In-depth analysis of distributed locks in Redis

This article mainly introduces you to the implementation and code analysis of distributed locks in Redis. I hope it will be helpful to you!

Redis Distributed Lock

Distributed lock handles are used in everyone’s projects and are usually used to order data. Operation scenarios, such as refunding an order (if it can be refunded multiple times). Or users place orders through multiple terminals. [Related recommendations: Redis video tutorial]

Maven dependency

I am mainly based on Spring-Boot 2.1.2 Jedis Implementation

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.2.RELEASE</version>
    </parent>

    <groupId>cn.edu.cqvie</groupId>
    <artifactId>redis-lock</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <java.version>1.8</java.version>
        <redis.version>2.9.0</redis.version>
        <spring-test.version>5.0.7</spring-test.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.data</groupId>
            <artifactId>spring-data-redis</artifactId>
        </dependency>
        <dependency>
            <groupId>redis.clients</groupId>
            <artifactId>jedis</artifactId>
            <version>${redis.version}</version>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-logging</artifactId>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>log4j-over-slf4j</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

Configuration file

application.properties The content of the configuration file is as follows:

spring.redis.host=127.0.0.1
spring.redis.port=6379
spring.redis.password=
spring.redis.timeout=30000
spring.redis.jedis.pool.max-active=8
spring.redis.jedis.pool.min-idle=2
spring.redis.jedis.pool.max-idle=4


logging.level.root=INFO

Interface definition

Interface definition, for locking our core, there is actually only one methodlock and unlock.

public interface RedisLock {

    long TIMEOUT_MILLIS = 30000;

    int RETRY_MILLIS = 30000;

    long SLEEP_MILLIS = 10;

    boolean tryLock(String key);

    boolean lock(String key);

    boolean lock(String key, long expire);

    boolean lock(String key, long expire, long retryTimes);

    boolean unlock(String key);
}

Distributed lock implementation

My implementation is through setnx. , if tryLock logic exists, it will retry by spin

// AbstractRedisLock.java 抽象类
public abstract class AbstractRedisLock implements RedisLock {

    @Override
    public boolean lock(String key) {
        return lock(key, TIMEOUT_MILLIS);
    }

    @Override
    public boolean lock(String key, long expire) {
        return lock(key, TIMEOUT_MILLIS, RETRY_MILLIS);
    }
}

// 具体实现
@Component
public class RedisLockImpl extends AbstractRedisLock {

    private Logger logger = LoggerFactory.getLogger(getClass());

    @Autowired
    private RedisTemplate<String, String> redisTemplate;
    private ThreadLocal<String> threadLocal = new ThreadLocal<String>();
    private static final String UNLOCK_LUA;

    static {
        StringBuilder sb = new StringBuilder();
        sb.append("if redis.call(\"get\",KEYS[1]) == ARGV[1] ");
        sb.append("then ");
        sb.append("    return redis.call(\"del\",KEYS[1]) ");
        sb.append("else ");
        sb.append("    return 0 ");
        sb.append("end ");
        UNLOCK_LUA = sb.toString();

    }

    @Override
    public boolean tryLock(String key) {
        return tryLock(key, TIMEOUT_MILLIS);
    }

    public boolean tryLock(String key, long expire) {
        try {
            return !StringUtils.isEmpty(redisTemplate.execute((RedisCallback<String>) connection -> {
                JedisCommands commands = (JedisCommands) connection.getNativeConnection();
                String uuid = UUID.randomUUID().toString();
                threadLocal.set(uuid);
                return commands.set(key, uuid, "NX", "PX", expire);
            }));
        } catch (Throwable e) {
            logger.error("set redis occurred an exception", e);
        }
        return false;
    }

    @Override
    public boolean lock(String key, long expire, long retryTimes) {
        boolean result = tryLock(key, expire);

        while (!result && retryTimes-- > 0) {
            try {
                logger.debug("lock failed, retrying...{}", retryTimes);
                Thread.sleep(SLEEP_MILLIS);
            } catch (InterruptedException e) {
                return false;
            }
            result = tryLock(key, expire);
        }
        return result;
    }

    @Override
    public boolean unlock(String key) {
        try {
            List<String> keys = Collections.singletonList(key);
            List<String> args = Collections.singletonList(threadLocal.get());
            Long result = redisTemplate.execute((RedisCallback<Long>) connection -> {
                Object nativeConnection = connection.getNativeConnection();

                if (nativeConnection instanceof JedisCluster) {
                    return (Long) ((JedisCluster) nativeConnection).eval(UNLOCK_LUA, keys, args);
                }
                if (nativeConnection instanceof Jedis) {
                    return (Long) ((Jedis) nativeConnection).eval(UNLOCK_LUA, keys, args);
                }
                return 0L;
            });
            return result != null && result > 0;
        } catch (Throwable e) {
            logger.error("unlock occurred an exception", e);
        }
        return false;
    }
}

Test code

Finally let’s take a look at how to use it. (The following is a simulated flash sale scenario)

@RunWith(SpringRunner.class)
@SpringBootTest
public class RedisLockImplTest {

    private Logger logger = LoggerFactory.getLogger(getClass());
    @Autowired
    private RedisLock redisLock;
    @Autowired
    private StringRedisTemplate redisTemplate;
    private ExecutorService executors = Executors.newScheduledThreadPool(8);

    @Test
    public void lock() {
        // 初始化库存
        redisTemplate.opsForValue().set("goods-seckill", "10");
        List<Future> futureList = new ArrayList<>();

        for (int i = 0; i < 100; i++) {
            futureList.add(executors.submit(this::seckill));
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        // 等待结果，防止主线程退出
        futureList.forEach(action -> {
            try {
                action.get();
            } catch (InterruptedException | ExecutionException e) {
                e.printStackTrace();
            }
        });

    }

    public int seckill() {
        String key = "goods";
        try {
            redisLock.lock(key);
            int num = Integer.valueOf(Objects.requireNonNull(redisTemplate.opsForValue().get("goods-seckill")));
            if (num > 0) {
                redisTemplate.opsForValue().set("goods-seckill", String.valueOf(--num));
                logger.info("秒杀成功，剩余库存：{}", num);
            } else {
                logger.error("秒杀失败，剩余库存：{}", num);
            }
            return num;
        } catch (Throwable e) {
            logger.error("seckill exception", e);
        } finally {
            redisLock.unlock(key);
        }
        return 0;
    }
}

Summary

This article is a simple method of Redis lock The implementation method implements the lock retry operation based on jedis. However, there are still disadvantages. It does not support automatic renewal of locks, lock reentrancy, and fairness (currently implemented through spin, which is equivalent to an unfair method).

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