Highlander

A prototype project to create the primitives for creating object oriented distributed systems.

There can be only one object for each object ID at a time.

Should I use this?

Probably not. This is a proof-of-concept for a crazy idea and hasn't been proved in production yet. I think a library like this should exist and I am willing to work towards getting this idea into production in time.

Features

This project currently has too many responsibilities and should probably become three different libraries:

• GenPersistedServer to persist state after updates
• ZookeeperRegistry to host the process registry in zk
• Highlander which would tie it together and provide the "object" macros to make creating applications easier

History

Why not use :global?

:global is great and is the MVP of cluster-wide process ids. It, however, is known to allow a "split brain" during a network partition. This is not a bad thing: maybe you don't mind having two of something for a short time.

http://erlang.org/doc/man/global.html explains about "name clashes" and the Resolve function. If it were consistent, then there would be no need for a Resolve.

Why not use :pg2?

:pg2 is explicitly not consistent and is pretty much AP, which can be a great thing. It also allows you to have a process under multiple ids and an id point at multiple processes.

Check out http://erlang.org/pipermail/erlang-questions/2012-June/067220.html where it says:

pg2 replicates all name lookup information in a way that doesn’t require consistency

Why not use Registry?

Registry is great and Highlander should use it instead of it's own internal state map for the on-node lookups. The problem is that Registry is not a multi-node solution. For a single node, it's really good.

What about :gproc?

So, I think it's not consistent. Read https://christophermeiklejohn.com/erlang/2013/06/05/erlang-gproc-failure-semantics.html where it says:

While gproc has been tested very thoroughly … its reliance on gen_leader is problematic.

I honestly don't know why there are so many negative articles about gen_leader, but it does seem like there is not community support for it being the correct solution to this problem. I don't know. I intend to write my own experiments with gen_leader to learn about it, but I have not had time.

What about riak_core?

Sure. I am sure one can use riak_* to achieve something similar, but I don't think it's wise for me to head down that road yet. I'd love to see someone do that.

Setup

Docker

You must have docker, docker-compose, and be able to $ docker ps.

Then you must be running all the necessary containers in the ./docker folder with docker-compose:

$ cd docker
$ docker-compose up -d

You can get to the logs of the containers with:

$ docker-compose logs

When you are finished developing or running tests you can terminate all containers with:

$ docker-compose down

Elixir

You must have elixir, erlang, etc installed.

Then install the necessary dependencies:

$ mix deps.get

Tests

$ epmd -daemon # to allow port mappings and things
$ mix test