[erlang-questions] Distributed Process Registry

[Roberto Ostinelli]

Dear list,
I have 3 interconnected nodes to which various devices connect to.
Once a device connects to one of those nodes, the related TCP socket events
are handled by a device_loop process on the node that it originally
connected to.

Every device is identified via its id (a binary). I need to enable
communication from one device to the other based on these ids, even within
different nodes. I have around 150k device processes per node (so up to
500k in total).

So, I basically need a global process registry. Not new, but haven't used
one in a while now.

As far as I can tell, my main options to send messages from one device
process to the other based on their id are the erlang global module, ulf's
gproc, or implement a custom solution based on, for instance, mnesia in ram
only.


I was first thinking of leaning towards using the erlang global module,
since register_name/2,3 now also allows general terms to be used as Name.
The advantages I see:

   - It is a simple built-in mechanism.
   - If a node goes down, the global names registered on that node are
   unregistered automatically.
   - If a new node is added, the global names registered are propagated
   automatically.

The cons:

   - I always feel that process registration should be used to identify
   long-running services.
   - I don't know if 500k is an acceptable number (i.e. if the global
   module is made to support my use case).


I also looked into gproc. The advantages I see:

   - Actively maintained, it seems to have been built for my use case.

The cons:

   - For the distributed part it relies on gen_leader. I've heard too many
   horror stories on gen_leader. Maybe that's not a thing anymore.
   - Not sure what happens if a node goes down / a new node is added.


I've considered a custom solution based on mnesia distributed ram-only
tables that would store the pids of the device loops based on their binary
id.The advantages I see:

   - Mnesia will take care of distributing, handling down events, etc.

The cons:

   - I need to reinvent the wheel and ensure that when a node goes down,
   all the device entries in the distributed mnesia tables related to that
   node are removed.



Has someone recently implemented a distributed process registry and can
shed some light for me?

Thank you in advance for your advice ^^_
r.
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