Subverting the BOSH release

Written on 2019-07-28

This is a post about doing bad things in order to do good things, using a BOSH release.


I often read posts about building and maintaining software well, and people doing things perfectly. I also read about the worst of the worst, absolute tyre-fires of software which limp into production, where they stay forever, much to the terror of everyone else: here is one of my favourites.

Less often I get to read about elegant (citation needed) hacks, that feel a bit dirty, but get the job done, whilst not being absolutely awful. The kind of hack you feel a bit bad about, but of which are also secretly proud. This post is about one of those; and is, IMO, especially relevant given all of the discourse about 10x and 1x engineers recently. Sharing our sins as well as our successes is important.


This work was done on the infrastructure for GOV.UK PaaS, whilst we were working on the single sign-on feature. The BOSH release source code is now open and can be found here, and is heavily based on an 18F BOSH release.


GOV.UK PaaS is a platform-as-a-service which service teams can use for building and maintaining online government services without needing much operations capability within the team/organisation. It is a highly customised deployment of Cloud Foundry designed to helping service teams quickly build excellent online government.

BOSH is a release engineering tool for deploying and managing virtual machines and the software deployed on virtual machines. BOSH manages the packaging of software and the lifecycle of virtual machines deployed in the Cloud (private or public). Software is distributed in a very opinionated way:

BOSH is a fantastic tool, and in my opinion does not get enough attention. Modern software development and operations is now focused on using (OCI) containers as the universal deployment artefact; however when you need to orchestrate servers instead of containers then BOSH is worth the initial setup cost and steep learning curve.

Within Cloud Foundry there is a component called UAA (User Account and Authentication Server) which is a Java application for managing access to other components within Cloud Foundry. UAA is itself both a service provider and an identity provider. Due to the complex nature of identity and access management, UAA is a complex piece of software with a large surface area; as such, it is updated quite often.

As alluded to earlier, there is an upfront cost to getting software built and deployed using BOSH, and sometimes it can be hard work to fork a repository, and build it yourself, just to make a few minor changes. This was the case, when we wanted to change how UAA was styled, and the content of the user facing pages.

Being a monolithic web application, UAA has the application logic and the assets for styling and content storeed together in the same codebase; built into the same BOSH package. This meant it was not possible to simply swap out the stylesheets and HTML templates for a customised set; nor was the BOSH release customisable enough to sufficiently change the content and styles of the application. Previously the team had done some very impressive (and impossible to change) CSS hacks to make the page look sufficiently GOV.UK-like to be palatable to users.

Essentially, we wanted to have our cake, and eat it too: which sometimes is possible in software development, although rarely. The result of which is due, in most part, to 18F’s uaa-customized-boshrelease which was the starting point for paas-uaa-customized-boshrelease. The latter is a fork of the former, although GOV.UK PaaS’s release has diverged significantly in content and scope.

How it works

Normally, UAA is deployed to a VM, and it is registered with a routing component (gorouter) using another piece of software called route-registrar. When UAA is being redeployed then the virtual machine is de-registered from the router, the software is updated, and then the virtual machine is re-registered. This is done via BOSH, and via a healthchecking script bundled with UAA, which is consumed by route-registrar.

Our BOSH release bundles its own healthcheck script, which is used by route registrar instead of UAA’s bundled healthchecking script. Once UAA has been updated and redeployed the instance is still not yet marked as healthy, it is stopped again, the WAR (web application resource) file which is used to distribute UAA is unpacked, some files (stylesheets and HTML templates) are updated. The WAR file is re-assembled, and then UAA is started again, healthchecked, and then the instance is re-registered.

This is a filthy hack and should not be used (which is coincidentally what it says in 18F’s original repository). However, like most awful hacks that end up getting used in production: it serves a purpose, and doing it this way was better than doing it properly (for some definition of better). Not all software can be beautiful, perenially clean distributions of excellence.


We do not have to maintain our own fork and release of UAA, which delays the speed at which we can ship security updates to UAA (which is unfortunately surprisingly necessary).

We learned a lot about the Thymeleaf templating language, and a lot about BOSH releases and the BOSH lifecycle.

We drastically improved the user experience of logging in to GOV.UK PaaS, which is perhaps worth a blog post on its own. However by having more control over just the stylesheets and HTML templates we were able to include the GOV.UK Design System, and considerably improve accessibility and usability.