Redis Lua script detailed explanation of lua script high frequency interview questions

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Redis stay 2.6 Launched the script function , have access to Lua Language script to Redis perform .

Advantages of scripting ：

1. Reduce network overhead ： Originally 5 Number of network requests , You can use a request to complete , original 5 The second request logic is sent to redis The server completes . Reduce network round-trip delay , This is similar to a pipe .
2. Atomic manipulation ：Redis The entire script is executed as a whole , The middle is not inserted by other commands .
3. replace Redis The transaction function of ：Redis The built-in transaction function is very weak , and redis Of lua Scripts implement almost normal transaction functions , It is officially recommended that if you want to use redis The transaction function of can be considered as lua replace .

Script instructions

Through the built-in lua Interpreter , have access to EVAL Command to Lua Script job search , The command format is as follows ：

#  Script format 
EVAL script numkeys key [key ...] arg [arg ...]
#  Example 
127.0.0.1:6379> EVAL "return {KEYS[1],KEYS[2],ARGV[1],ARGV[2]}" 2 k1 k2 a1 a2 
1) "k1"
2) "k2"
3) "a1"
4) "a2"
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Parameter description ：

first script The argument is a segment Lua Script program , He will be run in redis Server context , This script is not necessary （ Should not be ） Define as a lua function .

the second numkeys Parameter is used to specify the number of key name parameters .

Start with the third parameter （ The key parameters key ）, Represents those used in scripts Redis key （key）, These key name parameters can be found in Lua Through the global variable array KEYS from 1 Start access as base address KEYS[1],KEYS[2].

At the end of the order , Additional parameters that are not key name parameters arg [arg ...] , Can be in lua Through global variables ARGV Array , Subscripts are also derived from 1 Start .

stay Lua You can use redis.call() Function to execute redis command .

Jedis Deduction inventory case call example ：

try (Jedis jedis = jedisPool.getResource()) {
    //  Initialize product id by 100 Inventory of 
    jedis.set("product_stock_100", "10000");
    // @formatter:off
    String script = "local count = redis.call('get',KEYS[1]) " +
            "local a = tonumber(count) " + 
            "local b = tonumber(ARGV[1]) " + 
            "if a >= b then " + 
            "redis.call('set',KEYS[1],a-b) " + 
            "return 1 " + 
            "end " + 
            "return 0";
    // @formatter:on
    Object decrResult = jedis.eval(script, Arrays.asList("product_stock_100"), Arrays.asList("100"));
    System.out.println(decrResult);
}
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** Be careful ：** Not in lua There are dead loops and time-consuming operations in the script , otherwise **redis It will block **, No other commands will be accepted , Therefore, the above situation shall not occur during use .

Redis Is a single process single thread execution script , The pipe will not block redis .

Redis LUA Will script execution commands roll back ?

Use lua A script can execute a string of redis On command , Achieve certain atomicity （lua During the execution of multiple instructions in the script, new instructions will not be inserted ）, however It is not possible to roll back the previous results when there is an error in command execution .

We use the following code snippet to verify , Let's clear it before we execute the code redis data .

127.0.0.1:6379> FLUSHALL
OK
127.0.0.1:6379> KEYS *
(empty array)
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Then execute the following code , Full code for official account Exception of a program Send keywords LUA obtain .

try (Jedis jedis = jedisPool.getResource()) {
    // @formatter:off
    String script = " redis.call('SET', 'success','100') " 
                    //  simulation redis  An exception message appears ,-1 Index in bitmap There is no error reporting 
                    + " redis.call('SETBIT', 'bit_test','-1','1') "
                    + " redis.call('SET', 'fail','100') " 
                    + " return 1";
    // @formatter:on
    Object eval = jedis.eval(script);
    System.out.println(eval);
}
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View after execution redis The data in ：

127.0.0.1:6379> KEYS *
1) "success"
127.0.0.1:6379> get success
"100"
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Conclusion ： Use lua A script can execute a string of redis On command , Achieve certain atomicity （lua During the execution of multiple instructions in the script, new instructions will not be inserted ）, however It is not possible to roll back the previous results when there is an error in command execution .