Concurrency

When database is used by multiple processes/threads, we might run into some concurrency issues. Some of those might be solved by transactions, but that might not be enough (e.g. double booking) That’s why we use locking.

Locking

Exclusive Lock

Exlusive Lock allows to lock a row (or column) so that no other process is allowed to write or even read it. The other process would fail with an error if it tried to do that. Another name for it could be “write lock”, because it locks everyone from accessing the data until its written completely.

Shared Lock

With Shared Lock, only one process can write into something, but many processes can read it. Another name for it could be “read lock”, because it block any writes until all reads are done.