Custom domains for Multi-tenant web app

What you are wanting to do is fairly straight forward to implement. Your assumptions are correct about setting up the proxy. Nginx or haproxy will both work great for this (I personally would use haproxy). Here are some of the gotchas that you will run into though:

1. Changing the host header at a proxy server can cause the end web application to generate incorrect links. You can use relative paths to fix this, but it requires that the web application developer to be aware of the environment that they are running in.

   1. user connects to www.example.com (proxy server)
   2. proxy server connects to www.realdomain.com (web app)
   3. the web app has a link for a shopping cart. www.realdomain.com/shoppingcart
   4. the end user clicks on the link but the link is www.realdomain.com/shoppingcart instead of www.example.com/shoppingcart

2. The cost of the host acting as the proxy server. This can spiral out of control really quickly. For example, do you want redundancy, if so how are you planning on implementing that? Do you plan on having ssl termination? If so you will have to increase the CPU count to accommodate the additional load. Do you want to have a secure connection to heroku from your proxy? If you do then you will need to increase the CPU count for that as well. You may have to add additional ram as well depending on the number of concurrent connections.

3. Heroku also changes their load balancers regularly. This is important because your proxy service will need to reload the config / update the ip addresses of the heroku instances every 60 seconds. In my experience they may change once or twice a day, but the DNS entry that they use has a 60 second TTL. That means that you should make sure that you are capable of updating your config up to every 60 seconds.

My company has been doing something very similar to this for almost a year now. We use haproxy and simply have it reload the config regularly. We have never had an outage or an interruption to our end users. Nginx is also a very good product. It has built in DNS caching so if you go that route you will need to make sure that you configure it correctly so that the DNS cache TTL is 60 seconds.

Will many of your clients want to use your app on their domain apex? E.g. example.com rather than theapp.example.cpm? If not, I would recommend having them CNAME to proxy.myapp.com which CNAMEs to myapp.herokuapp.com. Then, you can update proxy.myapp.com without customer interruption.

If you do need apex or A record support, you would want to set up Nginx as a reverse proxy for your Heroku app. Keep in mind that if you need HTTPS support for client domains, you will need to do some sort of certificate management on your proxy.