how to create a high performance counter in c# using lua and redis with stackexchange

Question

I'm trying to create a simple method in c# to get a count of keys in my Redis instance. In redis, I have my keys like:

mykey:1
mykey:2
mykey:3

By using redis-cli, I can run:

eval "return #redis.pcall('keys', 'mykey:*')" 0

and it returns:

(integer) 3

So, I build this function in c#:

public void GetThatCounter()
    {
        var script=LuaScript.Prepare("redis.pcall('keys', 'mykey:*')");
        var result=dbCache.ScriptEvaluate(script);
        Console.WriteLine("result"+result.ToString());
    }

However, the result contains "isnull:true", meaning there are no results...

(The function is obviously returing nothing now, I'm just using it as a test and breaking code excecution on debug, to dig inside the result variable with the Visual Studio editor)

Any help ?

Answer 1

Ok, thanks to Aureliar's comments, here is the final answers:

• In order to get a result from a redis.pcall, you need to add a "return" before the redis.pcall
• In order to get the size of the array only, you need to add a # before the redis.call
• I also found that, in this case, you don't need a Lua script at all.

So the final function looks like this:

public int GetCount(string keyWithWildCards) //e.g. key:*
{           
 return (int)dbCache.ScriptEvaluate($"return #redis.pcall('keys', '{keyWithWildCards}')");
}

This will avoid to receive back millions of keys: the count will be made on redis side.

In case the pattern doesn't match any key, it will return 0.