Difference Between Pessimistic Concurrency and Optimistic Concurrency

In Pessimistic concurrency the server acquires locks to block access to data that another process is using. Pessimistic concurrency avoids conflicts by acquiring locks on data that is being read, so no other process can modify that data, it also acquires locks on data being modified so no other processes can access that data for either reading or modifying. Readers block writers and writers block readers.

In Optimistic Concurrency the server uses row versioning to allow data readers to see the state of the data before the modifications occur. Older versions of data rows are saved so a process reading data can see the data as it was when the process started reading and not be affected by any changes being made to that data. A process that modifies data is unaffected by processes reading the data because the readier is accessing a saved version of the data rows, readers do not block writers and writers do not block readers.