Script loading and caching in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of SCRIPT LOAD command

What does the SCRIPT LOAD command return when you load a Lua script into Redis?

Redis

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

A An integer representing the number of commands in the script

B A SHA1 hash string representing the loaded script

C The output of the script execution, e.g., 'PONG'

D The string 'OK' indicating the script loaded successfully

💡 Hint

Think about what Redis returns to identify a script uniquely after loading.

SCRIPT LOAD returns the SHA1 hash of the script, which can be used later to execute the script by reference.

❓ query_result

intermediate

Result of EVALSHA with missing script

What happens when you run EVALSHA with a SHA1 hash that is not loaded in Redis?

Redis

EVALSHA <nonexistent_sha1> 0

A Redis returns an error indicating the script is not found

B Redis automatically loads the script from the last executed scripts

C Redis returns nil without error

D Redis executes the script anyway by fetching it from disk

💡 Hint

Consider what Redis does if the script hash is unknown.

If the script SHA1 is not found in Redis cache, EVALSHA returns a NOSCRIPT error.

📝 Syntax

advanced

Identify the correct SCRIPT FLUSH usage

Which of the following commands correctly flushes all cached Lua scripts in Redis?

A SCRIPT DELETE ALL

B SCRIPT CLEAR

C SCRIPT FLUSH

D SCRIPT REMOVE

💡 Hint

Check the official Redis command for clearing scripts.

SCRIPT FLUSH clears all cached scripts from Redis memory.

❓ optimization

advanced

Best practice for script execution performance

To optimize performance when running the same Lua script multiple times in Redis, which approach is best?

A Use MULTI/EXEC transactions to run the script multiple times

B Run the script each time with EVAL to avoid caching issues

C Store the script in a Redis key and retrieve it before each execution

D Load the script once with SCRIPT LOAD, then run it multiple times with EVALSHA

💡 Hint

Think about how Redis caches scripts and how to reuse them efficiently.

Loading the script once and running it by SHA1 hash avoids repeated parsing and improves performance.

🧠 Conceptual

expert

Understanding script caching behavior on Redis restart

What happens to cached Lua scripts in Redis after a server restart?

A All cached scripts are lost and must be reloaded

B Cached scripts persist automatically across restarts

C Scripts are saved to disk and reloaded on startup

D Scripts remain cached if Redis is configured with AOF persistence

💡 Hint

Consider Redis memory and persistence behavior for scripts.

Cached scripts are stored in memory only and are lost on restart; they must be reloaded.

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