**Optimistic Locking** assumes that a database transaction conflict is very rare to happen. It uses a version number of the record to track the changes. It raise an error when other user tries to update the record while it is lock.

usage:

Just add a lock_version column to the table you want to place the lock and Rails will automatically check this column before updating the record.

**Pessimistic locking** assumes that database transaction conflict is very likely to happen. It locks the record until the transaction is done. If the record is currently lock and the other user make a transaction, that second transaction will wait until the lock in first transaction is release.

*usage*

`user = User.lock.find(1) #lock the record`
`user.name = 'ryan'`
`user.save! #release the lock`

or you can also use with_lock method to select and lock the record.

`user = User.find(1)`
`user.with_lock do #lock the record`
`  user.name = 'ryan'`
`  user.save`
`end`

### Reference
* https://www.ibm.com/support/knowledgecenter/SSPK3V_7.0.0/com.ibm.swg.im.soliddb.sql.doc/doc/pessimistic.vs.optimistic.concurrency.control.html
* http://api.rubyonrails.org/classes/ActiveRecord/Locking/Optimistic.html
* http://api.rubyonrails.org/classes/ActiveRecord/Locking/Pessimistic.html
* http://thelazylog.com/understanding-locking-in-rails-activerecord/