SCAN cursor [MATCH pattern] [COUNT count] [TYPE type]
Time complexity: O(1) for every call. O(N) for a complete iteration, including enough command calls for the cursor to return back to 0. N is the number of elements inside the collection.
SCAN command and the closely related commands
ZSCAN are used in order to incrementally iterate over a collection of elements.
SCANiterates the set of keys in the currently selected Dragonfly database.
SSCANiterates elements of Sets types.
HSCANiterates fields of Hash types and their associated values.
ZSCANiterates elements of Sorted Set types and their associated scores.
ZSCAN all work very similarly, so this documentation covers all the four commands. However an obvious difference is that in the case of
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 basic usage
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:
dragonfly> scan 0
2) 1) "key:12"
dragonfly> scan 17
2) 1) "key:5"
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.
SCAN command, and the other commands in the
SCAN family, are able to provide to the user a set of guarantees associated to full iterations.
- A full iteration always retrieves all the elements that were present in the collection from the start to the end of a full iteration. This means that if a given element is inside the collection when an iteration is started, and is still there when an iteration terminates, then at some point
SCANreturned it to the user.
- A full iteration never returns any element that was NOT present in the collection from the start to the end of a full iteration. So if an element was removed before the start of an iteration, and is never added back to the collection for all the time an iteration lasts,
SCANensures that this element will never be returned.
SCAN has very little state associated (just the cursor) it has the following drawbacks:
- A given element may be returned multiple times. It is up to the application to handle the case of duplicated elements, for example only using the returned elements in order to perform operations that are safe when re-applied multiple times.
- Elements that were not constantly present in the collection during a full iteration, may be returned or not: it is undefined.
Number of elements returned at every SCAN call
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.
The COUNT option
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.
- The default COUNT value is 10.
- When iterating the key space, or a Set, Hash or Sorted Set, assuming no MATCH option is used, the server will usually return count or a bit more than count elements per call.
- When iterating Sets encoded as intsets (small sets composed of just integers) usually all the elements are returned in the first
SCANcall regardless of the COUNT value.
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.
The MATCH option
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 its 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:
dragonfly> sadd myset 1 2 3 foo foobar feelsgood
dragonfly> sscan myset 0 match f*
2) 1) "foo"
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:
dragonfly> scan 0 MATCH *11*
2) 1) "key:911"
dragonfly> scan 288 MATCH *11*
2) (empty list or set)
dragonfly> scan 224 MATCH *11*
2) (empty list or set)
dragonfly> scan 80 MATCH *11*
2) (empty list or set)
dragonfly> scan 176 MATCH *11* COUNT 1000
2) 1) "key:611"
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.
The TYPE option
You can use the
!TYPE option to ask
SCAN to only return objects that match a given
type, allowing you to iterate through the database looking for keys of a specific type. The TYPE option is only available on the whole-database
type argument is the same string name that the
TYPE command returns.
dragonfly> ZADD zkey1 1000 value
dragonfly> ZADD zkey2 1000 value
dragonfly> TYPE zkey1
dragonfly> SCAN 0 TYPE zset
2) 1) "zkey1"
It is important to note that the TYPE filter is also applied after elements are retrieved from the database, so the option does not reduce the amount of work the server has to do to complete a full iteration, and for rare types you may receive no elements in many iterations.
Multiple parallel iterations
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. No server side state is taken at all.
Terminating iterations in the middle
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 corrupted cursor
SCAN with a broken, negative, out of range, or otherwise invalid cursor, will result in undefined behavior but never in a crash. What will be undefined is that the guarantees about the returned elements can no longer be ensured by the
The only valid cursors to use are:
- The cursor value of 0 when starting an iteration.
- The cursor returned by the previous call to SCAN in order to continue the iteration.
Guarantee of termination
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.
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.
SCANarray of elements is a list of keys.
SSCANarray of elements is a list of Set members.
HSCANarray of elements contain two elements, a field and a value, for every returned element of the Hash.
ZSCANarray of elements contain two elements, a member and its associated score, for every returned element of the sorted set.
Iteration of a Hash value.
dragonfly> hmset hash name Jack age 33
dragonfly> hscan hash 0
2) 1) "name"