In Redis, the BZMPOP command is the blocking variant of the ZMPOP command.

The BZMPOP command works exactly like ZMPOP when any of the sorted sets contain elements. It also works exactly like ZMPOP when used inside a MULTI/EXEC block. That is, it pops one or more elements from the first non-empty sorted set in the provided list of keys.

However, if all of the specified sorted sets are empty, then BZMPOP will block the connection until another client adds members to one of the keys or until the specified timeout elapses. It’s possible to block the connection indefinitely by using a timeout of zero.

The BZMPOP command was introduced in Redis 7.0.0.

Syntax

The syntax goes like this:

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

It’s basically the same as ZMPOP except that BZMPOP requires the timeout argument.

Example

Here’s an example of what happens when all keys we pass to the BZMPOP command contain empty sorted sets:

BZMPOP 1 2 birds cats MIN

Result:

(nil)
(1.02s)

Here, I provided a timeout of 1, which means that it waits for one second if all sorted sets are empty. We can see by the result that it took 1.02 seconds to run, and the result of the operation is nil.

Passing a timeout argument of 0 (zero) blocks the connection indefinitely.

This blocking feature is what distinguishes BZMPOP from ZMPOP. The ZMPOP command would simply return nil without waiting. To demonstrate this, here it is using ZMPOP:

ZMPOP 2 birds cats MIN

Result:

(nil)

When I ran that code, the result was returned immediately.

When the Key is Not Empty

When there’s a key that contains a non-empty sorted set, BZMPOP works exactly like ZMPOP. In the above example, both the birds and the cats keys didn’t exist. Let’s create one called cats:

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

Now let’s run BZMPOP again:

BZMPOP 1 2 birds cats MIN

Result:

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

This time my results were returned immediately. There was no waiting period like in the previous example.

Here I 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:

BZMPOP 1 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:

BZMPOP 1 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.

Wrong Data Type

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

BZMPOP 1 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.

Non-Blocking Variant

As mentioned, the BZMPOP command is the blocking variant of ZMPOP. Therefore, if you need a non-blocking variant, use the ZMPOP command instead.