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More in Lua Scripting
Why would you use a Lua script with Redis instead of just sending several separate commands from your application? How do you pass keys and other arguments into a Lua script when executing it with the EVAL command? How do you call other Redis commands from within a Lua script, and what restrictions apply to this? How do you manage and organize Lua scripts in a larger application to keep them maintainable as your codebase grows? How would you write a Lua script to implement an atomic rate limiter that checks and increments a request counter in a single operation? What are the performance and blocking implications of running a Lua script in Redis, especially for a script that takes a long time to execute?
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What is the difference between EVAL and EVALSHA, and why would you use EVALSHA in a production application?

Intermediate

Answer

EVAL sends the full text of the Lua script to Redis every single time it runs, while EVALSHA sends only a hash identifier of a script that has already been loaded and cached on the server beforehand, which reduces network bandwidth usage since you avoid repeatedly transmitting the same script text for every single execution.
SCRIPT LOAD "return redis.call('get', KEYS[1])"
-- returns a SHA hash
EVALSHA 'e0e1f9fabfc9d4800c877a703b823ac0578ff831' 1 mykey
Real-world example A high traffic application loads its frequently used Lua scripts once at startup, then calls them repeatedly using EVALSHA, saving meaningful network bandwidth compared to sending the full script text with every single call.

Common follow-ups: What happens if you call EVALSHA with a hash for a script that has not actually been loaded?;How long does Redis keep a loaded script cached?

Redis Performance Tuning & Benchmarking;Redis CLI & Basic Commands
All 7 Lua Scripting questions