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Add description for XREAD new '+' special ID for last stream entry (#… #2687

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Add description for XREAD new '+' special ID for last stream entry (#… #2687

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Add description for XREAD new '+' special ID for last stream entry (#…
ronen-kalish Mar 13, 2024
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Update xread.md
oranagra Mar 13, 2024
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22 changes: 21 additions & 1 deletion commands/xread.md
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Expand Up @@ -183,6 +183,26 @@ Once we get some replies, the next call will be something like:

And so forth.

## The special `+` ID

It is sometimes desirable to read the last entry in a stream. For a single
stream this can be easily achieved using the `XREVRANGE` command, like so:

```
> XREVRANGE stream + - COUNT 1
```
But if there are many streams involved, this approach is quickly becoming
too slow, having to issue a command per stream.
Instead, starting from redis 8.0, the `+` sign can be used as a special ID,
requesting the last available entry in a stream. For example:

```
> XREAD STREAM streamA streamB streamC streamD + + + +
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do we want to mention that the pattern is usually that you start iterating on a stream with + and then switch to $ on the next call?

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I don't know if this is the common use-case, maybe sometimes users just need to peep at the last element and don't continue from there. But I can add a mention if you think it's required.

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@majklik, @guybe7 please advise.

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One nice use case is the mentioned peek fro last value in multiple streams. In this case is perfect one time shoot with: XREAD STREAM streamA streamB streamC streamD + + + +
But for reliable reading of the content of streams, when I need to start from the last available value (which is my case), after the initial "XREAD STREAM streamA streamB streamC streamD + + + +" it is convenient to continue with the obtained IDs from the streams in the style: "XREAD STREAM streamA streamB streamC streamD idA idB idC idD". If the initial reading shows that one of the streams is empty, I will use 0 as the id for subsequent readings.

```

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.
Expand All @@ -199,4 +219,4 @@ data to the stream.

Reading the [Redis Streams introduction](/topics/streams-intro) is highly
suggested in order to understand more about the streams overall behavior
and semantics.
and semantics.
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