Detailed explanation of distributed transactions implemented by Redis

With the continuous growth of Internet transactions, distributed transactions have become an essential part of business systems. With the continuous enrichment of distributed transaction implementation methods, Redis, as a widely used in-memory database, is gradually becoming the first choice for distributed transaction implementation. This article mainly introduces how Redis implements distributed transactions.

Transaction model of Redis

The transaction model supported by Redis is batch operation. Within a transaction, Redis can execute multiple commands. Multiple commands all succeed or fail in the same transaction, ensuring the integrity of the transaction, so Redis can support transactionality well.

The basic principle of Redis transactions is to package multiple commands into one transaction and then execute them atomically. However, it should be noted that the execution process of Redis transactions is different from other relational databases. General transactions will execute the following Several steps:

1. Start transaction
2. Execute multiple commands
3. Submit transaction

But the execution process of Redis transaction is as follows:

1. Start transaction
2. Execute multiple commands
3. Queue cache command
4. Submit transaction

When Redis executes a transaction, multiple commands will be put into the queue , these commands will be executed atomically until the transaction is committed. If an error occurs during the execution of the transaction, Redis will return the error code to the client and will not execute the unexecuted commands in the transaction.

Instance of Redis transaction

The following code demonstrates the simple use of Redis transaction:

require 'redis'

# 连接Redis数据库
redis = Redis.new(host: 'localhost', port: 6379)

# 初始化一个Redis事务对象
redis.multi do
  redis.set('key1', 'value1')
  redis.set('key2', 'value2')
end

In the above code, we use a Redis transaction object to execute two commands, this The purpose of the transaction is to set the value of key1 to value1 and the value of key2 to value2. Use the multi method to start a Redis transaction, use the set method to set the key value, and use the exec method to submit the Redis transaction. Finally, Redis will automatically package these commands into a transaction for atomic execution.

Redis distributed transaction implementation

In a distributed system, transaction operation data may be distributed in different nodes, so a mechanism is needed to ensure the consistency of distributed transactions . Redis's distributed transactions have the following implementation methods:

1. Use Redis's WATCH command

By using the WATCH command, Redis can monitor one or more key variables. If these variables have been modified during previous command execution, the transaction will stop executing, which can avoid errors caused by data being updated while processing the transaction. The following is an example of using WATCH:

require 'redis'

# 连接Redis数据库
redis = Redis.new(host: 'localhost', port: 6379)

# 监视key
redis.watch('key')

# 开始事务
redis.multi do
  redis.set('key1', 'value1')
end

# 提交事务
redis.exec

In the above code, we use watch to monitor whether the key has been modified. When the transaction starts to execute, if it is found that the key has been modified, the execution of the transaction will be terminated. After execution, submit it through the exec method.

2. Use Redis transaction lock

If you use the WATCH command, you need to consider the situation of concurrent access by multiple clients, which will cause performance degradation. Therefore, Redis also provides a distributed lock solution, which can use Redis's SETNX instruction or SET instruction to implement transaction locks. The SETNX instruction will only be set successfully when the set key does not exist. This ensures that the key will only be obtained by one client. The expiration time of the key can be set through the SET instruction to ensure the uniqueness and security of the lock. The code is as follows:

require 'redis'

# 连接Redis数据库
redis = Redis.new(host: 'localhost', port: 6379)

# 设置Redis锁
redis.set('lock_key', 'value', ex: 10, nx: true)

# 执行事务
redis.multi do
  redis.set('key1', 'value1')
end

# 释放Redis锁
redis.del('lock_key')

# 提交事务
redis.exec

In the above code, we use the set instruction to set the lock, set the expiration time to 10 seconds, and set the nx parameter at the same time to avoid repeated acquisition of the lock.

3. Use Redis's Lua script

In addition to the above two methods, you can also use Redis's Lua script to implement the distributed transaction function. Lua scripts support atomic operations, and multiple commands can be executed in the same operation. By customizing Lua scripts, we can package multiple Redis commands into an atomic operation to achieve distributed transactions. The following is an example of using Lua script:

redis.call('WATCH', KEYS[1])

if redis.call('GET', KEYS[1]) == ARGV[1] then
    redis.call('MULTI')
    redis.call('SET', KEYS[1], ARGV[2])
    redis.call('SET', KEYS[2], ARGV[3])
    redis.call('EXEC')
end

redis.call('UNWATCH')

In the above code, we use Lua script to implement distributed transactions. Before the transaction starts, use the WATCH instruction to monitor the status of the key. After execution, use the UNWATCH instruction to cancel monitoring of the key. Through Lua scripts, we can package multiple Redis commands into an atomic operation to achieve distributed transactions.

Summary

The transaction model of Redis is a batch operation, which ensures the integrity of the transaction. In a distributed system, transaction operation data may be distributed among different nodes, so a mechanism is needed to ensure the consistency of distributed transactions. Redis provides three implementation methods: WATCH command, transaction lock and Lua script. Which method to choose needs to be chosen based on business needs.

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