--- acl_categories: - '@read' - '@stream' - '@slow' - '@blocking' arguments: - display_text: count name: count optional: true token: COUNT type: integer - display_text: milliseconds name: milliseconds optional: true token: BLOCK type: integer - arguments: - display_text: key key_spec_index: 0 multiple: true name: key type: key - display_text: id multiple: true name: id type: string name: streams token: STREAMS type: block arity: -4 categories: - docs - develop - stack - oss - rs - rc - oss - kubernetes - clients command_flags: - readonly - blocking - movablekeys complexity: 'For each stream mentioned: O(M) with M being the number of elements returned. If M is constant (for example, always asking for the first 10 elements with COUNT), you can consider it O(1). On the other side when XREADGROUP blocks, XADD will pay the O(N) time in order to serve the N clients blocked on the stream getting new data.' description: Returns messages from multiple streams with IDs greater than the ones requested. Blocks until a message is available otherwise. group: stream hidden: false key_specs: - RO: true access: true begin_search: spec: keyword: STREAMS startfrom: 1 type: keyword find_keys: spec: keystep: 1 lastkey: -1 limit: 2 type: range linkTitle: XREAD railroad_diagram: /images/railroad/xread.svg since: 5.0.0 summary: Returns messages from multiple streams with IDs greater than the ones requested. Blocks until a message is available otherwise. syntax_fmt: "XREAD [COUNT\_count] [BLOCK\_milliseconds] STREAMS\_key [key ...] id\n\ \ [id ...]" title: XREAD --- {{< 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 >}} Read data from one or multiple streams, only returning entries with an ID greater than the last received ID reported by the caller. This command has an option to block if items are not available, in a similar fashion to [`BRPOP`]({{< relref "/commands/brpop" >}}) or [`BZPOPMIN`]({{< relref "/commands/bzpopmin" >}}) and others. Please note that before reading this page, if you are new to streams, we recommend to read [our introduction to Redis Streams]({{< relref "/develop/data-types/streams" >}}). ## Non-blocking usage If the **BLOCK** option is not used, the command is synchronous, and can be considered somewhat related to [`XRANGE`]({{< relref "/commands/xrange" >}}): it will return a range of items inside streams, however it has two fundamental differences compared to [`XRANGE`]({{< relref "/commands/xrange" >}}) even if we just consider the synchronous usage: * This command can be called with multiple streams if we want to read at the same time from a number of keys. This is a key feature of `XREAD` because especially when blocking with **BLOCK**, to be able to listen with a single connection to multiple keys is a vital feature. * While [`XRANGE`]({{< relref "/commands/xrange" >}}) returns items in a range of IDs, `XREAD` is more suited in order to consume the stream starting from the first entry which is greater than any other entry we saw so far. So what we pass to `XREAD` is, for each stream, the ID of the last element that we received from that stream. For example, if I have two streams `mystream` and `writers`, and I want to read data from both the streams starting from the first element they contain, I could call `XREAD` like in the following example. Note: we use the **COUNT** option in the example, so that for each stream the call will return at maximum two elements per stream. ``` > XREAD COUNT 2 STREAMS mystream writers 0-0 0-0 1) 1) "mystream" 2) 1) 1) 1526984818136-0 2) 1) "duration" 2) "1532" 3) "event-id" 4) "5" 5) "user-id" 6) "7782813" 2) 1) 1526999352406-0 2) 1) "duration" 2) "812" 3) "event-id" 4) "9" 5) "user-id" 6) "388234" 2) 1) "writers" 2) 1) 1) 1526985676425-0 2) 1) "name" 2) "Virginia" 3) "surname" 4) "Woolf" 2) 1) 1526985685298-0 2) 1) "name" 2) "Jane" 3) "surname" 4) "Austen" ``` The **STREAMS** option is mandatory and MUST be the final option because such option gets a variable length of argument in the following format: STREAMS key_1 key_2 key_3 ... key_N ID_1 ID_2 ID_3 ... ID_N So we start with a list of keys, and later continue with all the associated IDs, representing *the last ID we received for that stream*, so that the call will serve us only greater IDs from the same stream. For instance in the above example, the last items that we received for the stream `mystream` has ID `1526999352406-0`, while for the stream `writers` has the ID `1526985685298-0`. To continue iterating the two streams I'll call: ``` > XREAD COUNT 2 STREAMS mystream writers 1526999352406-0 1526985685298-0 1) 1) "mystream" 2) 1) 1) 1526999626221-0 2) 1) "duration" 2) "911" 3) "event-id" 4) "7" 5) "user-id" 6) "9488232" 2) 1) "writers" 2) 1) 1) 1526985691746-0 2) 1) "name" 2) "Toni" 3) "surname" 4) "Morrison" 2) 1) 1526985712947-0 2) 1) "name" 2) "Agatha" 3) "surname" 4) "Christie" ``` And so forth. Eventually, the call will not return any item, but just an empty array, then we know that there is nothing more to fetch from our stream (and we would have to retry the operation, hence this command also supports a blocking mode). ## Incomplete IDs To use incomplete IDs is valid, like it is valid for [`XRANGE`]({{< relref "/commands/xrange" >}}). However here the sequence part of the ID, if missing, is always interpreted as zero, so the command: ``` > XREAD COUNT 2 STREAMS mystream writers 0 0 ``` is exactly equivalent to ``` > XREAD COUNT 2 STREAMS mystream writers 0-0 0-0 ``` ## Blocking for data In its synchronous form, the command can get new data as long as there are more items available. However, at some point, we'll have to wait for producers of data to use [`XADD`]({{< relref "/commands/xadd" >}}) to push new entries inside the streams we are consuming. In order to avoid polling at a fixed or adaptive interval the command is able to block if it could not return any data, according to the specified streams and IDs, and automatically unblock once one of the requested keys accept data. It is important to understand that this command *fans out* to all the clients that are waiting for the same range of IDs, so every consumer will get a copy of the data, unlike to what happens when blocking list pop operations are used. In order to block, the **BLOCK** option is used, together with the number of milliseconds we want to block before timing out. Normally Redis blocking commands take timeouts in seconds, however this command takes a millisecond timeout, even if normally the server will have a timeout resolution near to 0.1 seconds. This time it is possible to block for a shorter time in certain use cases, and if the server internals will improve over time, it is possible that the resolution of timeouts will improve. When the **BLOCK** command is passed, but there is data to return at least in one of the streams passed, the command is executed synchronously *exactly like if the BLOCK option would be missing*. This is an example of blocking invocation, where the command later returns a null reply because the timeout has elapsed without new data arriving: ``` > XREAD BLOCK 1000 STREAMS mystream 1526999626221-0 (nil) ``` ## The special `$` ID. When blocking sometimes we want to receive just entries that are added to the stream via [`XADD`]({{< relref "/commands/xadd" >}}) starting from the moment we block. In such a case we are not interested in the history of already added entries. For this use case, we would have to check the stream top element ID, and use such ID in the `XREAD` command line. This is not clean and requires to call other commands, so instead it is possible to use the special `$` ID to signal the stream that we want only the new things. It is **very important** to understand that you should use the `$` ID only for the first call to `XREAD`. Later the ID should be the one of the last reported item in the stream, otherwise you could miss all the entries that are added in between. This is how a typical `XREAD` call looks like in the first iteration of a consumer willing to consume only new entries: ``` > XREAD BLOCK 5000 COUNT 100 STREAMS mystream $ ``` Once we get some replies, the next call will be something like: ``` > XREAD BLOCK 5000 COUNT 100 STREAMS mystream 1526999644174-3 ``` And so forth. ## The special `+` ID You can read the last entry in a single stream easily using the `XREVRANGE` command, like so: ``` > XREVRANGE stream + - COUNT 1 ``` But this approach becomes slow as you add more streams because you must issue a separate command for each stream. Instead, starting from Redis 7.4, you can use the `+` sign as a special ID. This requests the last available entry in a stream. For example: ``` > XREAD STREAMS streamA streamB streamC streamD + + + + ``` Note that when using this special ID for a stream, the **COUNT** option will be ignored (for the specific stream) since only the last entry can be returned. ## How multiple clients blocked on a single stream are served Blocking list operations on lists or sorted sets have a *pop* behavior. Basically, the element is removed from the list or sorted set in order to be returned to the client. In this scenario you want the items to be consumed in a fair way, depending on the moment clients blocked on a given key arrived. Normally Redis uses the FIFO semantics in this use cases. However note that with streams this is not a problem: stream entries are not removed from the stream when clients are served, so every client waiting will be served as soon as an [`XADD`]({{< relref "/commands/xadd" >}}) command provides data to the stream. Reading the [Redis Streams introduction]({{< relref "/develop/data-types/streams" >}}) is highly suggested in order to understand more about the streams overall behavior and semantics. ## Redis Software and Redis Cloud compatibility | Redis
Software | Redis
Cloud | Notes | |:----------------------|:-----------------|:------| | ✅ Standard
✅ Active-Active | ✅ Standard
✅ Active-Active | | ## Return information {{< multitabs id="xread-return-info" tab1="RESP2" tab2="RESP3" >}} One of the following: * [Array reply](../../develop/reference/protocol-spec#arrays): an array where each element is an array composed of a two elements containing the key name and the entries reported for that key. The entries reported are full stream entries, having IDs and the list of all the fields and values. Field and values are guaranteed to be reported in the same order they were added by `XADD`. * [Nil reply](../../develop/reference/protocol-spec#bulk-strings): if the _BLOCK_ option is given and a timeout occurs, or if there is no stream that can be served. -tab-sep- One of the following: * [Map reply](../../develop/reference/protocol-spec#maps): A map of key-value elements where each element is composed of the key name and the entries reported for that key. The entries reported are full stream entries, having IDs and the list of all the fields and values. Field and values are guaranteed to be reported in the same order they were added by `XADD`. * [Null reply](../../develop/reference/protocol-spec#nulls): if the _BLOCK_ option is given and a timeout occurs, or if there is no stream that can be served. {{< /multitabs >}}