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

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

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

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

Hint

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

Practice

1. What does the Redis command EVALSHA primarily do?

easy

A. Uploads a new Lua script to Redis cache

B. Deletes a script from the cache by its SHA1 hash

C. Lists all cached scripts by their SHA1 hashes

D. Runs a cached Lua script using its SHA1 hash

Solution

1. Step 1: Understand the purpose of EVALSHA

   EVALSHA runs a Lua script already stored in Redis cache using its SHA1 hash instead of sending the full script text.

2. Step 2: Compare with other script commands

   Other commands like SCRIPT LOAD upload scripts, SCRIPT FLUSH deletes scripts, but EVALSHA executes cached scripts.

3. Final Answer:

   Runs a cached Lua script using its SHA1 hash -> Option D

4. Quick Check:

   EVALSHA = run cached script [OK]

Hint: EVALSHA runs scripts by hash, not by full text [OK]

Common Mistakes:

• Confusing EVALSHA with SCRIPT LOAD
• Thinking EVALSHA deletes scripts
• Assuming EVALSHA lists scripts

2. Which of the following is the correct syntax for using EVALSHA in Redis?

easy

A. EVALSHA <numkeys> <sha1> [key1 key2 ...] [arg1 arg2 ...]

B. EVALSHA <sha1> <numkeys> [key1 key2 ...] [arg1 arg2 ...]

C. EVALSHA <sha1> [key1 key2 ...] <numkeys> [arg1 arg2 ...]

D. EVALSHA <script> <numkeys> [key1 key2 ...] [arg1 arg2 ...]

Solution

1. Step 1: Recall EVALSHA syntax

   The correct syntax is: EVALSHA followed by the SHA1 hash, then the number of keys, then the keys, then the arguments.

2. Step 2: Check option formats

   EVALSHA <sha1> <numkeys> [key1 key2 ...] [arg1 arg2 ...] matches the correct order. Options A, B, and C have wrong order or use script text instead of SHA1.

3. Final Answer:

   EVALSHA <sha1> <numkeys> [key1 key2 ...] [arg1 arg2 ...] -> Option B

4. Quick Check:

   Syntax = EVALSHA sha1 numkeys keys args [OK]

Hint: SHA1 hash comes first, then number of keys [OK]

Common Mistakes:

• Putting script text instead of SHA1 hash
• Mixing order of numkeys and keys
• Omitting number of keys

3. Given the following Redis commands executed in order:

127.0.0.1:6379> SCRIPT LOAD "return {KEYS[1],ARGV[1]}"
"a1b2c3d4e5f6g7h8i9j0"
127.0.0.1:6379> EVALSHA a1b2c3d4e5f6g7h8i9j0 1 mykey myarg

What will be the output of the EVALSHA command?

medium

A. ["mykey", "myarg"]

B. ["myarg", "mykey"]

C. Error: NOSCRIPT No matching script

D. ["a1b2c3d4e5f6g7h8i9j0", "myarg"]

Solution

1. Step 1: Understand SCRIPT LOAD output

   SCRIPT LOAD returns the SHA1 hash of the script, here "a1b2c3d4e5f6g7h8i9j0".

2. Step 2: Analyze EVALSHA call

   EVALSHA runs the cached script with 1 key: "mykey" and 1 argument: "myarg". The script returns {KEYS[1], ARGV[1]} so output is ["mykey", "myarg"].

3. Final Answer:

   ["mykey", "myarg"] -> Option A

4. Quick Check:

   Return keys[1], argv[1] = [mykey, myarg] [OK]

Hint: SCRIPT LOAD returns SHA1; EVALSHA uses keys then args [OK]

Common Mistakes:

• Confusing keys and args order
• Expecting error without loading script
• Using script text instead of SHA1

4. You run the command EVALSHA deadbeef 2 key1 key2 arg1 but get the error NOSCRIPT No matching script. What is the most likely cause?

medium

A. The number of keys argument should be 1, not 2

B. You provided too many keys for the script

C. The SHA1 hash 'deadbeef' is not loaded in Redis script cache

D. The script syntax is invalid

Solution

1. Step 1: Understand NOSCRIPT error meaning

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

2. Step 2: Check command usage

   Since the hash 'deadbeef' is not loaded, Redis cannot run the script. Number of keys or syntax errors cause different errors.

3. Final Answer:

   The SHA1 hash 'deadbeef' is not loaded in Redis script cache -> Option C

4. Quick Check:

   NOSCRIPT = missing cached script [OK]

Hint: NOSCRIPT means script hash not cached yet [OK]

Common Mistakes:

• Assuming wrong number of keys causes NOSCRIPT
• Thinking syntax error causes NOSCRIPT
• Ignoring that script must be loaded first

5. You want to optimize a Redis Lua script execution by caching it. Which sequence correctly uses SCRIPT LOAD and EVALSHA to run the script multiple times with different keys and arguments?

hard

A. Load script once with SCRIPT LOAD, save SHA1, then call EVALSHA with SHA1, numkeys, keys, args each time

B. Call EVALSHA with script text each time, no need to load script

C. Use SCRIPT FLUSH before every EVALSHA to clear cache

D. Call EVALSHA with SHA1 but always set numkeys to 0

Solution

1. Step 1: Understand caching workflow

   First, SCRIPT LOAD uploads the script and returns its SHA1 hash. This hash is saved for reuse.

2. Step 2: Use EVALSHA for repeated execution

   Use EVALSHA with the saved SHA1, specifying number of keys, keys, and arguments to run the script efficiently multiple times.

3. Final Answer:

   Load script once with SCRIPT LOAD, save SHA1, then call EVALSHA with SHA1, numkeys, keys, args each time -> Option A

4. Quick Check:

   Load once, run many times with EVALSHA [OK]

Hint: Load script once, then run with EVALSHA and SHA1 [OK]

Common Mistakes:

• Calling EVALSHA with script text instead of SHA1
• Flushing scripts before every run
• Setting numkeys incorrectly