Question

I’m trying to debug some Redis issues I am experiencing and came by some inconclusive documentation about the SET command.

In my Redis config; I have the following lines (snippet):

# Note: with all the kind of policies, Redis will return an error on write
#       operations, when there are not suitable keys for eviction.
#
#       At the date of writing this commands are: set setnx setex append

On the documentation page for the SET command I found:

Status code reply: always OK since SET can't fail.

Any insights on the definitive behaviour?

Answer 1

tl;dr: SET will return an error response if the redis instance runs out of memory.

As far as I can tell from the source code in redis.c, esentially when a command is to be processed the flow goes like this (pseudo code):

IF memory is needed
    IF we can free keys
        Free keys
        Process the command
            SET -> process and return OK response
    ELSE return error response
ELSE
    Process command
        SET -> process and return OK response

It’s not exactly written this way, but the basic idea comes down to that: memory is being checked before the command is processed, so even if the command cannot fail, an error response will be returned if there’s no memory regardless the actual response of the command.