The first rule of BEAM Club is that we do not talk about the Process dictionary

Whoa. Now we’ve come full circle. I also briefly reference this thread in that talk!

Two places where popular elixir libraries use pdict are:

• logger - to store the metadata that is attached to each message. It would be hard to achieve current API without use of process dictionary here - you’d need to pass logger state around to each logger call.
• ecto - to keep track of current transaction. Inside the function passed to Repo.transaction a database connection is checked out and saved in process dictionary. Subsequent calls to other Repo functions look for the connection in the process dictionary before checking out another connection. It would be possible to rule out the use of pdict here with explicit connection passing. Because of ecto’s architecture connection cannot be shared between processes (to have a transaction spanning multiple processes) - because of this the use of pdict is not particularly limiting, but it does introduce a level of indirection and makes some use cases problematic (those that require dynamic connection management).