redis如何实现分布式限流

Redis可以使用令牌桶算法来实现分布式限流。令牌桶算法是一种常用的限流算法，它通过维护一个固定容量的令牌桶，每秒钟往桶里放入一定数量的令牌。当请求到达时，如果令牌桶中有足够的令牌，那么允许请求通过并消耗一个令牌；如果令牌桶中没有足够的令牌，则拒绝请求。

以下是使用Redis实现分布式限流的步骤：

使用Redis的Lua脚本编写一个令牌桶算法的限流器。Lua脚本可以在Redis服务器端执行，可以保证原子性。以下是一个简单的令牌桶算法的Lua脚本示例：

local key = KEYS[1]
local capacity = tonumber(ARGV[1])
local rate = tonumber(ARGV[2])
local now = tonumber(ARGV[3])
local tokens_key = key .. ":tokens"
local timestamp_key = key .. ":timestamp"
local tokens = tonumber(redis.call("get", tokens_key))
if not tokens then
tokens = capacity
end
local last_refreshed = tonumber(redis.call("get", timestamp_key))
if not last_refreshed then
last_refreshed = now
end
local delta = math.max(0, now - last_refreshed)
local filled_tokens = math.min(capacity, tokens + delta * rate)
local allowed = filled_tokens >= 1
local new_tokens = filled_tokens
local new_timestamp = last_refreshed
if allowed then
new_tokens = filled_tokens - 1
new_timestamp = now
end
redis.call("set", tokens_key, new_tokens)
redis.call("set", timestamp_key, new_timestamp)
return allowed

在代码中调用该Lua脚本，传入限流器的唯一标识符、限流器的容量、限流器的速率以及当前时间戳作为参数，获取限流结果。

import redis.clients.jedis.Jedis;
import redis.clients.jedis.JedisPool;
public class RedisRateLimiter {
private JedisPool jedisPool;
public RedisRateLimiter(JedisPool jedisPool) {
this.jedisPool = jedisPool;
}
public boolean allowRequest(String key, int capacity, int rate) {
try (Jedis jedis = jedisPool.getResource()) {
long now = System.currentTimeMillis();
Object result = jedis.eval(
"local key = KEYS[1]\n" +
"local capacity = tonumber(ARGV[1])\n" +
"local rate = tonumber(ARGV[2])\n" +
"local now = tonumber(ARGV[3])\n" +
"\n" +
"local tokens_key = key .. \":tokens\"\n" +
"local timestamp_key = key .. \":timestamp\"\n" +
"\n" +
"local tokens = tonumber(redis.call(\"get\", tokens_key))\n" +
"if not tokens then\n" +
"    tokens = capacity\n" +
"end\n" +
"\n" +
"local last_refreshed = tonumber(redis.call(\"get\", timestamp_key))\n" +
"if not last_refreshed then\n" +
"    last_refreshed = now\n" +
"end\n" +
"\n" +
"local delta = math.max(0, now - last_refreshed)\n" +
"local filled_tokens = math.min(capacity, tokens + delta * rate)\n" +
"local allowed = filled_tokens >= 1\n" +
"\n" +
"local new_tokens = filled_tokens\n" +
"local new_timestamp = last_refreshed\n" +
"if allowed then\n" +
"    new_tokens = filled_tokens - 1\n" +
"    new_timestamp = now\n" +
"end\n" +
"\n" +
"redis.call(\"set\", tokens_key, new_tokens)\n" +
"redis.call(\"set\", timestamp_key, new_timestamp)\n" +
"\n" +
"return allowed",
1,
key,
Integer.toString(capacity),
Integer.toString(rate),
Long.toString(now)
);
return (Boolean) result;
}
}
}

在需要进行限流的地方调用RedisRateLimiter的allowRequest方法进行

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