at arbitrary times, so Thread a may be sleeping forever waiting for data whereas Thread b constantly gets data from the socket and goes on with its processing. That’s the background.

Now suppose they need to share a dictionary. When Thread a gets some data (if ever) it adds a key value pair into the dictionary after some processing, and then continues to wait for more data. When Thread b receives data from its socket it first queries the dictionary to see if there is information related to the data it has received before going on with its processing. There are no deletions to the dictionary, only inserts and queries (I’d be interested if this makes a difference in the end solution).

In a standard imperative language like python or c, this is pretty easy to do by making the dictionary available in both scopes and only querying it after a thread has acquired a lock, so Thread B always sees the most (well almost) up to date dictionary.

In Haskell, I seem to be struggling to come up with a good implementation of this pattern. MVars, can only have one item at a time so it can’t be that Thread a puts in the dictionary, since a new update might occur and it would not be able to push that new dictionary until Thread b fetched it from the MVar. On the other hand if thread b uses an MVar to send a ready signal “ok!” to thread a, it may be the case that Thread a is sleeping on its read socket, so it would be unable to send back data until its read socket unblocked! There are also channels, but that seems messy since I would have to keep sending new dictionaries and Thread B would discard all but the last one.

The alternative solution that would work is to simply send the updates down a channel, and have thread B construct the dictionary for itself. However I’m wondering if there are better alternative solutions.

Thanks for taking the time to read this very long question!

Answer 1

You can use an MVar in the following way:

• When thread A gets new data, it tries to get the dictionary with takeMVar. When that succeeds, it updates the dictionary and puts it back into the MVar
• When thread B gets data, it tries to get the dictionary with takeMVar – in the above scenario where A seldom gets data that should succeed rather quickly on average. Then it does the lookup and puts the dictionary back.

As hammar pointed out, it’s probably better to not directly use takeMVar and putMVar but rather wrap them in modifyMVar_ resp. modifyMVar to not leave the MVar empty if one thread gets an exception while using the dictionary.

In thread A, something like

modifyMVar_ mvar (\dict -> putMVar mvar (insert newStuff dict))

in thread B all you need is a simple readMVar (thanks to @hammar again for pointing that out).