StackExchange.Redis - LockTake / LockRelease Usage

Question

I am using Redis with StackExchange.Redis. I have multiple threads that will at some point access and edit the value of the same key, so I need to synchronize the manipulation of the data.

Looking at the available functions, I see that there are two functions, TakeLock and ReleaseLock. However, these functions take both a key and a value parameter rather than the expected single key to be locked. The intellisene documentation and source on GitHub don't explain how to use the LockTake and LockRelease functions or what to pass in for the key and value parameters.

Q: What is the correct usage of LockTake and LockRelease in StackExchange.Redis?

Pseudocode example of what I'm aiming to do:

//Add Items Before Parallel Execution
redis.StringSet("myJSONKey", myJSON);

//Parallel Execution
Parallel.For(0, 100, i =>
    {
        //Some work here
        //....

        //Lock
        redis.LockTake("myJSONKey");

        //Manipulate
        var myJSONObject = redis.StringGet("myJSONKey");
        myJSONObject.Total++;
        Console.WriteLine(myJSONObject.Total);
        redis.StringSet("myJSONKey", myNewJSON);

        //Unlock
        redis.LockRelease("myJSONKey");

        //More work here
        //...
    });

Answer 1

There are 3 parts to a lock:

• the key (the unique name of the lock in the database)
• the value (a caller-defined token which can be used both to indicate who "owns" the lock, and to check that releasing and extending the lock is being done correctly)
• the duration (a lock intentionally is a finite duration thing)

If no other value comes to mind, a guid might make a suitable "value". We tend to use the machine-name (or a munged version of the machine name if multiple processes could be competing on the same machine).

Also, note that taking a lock is speculative, not blocking. It is entirely possible that you fail to obtain the lock, and hence you may need to test for this and perhaps add some retry logic.

A typical example might be:

RedisValue token = Environment.MachineName;
if(db.LockTake(key, token, duration)) {
    try {
        // you have the lock do work
    } finally {
        db.LockRelease(key, token);
    }
}

Note that if the work is lengthy (a loop, in particular), you may want to add some occasional LockExtend calls in the middle - again remembering to check for success (in case it timed out).

Note also that all individual redis commands are atomic, so you don't need to worry about two discreet operations competing. For more complexing multi-operation units, transactions and scripting are options.