Rate limiter with INCR and EXPIRE in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does the INCR command help in rate limiting?

In Redis, the INCR command is often used in rate limiting. What is the primary role of INCR in this context?

A It resets the counter to zero after each request.

B It increments a counter for each request to track usage.

C It deletes the key after a certain number of requests.

D It sets an expiration time for the key automatically.

💡 Hint

Think about how you count the number of times something happens.

The INCR command increases the value of a key by one. In rate limiting, it counts how many requests have been made.

❓ Architecture

intermediate

What is the role of EXPIRE in a Redis rate limiter?

In a Redis-based rate limiter, why is the EXPIRE command used alongside INCR?

A To automatically reset the counter after a fixed time window.

B To increase the counter value by a fixed amount.

C To permanently store the counter without expiration.

D To block requests once the limit is reached.

💡 Hint

Consider how to limit requests within a time frame.

EXPIRE sets a time-to-live on the key so the counter resets after the time window, enabling fixed window rate limiting.

❓ scaling

advanced

How to handle race conditions when using INCR and EXPIRE for rate limiting?

When multiple clients send requests simultaneously, what is the best way to ensure atomicity of INCR and EXPIRE commands in Redis rate limiting?

A Ignore race conditions as Redis handles them automatically.

B Send INCR and EXPIRE commands separately without any atomic guarantees.

C Use a client-side lock to prevent concurrent requests.

D Use a Redis transaction (MULTI/EXEC) or Lua script to combine INCR and EXPIRE atomically.

💡 Hint

Think about how to make multiple commands run as one indivisible operation.

Using MULTI/EXEC or Lua scripts ensures INCR and EXPIRE run together atomically, preventing race conditions.

❓ tradeoff

advanced

What is a limitation of using INCR and EXPIRE for rate limiting in Redis?

Which of the following is a known limitation when implementing rate limiting using only INCR and EXPIRE in Redis?

A It cannot handle sliding window rate limiting precisely.

B It requires complex client-side logic to increment counters.

C It automatically blocks requests beyond the limit without configuration.

D It stores user data permanently without expiration.

💡 Hint

Consider the difference between fixed window and sliding window rate limiting.

INCR and EXPIRE implement fixed window counters, which can cause bursts at window edges and do not support precise sliding windows.

❓ estimation

expert

Estimate Redis memory usage for rate limiting 1 million users

You want to rate limit 1 million users with a 1-minute window using Redis keys with INCR and EXPIRE. Each key stores a counter as a 64-bit integer. Estimate the approximate memory usage in megabytes (MB) for storing all counters simultaneously.

A About 1 GB

B About 10 MB

C About 100 MB

D About 500 MB

💡 Hint

Consider key overhead, value size, and approximate Redis memory usage per key.

Each key-value pair in Redis has overhead (~50 bytes) plus 8 bytes for the integer. For 1 million keys, ~58 MB plus additional overhead (hash table, key strings) typically totals about 100 MB.