How Redis achieves sub-millisecond latency - Practice Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why does Redis use an in-memory data store?

Redis stores data primarily in memory rather than on disk. What is the main reason this design helps Redis achieve sub-millisecond latency?

A Because Redis does not support disk persistence

B Because memory is cheaper than disk storage

C Because disk storage is more reliable than memory

D Because accessing data in memory is much faster than reading from disk

💡 Hint

Think about the speed difference between memory and disk access.

Redis keeps data in memory to avoid slow disk reads and writes. Memory access is much faster, enabling quick data retrieval and updates.

🧠 Conceptual

intermediate

How does Redis handle commands to maintain speed?

Redis processes commands using a single-threaded event loop. Why does this design help keep latency low?

A It queues commands to process them in batches later

B It allows Redis to use multiple CPU cores simultaneously

C It avoids the overhead of managing multiple threads and context switching

D It stores commands on disk to process them faster

💡 Hint

Think about what happens when a program switches between many tasks.

Using a single thread avoids delays caused by switching between threads, which can slow down processing. This keeps command execution fast and predictable.

❓ query_result

advanced

What is the output of this Redis command sequence?

Given the following Redis commands executed in order, what is the output of the final command? 1. SET user:1 "Alice" 2. INCR visits:1 3. INCR visits:1 4. GET visits:1

Redis

SET user:1 "Alice"
INCR visits:1
INCR visits:1
GET visits:1

A "2"

B "1"

C Error: visits:1 is not an integer

D nil

💡 Hint

INCR increases the integer value stored at the key by 1.

The key visits:1 is incremented twice starting from 0 (default). So after two increments, its value is 2.

❓ optimization

advanced

Which Redis feature helps reduce latency by avoiding disk writes on every command?

Redis can be configured to save data to disk periodically instead of after every command. Which feature allows this behavior to help maintain sub-millisecond latency?

A Snapshotting (RDB)

B Append-only file (AOF) with fsync always

C Synchronous disk writes after every command

D Disabling persistence completely

💡 Hint

Think about saving data in batches rather than immediately.

Snapshotting saves the dataset to disk at intervals, reducing disk I/O during normal operations and keeping latency low.

🔧 Debug

expert

Why does this Redis Lua script cause latency spikes?

A Redis Lua script runs a long loop that processes 1 million items in one call. What is the main reason this causes latency spikes for other clients?

A Because Lua scripts run in parallel threads causing race conditions

B Because Redis is single-threaded and the script blocks all other commands until it finishes

C Because Redis automatically splits long scripts into smaller chunks

D Because the script writes data to disk after each iteration

💡 Hint

Consider how Redis handles commands when one takes a long time to run.

Redis runs Lua scripts atomically and single-threaded, so a long-running script blocks all other commands, causing latency spikes.