The application the editor is responsible for is a management background application. Shiro framework is used for permission management. Since there are multiple nodes, distributed Session needs to be used, so Redis is used to store Session information.
Since Shiro does not directly provide the Redis storage Session component, Afan had to use shiro-redis, an open source component from Github.
Since the Shiro framework needs to regularly verify whether the Session is valid, the bottom layer of Shiro will call SessionDAO#getActiveSessions
to obtain all Session information.
And shiro-redis
happens to inherit the SessionDAO
interface. The bottom layer uses the keys
command to find all the Session## stored in Redis. #key.
public Set<byte[]> keys(byte[] pattern){ checkAndInit(); Set<byte[]> keys = null; Jedis jedis = jedisPool.getResource(); try{ keys = jedis.keys(pattern); }finally{ jedis.close(); } return keys; }
shiro-redis to the latest version.
shiro-redis uses the
scan command instead of
keys to fix this problem.
public Set<byte[]> keys(byte[] pattern) { Set<byte[]> keys = null; Jedis jedis = jedisPool.getResource(); try{ keys = new HashSet<byte[]>(); ScanParams params = new ScanParams(); params.count(count); params.match(pattern); byte[] cursor = ScanParams.SCAN_POINTER_START_BINARY; ScanResult<byte[]> scanResult; do{ scanResult = jedis.scan(cursor,params); keys.addAll(scanResult.getResult()); cursor = scanResult.getCursorAsBytes(); }while(scanResult.getStringCursor().compareTo(ScanParams.SCAN_POINTER_START) > 0); }finally{ jedis.close(); } return keys; }
keys command cause other commands to execute slower?
Keys command query is so slow?
Scan command?
keys instruction cause the execution of other commands to slow down?
Keys command query so slow?
keys The command needs to return all Redis keys that match the given pattern
pattern. In order to achieve this purpose, Redis has to traverse the dictionary
ht[ 0]The underlying array of the hash table, this time complexity is
『O(N)』 (N is the number of keys in Redis).
keys to query all keys. This query will block for about ten seconds. .
eval "for i=1,100000 do redis.call('set',i,i+1) end" 0
Here Afan uses Docker to deploy Redis, and the performance may be slightly worse.SCAN PrincipleFinally let’s look at the third question, why is there no problem with the
scan command?
scan command uses a black technology-
"cursor-based iterator".
scan command is called, Redis will return a new cursor and a certain number of keys to the user. If you want to continue to obtain the remaining keys next time, you need to pass this cursor to the scan command to continue the previous iteration process.
scan command uses paging to query redis.
scan The command uses cursors to cleverly split a full query into multiple times, reducing the complexity of the query. Spend.
scan command is the same as
keys, both are
「O(N)」, but due to scan The command only needs to return a small number of keys, so the execution speed will be very fast.
scan command solves the
keys shortcomings, it also introduces some other defects:
During the iteration process, elements being added to Redis or elements being deleted may be returned, or they may not be returned.
The above defects need to be considered in our development.
In addition to scan
, redis has several other commands for incremental iteration:
sscan
: use Used to iterate the database keys in the current database, used to solve the smembers
possible blocking problem
hscan
command is used to iterate the hash keys Key-value pairs are used to solve hgetall
possible blocking problems.
zscan
: The command is used to iterate the elements in the ordered set (including element members and element scores), and is used to generate zrange
Blocking problems may occur.
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