Introduction to Redis strings
Redis strings store sequences of bytes, including text, serialized objects, and binary arrays. As such, strings are the simplest type of value you can associate with a Redis key. They're often used for caching, but they support additional functionality that lets you implement counters and perform bitwise operations, too.
Since Redis keys are strings, when we use the string type as a value too, we are mapping a string to another string. The string data type is useful for a number of use cases, like caching HTML fragments or pages.
As you can see using the
SET and the
GET commands are the way we set
and retrieve a string value. Note that
SET will replace any existing value
already stored into the key, in the case that the key already exists, even if
the key is associated with a non-string value. So
SET performs an assignment.
Values can be strings (including binary data) of every kind, for instance you can store a jpeg image inside a value. A value can't be bigger than 512 MB.
SET command has interesting options, that are provided as additional
arguments. For example, I may ask
SET to fail if the key already exists,
or the opposite, that it only succeed if the key already exists:
There are a number of other commands for operating on strings. For example
GETSET command sets a key to a new value, returning the old value as the
result. You can use this command, for example, if you have a
system that increments a Redis key using
every time your web site receives a new visitor. You may want to collect this
information once every hour, without losing a single increment.
GETSET the key, assigning it the new value of "0" and reading the
old value back.
MGET is used, Redis returns an array of values.
Strings as counters
Even if strings are the basic values of Redis, there are interesting operations you can perform with them. For instance, one is atomic increment:
INCR command parses the string value as an integer,
increments it by one, and finally sets the obtained value as the new value.
There are other similar commands like
DECRBY. Internally it's
always the same command, acting in a slightly different way.
What does it mean that INCR is atomic? That even multiple clients issuing INCR against the same key will never enter into a race condition. For instance, it will never happen that client 1 reads "10", client 2 reads "10" at the same time, both increment to 11, and set the new value to 11. The final value will always be 12 and the read-increment-set operation is performed while all the other clients are not executing a command at the same time.
By default, a single Redis string can be a maximum of 512 MB.
Getting and setting Strings
SETstores a string value.
SETNXstores a string value only if the key doesn't already exist. Useful for implementing locks.
GETretrieves a string value.
MGETretrieves multiple string values in a single operation.
INCRBYatomically increments (and decrements when passing a negative number) counters stored at a given key.
- Another command exists for floating point counters:
To perform bitwise operations on a string, see the bitmaps data type docs.
See the complete list of string commands.
Most string operations are O(1), which means they're highly efficient.
However, be careful with the
SETRANGE commands, which can be O(n).
These random-access string commands may cause performance issues when dealing with large strings.