APPEND

Introduction and Use Case(s)

The APPEND command in Redis is used to append a value to the end of an existing string key. If the key does not exist, it creates the key with the given value. This command is particularly useful for logging or accumulating text data without overwriting the existing content.

Syntax

APPEND key value

Parameter Explanations

• key: The name of the string key to which the value will be appended. If the key doesn’t exist, it will be created.
• value: The string value that you want to append to the existing value of the key.

Return Values

The APPEND command returns the total length of the string after the append operation.

Examples of possible outputs:

• If the initial value of the key is “Hello” and you append “, World!”, the return value will be 13 because the resulting string “Hello, World!” has 13 characters.

Code Examples

dragonfly> SET mykey "Hello"
OK
dragonfly> APPEND mykey ", World!"
(integer) 13
dragonfly> GET mykey
"Hello, World!"
dragonfly> APPEND mykey " How are you?"
(integer) 24
dragonfly> GET mykey
"Hello, World! How are you?"

Best Practices

• Ensure that the key you’re appending to is intended to be a string. Using APPEND on non-string keys will result in a type error.
• Use APPEND in scenarios where continuous accumulation of text data is required, such as logging messages or building simple reports.

Common Mistakes

• Trying to append using non-string keys. This will generate an error since APPEND only works with string keys.
• Not checking the length of the string after multiple append operations, which could lead to the string becoming very large and potentially impacting performance.

FAQs

What happens if the key does not exist?

If the key does not exist, the APPEND command will create a new key with the specified value.

Can I use APPEND with other data types like lists or hashes?

No, APPEND is specifically designed for string values. Using it with other data types will result in an error.