Testing Concurrency

Kendra Little walks through using OStress to test concurrency:

Even though I’m taking out UPDLOCKS, the following race condition pattern can still occur

  • Session A takes out an UPDLOCK, sees that key 266 does not exist, releases its UPDLOCK, and prepares to insert
  • Session B takes out an UPDLOCK, sees that key 266 does not exist, releases its UPDLOCK, and prepares to insert
  • Session A runs its  insert
  • Session B attempts to run its insert, but fails because of a duplicate key error

We need to hold that lock.

Understanding concurrency is one of the toughest parts of being a database developer, especially because it’s historically been difficult to test it.  I like what Kendra’s done here, making the process easy to follow.

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