Redis ZMPOP Command Explained

by

In Redis, the ZMPOP command removes and returns one or more elements from the first non-empty sorted set in the provided list of keys.

The ZMPOP command was introduced in Redis 7.0.0.

Syntax

The syntax goes like this:

ZMPOP numkeys key [key ...] <MIN | MAX> [COUNT count]

Example

Suppose we create the following sorted set:

ZADD cats 1 Meow 2 Fluffy 3 Scratch 4 Purr 5 Bite 6 Bunting

Let’s use the ZMPOP command to pop an element from that sorted set:

ZMPOP 2 birds cats MIN

Result:

1) "cats"
2) 1) 1) "Meow"
      2) "1"

Here I actually provided two sorted sets; birds and cats. The birds key didn’t exist and so it was treated as an empty sorted set. The cats key did exist with a sorted set, and so the operation was performed against that key.

In this case I specified MIN, which means that the elements with the lowest scores are popped. By default, only one element is popped and so the element with the lowest score was popped. Using MAX would’ve popped the element with the highest score.

Let’s take a look at the cats sorted set now:

ZRANGE cats 0 -1

Result:

1) "Fluffy"
2) "Scratch"
3) "Purr"
4) "Bite"
5) "Bunting"

We can see that Meow no longer exists in the sorted set.

Pop Multiple Elements

We can use the optional COUNT argument to pop multiple elements. By default, this is set to 1 but we can specify a different number.

Here’s an example of using the COUNT argument:

ZMPOP 1 cats MIN COUNT 2

Result:

1) "cats"
2) 1) 1) "Fluffy"
      2) "2"
   2) 1) "Scratch"
      2) "3"

This time two elements were popped because I specified a count of 2.

I also removed the birds key from the list of keys and so I changed the first argument to 1 instead of 2.

Let’s take a look at the cats sorted set now:

ZRANGE cats 0 -1

Result:

1) "Purr"
2) "Bite"
3) "Bunting"

Now there’s only three elements left, because the others have been popped.

When COUNT is Greater than the Sorted Set’s Cardinality

If the COUNT argument is a larger number than the number of elements in the sorted set, then all elements are popped.

Example:

ZMPOP 1 cats MAX COUNT 10

Result:

1) "cats"
2) 1) 1) "Bunting"
      2) "6"
   2) 1) "Bite"
      2) "5"
   3) 1) "Purr"
      2) "4"

We can see that all elements were popped from our sorted set. In this case I used the MAX argument and so it popped the elements with the highest scores. However, given we used a COUNT that was greater than the sorted set’s cardinality, all elements were popped.

When None of the Keys Exist

If none of the keys we pass actually exist, we get nil:

ZMPOP 2 oops1 oops2 MIN

Result:

(nil)

Wrong Data Type

When the key contains the wrong data type, an error is returned:

ZMPOP 1 countries MIN

Result:

(error) WRONGTYPE Operation against a key holding the wrong kind of value

In this case, the countries key contains a set (not a sorted set), and so we got an error.

We can use the TYPE command to check the key’s data type:

TYPE countries

Result:

set

As suspected, it’s a set.