How to operate Redis through Lua script in Go language

Preface

In order to reduce the communication cost with Redis in one of my basic libraries, I encapsulated a series of operations into a LUA script and used the EVAL command provided by Redis to simplify operate.

Features that EVAL can provide:

• Can encapsulate several operations in a LUA script. If there are multiple Redis instructions, encapsulation After that, just send all the parameters to Redis at once to get the result

• Redis can guarantee that no other commands will be inserted and executed during the running of the Lua script, providing atomicity like database transactions.

• Redis will cache the script according to the SHA value of the script. The cached script does not need to transmit the Lua code again, which reduces the communication cost. In addition, when changing the Lua script in your own code, when executing Redis will also use the latest code.

Import common Go libraries such as "github.com/go-redis/redis" to implement the following code.

Generate a Lua script

// KEYS: key for record
// ARGV: fieldName, currentUnixTimestamp, recordTTL
// Update expire field of record key to current timestamp, and renew key expiration
var updateRecordExpireScript = redis.NewScript(`
redis.call("EXPIRE", KEYS[1], ARGV[3])
redis.call("HSET", KEYS[1], ARGV[1], ARGV[2])
return 1
`)

When this variable is created, the Lua code will not be executed, and there is no need to have an existing Redis connection.

The Lua script support provided by Redis has two arrays, KEYS and ARGV, by default. KEYS represents several key values ​​passed in when the script is run, and ARGV represents several parameters passed in. Since the Lua code needs to be kept concise and difficult to read, it is best to write some comments for these parameters

Note: The above code uses `` across lines, although the line where ` is located A blank carriage return will also be considered a line. Don't misread the code line number when reporting an error.

Run a Lua script

 updateRecordExpireScript.Run(c.Client, []string{recordKey(key)}, 
         expireField,
         time.Now().UTC().UnixNano(), int64(c.opt.RecordTTL/time.Second)).Err()

When running, Run will first try to run the script through the cache through EVALSHA. If there is no cache, use EVAL to run, and then the entire Lua script will be passed to Redis.

Limitations of Lua scripts

• Redis does not provide the introduction of additional packages, such as os, etc. Only the redis package is available.

• The Lua script will be run in a function, and all variables must be declared using local

• When return returns multiple values, Redis will Only give you the first

Type restrictions in the script

• What you get in Go when the script returns nil Yes err = redis.Nil (Same as Get cannot find the value)

• When the script returns false, what is obtained in Go is nil, and when the script returns true, What is obtained in Go is the int64 type 1

• When the script returns {"ok": ...}, what is obtained in Go is the status type of redis (true/false)

• When the script returns {"err": ...}, the err value is obtained in Go. You can also use return redis.error_reply("My Error") Achieved

• When the script returns the number type, what is obtained in Go is the int64 type

• In the script, if necessary, the incoming KEYS/ To convert the value in ARGV from string type to number type, you should use to_number function

What happens if the script runs for a long time?

During the running of the Lua script, in order to avoid data contamination by other operations, other commands cannot be executed during this period, and other requests cannot be continued until execution is completed. When the Lua script execution time exceeds lua-time-limit, other requests will receive Busy errors, unless these requests are SCRIPT KILL (kill the script) or SHUTDOWN NOSAVE (close Redis without saving the results)

For more information, please refer to the following address. Here I mainly provide some summaries based on my experience in using Go. https://redis.io/commands/eval

A more "complex" script, which requires that when obtaining a key value, if the value is accessed more often, the life cycle will be extended. In addition, the update time must be compared. If no update is required, the obtained value will be returned directly. Otherwise, redis.Nil

// KEYS: rec:key, key
// ARGV: currentUnixTimestamp, hotHit, recordTTL, ttl
// When there's a hit,
var fetchRecordScript = redis.NewScript(local value = redis.call("GET", KEYS[2]) if(value == nil) then return nil end local hit = redis.call("HINCRBY", KEYS[1], "hit", 1) redis.call("EXPIRE", KEYS[1], ARGV[3]) local minHotHit = tonumber(ARGV[2]) local keyTTL = tonumber(ARGV[4]) if(hit > minHotHit)then keyTTL = keyTTL * 2 end redis.call("EXPIRE", KEYS[2], keyTTL) local expire = tonumber(redis.call("HGET", KEYS[1], "expire")) local unixTime = tonumber(ARGV[1]) if(expire == nil or expire < unixTime) then return nil else return value end)
// KEYS: key for record
// ARGV: fieldName, currentUnixTimestamp, recordTTL
// Update expire field of record key to current timestamp, and renew key expiration
var updateRecordExpireScript = redis.NewScript(redis.call("EXPIRE", KEYS[1], ARGV[3]) redis.call("HSET", KEYS[1], ARGV[1], ARGV[2]) return 1)

will be returned.

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