RPUSHX
Introduction and Use Case(s)
The RPUSHX
command in Redis is used to append a value to the end of a list, but only if the list already exists. This command is useful for maintaining data integrity by ensuring that no new lists are created accidentally. Typical use cases include logging systems where entries are appended to existing logs, or queues where tasks are only added to pre-existing task lists.
Syntax
RPUSHX key value [value ...]
Parameter Explanations
- key: The name of the list to which you want to append the value(s). This list must exist; otherwise, the command will have no effect.
- value [value …]: One or more values to append to the list. Multiple values can be appended in a single command, separated by spaces.
Return Values
- (integer): The length of the list after the
RPUSHX
operation, or 0 if the list does not exist.
Example:
If the list does not exist:
dragonfly> RPUSHX mylist "world"
(integer) 0
If the list exists:
dragonfly> RPUSH mylist "hello"
(integer) 1
dragonfly> RPUSHX mylist "world"
(integer) 2
Code Examples
Appending to a non-existent list:
dragonfly> RPUSHX mylist "item1"
(integer) 0
Appending to an existing list:
dragonfly> RPUSH mylist "start"
(integer) 1
dragonfly> RPUSHX mylist "middle"
(integer) 2
dragonfly> RPUSHX mylist "end"
(integer) 3
dragonfly> LRANGE mylist 0 -1
1) "start"
2) "middle"
3) "end"
Appending multiple values:
dragonfly> RPUSH mylist "initial"
(integer) 1
dragonfly> RPUSHX mylist "one" "two" "three"
(integer) 4
dragonfly> LRANGE mylist 0 -1
1) "initial"
2) "one"
3) "two"
4) "three"
Best Practices
- Ensure the list exists before using
RPUSHX
to avoid unnecessary command executions. - Use
EXISTS
command to check for list existence if unsure.
Common Mistakes
- Using
RPUSHX
on a non-existent list, resulting in no elements being added and the return value being 0.
FAQs
How is RPUSHX
different from RPUSH
?
RPUSHX
only appends elements if the list already exists, whereas RPUSH
will create the list if it does not exist.
Can I append multiple values using RPUSHX
?
Yes, you can append multiple values in one command by specifying them consecutively.