EVALSHA for cached scripts in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis EVALSHA command?

Given the Lua script SHA1 hash is abcd1234 and the script returns the sum of two keys' integer values, what will be the output of this command if key1=5 and key2=3?

Redis

EVALSHA abcd1234 2 key1 key2

A TypeError

B 8

C Error: NOSCRIPT No matching script. Please use EVAL.

D 5 3

💡 Hint

EVALSHA runs a cached Lua script by its SHA1 hash and returns the script's result.

The script sums the integer values of key1 and key2, which are 5 and 3, so the output is 8.

🧠 Conceptual

intermediate

Why use EVALSHA instead of EVAL in Redis?

Choose the best reason why Redis clients prefer using EVALSHA over EVAL after the script is loaded.

A EVALSHA is faster because it uses the cached script by its SHA1 hash, avoiding re-sending the script code.

B EVALSHA can run scripts without any keys or arguments.

C EVALSHA automatically updates the script if it changes on the server.

D EVALSHA returns the script source code instead of executing it.

💡 Hint

Think about network efficiency and script caching.

EVALSHA uses the cached script identified by its SHA1 hash, so it avoids sending the full script code every time, making it faster.

📝 Syntax

advanced

Which EVALSHA command syntax is correct to run a script with 3 keys and 2 arguments?

Select the correct Redis EVALSHA command syntax to run a cached script with SHA1 hash f1e2d3c4, passing 3 keys and 2 arguments.

A EVALSHA f1e2d3c4 5 key1 key2 key3 arg1 arg2

B EVALSHA f1e2d3c4 key1 key2 key3 3 arg1 arg2

C EVALSHA f1e2d3c4 3 key1 key2 key3 arg1 arg2

D EVALSHA f1e2d3c4 2 key1 key2 key3 arg1 arg2

💡 Hint

Remember the syntax: EVALSHA ... ...

The second argument is the number of keys (3), followed by the keys, then the arguments.

🔧 Debug

advanced

What error occurs if you run EVALSHA with an unknown SHA1 hash?

You run EVALSHA deadbeef 1 key1 arg1 but the script is not cached on the server. What error message will Redis return?

A SyntaxError: invalid command

B OK

C TypeError: missing arguments

D Error: NOSCRIPT No matching script. Please use EVAL.

💡 Hint

Think about what happens when the script is not found in cache.

Redis returns a NOSCRIPT error indicating the script is not cached and suggests using EVAL to load it.

❓ optimization

expert

How to optimize repeated Lua script execution in Redis using EVALSHA?

You have a Lua script that you run many times with different keys and arguments. Which approach best optimizes performance?

A Load the script once with EVAL, then run it repeatedly with EVALSHA using the cached SHA1 hash.

B Run the script every time with EVAL to ensure the latest version is used.

C Use EVALSHA without loading the script first; Redis will auto-load it.

D Store the script in a Redis string and call it by GET command.

💡 Hint

Consider network traffic and script caching benefits.

Loading the script once with EVAL caches it on the server. Subsequent calls with EVALSHA use the cached script, reducing network overhead and improving speed.

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