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I have a Procfile like so:
web: bundle exec rails server -p $PORT
em: script/eventmachine
The em process fires up an eventmachine with start_server (port ENV['PORT']) and my web process occasionally needs to communicate with it.
My question is how does the web process know what port to communicate with it on? If I understand heroku correctly it assigns you a random port when the process starts up (and it can change if the ps is killed or restarted). Thanks!
Processes are isolated and cannot communicate directly with each other.
http://www.12factor.net/processes
There are, however, a few other ways. One is to use a backing service such as Redis, or Postgres to act as an intermediary – another is to use FIFO to communicate.
http://en.wikipedia.org/wiki/FIFO
It is a good thing that your processes are isolated and share-nothing, but you do need to architecture your application slightly differently to accommodate this.