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

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

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

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

Hint

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

Practice

1. What does the SCRIPT LOAD command do in Redis?

easy

A. It executes a Lua script immediately without caching.

B. It lists all cached Lua scripts in Redis.

C. It deletes a cached Lua script by its SHA1 hash.

D. It loads a Lua script into Redis and returns its SHA1 hash.

Solution

1. Step 1: Understand SCRIPT LOAD purpose

   The SCRIPT LOAD command stores a Lua script in Redis and returns a unique SHA1 hash for it.

2. Step 2: Differentiate from other commands

   Unlike EVAL, SCRIPT LOAD does not execute the script but prepares it for later use.

3. Final Answer:

   It loads a Lua script into Redis and returns its SHA1 hash. -> Option D

4. Quick Check:

   SCRIPT LOAD returns SHA1 hash [OK]

Hint: SCRIPT LOAD stores script and returns SHA1 hash [OK]

Common Mistakes:

• Confusing SCRIPT LOAD with EVAL which runs scripts immediately
• Thinking SCRIPT LOAD deletes scripts
• Assuming SCRIPT LOAD lists scripts

2. Which of the following is the correct syntax to load a Lua script into Redis and get its SHA1 hash?

easy

A. EVALSHA "return redis.call('PING')" 0

B. SCRIPT LOAD "return redis.call('PING')"

C. SCRIPT EXECUTE "return redis.call('PING')"

D. LOAD SCRIPT "return redis.call('PING')"

Solution

1. Step 1: Identify correct command syntax

   The correct command to load a script is SCRIPT LOAD followed by the Lua script as a string.

2. Step 2: Eliminate incorrect options

   EVALSHA runs a cached script by SHA, SCRIPT EXECUTE and LOAD SCRIPT are invalid Redis commands.

3. Final Answer:

   SCRIPT LOAD "return redis.call('PING')" -> Option B

4. Quick Check:

   SCRIPT LOAD syntax is SCRIPT LOAD "script" [OK]

Hint: SCRIPT LOAD "script" loads script and returns SHA1 [OK]

Common Mistakes:

• Using EVALSHA instead of SCRIPT LOAD to load script
• Using non-existent commands like SCRIPT EXECUTE
• Incorrect command order or missing quotes

3. Given the following commands executed in Redis:

SCRIPT LOAD "return ARGV[1]"
EVALSHA <sha1> 0 Hello

What will be the output of the EVALSHA command?

medium

A. 0

B. An error because the script is not cached

C. "Hello"

D. The SHA1 hash string

Solution

1. Step 1: Understand SCRIPT LOAD and EVALSHA interaction

   SCRIPT LOAD stores the script and returns its SHA1 hash. EVALSHA runs the cached script using that SHA1.

2. Step 2: Analyze the script and arguments

   The script returns ARGV[1], and EVALSHA is called with argument "Hello" as ARGV[1]. So output is "Hello".

3. Final Answer:

   "Hello" -> Option C

4. Quick Check:

   EVALSHA runs cached script returning ARGV[1] = "Hello" [OK]

Hint: EVALSHA runs cached script with ARGV arguments [OK]

Common Mistakes:

• Confusing SCRIPT LOAD output with EVALSHA output
• Forgetting to pass correct number of keys (0 here)
• Expecting SHA1 hash as output of EVALSHA

4. You run the following Redis commands:

SCRIPT LOAD "return redis.call('GET', KEYS[1])"
EVALSHA <sha1> 1 mykey

But you get an error: NOSCRIPT No matching script. Please use EVAL. What is the most likely cause?

medium

A. The SHA1 hash used in EVALSHA is incorrect or from a different script.

B. The KEYS argument count is wrong; it should be 0 instead of 1.

C. SCRIPT LOAD does not cache scripts, so EVALSHA fails.

D. Redis does not support EVALSHA for GET commands.

Solution

1. Step 1: Understand NOSCRIPT error meaning

   This error means Redis does not have the script cached under the given SHA1 hash.

2. Step 2: Identify common causes

   Most often, the SHA1 hash passed to EVALSHA is wrong or from a different script than loaded.

3. Final Answer:

   The SHA1 hash used in EVALSHA is incorrect or from a different script. -> Option A

4. Quick Check:

   NOSCRIPT error means SHA1 mismatch [OK]

Hint: NOSCRIPT means SHA1 hash mismatch or missing script [OK]

Common Mistakes:

• Assuming KEYS count is always zero
• Thinking SCRIPT LOAD does not cache scripts
• Believing Redis disallows EVALSHA for certain commands

5. You want to optimize Redis script execution by caching a Lua script that increments a key's value by a given amount. Which sequence of commands correctly loads and runs this script multiple times efficiently?

hard

A. Use SCRIPT LOAD to get SHA1, then call EVALSHA with the SHA1 and arguments for each increment.

B. Use EVAL with the full script every time you want to increment the key.

C. Use SCRIPT FLUSH before every increment to clear cached scripts, then EVALSHA.

D. Use SCRIPT LOAD once, then call EVAL with the full script every time.

Solution

1. Step 1: Understand caching for efficiency

   Loading the script once with SCRIPT LOAD caches it and returns SHA1 for reuse.

2. Step 2: Use EVALSHA for repeated execution

   Calling EVALSHA with the SHA1 runs the cached script efficiently without sending full script each time.

3. Step 3: Eliminate inefficient options

   Using EVAL every time sends full script, SCRIPT FLUSH clears cache unnecessarily.

4. Final Answer:

   Use SCRIPT LOAD to get SHA1, then call EVALSHA with the SHA1 and arguments for each increment. -> Option A

5. Quick Check:

   Cache script once, run with EVALSHA repeatedly [OK]

Hint: Load once with SCRIPT LOAD, run many times with EVALSHA [OK]

Common Mistakes:

• Using EVAL repeatedly instead of EVALSHA
• Flushing scripts unnecessarily with SCRIPT FLUSH
• Reloading script before every execution