How Redis achieves the consistency of distributed transactions

Redis is a high-performance, distributed memory database that is widely used in distributed systems. In distributed systems, how to achieve transaction consistency has always been a problem, and the transaction mechanism provided by Redis can help developers solve this problem. This article will introduce how Redis achieves the consistency of distributed transactions and show code examples.

1. Introduction to Redis transaction mechanism

Redis provides a transaction mechanism in version 2.0, which is implemented through five commands: MULTI, EXEC, WATCH, DISCARD and UNWATCH. The operations in the transaction will be recorded sequentially in a queue and executed in batches when the EXEC command is called. If the entire transaction is submitted successfully, all operations in the record queue will be executed in sequence; if an operation fails, the entire transaction will be rolled back. Multiple clients can start their own transactions at the same time. Since they are executed in the operation record queue, the transactions are independent of each other.

2. Redis distributed transaction implementation principle

In a Redis stand-alone transaction, each client is processed by the same process, but in a distributed situation, each client may Corresponding to different Redis instances, it is necessary to implement distributed transaction consistency to ensure the correctness of the data.

The key to Redis implementing distributed transactions lies in the WATCH and UNWATCH commands. Each client can mark some key data in Redis through the WATCH command. When these data are modified by other clients, the client's transaction will be terminated. This mark can be removed through the UNWATCH command. The reason for this is that when the user starts a transaction, if there is the same write competition with other clients, the transaction will be rolled back and a transaction failure signal will be set. In this process, the client needs to uniquely identify all its key data that needs to be monitored. When a conflict occurs, the client will use these identifications to determine whether the transaction needs to be rolled back. If a rollback is required, the client retries the transaction.

3. Code Example

Below we use Python to implement a simple distributed transaction, simulating two clients to execute transactions on different Redis instances to implement transfer operations, requiring that the transfer must Successfully, use the WATCH/UNWATCH command to achieve consistency control.

Prerequisites:

• Python 3.x
• Redis-py

The code is as follows:

import redis

# 新建两个 Redis 实例 
redis1 = redis.StrictRedis(host="localhost", port=6379, db=0)
redis2 = redis.StrictRedis(host="localhost", port=6380, db=0)

# 我们模拟一下一个转帐操作
def transfer(from_user, to_user, value):
    # 两个实例都要执行事务
    tx = redis1.pipeline()
    tx2 = redis2.pipeline()

    # Watch 监控 from_user 和 to_user 的 balance 值
    tx.watch(from_user, to_user)
    tx2.watch(from_user, to_user)

    # 如果 from_user 的 balance 值减去转账数值，小于0
    if tx.get(from_user) < int(value):
        tx.unwatch()
    else:
        tx.multi()
        tx.decrby(from_user, int(value))

        # 通过2个实例之间的网络通信，将 balance 放入另一个
        tx2.multi()
        tx2.incrby(to_user, int(value))

        print(tx.execute())
        print(tx2.execute())

transfer('user1', 'user2', '100') #执行转账操作

In the code Two new Redis instances were created. Then a transfer function is defined, which simulates a transfer operation and requires the from_user, to_user and value parameters of the transfer to be passed in. Within the function, the core part is to use the WATCH command to monitor the balance values ​​of from_user and to_user on two Redis instances to avoid race conditions during the transfer process. Then use the transaction to change the balance on the two Redis instances to ensure the consistency of the transfer operation.

Summary

Redis supports transaction mechanism to ensure consistency on a single Redis instance. However, in a distributed environment, in order to ensure consistency on multiple Redis instances, a distributed transaction mechanism needs to be introduced. Redis implements this mechanism through the WATCH and UNWATCH commands. We can better understand the implementation principles of Redis distributed transactions through code examples.

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