LUA Example? How would I do this?

[marc perkel]

Here's what I would like to do if Redis has the feature using LUA to do it.

I create or update a hash as follows:

HINCRBY myhash count 1

I'd like to have a background process with would set "myhash lastaccess" to the current time. (seconds in eopch).

Is there some trick in Redis/LUA that I'm missing that will do something like that?

[marc perkel]

Another example.

I want to decrement the count by 1 and if the count reaches 0 then delete the key.

I just cant seem to wrap my brain around how this is done. Just need a simple example to get started.

[Itamar Haber]

There's only one way (currently) to invoke Lua scripts in Redis, and that's by explicitly rubbing them using EVAL (or EVALSHA) - no tricks nor trigger-like/time-based manners. This means that your first example is solvable only with a client application that periodically updates the relevant key.

As for your second example, Lua can definitely help with that. Instead of directly calling INCRBY to do the decrement, call a script that wraps it and the logic (if + DEL). That script would probably look something like:

    if redis.call('INCRBY', KEYS[1], -1) == 0) then redis.call('DEL', KEYS[1]) end

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[Itamar Haber]

Edits:
^rubbing^running
Extra unneeded closing parenthesis in snippet, should be:

        if redis.call('INCRBY', KEYS[1], -1) == 0 then redis.call('DEL', KEYS[1]) end