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Heroku deployment

First time

  1. Install and configure the Heroku Toolbelt.
  2. Clone Enketo Express.
  3. Create your Heroku application from your Enketo Express folder with heroku create.
  4. Configure Enketo (see next section). The absolute minimum is setting ENKETO_LINKED_FORM_AND_DATA_SERVER_SERVER_URL=. This could be an empty value.
  5. Create main database with heroku addons:create heroku-redis:premium-0 --as enketo_redis_main.
  6. Create cache database with heroku addons:create heroku-redis:premium-0 --as enketo_redis_cache. Note that heroku-redis:premium-0 is persistent which is actually not necessary, but won't hurt either.
  7. Start web server with heroku ps:scale web=1. For multiple dynos upgrade to Standard or Performance first. You'll likely have to upgrade your heroku redis addons as well, at least the one containing the main database (e.g. with heroku addons:upgrade enketo_redis_main:premium-1 - this takes several minutes to complete).
  8. Push code to Heroku with git push heroku master.
  9. Check the logs while it's running. You may have to upgrade the redis addons (error: too many connections) or memory (error: memory quota exceeded)!

Heroku configuration

On Heroku, the regular config.json configuration should not be used (and not be created). Instead Enketo is configured with environment variables using heroku config:set. See README

Enketo's JS and CSS build process uses configuration variables. This means that every time an environment variable (that is used in browser scripts) is changed, Enketo needs to rebuild. In Heroku rebuilds are always triggered with a git push. If there is nothing to push you'll therefore have to trick Heroku into rebuilding by pushing an empty commit like this: git commit --allow-empty -m "empty commit".

Redis connections

The key sizing criterion for redis addons seems to be the number of simultaneous database connections. It seems Enketo on Heroku is using about 26 connections for enketo_redis_main and 18 for enketo_redis_cache per dyno (8 threads). Make sure to check your logs to see if the app is exceeding the amount of allowed redis connections. You'll want to immediately address this critical error state by either reducing the number of dynos or upgrading your redis addon.

Warning: before upgrading or downgrading heroku-redis

Before upgrading or downgrading your heroku-redis addon, shut down your server with heroku ps:scale web=0. This avoids 2 potential issues:

  1. Any data written to redis during the upgrade/downgrade might get lost forever.
  2. In the past there have been issues with permanent loss of data when upgrading the addon when the app was in a 'maximum allowed connections exceeded' state. This was fixed by Heroku on June 30th 2016, but the experience means it is wise to take extra care.

Advantages of using Heroku

  1. Fastest possible deployment of your own Enketo server.
  2. Easy scaling.
  3. Reliable, presumably.

Disadvantages of using Heroku

  1. Expensive.
  2. Initial Enketo configuration is more cumbersome (editing a structured json file is just much easier).
  3. Little control. Requires trust (e.g. for database backups) and relying on support (e.g. for database restoration).