A single application on Heroku can have any number of domains assigned to it, but you can only add a domain to one app. This means that by default you can’t serve example.com from the example-1 while example.com/blog is served from example-2 .

We ran into this problem with pilot.co recently where we have a constellation of apps ( pilot-co , pilot-blog , pilot-stories , etc.) which we wanted to host under a single domain.

We found a way to do that by putting a custom HAProxy instance, also hosted on Heroku, in front of all other Heroku apps we use.

Set up

Let’s say you have two apps on Heroku already:

example-com running https://example.com

running example-blog running https://blog.example.com

We will need a new app for your load balancer:

$ mkdir load-balancer $ cd load-balancer $ git init .

Then create an app on Heroku:

$ heroku apps:create example-lb Creating example-lb... done, stack is cedar-14 http://example-lb.herokuapp.com/ | https://git.heroku.com/example-lb.git Git remote heroku added

Installing Docker

You will deploy it to Heroku using Docker. We found it to be easier to manage than creating a custom buildpack.

Luckily, installing Docker on your machine is easy. Get Docker Toolbox and follow its setup instructions.

To verify that you have a working Docker installation, open your terminal and run:

docker ps CONTAINER ID IMAGE COMMAND ... $ docker-compose --version docker-compose version: 1.4.0

To deploy a Docker container to Heroku you will need heroku-docker :

$ heroku plugins:install heroku-docker

Heroku requires an app.json and Procfile manifests to be able to run your app.

{ "name": "Pilot Load Balancer", "description": "A load balancer for pilot.co", }

Your Procfile should look something like this:

web: sbin/haproxy -f haproxy.cfg

Then initialise Docker assets for the app:

$ heroku docker:init Wrote Dockerfile Wrote docker-compose.yml

Configuring HAProxy

Your Dockerfile is where we add instructions for Heroku on how to compile HAProxy:

FROM heroku/cedar:14 RUN mkdir -p /app/user WORKDIR /app/user # Install HAProxy RUN apt-get update && apt-get install -y libssl1.0.0 libpcre3 --no-install-recommends && rm -rf /var/lib/apt/lists/* ENV HAPROXY_MAJOR 1.5 ENV HAPROXY_VERSION 1.5.14 ENV HAPROXY_MD5 ad9d7262b96ba85a0f8c6acc6cb9edde # see http://sources.debian.net/src/haproxy/1.5.8-1/debian/rules/ for some helpful navigation of the possible "make" arguments RUN buildDeps='curl gcc libc6-dev libpcre3-dev libssl-dev make' \ && set -x \ && apt-get update && apt-get install -y $buildDeps --no-install-recommends && rm -rf /var/lib/apt/lists/* \ && curl -SL "http://www.haproxy.org/download/${HAPROXY_MAJOR}/src/haproxy-${HAPROXY_VERSION}.tar.gz" -o haproxy.tar.gz \ && echo "${HAPROXY_MD5} haproxy.tar.gz" | md5sum -c \ && mkdir -p /app/user/src/haproxy \ && tar -xzf haproxy.tar.gz -C /app/user/src/haproxy --strip-components=1 \ && rm haproxy.tar.gz \ && make -C /app/user/src/haproxy \ TARGET=linux2628 \ USE_PCRE=1 PCREDIR= \ USE_OPENSSL=1 \ USE_ZLIB=1 \ PREFIX=/app/user \ all \ install-bin \ && rm -rf /app/user/src/haproxy \ && apt-get purge -y --auto-remove $buildDeps COPY haproxy.cfg /app/user/haproxy.cfg

One last thing we need to do is configure HAProxy to route requests from our main app (called frontend) to all other apps (called backends).

HAProxy’s configuration manual is relatively easy to understand, and after some fine-tuning you should end up with something like this:

global maxconn 256 defaults mode http timeout connect 5000ms timeout client 50000ms timeout server 50000ms frontend http bind 0.0.0.0:$PORT option forwardfor # Force SSL redirect scheme https code 301 if ! { hdr(x-forwarded-proto) https } # Redirect all requests to /blog* to the `example-blog` app. use_backend example-blog if { path_beg /blog } # And all other requests to `example-com`. default_backend pilot-com backend pilot-com http-request set-header X-Forwarded-Host example.com http-request set-header X-Forwarded-Port %[dst_port] reqirep ^Host: Host:\ example-com.herokuapp.com server example-com example-com.herokuapp.com:443 ssl verify none backend example-blog http-request set-header X-Forwarded-Host example.com http-request set-header X-Forwarded-Port %[dst_port] reqirep ^Host: Host:\ example-blog.herokuapp.com server example-blog example-blog.herokuapp.com:443 ssl verify none

You can verify your setup locally by starting Docker:

$ docker-compose up web

and opening the browser:

$ open "http://$(docker-machine ip default):8080"

Deploying your load balancer to Heroku

If you’re satisfied with the outcome, it’s time to deploy it to Heroku:

heroku docker:release heroku open

After you verified that your new setup works on https://example-lb.herokuapp.com you can remove the example.com domain from example-com and attach it to example-lb .

After you’re done

Requests to https://example.com will go through example-lb and be served from example-com .

will go through and be served from . Requests to https://example.com/blog will also go through example-lb but be served from example-blog instead.

will also go through but be served from instead. All this will be completely hidden from your users. At no point they should see example-blog.herokuapp.com or any domain other than example.com .

If you’re using SSL (which this guide assumes you were) you can safely remove the SSL add on from all apps other than example-lb . Traffic between Heroku apps will be encrypted using their *.herokuapp.com certificate.

Additional resources