Counters

Integers stored as strings

Redis stores everything as a string, but it recognises when a string is a valid integer (or float) and lets you do arithmetic on it with dedicated commands. The key insight is that these operations are atomic — Redis executes each one as a single, uninterruptible step.

INCR and DECR

INCR key increments the integer value by 1. DECR key decrements by 1. If the key does not exist, Redis treats the missing value as 0 before applying the operation.

127.0.0.1:6379> SET pageviews:home 0
OK
127.0.0.1:6379> INCR pageviews:home
(integer) 1
127.0.0.1:6379> INCR pageviews:home
(integer) 2
127.0.0.1:6379> INCR pageviews:home
(integer) 3
127.0.0.1:6379> DECR pageviews:home
(integer) 2

SET pageviews:home 0
INCR pageviews:home
INCR pageviews:home
INCR pageviews:home
DECR pageviews:home

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INCRBY and DECRBY

When you need to add or subtract values other than 1, use INCRBY and DECRBY:

127.0.0.1:6379> SET score:player1 100
OK
127.0.0.1:6379> INCRBY score:player1 50
(integer) 150
127.0.0.1:6379> DECRBY score:player1 30
(integer) 120
127.0.0.1:6379> INCRBY score:player1 -10
(integer) 110

SET score:player1 100
INCRBY score:player1 50
DECRBY score:player1 30
INCRBY score:player1 -10

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INCRBYFLOAT

For floating-point arithmetic (prices, averages, sensor data), use INCRBYFLOAT. There is no DECRBYFLOAT — just pass a negative delta.

127.0.0.1:6379> SET temperature 22.5
OK
127.0.0.1:6379> INCRBYFLOAT temperature 1.3
"23.8"
127.0.0.1:6379> INCRBYFLOAT temperature -5.0
"18.8"
127.0.0.1:6379> GET temperature
"18.8"

SET temperature 22.5
INCRBYFLOAT temperature 1.3
INCRBYFLOAT temperature -5.0
GET temperature

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Real-world pattern: page-view counter

Because INCR initialises a missing key to 0 before incrementing, you do not need a SET 0 first. Combine with EXPIRE for a rolling-window counter:

127.0.0.1:6379> INCR views:article:42
(integer) 1
127.0.0.1:6379> INCR views:article:42
(integer) 2
127.0.0.1:6379> INCR views:article:42
(integer) 3
127.0.0.1:6379> GET views:article:42
"3"

INCR views:article:42
INCR views:article:42
INCR views:article:42
GET views:article:42

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Real-world pattern: rate limiter

Track how many requests an IP has made within a window. Use INCR plus EXPIRE set only on the first increment:

127.0.0.1:6379> INCR ratelimit:ip:203.0.113.5
(integer) 1
127.0.0.1:6379> EXPIRE ratelimit:ip:203.0.113.5 60
(integer) 1
127.0.0.1:6379> INCR ratelimit:ip:203.0.113.5
(integer) 2
127.0.0.1:6379> TTL ratelimit:ip:203.0.113.5
(integer) 57

In application code you check: if the result of INCR exceeds your limit (e.g., 100), reject the request. The key auto-deletes after 60 seconds, resetting the window.

INCR ratelimit:ip:203.0.113.5
EXPIRE ratelimit:ip:203.0.113.5 60
INCR ratelimit:ip:203.0.113.5
TTL ratelimit:ip:203.0.113.5

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🟥 INCR is atomic — no read-modify-write race

In a multi-threaded application, a naive counter looks like: read the value, add 1, write it back. Two threads doing this simultaneously can both read the same value and produce the wrong total (a classic race condition).

INCR sidesteps this entirely. Redis is single-threaded in its command execution, so INCR is an indivisible operation — no other command can interleave between the read and the write. Ten thousand concurrent requests all calling INCR pageviews will produce exactly 10,000 with no losses.

What value does INCR use when the key does not exist yet?

1 0 nil It returns an error

Which command increments a floating-point value?

INCR INCRBY INCRBYFLOAT INCRFLOAT

Why is INCR safe for concurrent counters?

It uses a client-side lock It is an atomic operation executed as a single step by Redis It queues requests and processes them serially in the client It uses Redis transactions (MULTI/EXEC) internally

How do you subtract 5 using INCRBYFLOAT?

DECRBYFLOAT key 5 INCRBYFLOAT key -5 INCRBY key -5.0 You cannot subtract with INCRBYFLOAT

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