This article will cover some of the most common Redis interview questions and answers. If you want to learn more about Redis, check out the Understanding Redis: Basic concepts.

Why Redis is so fast?

  • Stores data in RAM rather than on disk: This allows for extremely fast read and write operations, as accessing data in memory is significantly faster than accessing it on disk.
  • Using a single-threaded model combined with I/O multiplexing: Redis can handle multiple connections simultaneously without the overhead of thread management.
  • Efficient data structures: Redis uses highly optimized data structures that are designed for speed and efficiency such as Lists, Sets, Sorted Sets, Hashes, and etc.

Redis Scan vs. Keys: Which One Should You Use?

SCAN and KEYS are both used for searching keys in Redis, but they work differently. KEYS returns all matched keys in one go, while SCAN uses a cursor for incremental iteration.

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# KEYS pattern (O(n))
redis> KEYS *name*
1) "firstname"
2) "lastname"
3) "nickname"

# SCAN cursor [MATCH pattern] [COUNT count] (O(1))
SCAN 0 MATCH *name*
1) "4"
2) 1) "firstname"
SCAN 0 MATCH *name*
1) "0"
2) 1) "lastname"
   2) "nickname"

Both commands iterate over all keys. The advantage of SCAN is its ability to return only a small number of elements per call, avoiding long server-blocking periods like KEY, especially with large datasets. However, while the KEY command can provide all the elements that are part of a Set in a given moment, SCAN provides limited guarantees about the returned elements due to potential changes in the collection during iteration.

  • KEYS: Use this command when you need to find all keys matching a specific pattern. However, be cautious with large datasets, as it can block the server for a long time.
  • SCAN: Use this command for large datasets or when you want to avoid blocking the server. It’s more efficient and allows for incremental iteration.

What technique does Redis use to avoid running out of memory when creating RDB snapshots?

Redis Forking

Redis uses the copy-on-write (COW) technique during the fork process to avoid running out of memory when creating RDB snapshots.

  • When forking a process, the parent and child share the same physical memory.
  • The copy-on-write (COW) technique ensures that both the parent and child processes share the same memory pages until one of them modifies a page.
  • When either the parent process attempts to change to a shared page, the operating system makes a copy of the page. The child process is fully unaware of the change and has a consistent memory snapshot. This means that only the pages that are modified after the fork will consume additional memory.

How should we structure our Redis keys?

[TBU]

How to handle atomic operations in Redis?

[TBU]

Pub/Sub model in Redis and how’s it implemented?

[TBU]

What is Redis transaction?

[TBU]

Redis List and blocking operation in Redis List?

[TBU]

How to mointor Redis performance?

[TBU]