With the ever more frequent use of multiple servers and worker processes, issues which only occur when specific tasks are running in parallel become ever more likely to happen.

In this talk Marcos talks about some rails concurrency limitations you need to be aware of, including why you should never trust rails to check for uniqueness, why mysql can't do optimistic locking with retrying by default and how to fix it, how to properly choose between optimistic and pessimistic locking, how parallel processes can cause deadlocks even if you're not using locks, why you should never have serialized attributes in rows that might be edited in parallel, and why rails commits your transactions before they finish in case your multi threaded process is killed and how to prevent it from leaving your database on an inconsistent state.