Introduction to Redict Strings #

Redict 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 Redict key. They’re often used for caching, but they support additional functionality that lets you implement counters and perform bitwise operations, too.

Since Redict 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.

> SET bike:1 Deimos
OK
> GET bike:1
"Deimos"

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.

The 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:

> set bike:1 bike nx
(nil)
> set bike:1 bike xx
OK

There are a number of other commands for operating on strings. For example the 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 Redict key using INCR every time your web site receives a new visitor. You may want to collect this information once every hour, without losing a single increment. You can GETSET the key, assigning it the new value of “0” and reading the old value back.

The ability to set or retrieve the value of multiple keys in a single command is also useful for reduced latency. For this reason there are the MSET and MGET commands:

> mset bike:1 "Deimos" bike:2 "Ares" bike:3 "Vanth"
OK
> mget bike:1 bike:2 bike:3
1) "Deimos"
2) "Ares"
3) "Vanth"

When MGET is used, Redict returns an array of values.

Strings as counters #

Even if strings are the basic values of Redict, there are interesting operations you can perform with them. For instance, one is atomic increment:

> set total_crashes 0
OK
> incr total_crashes
(integer) 1
> incrby total_crashes 10
(integer) 11

The 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 INCRBY, DECR and 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.

Limits #

By default, a single Redict string can be a maximum of 512 MB.

Basic commands #

Getting and setting Strings #

SET stores a string value.
SETNX stores a string value only if the key doesn’t already exist. Useful for implementing locks.
GET retrieves a string value.
MGET retrieves multiple string values in a single operation.

Managing counters #

INCRBY atomically increments (and decrements when passing a negative number) counters stored at a given key.
Another command exists for floating point counters: INCRBYFLOAT.

Bitwise operations #

To perform bitwise operations on a string, see the bitmaps data type docs.

See the complete list of string commands.

Performance #

Most string operations are O(1), which means they’re highly efficient.
However, be careful with the SUBSTR, GETRANGE, and SETRANGE commands, which can be O(n). These random-access string commands may cause performance issues when dealing with large strings.