StringSetAsync with When.NotExists returns false when key doesn't exist

Question

I am using StackExchange.Redis with a replica cluster (3 nodes) configured to use Sentinel.

I get multiple requests trying to update the cache with the same data, so to avoid multiple writes, I am using When.NotExists in StringSetAsync. My understanding is that this will prevent the set from happening if the key already exists. I am expecting StringSetAsync to only return true in cases where the set actually happened.

Example:

var inserted = await connectionMultiplexer.GetDatabase().StringSetAsync("my-key", "my-value", myTimeout, When.NotExists);

if(inserted == false)
{
  var keyExists = await connectionMultiplexer.GetDatabase().KeyExistsAsync("my-key");
  if(keyExists == false)
  {
    // I end up here, which I assumed should not happen. 
  }
}

To my surprise I am seeing that StringSetAsync returns false even if the key didn't already exist based on the sanity check where I manually check for the existence of the key.

Notes: My environment uses 2 replica nodes and one primary. StackExchange.Redis ver: 2.1.58

Answer 1

What I typically do in these cases is temporarily increase the expiry (myTimeout) parameter to a bigger value just to remove the time-out from the equation. Next steps would be checking the access rights for all system boundaries between your application and the database server.

Also bear in mind that if the operation throws an exception, because of time-out or another kind of failure, depending on the way you are applying the async/await stack, in some cases the timeout exception can not be thrown to the calling thread and you need to query the Task.Exception aggregate to check the source of the problem.

It is suspicious to me that the result is false (the default value for boolean) and potentially you have the answer to your problem in the Task.Exception aggregate.

To find out you can try it synchronously:

try
{
    var inserted = connectionMultiplexer.GetDatabase().StringSetAsync("my-key", "my-value", myTimeout, When.NotExists).Result;
    if (inserted == false)
    {
        var keyExists = connectionMultiplexer.GetDatabase().KeyExistsAsync("my-key").Result;
        if (keyExists == false)
        {
            // I end up here, which I assumed should not happen. 
        }
    }
}
catch (AggregateException aex)
{
    foreach (var ex in aex.InnerExceptions)
    {
        Console.WriteLine(ex);
    }
}

Please tell me how it goes!