Shared Lock

About

A shared lock is a lock acquired in the shared mode over a data structure.

The data structure can be any:

• from a file
• to any in-memory structure (ie variable with a complex type, generally a collection)

A shared lock:

• allows two or more process/thread to read from the data structure at the same time.
• prevents another process from writing to the data structure while a process is reading it

Shared locks are usually used while the process is reading data (SQL SELECT operation , …)

A process/thread gets ownership of the data structure by obtaining the shared lock.

This is the first step step toward reading a file that can be accessed concurrently. For instance, a database application will acquire a shared lock on its database file before reading it.

Why ?

Without this lock, if a process is writing in the data structure (file, map, collection, …), a reading process might read:

• some data before the change
• and other data after the change.

The read would then not be consistent and the write change will then appear not atomic (even if it's).

In the context of a transaction, this read are called dirty read.