INCR and DECR for counters in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command INCR do?

INCR increases the integer value of a key by 1. If the key does not exist, it is set to 0 before incrementing.

beginner

What happens if you use DECR on a key that does not exist in Redis?

The key is set to 0 first, then decreased by 1, resulting in a value of -1.

intermediate

Can INCR and DECR be used on keys holding non-integer values?

No. INCR and DECR only work on keys holding integer values. Using them on non-integer values causes an error.

intermediate

How would you atomically increase a counter by 5 in Redis?

Use the INCRBY command with the key and 5 as arguments, e.g., INCRBY key 5.

beginner

Why are INCR and DECR commands useful for counters in Redis?

They provide a simple, fast, and atomic way to increase or decrease counters without race conditions.

What is the result of INCR on a key that does not exist?

A Does nothing

B Returns an error

C Sets the key to 0

D Sets the key to 1

INCR sets the key to 0 first, then increments it by 1, so the final value is 1.

Which command decreases the value of a Redis key by 1?

A INCR

B DECR

C INCRBY

D DECRBY

DECR decreases the integer value of a key by 1.

What happens if you run INCR on a key holding the string 'hello'?

A An error is returned

B The value becomes 1

C The value becomes 'hello1'

D The key is deleted

INCR only works on integer values; using it on a non-integer string causes an error.

How can you increase a Redis counter by 10 in one command?

A INCR key 10

B DECRBY key 10

C INCRBY key 10

D INCR key; INCR key; ... (10 times)

INCRBY key 10 increases the key's value by 10 atomically.

Why is using INCR and DECR commands better than getting and setting values manually?

A They are atomic and avoid race conditions

B They are slower but safer

C They use less memory

D They allow floating point increments

INCR and DECR are atomic operations, so they prevent race conditions in concurrent environments.

Explain how INCR and DECR commands work in Redis and why they are useful for counters.

Think about how counters change and why atomicity matters.

Describe what happens if you use INCR or DECR on a key with a non-integer value in Redis.

Consider data types and command restrictions.