Why is spop not an allowed script command?

[ssoroka-se]

I'm trying to write a lua script for redis that includes this line:

local id = redis.call('spop', 'processed_ids')

but I'm getting an error This Redis command is not allowed from scripts. It looks like this error is coming from this line in the source.
As per https://github.com/antirez/redis/blob/unstable/src/redis.c#L163 , spop has the 's' flag set, which is REDIS_CMD_NOSCRIPT. Why was this set?

[michael-grunder]

Hey,

That's because the SPOP command isn't deterministic (it will pop a random member), which would break replication from inside a script. If you look at the source code for SPOP you'll see that it converts into an SREM command for the purposes of replication.

https://github.com/antirez/redis/blob/unstable/src/t_set.c#L399-L402

Cheers! 😺
Mike

[ssoroka-se]

So a work-around would be to use srandmember and srem, or sscan and srem.

On Wednesday, November 12, 2014, Michael Grunder notifications@github.com
wrote:

Hey,

That's because the SPOP command isn't deterministic (it will pop a random
member), which would break replication from inside a script. If you look at
the source code for SPOP you'll see that it converts into an SREM command
for the purposes of replication.

https://github.com/antirez/redis/blob/unstable/src/t_set.c#L399-L402

Cheers! [image: 😺]
Mike

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#2139 (comment).

[michael-grunder]

Yeah either of those strategies should work for you. Which one you pick would just have to do with your use case specifically.

[itapita]

Actually no, you can't do a write after a nondeterministic command for the
same reason (breaking replication) and Redis will block you from doing that
with an error.
On Nov 13, 2014 12:53 AM, "Michael Grunder" notifications@github.com
wrote:

Yeah either of those strategies should work for you. Which one you pick
would just have to do with your use case specifically.

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#2139 (comment).

[michael-grunder]

Yeah, @itapita is correct. I spaced out on that one for some reason. I need more (or maybe less) coffee.

[ssoroka-se]

That's kind of a major draw-back. I guess I'm left with sorted sets.

[ssoroka-se]

For anyone else looking at this in the future, I worked around the problem by doing srandmember outside of the script, then srem inside of the script, passing the member in, and that met my needs.