I just found the greatest analogy of rdbms locks

Feb 1, 2019

I had looked for a great analogy of an exclusive lock and a shared lock for a long time and today I just found this impressive analogy on the Stackoverflow answered by ArjunShankar !

For people who don't understand why it's really useful to understand a locked mechanism, you are going to see this topic in various database topics such as Serializability, Consistency, Linearizability and two phased commits.

I wrote this answer down because I thought this would be a fun (and fitting) analogy:

Think of a lockable object as a blackboard (lockable) in a class room containing a teacher (writer) and many students (readers).

While a teacher is writing something (exclusive lock) on the board:

  1. Nobody can read it, because it's still being written, and she's blocking your view => If an object is exclusively locked, shared locks cannot be obtained.
  1. Other teachers won't come up and start writing either, or the board becomes unreadable, and confuses students => If an object is exclusively locked, other exclusive locks cannot be obtained.

When the students are reading (shared locks) what is on the board:

  1. They all can read what is on it, together => Multiple shared locks can co-exist.
  1. The teacher waits for them to finish reading before she clears the board to write more => If one or more shared locks already exist, exclusive locks cannot be obtained.


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