EVALSHA for cached scripts in Redis - Mini Project: Build & Apply

Using EVALSHA for Cached Scripts in Redis

📖 Scenario: You are managing a Redis database for a small online store. You want to use Lua scripts to perform operations efficiently. To avoid sending the full script every time, you will cache the script in Redis and use its SHA1 hash to run it quickly.

🎯 Goal: Build a Redis Lua script caching workflow using EVALSHA to run a cached script that increments a product's stock count.

📋 What You'll Learn

Create a Lua script that increments a product stock by a given amount.

Load the script into Redis and get its SHA1 hash.

Store the SHA1 hash in a variable called script_sha.

Use EVALSHA with script_sha to increment the stock count.

Handle the case where the script is not cached by loading it again.

💡 Why This Matters

🌍 Real World

Caching Lua scripts in Redis is common in real-world applications to speed up repeated script execution and reduce network traffic.

💼 Career

Understanding <code>EVALSHA</code> and script caching is valuable for backend developers and database administrators working with Redis.

1

Create the Lua script to increment stock

Create a variable called lua_script and assign it this exact Lua script as a string: return redis.call('INCRBY', KEYS[1], ARGV[1])

Redis

# Create a variable called lua_script with the Lua code string
# Your code here

Need a hint?

This Lua script uses redis.call('INCRBY', ...) to increase a key's value by a given number.

2

Load the Lua script into Redis and get SHA1 hash

Use the Redis command SCRIPT LOAD with lua_script to load the script and store the returned SHA1 hash in a variable called script_sha.

Redis

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

Need a hint?

Use redis.call('SCRIPT', 'LOAD', lua_script) to load the script and get its SHA1 hash.

3

Use EVALSHA to increment stock count

Use redis.call with 'EVALSHA', the variable script_sha, 1 for the number of keys, the key 'product_stock', and the increment amount '5' as arguments to run the cached script.

Redis

lua_script = "return redis.call('INCRBY', KEYS[1], ARGV[1])"
script_sha = redis.call('SCRIPT', 'LOAD', lua_script)
# Use EVALSHA with script_sha to increment 'product_stock' by 5
# Your code here

Need a hint?

Use redis.call('EVALSHA', script_sha, 1, 'product_stock', '5') to run the cached script.

4

Handle missing cached script by loading again

Add a check: if redis.call('EVALSHA', script_sha, 1, 'product_stock', '5') fails with a NOSCRIPT error, reload the script with SCRIPT LOAD and run EVALSHA again.

Redis

lua_script = "return redis.call('INCRBY', KEYS[1], ARGV[1])"
script_sha = redis.call('SCRIPT', 'LOAD', lua_script)
result = redis.call('EVALSHA', script_sha, 1, 'product_stock', '5')
# Add error handling for NOSCRIPT error to reload script and rerun EVALSHA
# Your code here

Need a hint?

Use pcall to catch errors and check if the error message contains 'NOSCRIPT'. If yes, reload the script and run EVALSHA again.