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|
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` or `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](/topics/streams-intro).
## Non-blocking usage
If the **BLOCK** option is not used, the command is synchronous, and can
be considered somewhat related to `XRANGE`: it will return a range of items
inside streams, however it has two fundamental differences compared to `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` 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`. 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` 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 `$` ID.
When blocking sometimes we want to receive just entries that are added
to the stream via `XADD` starting from the moment we block. In such a case
we are not interested in the history of already added entries. For
this use case, we would have to check the stream top element ID, and use
such ID in the `XREAD` command line. This is not clean and requires to
call other commands, so instead it is possible to use the special `$`
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.
## 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` command provides
data to the stream.
@return
@array-reply, specifically:
The command returns an array of results: each element of the returned
array is an array composed of a two element 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`.
When **BLOCK** is used, on timeout a null reply is returned.
Reading the [Redis Streams introduction](/topics/streams-intro) is highly
suggested in order to understand more about the streams overall behavior
and semantics.
|
