EVALSHA for cached scripts in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command EVALSHA do?

EVALSHA runs a Lua script that is already cached in Redis using its SHA1 hash. It avoids sending the full script again.

beginner

Why use EVALSHA instead of EVAL in Redis?

EVALSHA is faster because it runs a cached script by its hash, so you don't resend the whole script text every time.

intermediate

What happens if you run EVALSHA with a SHA1 hash that Redis does not recognize?

Redis returns an error saying the script is not found. You must load the script first with SCRIPT LOAD or use EVAL to run it.

beginner

How do you get the SHA1 hash of a Lua script to use with EVALSHA?

Use the Redis command SCRIPT LOAD <script>. It returns the SHA1 hash of the script.

intermediate

What are the arguments passed to EVALSHA after the SHA1 hash?

First, the number of keys the script will access, then the keys themselves, followed by any additional arguments the script needs.

What does EVALSHA require to run a script?

A The SHA1 hash of a cached script

B The full Lua script text

C The script filename

D The script execution time

EVALSHA runs a script by its SHA1 hash, which must be cached in Redis.

If EVALSHA returns a 'NOSCRIPT' error, what should you do?

A Use <code>SCRIPT LOAD</code> to load the script or run <code>EVAL</code> first

B Change the script name

C Restart Redis server

D Increase Redis memory

The 'NOSCRIPT' error means Redis does not have the script cached. Load it or run EVAL to cache it.

Which command returns the SHA1 hash of a Lua script in Redis?

A SCRIPT HASH

B SCRIPT LOAD

C EVALSHA

D SCRIPT GET

SCRIPT LOAD loads a script and returns its SHA1 hash.

What is the first argument after the SHA1 hash in EVALSHA?

A Number of arguments

B Script timeout

C Number of keys the script will access

D Script version

The first argument after the SHA1 hash is the number of keys the script will access.

Why is EVALSHA preferred over EVAL for repeated script execution?

A It automatically updates scripts

B It uses less memory

C It supports more Lua features

D It runs scripts faster by avoiding re-sending the script text

EVALSHA runs cached scripts by hash, so it avoids sending the full script text each time, improving speed.

Explain how EVALSHA works in Redis and why it is useful.

Think about caching and script reuse.

Describe the steps to run a Lua script repeatedly using EVALSHA in Redis.

Consider script loading, running, and error handling.