All 7 Rate Limiting with Redis questions
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What is rate limiting, and why is Redis a good fit for implementing it? What is the sliding window log algorithm for rate limiting, and how can it be implemented using a Redis sorted set? What is the token bucket algorithm for rate limiting, and what advantage does it offer over a simple fixed window counter? How do you provide helpful feedback to API consumers about their current rate limit status, such as how many requests they have remaining? How would you implement a distributed rate limiter using Redis that works correctly across multiple application servers handling requests for the same user? How would you design a tiered rate limiting system that applies different limits based on a user's subscription plan?
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Redis › Rate Limiting with Redis

How do you implement a basic fixed window rate limiter using Redis's INCR and EXPIRE commands?

Beginner

Answer

You increment a counter key representing the current time window each time a request comes in, and set an expiration on that key only the first time it is created, so the counter automatically resets once the time window passes, letting you reject any request once the counter exceeds your configured limit within that window.
local count = redis.call('incr', KEYS[1])
if count == 1 then
  redis.call('expire', KEYS[1], 60)
end
return count <= 100
Real-world example A login endpoint limits attempts to five per minute per IP address using a simple fixed window counter, automatically resetting the count once each minute passes.

Common follow-ups: What is a known weakness of the fixed window approach near the boundary between two windows?;How do you handle a burst of requests right at the start of a new window?

Lua Scripting;Expiration & Eviction
All 7 Rate Limiting with Redis questions