Thanks for reporting this. Out of curiosity how do you use this command? I am asking because I have not seen much traction with RAND commands of Redis API.

Sure thing! Thanks for the quick response.

In this case, I am keeping global set of unique identifiers and the timestamp they were last updated in a handful of other data structures that all tie back to that identifier to track when they can be safely expired. The rand usage is probably somewhat niche - I use it to facilitate a couple of kinds of load testing of the datastore and the soft real-time API that sits in front of it. One such scenario is generating a constant read load in a production setting alongside normal traffic. The randomness is useful for generating synthetic read load that is dynamic to data that is known to be in the system at any given time.

There are, of course, a thousand other ways to skin that cat, but I've found that this one to be one of the simplest and effective for the use cases (real-time, high data volume) I tend to have.

I would like to add that SRANDMEMBER and SPOP have the same problem. Both commands return/remove only the lowest scoring element(s).

SRANDMEMBER

The steps to reproduce the behavior of the SRANDMEMBER command are the same as in the case of ZRANDMEMBER:

1. Inserting elements into a set: redis-cli SADD myset one two three four five six seven eight nine ten
2. Сalling SRANDMEMBER multiple times: seq 20 | xargs -I {} redis-cli SRANDMEMBER myset

The result is always "eight".

SPOP

The steps to reproduce the SPOP behavior are more tricky. After each deletion of an element, we must return it back to the set in order to keep the same set of elements:

1. Insert elements into a set: redis-cli SADD myset one two three four five six seven eight nine ten

2. Remove the element and return it back multiple times: for i in {1..30}; do redis-cli SPOP myset | tee -a removed_members.txt | xargs -I {} redis-cli SADD myset {}; done

   • Notice: between running SPOP and SADD, we write the SPOP result to the file removed_members.txt. As a result, in removed_elements.txt we can see which elements were deleted during the SPOP commands.

In removed_element.txt we can see that "eight" was deleted each time.

Expected behavior

Redis always removes a pseudo-random element. So, the result may differ between runs:

$ cat removed_members.txt
ten
nine
four
five
eight
eight
one
nine
two
nine
three
five
four
nine
eight
five
four
two
six
two
two
ten
eight
two
five
ten
two
four
six
nine