XRANGE
Syntax
XRANGE key start end [COUNT count]
Time complexity: O(N) with N being the number of elements being returned. If N is constant (e.g. always asking for the first 10 elements with COUNT), you can consider it O(1).
The command returns the stream entries matching a given range of IDs. The range is specified by a minimum and maximum ID. All the entries having an ID between the two specified or exactly one of the two IDs specified (closed interval) are returned.
The XRANGE command has a number of applications:
- Returning items in a specific time range. This is possible because Stream IDs are related to time.
- Iterating a stream incrementally, returning just
a few items at every iteration. However it is semantically much more
robust than the
SCANfamily of functions. - Fetching a single entry from a stream, providing the ID of the entry to fetch two times: as start and end of the query interval.
The command also has a reciprocal command returning items in the
reverse order, called XREVRANGE, which is otherwise identical.
- and + special IDs
The - and + special IDs mean respectively the minimum ID possible
and the maximum ID possible inside a stream, so the following command
will just return every entry in the stream:
> XRANGE somestream - +
1) 1) 1526985054069-0
2) 1) "duration"
2) "72"
3) "event-id"
4) "9"
5) "user-id"
6) "839248"
2) 1) 1526985069902-0
2) 1) "duration"
2) "415"
3) "event-id"
4) "2"
5) "user-id"
6) "772213"
... other entries here ...
The - and + special IDs mean, respectively, the minimal and maximal range IDs.
Incomplete IDs
Stream IDs are composed of two parts, a Unix millisecond time stamp and a
sequence number for entries inserted in the same millisecond. It is possible
to use XRANGE specifying just the first part of the ID, the millisecond time,
like in the following example:
XRANGE somestream 1526985054069 1526985055069
In this case, XRANGE will auto-complete the start interval with -0
and end interval with -18446744073709551615 (264-1), in order to return all the
entries that were generated between a given millisecond and the end of
the other specified millisecond. This also means that repeating the same
millisecond two times, we get all the entries within such millisecond,
because the sequence number range will be from zero to the maximum.
Used in this way XRANGE works as a range query command to obtain entries
in a specified time. This is very handy in order to access the history
of past events in a stream.
Exclusive ranges
The range is closed (inclusive) by default, meaning that the reply can include
entries with IDs matching the query's start and end intervals (i.e., the interval [start, end]). It is possible
to specify an open interval (exclusive) by prefixing the ID with the
character (. This is useful for iterating the stream, as explained below.
Returning a maximum number of entries
Using the COUNT option it is possible to reduce the number of entries reported. This is a very important feature even if it may look marginal, because it allows, for instance, to model operations such as give me the entry greater or equal to the following:
dragonfly> XRANGE somestream 1526985054069-0 + COUNT 1
1) 1) 1526985054069-0
2) 1) "duration"
2) "72"
3) "event-id"
4) "9"
5) "user-id"
6) "839248"
In the above case the entry 1526985054069-0 exists, otherwise the server
would have sent us the next one. Using COUNT is also the base in order to
use XRANGE as an iterator.
Iterating a stream
In order to iterate a stream, we can proceed as follows. Let's assume that we want two elements per iteration. We start fetching the first two elements, which is trivial:
dragonfly> XRANGE writers - + COUNT 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"
Then instead of starting the iteration again from -, as the start
of the range we use the entry ID of the last entry returned by the
previous XRANGE call as an exclusive interval.
The ID of the last entry is 1526985685298-0, so we just prefix it
with a '(', and continue our iteration:
dragonfly> XRANGE writers (1526985685298-0 + COUNT 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 this will allow to visit all the entries in the
stream. Obviously, we can start the iteration from any ID, or even from
a specific time, by providing a given incomplete start ID. Moreover, we
can limit the iteration to a given ID or time, by providing an end
ID or incomplete ID instead of +.
The command XREVRANGE can iterate the stream reverse, from higher IDs
(or times) to lower IDs (or times).
Fetching single items
If you look for an XGET command you'll be disappointed because XRANGE
is effectively the way to go in order to fetch a single entry from a
stream. All you have to do is to specify the ID two times in the arguments
of XRANGE:
dragonfly> XRANGE mystream 1526984818136-0 1526984818136-0
1) 1) 1526984818136-0
2) 1) "duration"
2) "1532"
3) "event-id"
4) "5"
5) "user-id"
6) "7782813"
Return
Array reply, specifically:
The command returns the entries with IDs matching the specified range.
The returned entries are complete, that means that the ID and all the fields
they are composed are returned. Moreover, the entries are returned with
their fields and values in the exact same order as XADD added them.
Examples
dragonfly> XADD writers * name Virginia surname Woolf
"1676903940637-0"
dragonfly> XADD writers * name Jane surname Austen
"1676903940637-1"
dragonfly> XADD writers * name Toni surname Morrison
"1676903940637-2"
dragonfly> XADD writers * name Agatha surname Christie
"1676903940637-3"
dragonfly> XADD writers * name Ngozi surname Adichie
"1676903940638-0"
dragonfly> XLEN writers
(integer) 5
dragonfly> XRANGE writers - + COUNT 2
1) 1) "1676903940637-0"
2) 1) "name"
2) "Virginia"
3) "surname"
4) "Woolf"
2) 1) "1676903940637-1"
2) 1) "name"
2) "Jane"
3) "surname"
4) "Austen"