Script loading and caching in Redis - Mini Project: Build & Apply

Script Loading and Caching in Redis

📖 Scenario: You are managing a Redis database for a web application. To improve performance, you want to load a Lua script into Redis and cache it for repeated use without sending the full script every time.

🎯 Goal: Build a Redis setup where you load a Lua script, cache its SHA1 hash, and then execute the script using the cached hash.

📋 What You'll Learn

Create a Lua script that increments a counter key by 1

Load the Lua script into Redis and store its SHA1 hash in a variable

Use the cached SHA1 hash to execute the script

Handle the case where the script is not cached and needs to be loaded again

💡 Why This Matters

🌍 Real World

Caching Lua scripts in Redis improves performance by avoiding sending full scripts repeatedly. This is useful in web apps, caching layers, and real-time systems.

💼 Career

Understanding script loading and caching in Redis is important for backend developers, DevOps engineers, and database administrators working with Redis to optimize performance.

1

Create the Lua script to increment a counter

Create a variable called lua_script that contains the Lua code to increment a Redis key by 1. The script should use redis.call('INCR', KEYS[1]).

Redis

# Create a variable called lua_script with the Lua code to increment a key
# Your code here

Need a hint?

Use a string variable to hold the Lua script exactly as: return redis.call('INCR', KEYS[1])

2

Load the Lua script into Redis and cache its SHA1 hash

Use the Redis command SCRIPT LOAD to load the lua_script into Redis. Store the returned SHA1 hash in a variable called script_sha.

Redis

lua_script = "return redis.call('INCR', KEYS[1])"
# Load the lua_script into Redis and store the SHA1 hash in script_sha
# Your code here

Need a hint?

Use redis.call('SCRIPT', 'LOAD', lua_script) to load the script and assign it to script_sha.

3

Execute the cached Lua script using its SHA1 hash

Use redis.call('EVALSHA', script_sha, 1, 'counter') to execute the cached Lua script on the key 'counter'.

Redis

lua_script = "return redis.call('INCR', KEYS[1])"
script_sha = redis.call('SCRIPT', 'LOAD', lua_script)
# Execute the cached script using EVALSHA on key 'counter'
# Your code here

Need a hint?

Use redis.call('EVALSHA', script_sha, 1, 'counter') to run the script on the key named 'counter'.

4

Handle missing cached script by loading and executing

Write code that tries to execute the script using EVALSHA. If it fails because the script is missing, load the script again with SCRIPT LOAD and then execute it with EVALSHA. Use variables lua_script and script_sha.

Redis

lua_script = "return redis.call('INCR', KEYS[1])"
script_sha = redis.call('SCRIPT', 'LOAD', lua_script)
result = redis.call('EVALSHA', script_sha, 1, 'counter')
# Add code to catch missing script error, reload script, and execute again
# Your code here

Need a hint?

Use pcall to catch errors when running EVALSHA. If it fails, reload the script and run EVALSHA again.