EVAL

Introduction and Use Case(s)

The EVAL command in Redis is used to execute Lua scripts on the server side. This allows for complex operations to be performed atomically, which can enhance performance and consistency. Typical scenarios include transactions, batch operations, and custom commands that aren’t natively supported by Redis.

Syntax

  1. EVAL script numkeys key [key ...] arg [arg ...]

Parameter Explanations

  • script: The Lua script to execute.
  • numkeys: The number of keys that the script will interact with.
  • key [key …]: The actual keys that the script will operate on.
  • arg [arg …]: Additional arguments to pass to the script, accessible within Lua.

Return Values

The return value of EVAL depends on what the Lua script returns:

  • If the script returns a Redis data type, it’s converted to its respective type (e.g., string, list, set).
  • If the script returns an error, EVAL will propagate this error to the client.

Example outputs:

  • A number: (integer) 10
  • A string: "Hello, World!"
  • A table: [ "1", "2", "3" ]

Code Examples

  1. dragonfly> EVAL "return redis.call('SET', KEYS[1], ARGV[1])" 1 mykey "myvalue"
  2. "OK"
  4. dragonfly> EVAL "return redis.call('GET', KEYS[1])" 1 mykey
  5. "myvalue"
  7. dragonfly> EVAL "return {ARGV[1], ARGV[2]}" 0 "arg1" "arg2"
  8. 1) "arg1"
  9. 2) "arg2"

Best Practices

  • Always validate and sanitize your Lua scripts to avoid security vulnerabilities.
  • Minimize the complexity of Lua scripts to improve performance and maintainability.
  • Use the redis.call function for Redis commands to ensure atomicity within scripts.

Common Mistakes

  • Not setting numkeys correctly, which leads to errors.
  • Ignoring error handling within Lua scripts.
  • Using blocking commands like BLPOP inside Lua scripts, which can cause performance issues.

FAQs

What happens if the script has an error?

If the Lua script encounters an error, the EVAL command will return an error message describing the issue.

Can I use all Redis commands within a Lua script?

Most Redis commands can be used within Lua scripts through redis.call, but some blocking commands are not allowed.

How do I debug a failing Lua script?

To debug, you can use redis.log within your Lua script to output messages to the Redis log.