Counters

Atomic Counters

Redis string commands include atomic integer operations. Because they are atomic, multiple clients can increment a counter simultaneously without race conditions — no transactions needed.

INCR — Increment by 1

SET pageviews 0
INCR pageviews
INCR pageviews
INCR pageviews
GET pageviews

Output:

OK
(integer) 1
(integer) 2
(integer) 3
"3"

If the key doesn't exist, INCR starts from 0 and increments to 1.

INCRBY — Increment by N

INCRBY pageviews 10

DECR and DECRBY — Decrement

DECR pageviews
DECRBY pageviews 5

INCRBYFLOAT — Floating-Point Increment

SET price 10.50
INCRBYFLOAT price 2.25
GET price

Output:

OK
"12.75"
"12.75"

Pattern: Rate Limiting

The INCR pattern is ideal for rate limiting:

# Allow 100 requests per minute per user
INCR rate:user123
EXPIRE rate:user123 60

# On each request:
# If GET rate:user123 > 100, reject the request

The key expires automatically after 60 seconds, resetting the counter.

Pattern: Page View Counters

# Increment on each page load
INCR views:article:42

# Get current count
GET views:article:42

Pattern: Inventory Management

SET inventory:item:5 100
DECRBY inventory:item:5 3
GET inventory:item:5

Your Task

Set a key score to 0. Increment it 3 times with INCR. Then add 10 more with INCRBY. The final value should be 13.