EVALSHA for cached scripts in Redis - Step-by-Step Execution

Concept Flow - EVALSHA for cached scripts

Write Lua script

↓

Load script with SCRIPT LOAD

↓

Get SHA1 hash of script

↓

Run script with EVALSHA using SHA1

↓

If SHA1 not found -> Error

↓

Optionally reload script and retry

You write a Lua script, load it into Redis to get a SHA1 hash, then run it using EVALSHA with that hash. If Redis doesn't have the script cached, you get an error and can reload it.

Execution Sample

Redis

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

Load a Lua script that gets a key's value, then run it using its SHA1 hash.

Execution Table

Step	Command	Input	Action	Output/Result
1	SCRIPT LOAD	"return redis.call('GET', KEYS[1])"	Load script into Redis cache	Returns SHA1 hash: abc123...
2	EVALSHA	abc123... 1 mykey	Run cached script with SHA1	Returns value of 'mykey' if exists
3	EVALSHA	wrongsha 1 mykey	Try running with unknown SHA1	Error: NOSCRIPT No matching script. Please use EVAL.
4	SCRIPT LOAD	"return redis.call('GET', KEYS[1])"	Reload script after error	Returns SHA1 hash: abc123...
5	EVALSHA	abc123... 1 mykey	Run script again after reload	Returns value of 'mykey'

💡 Execution stops when script runs successfully or error occurs due to missing SHA1.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
SHA1	null	abc123...	abc123...	wrongsha	abc123...	abc123...
Script Cached?	false	true	true	false	true	true
Script Result	null	null	value of mykey or nil	Error NOSCRIPT	null	value of mykey or nil

Key Moments - 2 Insights

Why do we get an error when using EVALSHA with a SHA1 that Redis doesn't know?

Because Redis only runs scripts it has cached. If the SHA1 is unknown, Redis returns a NOSCRIPT error as shown in step 3 of the execution_table.

What does SCRIPT LOAD do and why is it important before EVALSHA?

SCRIPT LOAD stores the Lua script in Redis and returns its SHA1 hash. This hash is needed to run the script with EVALSHA, as shown in step 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of SCRIPT LOAD in step 1?

A Returns the SHA1 hash of the script

B Returns the value of 'mykey'

C Returns an error NOSCRIPT

D Returns null

💡 Hint

Check the 'Output/Result' column in step 1 of the execution_table

At which step does Redis return an error because the SHA1 is unknown?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the 'Error: NOSCRIPT' message in the execution_table

If you skip SCRIPT LOAD and run EVALSHA with a random SHA1, what happens?

A Script runs successfully

B Redis returns NOSCRIPT error

C Redis loads the script automatically

D Redis returns null

💡 Hint

Refer to step 3 in the execution_table where unknown SHA1 causes error

Concept Snapshot

EVALSHA runs a cached Lua script by its SHA1 hash.
Use SCRIPT LOAD to cache script and get SHA1.
If SHA1 unknown, Redis returns NOSCRIPT error.
Reload script with SCRIPT LOAD before retrying EVALSHA.
This improves performance by avoiding sending full script each time.

Full Transcript

This visual execution shows how Redis runs Lua scripts using EVALSHA. First, you write a Lua script and load it into Redis with SCRIPT LOAD. This returns a SHA1 hash representing the script. You then run the script by calling EVALSHA with that SHA1 and the keys/arguments. If Redis does not have the script cached (unknown SHA1), it returns a NOSCRIPT error. You must reload the script with SCRIPT LOAD to cache it again before retrying EVALSHA. The execution table traces these steps, showing the SHA1 variable, caching state, and results. Key moments clarify why the NOSCRIPT error happens and the role of SCRIPT LOAD. The quiz tests understanding of these steps and error conditions. This method improves efficiency by sending only the SHA1 hash after caching the script once.