前面已经写了一篇Redisson的分布式限流的使用，Redisson分布式限流的简单实践，对其中的原理很好奇。

一、使用

// 1、 声明一个限流器
RRateLimiter rateLimiter = redissonClient.getRateLimiter(key);

// 2、 设置速率，5秒中产生3个令牌
rateLimiter.trySetRate(RateType.OVERALL, 3, 5, RateIntervalUnit.SECONDS);

// 3、试图获取一个令牌，获取到返回true
rateLimiter.tryAcquire(1)

二、原理

1. getRateLimiter

// 声明一个限流器 名称 叫key
redissonClient.getRateLimiter(key)

2. RedissonRateLimiter#trySetRate。5秒中产生3个令牌。rateInterval指的是时间间隔，rate指的是指定时间间隔产生的令牌数。

@Override
public boolean trySetRate(RateType type, long rate, long rateInterval, RateIntervalUnit unit) {
    return get(trySetRateAsync(type, rate, rateInterval, unit));
}

@Override
public RFuture<Boolean> trySetRateAsync(RateType type, long rate, long rateInterval, RateIntervalUnit unit) {
    return commandExecutor.evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
            "redis.call('hsetnx', KEYS[1], 'rate', ARGV[1]);"
          + "redis.call('hsetnx', KEYS[1], 'interval', ARGV[2]);"
          + "return redis.call('hsetnx', KEYS[1], 'type', ARGV[3]);",
            Collections.singletonList(getRawName()), rate, unit.toMillis(rateInterval), type.ordinal());
}

3. RedissonRateLimiter#tryAcquire()。
   • key的数组是 Arrays.asList(getRawName(), getValueName(), getClientValueName(), getPermitsName(), getClientPermitsName())。
   • 如果name是rate.limiter，那么lua脚本中的valueName是{rate.limiter}:value；permitsName是{rate.limiter}:permits。key[3]的结果是getClientValueName()，{rate.limiter}:value:4802866e-25b3-4482-aa74-61aa947d6f7a，需要拼接机器的唯一id。key[5]的结果是{rate.limiter}:permits:4802866e-25b3-4482-aa74-61aa947d6f7a。
   • tonumber(rate) >= tonumber(ARGV[1])，表明rate要比请求的令牌数大。
   • 如果首次获取，设置valueName为rate，设置permitsName的score为当前时间戳，设置值为随机数和获取的令牌数，更新valueName，减去需要获取的令牌数。
   • 第二次获取令牌执行，获取0-（当前时间-生成令牌间隔interval)时间内的数据。获取之前所有的请求数released，如果released>0，更新valueName为当前值+释放令牌数。之前的请求令牌数 > 0, 例如10s产生3个令牌，现在超过10s了，重置周期并计算剩余令牌数。
   • 如果当前可提供的令牌数小于获取的令牌数，获取最近一次的记录。返回当前key的剩余过期时间。上一次请求的时间戳 - (当前时间戳 - 令牌生成的时间间隔) ，这个值表示还需要多久才能生产出足够的令牌。
   • 如果当前令牌数 ≥ 请求的令牌数，表示令牌够多，直接更新zset。

    @Override
    public boolean tryAcquire(long permits) {
        return get(tryAcquireAsync(RedisCommands.EVAL_NULL_BOOLEAN, permits));
    }

    private <T> RFuture<T> tryAcquireAsync(RedisCommand<T> command, Long value) {
        return commandExecutor.evalWriteAsync(getRawName(), LongCodec.INSTANCE, command,
                "local rate = redis.call('hget', KEYS[1], 'rate');"
              + "local interval = redis.call('hget', KEYS[1], 'interval');"
              + "local type = redis.call('hget', KEYS[1], 'type');"
              + "assert(rate ~= false and interval ~= false and type ~= false, 'RateLimiter is not initialized')"
              
              + "local valueName = KEYS[2];"
              + "local permitsName = KEYS[4];"
              + "if type == '1' then "
                  + "valueName = KEYS[3];"
                  + "permitsName = KEYS[5];"
              + "end;"

              + "assert(tonumber(rate) >= tonumber(ARGV[1]), 'Requested permits amount could not exceed defined rate'); "

              + "local currentValue = redis.call('get', valueName); "
              + "if currentValue ~= false then "
                     + "local expiredValues = redis.call('zrangebyscore', permitsName, 0, tonumber(ARGV[2]) - interval); "
                     + "local released = 0; "
                     + "for i, v in ipairs(expiredValues) do "
                          + "local random, permits = struct.unpack('fI', v);"
                          + "released = released + permits;"
                     + "end; "

                     + "if released > 0 then "
                          + "redis.call('zremrangebyscore', permitsName, 0, tonumber(ARGV[2]) - interval); "
                          + "currentValue = tonumber(currentValue) + released; "
                          + "redis.call('set', valueName, currentValue);"
                     + "end;"

                     + "if tonumber(currentValue) < tonumber(ARGV[1]) then "
                         + "local nearest = redis.call('zrangebyscore', permitsName, '(' .. (tonumber(ARGV[2]) - interval), '+inf', 'withscores', 'limit', 0, 1); "
                         + "return tonumber(nearest[2]) - (tonumber(ARGV[2]) - interval);"
                     + "else "
                         + "redis.call('zadd', permitsName, ARGV[2], struct.pack('fI', ARGV[3], ARGV[1])); "
                         + "redis.call('decrby', valueName, ARGV[1]); "
                         + "return nil; "
                     + "end; "
              + "else "
                     + "redis.call('set', valueName, rate); "
                     + "redis.call('zadd', permitsName, ARGV[2], struct.pack('fI', ARGV[3], ARGV[1])); "
                     + "redis.call('decrby', valueName, ARGV[1]); "
                     + "return nil; "
              + "end;",
                Arrays.asList(getRawName(), getValueName(), getClientValueName(), getPermitsName(), getClientPermitsName()),
                value, System.currentTimeMillis(), ThreadLocalRandom.current().nextLong());
    }

RedissonRateLimiter#getValueName，生成valueName。

    String getValueName() {
        return suffixName(getRawName(), "value");
    }  

    String getPermitsName() {
        return suffixName(getRawName(), "permits");
    }

	public static String suffixName(String name, String suffix) {
        if (name.contains("{")) {
            return name + ":" + suffix;
        }
        return "{" + name + "}:" + suffix;
    }

RedissonRateLimiter#tryAcquire(long, long, java.util.concurrent.TimeUnit)，带时限的获取令牌。delay就是lua脚本返回的，还需要多久才会有令牌。如果获取令牌的时间比设置的超时时间还要大的话，直接就false了，否则会再次尝试获取令牌。

	@Override
	public boolean tryAcquire(long permits, long timeout, TimeUnit unit) {
    	return get(tryAcquireAsync(permits, timeout, unit));
	}

    @Override
    public RFuture<Boolean> tryAcquireAsync(long permits, long timeout, TimeUnit unit) {
        RPromise<Boolean> promise = new RedissonPromise<Boolean>();
        long timeoutInMillis = -1;
        if (timeout >= 0) {
            timeoutInMillis = unit.toMillis(timeout);
        }
        tryAcquireAsync(permits, promise, timeoutInMillis);
        return promise;
    }
    
    private void tryAcquireAsync(long permits, RPromise<Boolean> promise, long timeoutInMillis) {
        long s = System.currentTimeMillis();
        RFuture<Long> future = tryAcquireAsync(RedisCommands.EVAL_LONG, permits);
        future.onComplete((delay, e) -> {
            if (e != null) {
                promise.tryFailure(e);
                return;
            }
            
            if (delay == null) {
                promise.trySuccess(true);
                return;
            }
            
            if (timeoutInMillis == -1) {
                commandExecutor.getConnectionManager().getGroup().schedule(() -> {
                    tryAcquireAsync(permits, promise, timeoutInMillis);
                }, delay, TimeUnit.MILLISECONDS);
                return;
            }
            
            long el = System.currentTimeMillis() - s;
            long remains = timeoutInMillis - el;
            if (remains <= 0) {
                promise.trySuccess(false);
                return;
            }
            if (remains < delay) {
                commandExecutor.getConnectionManager().getGroup().schedule(() -> {
                    promise.trySuccess(false);
                }, remains, TimeUnit.MILLISECONDS);
            } else {
                long start = System.currentTimeMillis();
                commandExecutor.getConnectionManager().getGroup().schedule(() -> {
                    long elapsed = System.currentTimeMillis() - start;
                    if (remains <= elapsed) {
                        promise.trySuccess(false);
                        return;
                    }
                    
                    tryAcquireAsync(permits, promise, remains - elapsed);
                }, delay, TimeUnit.MILLISECONDS);
            }
        });
    }