Redis Demo系列之(三)累加器

我就是我 2022-05-18 00:10 299阅读 0赞

### 前言 ###

累加器也是Redis场见的应用之一。值得注意的是Redis内的命令诸如`incr`等命令皆为原子操作。但是多命令组合而成的操作，为非原子操作。（如abc=incr(hello) get(hello) set(hello,abc)。此类操作需要使用`Multi`或`Lua`脚本保证Redis内操作的正确性。

本文相关代码，可在我的Github项目[https://github.com/SeanYanxml/bigdata/tree/master/redis][https_github.com_SeanYanxml_bigdata_tree_master_redis] 目录下可以找到。  
PS: (如果觉得项目不错，可以给我一个Star。)

--------------------

### Demos ###

/** * 记录Redis累加器。 * * */
    
    public class RedisCounterDemo { 
    
        // demo1: first not perfect
        public static Jedis jedis = new JedisPoolManager().getJedis();
    
        public static long INCR_END_COUNT = 10;
        public static long DECR_END_COUNT = 0;
    
    
        // 建议操作为 自增在前 操作在后。
        // 如果 操作在前 自增在后。会导致先执行操作，后判断。易导致数据总值变多。(即秒杀时，显示无库存。仍然认为有。造成损失。)
        public static boolean increase(String key){
            Long count = jedis.incr(key);
            if(count > INCR_END_COUNT){
                // do something
                return true;
            }
            return false;
        }
    
        public static boolean decrease(String key){
            Long count = jedis.decr(key);
            if(count > DECR_END_COUNT){
                // do something
                return true;
            }
            return false;
        }
    
        // 使用LUA 判断 
        // < LUA ERROR ? why? warn the type
        // have the string key first. 
        public static boolean execLua(String key){
            String script = "if tonumber(redis.call('get', KEYS[1])) > 100 then return redis.call('incr',KEYS[1]) else return 0 end";
            Object result = jedis.eval(script, Collections.singletonList(key), Collections.singletonList(key));
            if ((long)result > 0l) {
                // do something
                System.out.println("Hello");
                return true;
            }
            return false;
        }
    
        // 使用multi事务进行处理
        public static boolean execMulti(String key){
            return false;
        }
    
        public static void main(String[] args) {
            execLua("hello");
        }
    
    }
    
    //to number
    //https://blog.csdn.net/u013916933/article/details/54962768
    
    //https://stackoverflow.com/questions/21707021/lua-script-for-redis-which-sums-the-values-of-keys
    
    //Exception in thread "main" redis.clients.jedis.exceptions.JedisDataException: ERR Error running script (call to f_1fb04376fc5dc524aeac5a008daa880db5104604): @user_script:1: user_script:1: attempt to compare number with nil 
    //at redis.clients.jedis.Protocol.processError(Protocol.java:127)
    //at redis.clients.jedis.Protocol.process(Protocol.java:161)
    //at redis.clients.jedis.Protocol.read(Protocol.java:215)
    //at redis.clients.jedis.Connection.readProtocolWithCheckingBroken(Connection.java:340)
    //at redis.clients.jedis.Connection.getOne(Connection.java:322)
    //at redis.clients.jedis.Jedis.getEvalResult(Jedis.java:2731)
    //at redis.clients.jedis.Jedis.eval(Jedis.java:2671)
    //at redis.clients.jedis.Jedis.eval(Jedis.java:2719)
    //at com.yanxml.redis.demos.count.RedisCounterDemo.execLua(RedisCounterDemo.java:47)
    //at com.yanxml.redis.demos.count.RedisCounterDemo.main(RedisCounterDemo.java:62)

--------------------

### Reference ###

\[1\]. [redis 的incr/decr 的原子性是什么意思？][redis _incr_decr]  
\[2\]. [Redis研究（七）—如何判断set/get是否为原子操作][Redis_set_get]  
\[3\]. [ Redis 原子操作INCR ][Redis _INCR]  
\[4\]. [Redis 的操作为什么是的原子性的详解][Redis]  
\[5\]. [redis如何实现原子性-LPOPRPUSH][redis_-LPOPRPUSH]

[https_github.com_SeanYanxml_bigdata_tree_master_redis]: https://github.com/SeanYanxml/bigdata/tree/master/redis
[redis _incr_decr]: https://blog.csdn.net/u010412301/article/details/60595435
[Redis_set_get]: https://blog.csdn.net/wtyvhreal/article/details/42750791
[Redis _INCR]: https://www.cnblogs.com/mengjianzhou/p/6903341.html
[Redis]: https://blog.csdn.net/ahjxhy2010/article/details/80520434
[redis_-LPOPRPUSH]: https://bbs.csdn.net/topics/392021207