SCAN cursor [MATCH pattern] [COUNT count]
Since these commands allow for incremental iteration, returning only a small number of elements per call, they can be used in production without the downside of commands like KEYS or SMEMBERS that may block the server for a long time (even several seconds) when called against big collections of keys or elements.
However while blocking commands like SMEMBERS are able to provide all the elements that are part of a Set in a given moment, The SCAN family of commands only offer limited guarantees about the returned elements since the collection that we incrementally iterate can change during the iteration process.
Note that SCAN, SSCAN, HSCAN and ZSCAN all work very similarly, so this documentation covers all the four commands. However an obvious difference is that in the case of SSCAN, HSCAN and ZSCAN the first argument is the name of the key holding the Set, Hash or Sorted Set value. The SCAN command does not need any key name argument as it iterates keys in the current database, so the iterated object is the database itself.
SCAN is a cursor based iterator. This means that at every call of the command, the server returns an updated cursor that the user needs to use as the cursor argument in the next call.
An iteration starts when the cursor is set to 0, and terminates when the cursor returned by the server is 0. The following is an example of SCAN iteration:
redis 127.0.0.1:6379> scan 0 1) "17" 2) 1) "key:12" 2) "key:8" 3) "key:4" 4) "key:14" 5) "key:16" 6) "key:17" 7) "key:15" 8) "key:10" 9) "key:3" 10) "key:7" 11) "key:1" redis 127.0.0.1:6379> scan 17 1) "0" 2) 1) "key:5" 2) "key:18" 3) "key:0" 4) "key:2" 5) "key:19" 6) "key:13" 7) "key:6" 8) "key:9" 9) "key:11"
In the example above, the first call uses zero as a cursor, to start the iteration. The second call uses the cursor returned by the previous call as the first element of the reply, that is, 17.
As you can see the SCAN return value is an array of two values: the first value is the new cursor to use in the next call, the second value is an array of elements.
Since in the second call the returned cursor is 0, the server signaled to the caller that the iteration finished, and the collection was completely explored. Starting an iteration with a cursor value of 0, and calling SCAN until the returned cursor is 0 again is called a full iteration.
However because SCAN has very little state associated (just the cursor) it has the following drawbacks:
SCAN family functions do not guarantee that the number of elements returned per call are in a given range. The commands are also allowed to return zero elements, and the client should not consider the iteration complete as long as the returned cursor is not zero.
However the number of returned elements is reasonable, that is, in practical terms SCAN may return a maximum number of elements in the order of a few tens of elements when iterating a large collection, or may return all the elements of the collection in a single call when the iterated collection is small enough to be internally represented as an encoded data structure (this happens for small sets, hashes and sorted sets).
However there is a way for the user to tune the order of magnitude of the number of returned elements per call using the COUNT option.
While SCAN does not provide guarantees about the number of elements returned at every iteration, it is possible to empirically adjust the behavior of SCAN using the COUNT option. Basically with COUNT the user specified the amount of work that should be done at every call in order to retrieve elements from the collection. This is just a hint for the implementation, however generally speaking this is what you could expect most of the times from the implementation.
Important: there is no need to use the same COUNT value for every iteration. The caller is free to change the count from one iteration to the other as required, as long as the cursor passed in the next call is the one obtained in the previous call to the command.
It is possible to only iterate elements matching a given glob-style pattern, similarly to the behavior of the KEYS command that takes a pattern as only argument.
To do so, just append the
MATCH <pattern> arguments at the end of the SCAN command (it works with all the SCAN family commands).
This is an example of iteration using MATCH:
redis 127.0.0.1:6379> sadd myset 1 2 3 foo foobar feelsgood (integer) 6 redis 127.0.0.1:6379> sscan myset 0 match f* 1) "0" 2) 1) "foo" 2) "feelsgood" 3) "foobar" redis 127.0.0.1:6379>
It is important to note that the MATCH filter is applied after elements are retrieved from the collection, just before returning data to the client. This means that if the pattern matches very little elements inside the collection, SCAN will likely return no elements in most iterations. An example is shown below:
redis 127.0.0.1:6379> scan 0 MATCH *11* 1) "288" 2) 1) "key:911" redis 127.0.0.1:6379> scan 288 MATCH *11* 1) "224" 2) (empty list or set) redis 127.0.0.1:6379> scan 224 MATCH *11* 1) "80" 2) (empty list or set) redis 127.0.0.1:6379> scan 80 MATCH *11* 1) "176" 2) (empty list or set) redis 127.0.0.1:6379> scan 176 MATCH *11* COUNT 1000 1) "0" 2) 1) "key:611" 2) "key:711" 3) "key:118" 4) "key:117" 5) "key:311" 6) "key:112" 7) "key:111" 8) "key:110" 9) "key:113" 10) "key:211" 11) "key:411" 12) "key:115" 13) "key:116" 14) "key:114" 15) "key:119" 16) "key:811" 17) "key:511" 18) "key:11" redis 127.0.0.1:6379>
As you can see most of the calls returned zero elements, but the last call where a COUNT of 1000 was used in order to force the command to do more scanning for that iteration.
It is possible for an infinite number of clients to iterate the same collection at the same time, as the full state of the iterator is in the cursor, that is obtained and returned to the client at every call. Server side no state is taken at all.
Since there is no state server side, but the full state is captured by the cursor, the caller is free to terminate an iteration half-way without signaling this to the server in any way. An infinite number of iterations can be started and never terminated without any issue.
Calling SCAN with a broken, negative, out of range, or otherwise invalid cursor, will result into undefined behavior but never into a crash. What will be undefined is that the guarantees about the returned elements can no longer be ensured by the SCAN implementation.
The only valid cursors to use are:
The SCAN algorithm is guaranteed to terminate only if the size of the iterated collection remains bounded to a given maximum size, otherwise iterating a collection that always grows may result into SCAN to never terminate a full iteration.
This is easy to see intuitively: if the collection grows there is more and more work to do in order to visit all the possible elements, and the ability to terminate the iteration depends on the number of calls to SCAN and its COUNT option value compared with the rate at which the collection grows.
COUNT option documentation, we state that sometimes this family of commands may return all the elements of a Set, Hash or Sorted Set at once in a single call, regardless of the
COUNT option value. The reason why this happens is that the cursor-based iterator can be implemented, and is useful, only when the aggregate data type that we are scanning is represented as an hash table. However Redis uses a memory optimization where small aggregate data types, until they reach a given amount of items or a given max size of single elements, are represented using a compact single-allocation packed encoding. When this is the case, SCAN has no meaningful cursor to return, and must iterate the whole data structure at once, so the only sane behavior it has is to return everything in a call.
However once the data structures are bigger and are promoted to use real hash tables, the SCAN family of commands will resort to the normal behavior. Note that since this special behavior of returning all the elements is true only for small aggregates, it has no effects on the command complexity or latency. However the exact limits to get converted into real hash tables are user configurable, so the maximum number of elements you can see returned in a single call depends on how big an aggregate data type could be and still use the packed representation.
SCAN, SSCAN, HSCAN and ZSCAN return a two elements multi-bulk reply, where the first element is a string representing an unsigned 64 bit number (the cursor), and the second element is a multi-bulk with an array of elements.
Iteration of a Hash value.
redis 127.0.0.1:6379> hmset hash name Jack age 33 OK redis 127.0.0.1:6379> hscan hash 0 1) "0" 2) 1) "name" 2) "Jack" 3) "age" 4) "33"
© 2009–2018 Salvatore Sanfilippo
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