Tutorial

Getting started with Redis Functions

February 25, 20266 minute read

Elena Kolevska

TL;DR:

Redis Functions, introduced in Redis 7.0, replace ad-hoc EVAL Lua scripts with persistent, modular server-side code. Functions are stored in the database, replicated automatically, and loaded via FUNCTION LOAD. You call them with FCALL instead of EVAL, eliminating the need to ship script source on every invocation.

#What you'll learn

What Redis Functions are and how they improve on EVAL scripts
How to create a function library in Lua and load it into Redis
How to call functions with FCALL
How to use functions to maintain a consistent data schema with automatic timestamps

#Prerequisites

Redis 7.0 or later — Redis Functions are not available in earlier versions. Follow the Redis quick start to get set up.
A terminal with access to redis-cli.

#What are Redis Functions?

The most impactful addition to Redis version 7.0 is Redis Functions — a new programmability option, improving on scripts by adding modularity, reusability, and better overall developer experience.

Functions are, in contrast to scripts, persisted in the .rdb and .aof files as well as automatically replicated to all the replicas, which makes them a first-class citizen of Redis.

Redis has the capability of supporting multiple execution engines so in one of the future releases we'll be able to write Redis Functions in Lua, Javascript, and more languages, but at the moment (Redis v7.0) the only supported language is Lua.

#How do Redis Functions compare to EVAL scripts?

If you have used EVAL to run Lua scripts in Redis before, you may wonder why you should switch to Redis Functions. The table below summarizes the key differences:

Feature	`EVAL` scripts	Redis Functions
Persistence	Not persisted; must be resent on every call or cached via `SCRIPT LOAD` / `EVALSHA`	Persisted in RDB/AOF and replicated automatically
Modularity	Single monolithic script per call	Libraries can contain multiple named functions
Invocation	`EVAL <script> numkeys ...` or `EVALSHA <sha1> ...`	`FCALL <function> numkeys ...`
Deployment	Client must manage script source/SHA	Load once with `FUNCTION LOAD`; available after restart
Replication	Scripts replicated as commands	Functions replicated as part of the data set
Upgrades	Replace by sending new script text	`FUNCTION LOAD` with `REPLACE` flag

In short, Redis Functions give you stored-procedure-like behavior — load your code once and call it by name from any client, without managing SHA hashes or script source distribution.

#How do you create a Redis Function library?

A common pain point for developers is to maintain a consistent view of data entities through a logical schema. Redis Functions are ideally suited for solving this problem and in this tutorial, we will demonstrate just that — we'll create a library with two functions; the first one will make sure that we can automatically set _created_at and _updated_at timestamps for hash keys and the second one will simply update the _updated_at timestamp without changing the other elements, simulating the "touch" Unix function. Let's go!

NOTE

In the rest of this tutorial we'll use the $ character to indicate that the command needs to be run on the command prompt and redis-cli> to indicate the same for a redis-cli prompt.

#Warm-up: your first function

Now that we have our Redis server running, we can create a file named mylib.lua and in it create a function named hset that would receive the keys and arguments we pass on the command line as parameters. Functions in Redis are always a part of a library, and a single library can have multiple functions.

For starters, let's create a simple function that returns "Hello Redis 7.0" and save it in the mylib.lua file.

#Adding automatic timestamps with HSET

We're now ready to start working on the requirement. First, let's implement the part that adds an _updated_at timestamp:

#Frequently asked questions

#What Redis version do I need for Functions?

Redis Functions require Redis 7.0 or later. If you are running an older version, you will need to upgrade before using FUNCTION LOAD and FCALL.

#Can I use languages other than Lua?

At the time of writing, Lua is the only supported scripting engine. Redis is designed to support multiple execution engines, so additional languages may be available in future releases.

#Are Redis Functions replicated to replicas?

Yes. Unlike EVAL scripts, Redis Functions are persisted in RDB and AOF files and automatically replicated to all replicas, making them a first-class part of your data set.

#Next steps

Read the Redis Functions documentation for the full command reference.
Learn about Redis Lua scripting if you need to support older Redis versions.