Atomic Get and Delete

Credit: Gleicon Moraes, Dov Murik, Matti Paksula

Problem

You want to atomically GET and then DELETE an object from Redis.

Solution

Make use of Redis' built-in atomic functions, and MULTI-EXEC functionalty.

One approach to the problem might go something like this. In pseudo-code:

success = RENAME key key:tmp
if success
  value = GET key:tmp
  DELETE key:tmp
  return value
end

This is a simple one, but makes good use of Redis' atomic features. The RENAME function will succeed for first caller, subsequent callers will fail because the key was already renamed. GET and DEL benefit from the RENAME function in order to keep other clients from reading the object data between operations.

However, there is an potential problem lurking here. If the execution is interrupted on line 2 (if success) or line 3 (value = GET key:tmp), then that key stays renamed as key:tmp for good in the database.

Using Redis' MULTI-EXEC function provides a solution.

First, in pseudocode:

MULTI
value = GET key
DELETE key
EXEC    
return value

And using redis-cli:

redis> SET TOTO 1
OK
redis> GET TOTO
1
redis> MULTI
OK
redis> GET TOTO
QUEUED
redis> DEL TOTO
QUEUED
redis> EXEC
1. 1
2. (integer) 1
redis> GET TOTO
(nil)

Atomic Get and Delete

Problem

Solution

Discussion

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