浅谈限流组件的应用和设计原则

## Redisson源码片段[Java]：获取令牌。

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());

## Sentinel源码片段[Java]：利用ConcurrentLinkedHashMap实现LRU，可以淘汰老数据。

public class ConcurrentLinkedHashMapWrapper<T, R> implements CacheMap<T, R> {

    private final ConcurrentLinkedHashMap<T, R> map;

    public ConcurrentLinkedHashMapWrapper(long size) {
        if (size <= 0) {
            throw new IllegalArgumentException("Cache max capacity should be positive: " + size);
        }
        this.map = new ConcurrentLinkedHashMap.Builder<T, R>()
            .concurrencyLevel(DEFAULT_CONCURRENCY_LEVEL)
            .maximumWeightedCapacity(size)
            .weigher(Weighers.singleton())
            .build();
    }
    ...
}

## Redis-cell源码[rust]：获取令牌前，主动同步当前时间。    

	/// Gets the given key's value and the current time as dictated by the
    /// store (this is done so that rate limiters running on a variety of
    /// different nodes can operate with a consistent clock instead of using
    /// their own). If the key was unset, -1 is returned.
    fn get_with_time(&self, key: &str) -> Result<(i64, time::Tm), CellError>;

    fn get_with_time(&self, key: &str) -> Result<(i64, time::Tm), CellError> {
        match self.map.get(key) {
            Some(n) => Ok((*n, time::now_utc())),
            None => Ok((-1, time::now_utc())),
        }
    }

	// 下面的代码是rust库函数，获取时间

	/// Returns the current time in UTC
	pub fn now_utc() -> Tm {
    	at_utc(get_time())
	}

    /**
     * Returns the current time as a `timespec` containing the seconds and
     * nanoseconds since 1970-01-01T00:00:00Z.
     */
    pub fn get_time() -> Timespec {
        let (sec, nsec) = sys::get_time();
        Timespec::new(sec, nsec)
    }

    pub fn get_time() -> (i64, i32) {
        let mut tv = libc::timespec { tv_sec: 0, tv_nsec: 0 };
        unsafe { libc::clock_gettime(libc::CLOCK_REALTIME, &mut tv); }
        (tv.tv_sec as i64, tv.tv_nsec as i32)
    }