--- acl_categories: - '@write' - '@list' - '@slow' - '@blocking' arguments: - display_text: key key_spec_index: 0 multiple: true name: key type: key - display_text: timeout name: timeout type: double arity: -3 categories: - docs - develop - stack - oss - rs - rc - oss - kubernetes - clients command_flags: - write - blocking complexity: O(N) where N is the number of provided keys. description: Removes and returns the first element in a list. Blocks until an element is available otherwise. Deletes the list if the last element was popped. group: list hidden: false history: - - 6.0.0 - '`timeout` is interpreted as a double instead of an integer.' key_specs: - RW: true access: true begin_search: spec: index: 1 type: index delete: true find_keys: spec: keystep: 1 lastkey: -2 limit: 0 type: range linkTitle: BLPOP railroad_diagram: /images/railroad/blpop.svg since: 2.0.0 summary: Removes and returns the first element in a list. Blocks until an element is available otherwise. Deletes the list if the last element was popped. syntax_fmt: BLPOP key [key ...] timeout title: BLPOP --- {{< note >}} This command's behavior varies in clustered Redis environments. See the [multi-key operations]({{< relref "/develop/using-commands/multi-key-operations" >}}) page for more information. {{< /note >}} `BLPOP` is a blocking list pop primitive. It is the blocking version of [`LPOP`]({{< relref "/commands/lpop" >}}) because it blocks the connection when there are no elements to pop from any of the given lists. An element is popped from the head of the first list that is non-empty, with the given keys being checked in the order that they are given. ## Non-blocking behavior When `BLPOP` is called, if at least one of the specified keys contains a non-empty list, an element is popped from the head of the list and returned to the caller together with the `key` it was popped from. Keys are checked in the order that they are given. Let's say that the key `list1` doesn't exist and `list2` and `list3` hold non-empty lists. Consider the following command: ``` BLPOP list1 list2 list3 0 ``` `BLPOP` guarantees to return an element from the list stored at `list2` (since it is the first non empty list when checking `list1`, `list2` and `list3` in that order). ## Blocking behavior If none of the specified keys exist, `BLPOP` blocks the connection until another client performs an [`LPUSH`]({{< relref "/commands/lpush" >}}) or [`RPUSH`]({{< relref "/commands/rpush" >}}) operation against one of the keys. Once new data is present on one of the lists, the client returns with the name of the key unblocking it and the popped value. When `BLPOP` causes a client to block and a non-zero timeout is specified, the client will unblock returning a `nil` multi-bulk value when the specified timeout has expired without a push operation against at least one of the specified keys. **The timeout argument is interpreted as a double value specifying the maximum number of seconds to block**. A timeout of zero can be used to block indefinitely. ## What key is served first? What client? What element? Priority ordering details. * If the client tries to block for multiple keys, but at least one key contains elements, the returned key/element pair is the first key from left to right that has one or more elements. In this case, the client is not blocked. For example, `BLPOP key1 key2 key3 key4 0`, assuming that both `key2` and `key4` are non-empty, will always return an element from `key2`. * If multiple clients are blocked for the same key, the first client to be served is the one that has been waiting the longest (the first that blocked for the key). Once a client is unblocked it does not retain any priority; when it blocks again with another `BLPOP` call, it will be served according to its new position in the queue. * When a client is blocking for multiple keys and multiple keys become non-empty as a result of the same command, transaction, or Lua script, the client is served according to the **order of keys in its `BLPOP` call**, not the order of the write operations. After the writes occur, Redis reprocesses the blocking command for the client and pops from the **first non-empty key** in the key list. For example, a client blocked on `BLPOP key1 key2 0` will pop from `key1` if both `key1` and `key2` receive elements during the same `MULTI`/`EXEC` transaction, because `key1` appears first. Internally, when a write makes progress possible for a blocked client, Redis marks that client as unblocked (adding it to an internal queue). No reply is sent immediately. Before the server goes idle (in the `beforeSleep()` phase), Redis rechecks the blocking command and serves the client based on the first non-empty key in its provided key list. #### Example **Client A** ``` BLPOP key1 key2 0 ``` **Client B** ``` MULTI RPUSH key2 1 2 3 4 RPUSH key1 5 6 7 EXEC ``` Although `key2` is pushed first inside the transaction, Redis reprocesses the blocking command for Client A after the transaction completes. At that time both lists are non-empty, and `BLPOP` pops from the first non-empty key in its argument list (`key1`): ``` "key1" "5" ``` To make Redis pop from `key2` instead, reverse the order of the keys in the blocking call: ``` BLPOP key2 key1 0 ``` ## Behavior of `BLPOP` when multiple elements are pushed inside a list. There are times when a list can receive multiple elements in the context of the same conceptual command: * Variadic push operations such as `LPUSH mylist a b c`. * After an [`EXEC`]({{< relref "/commands/exec" >}}) of a [`MULTI`]({{< relref "/commands/multi" >}}) block with multiple push operations against the same list. * Executing a Lua Script with Redis 2.6 or newer. When multiple elements are pushed inside a list where there are clients blocking, the behavior is different for Redis 2.4 and Redis 2.6 or newer. For Redis 2.6 what happens is that the command performing multiple pushes is executed, and *only after* the execution of the command the blocked clients are served. Consider this sequence of commands. Client A: BLPOP foo 0 Client B: LPUSH foo a b c If the above condition happens using a Redis 2.6 server or greater, Client **A** will be served with the `c` element, because after the [`LPUSH`]({{< relref "/commands/lpush" >}}) command the list contains `c,b,a`, so taking an element from the left means to return `c`. Instead Redis 2.4 works in a different way: clients are served *in the context* of the push operation, so as long as `LPUSH foo a b c` starts pushing the first element to the list, it will be delivered to the Client **A**, that will receive `a` (the first element pushed). The behavior of Redis 2.4 creates a lot of problems when replicating or persisting data into the AOF file, so the much more generic and semantically simpler behavior was introduced into Redis 2.6 to prevent problems. Note that for the same reason a Lua script or a `MULTI/EXEC` block may push elements into a list and afterward **delete the list**. In this case the blocked clients will not be served at all and will continue to be blocked as long as no data is present on the list after the execution of a single command, transaction, or script. ## `BLPOP` inside a `MULTI` / `EXEC` transaction `BLPOP` can be used with pipelining (sending multiple commands and reading the replies in batch), however this setup makes sense almost solely when it is the last command of the pipeline. Using `BLPOP` inside a [`MULTI`]({{< relref "/commands/multi" >}}) / [`EXEC`]({{< relref "/commands/exec" >}}) block does not make a lot of sense as it would require blocking the entire server in order to execute the block atomically, which in turn does not allow other clients to perform a push operation. For this reason the behavior of `BLPOP` inside [`MULTI`]({{< relref "/commands/multi" >}}) / [`EXEC`]({{< relref "/commands/exec" >}}) when the list is empty is to return a `nil` multi-bulk reply, which is the same thing that happens when the timeout is reached. If you like science fiction, think of time flowing at infinite speed inside a [`MULTI`]({{< relref "/commands/multi" >}}) / [`EXEC`]({{< relref "/commands/exec" >}}) block... ## Examples ``` redis> DEL list1 list2 (integer) 0 redis> RPUSH list1 a b c (integer) 3 redis> BLPOP list1 list2 0 1) "list1" 2) "a" ``` ## Reliable queues When `BLPOP` returns an element to the client, it also removes the element from the list. This means that the element only exists in the context of the client: if the client crashes while processing the returned element, it is lost forever. This can be a problem with some application where we want a more reliable messaging system. When this is the case, please check the [`BRPOPLPUSH`]({{< relref "/commands/brpoplpush" >}}) command, that is a variant of `BLPOP` that adds the returned element to a target list before returning it to the client. ## Pattern: Event notification Using blocking list operations it is possible to mount different blocking primitives. For instance for some application you may need to block waiting for elements into a Redis Set, so that as far as a new element is added to the Set, it is possible to retrieve it without resort to polling. This would require a blocking version of [`SPOP`]({{< relref "/commands/spop" >}}) that is not available, but using blocking list operations we can easily accomplish this task. The consumer will do: ``` LOOP forever WHILE SPOP(key) returns elements ... process elements ... END BRPOP helper_key END ``` While in the producer side we'll use simply: ``` MULTI SADD key element LPUSH helper_key x EXEC ``` ## Redis Software and Redis Cloud compatibility | Redis
Software | Redis
Cloud | Notes | |:----------------------|:-----------------|:------| | ✅ Standard
✅ Active-Active | ✅ Standard
✅ Active-Active | | ## Return information {{< multitabs id="blpop-return-info" tab1="RESP2" tab2="RESP3" >}} One of the following: * [Nil reply](../../develop/reference/protocol-spec#bulk-strings): no element could be popped and the timeout expired * [Array reply](../../develop/reference/protocol-spec#arrays): the key from which the element was popped and the value of the popped element. -tab-sep- One of the following: * [Null reply](../../develop/reference/protocol-spec#nulls): no element could be popped and the timeout expired * [Array reply](../../develop/reference/protocol-spec#arrays): the key from which the element was popped and the value of the popped element. {{< /multitabs >}}