Using scan in redistemplate

SCAN brief introduction

SCAN  Orders and related  SSCAN  command 、 HSCAN  Command and  ZSCAN  Commands are used to iterate incrementally （incrementally iterate） An episode of elements （a collection of elements）：

• SCAN  The command is used to iterate over the database keys in the current database .
• SSCAN  The command is used to iterate over the elements in the set key .
• HSCAN  The command is used to iterate over key value pairs in hash keys .
• ZSCAN  The command is used to iterate over elements in an ordered set （ Include element members and element scores ）.

 

SCAN and KEYS The difference between

When  KEYS  When the command is used to process a large database , Or,  SMEMBERS  When the command is used to handle a large set key , They lock redis library , May block the server for several seconds . High concurrency can cause a large number of requests to pile up and cause service avalanche . Some companies directly disable it in the production environment kyes * command . But in redis The server key When the number is small , Use keys There's no problem .

SCAN  Orders and related  SSCAN  command 、 HSCAN  Command and  ZSCAN  Commands are used to iterate incrementally , They return only a few elements each time they execute , It doesn't block the server , So these commands can be used in the production environment , But not like  KEYS  command 、 SMEMBERS  Problems with orders .

SCAN It has its own problems ：

1. Because it's segmentation key, So it will ask for redis The server , This is bound to take the same key,scan Takes longer .
2. In the process of incrementally iterating on the bond , The key may be modified , So the incremental iteration command can only provide limited guarantee for the returned elements
3. 
   

   /**
    * redis Extension tools 
    *
    * @author yuhao.wang3
    * @since 2020/2/21 23:35
    */
   public abstract class RedisHelper {
       private static Logger logger = LoggerFactory.getLogger(RedisHelper.class);
   
       /**
        * scan  Realization 
        *
        * @param redisTemplate redisTemplate
        * @param pattern        expression , Such as ：abc*, Find out everything to abc The start key 
        */
       public static Set<String> scan(RedisTemplate<String, Object> redisTemplate, String pattern) {
           return redisTemplate.execute((RedisCallback<Set<String>>) connection -> {
               Set<String> keysTmp = new HashSet<>();
               try (Cursor<byte[]> cursor = connection.scan(new ScanOptions.ScanOptionsBuilder()
                       .match(pattern)
                       .count(10000).build())) {
   
                   while (cursor.hasNext()) {
                       keysTmp.add(new String(cursor.next(), "Utf-8"));
                   }
               } catch (Exception e) {
                   logger.error(e.getMessage(), e);
                   throw new RuntimeException(e);
               }
               return keysTmp;
           });
       }
   }