Let's Talk Process Dictionary

A tale of heated opinions, information whispered in the shadows, and mutants living in your BEAM at this very moment.
From Lonestar ElixirConf 2018
Video available: https://www.youtube.com/watch?v=zDIoFWwfBO0
Code available: https://github.com/gvaughn/process_dictionary_talk

Transcript

1. LET'S TALK PROCESS DICTIONARY Greg Vaughn @gregvaughn
2. We're hiring!*
3. WHAT IS A PROCESS DICTIONARY? A per-process key/value map Straightforward API Process.get Process.put Process.delete Process.get_keys Process.info
4. iex(1)> f = fn -> ...(1)> IO.inspect(Process.get_keys(), label: "get_keys()") ...(1)> IO.inspect(Process.put(:lonestar, :elixirconf), label: "put(:lonestar, :elixirconf") ...(1)> IO.inspect(Process.get_keys(), label: "get_keys()") ...(1)> IO.inspect(Process.get(:lonestar), label: "get(:lonestar)") ...(1)> IO.inspect(Process.put(:lonestar, :rocks), label: "put(:lonestar, :rocks") ...(1)> end #Function<20.99386804/0 in :erl_eval.expr/5> iex(2)> spawn(f) get_keys(): [] put(:lonestar, :elixirconf): nil get_keys(): [:lonestar] get(:lonestar): :elixirconf put(:lonestar, :rocks): :elixirconf
5. DEMO
6. PEOPLE ON THE INTERNET HAVE STRONG OPINIONS
7. • Process Dictionary erases all the functional benefits we get from Elixir! • It should be removed from the language! • You should never use it! OH NO!
8. • Process Dictionary helps Elixir be multi-paradigm! • There's plenty of examples of its use by responsible programmers! • Use it whenever it is convenient!
9. DEBUNK (or at least apply context and nuance)
10. It erases all the functional benefits we get from Elixir! It helps Elixir be multi-paradigm!
11. Elixir is not pure Side effects are essential for useful programs – a receive loop is useless without them Multi-paradigm It means the language refuses to promote one approach over another and leads to unmaintainable messes Isolating where side effects happen and keeping the rest of your program functional/immutable is The Elixir Way™
12. It should be removed from the language! There's plenty of examples of its use by responsible programmers!
13. • Erlang's rand and crypto modules store the seed • proc_lib stores $initial_call and $ancestors • mnesia stores a transaction identifier • Logger stores logging metadata • Gettext stores locale • Ecto transactions store a connection • iex stores history
14. DEMO
15. You should never use it! Use it whenever it is convenient!
16. Default to not using the Process Dictionary. Ensure you know how to solve your problem without it. In a GenServer, use the state you already have. Consider reserving part of that state to pass into some stateful helper module. Safest: a write once variable, avoiding mutation. Do: centralize where reads/writes happen near each other. Space and time. Caution: it is not garbage collected and will be copied via Process.info/2 so don't put large datasets in there.
17. PROCESS DICTIONARY FIRST RULE OF MACRO ^ CLUB 1. Don't do it 2. Do it responsibly
18. LET'S TALK PROCESS DICTIONARY Greg Vaughn @gregvaughn QUESTIONS?