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INCR

Introduction​

In Dragonfly, as well as in Redis and Valkey, the INCR command is used to atomically increment the integer value of a key by one. This is helpful for implementing counters, tracking requests, or managing sequence numbers since it is thread-safe and operates in constant time O(1).

Syntax​

INCR key
  • Time complexity: O(1)
  • ACL categories: @write, @string, @fast

Parameter Explanations​

  • key: The key whose value will be incremented by one. If the key doesn't exist, it will be automatically created with a value of 0 before being incremented.

Return Values​

Returns the new value of the key after the increment as an integer.

Code Examples​

Basic Example​

Increment an integer stored in a key:

dragonfly> SET mykey 10
OK
dragonfly> INCR mykey
(integer) 11

Automatically Creating and Incrementing a Key​

If the key does not exist, INCR will set it to 0 before incrementing it:

dragonfly> EXISTS counter
(integer) 0
dragonfly> INCR counter
(integer) 1

Using INCR with Negative Numbers​

INCR can handle negative numbers too. It will still increase the value by one in this case:

dragonfly> SET mykey -5
OK
dragonfly> INCR mykey
(integer) -4

Incrementing in a Loop​

You can repeatedly call INCR to track a counter of operations:

dragonfly> SET request_count 100
OK
dragonfly> INCR request_count
(integer) 101
dragonfly> INCR request_count
(integer) 102

Best Practices​

  • Use INCR to implement performance-efficient counters due to its atomic nature.
  • Consider using the EXPIRE command alongside to set TTLs for temporary counters, such as tracking hits in a web application.
  • If you need to increment a value by something other than 1, use the closely related INCRBY command, which allows you to specify the increment amount.

Common Mistakes​

  • Trying to use INCR on non-integer or non-string values (e.g., lists or sets). This will result in an error, as INCR expects the value stored at key to be a string representation of an integer.

    dragonfly> LPUSH mylist 1
    (integer) 1
    dragonfly> INCR mylist
    (error) WRONGTYPE Operation against a key holding the wrong kind of value

  • Not realizing that INCR only creates keys as integer strings when they don't exist. If you want to manipulate more complex data types, use other commands like INCRBYFLOAT for floats.

FAQs​

What happens if the key contains a non-integer value?​

You will get an error. INCR expects the value at key to be a valid integer or an integer-convertible string (e.g., "10").

Will INCR work with floating-point numbers?​

No, INCR is meant for integer manipulation only. For floating-point values, use the INCRBYFLOAT command, which is designed specifically for that purpose.

What happens if the key does not exist?​

If the key does not exist, INCR creates the key with a value of 0, and then increments it by 1. So the first call to INCR will result in the value 1.