All 7 Rate Limiting with Redis questions
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How do you implement a basic fixed window rate limiter using Redis's INCR and EXPIRE commands? 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 design a tiered rate limiting system that applies different limits based on a user's subscription plan?
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How would you implement a distributed rate limiter using Redis that works correctly across multiple application servers handling requests for the same user?

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You centralize all rate limit counting in Redis rather than in each individual application server's local memory, using an atomic Lua script to check and increment the shared counter, ensuring that no matter which specific application server receives a given request, they are all checking against the exact same shared, consistent count for that user.
-- All application servers call the same atomic Lua script
-- against the same shared Redis key for a given user
EVALSHA rate_limit_script_sha 1 'user:123:limit' 100 60
Real-world example A company running dozens of application servers behind a load balancer uses a shared Redis based rate limiter, ensuring a user cannot bypass their rate limit simply by having requests routed to different application servers.

Common follow-ups: What happens to rate limiting accuracy if the Redis instance itself becomes temporarily unavailable?;How do you handle rate limiting during a Redis failover event?

Distributed Locks with Redis;Redis Sentinel & High Availability
All 7 Rate Limiting with Redis questions