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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。

    @Overridepublic 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。

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