Resource Page Explained

The purpose of this document is to introduce and justify the existence of the resource page in its current incarnation, and to preview some upcoming changes to the page.

Why does this page exist? What does it do right now?

Concourse Resource Page for credhub-release resource in prod pipeline — currently employing the “disable everything and pause” kludge described below
  1. Surface the resource check status and age — show check errors, if any.
  2. Radiate information about a resource’s versions — at a glance, you can see the list of versions (that have been found by checking) in reverse chronological order. For example, you can see the equivalent of running git log on a git resource to understand which commits your Concourse instance has discovered.
  3. Get version metadata — sometimes the individual versions are not especially expressive; the git resource is a great example of this. Only seeing the commit hash doesn’t make it very easy for a human to identify the commit, but when you expand a version that has been retrieved via a get step in a job, you can see the commit time, author, and message, so you don’t need to cross reference against GitHub or your local git log output to figure out what is going on.
  4. Get inputs to/outputs of — this provides another means of visualizing how this resource interacts with your jobs, and a jumping off point to go to specific builds for those jobs — if all the builds for a given version are red, maybe it’s because there is some breaking change in that version?
  5. Pause checking—this button was introduced with use cases like the scenario below in mind. When an upstream dependency introduces a breaking change, you may want to stop using new versions until you bring your code back into compatibility.
  6. Enable/disable versions — this feature was created with the “Bad Version” scenario (to be described in another story) in mind.

The “Pinned Version” Scenario

In the following scenario, let’s assume we are a development team delivering software via a a so-called “three-box” continuous delivery pipeline like this (some fine details intentionally left out):

- name: third-party-dependency
type: gcs
- name: my-source-code
type: git
- name: my-build-artifacts
type: gcs
- name: my-app
type: cf
- name: build
— get: my-source-code
trigger: true
— get: third-party-dependency
— task: build
— put: my-build-artifacts
- name: test
— get: my-build-artifacts
trigger: true
— task: test
- name: deploy
— get: my-build-artifacts
trigger: true
passed: [test]
— put: my-app
Rendered view of the delivery pipeline in question

Suppose that the authors of third-party-dependency release a new major version that breaks compatibility with my-source-code in a way that we’re not ready to adapt to yet because we have more pressing issues in our backlog.

That version might be perfect and bug-free in and of itself, but it causes the test job to fail. However we’ve still got features to deliver — we’d like to just pin third-party-dependency at the last known working version so that we’re not prevented from deploying (by the passed: constraint on the get
step in our deploy job) for the time being, and we’ll pay down the technical debt of bringing our dependency back up to date later.

Now, strictly speaking, Concourse does provide an official solution to this problem: you can define a resource in the pipeline config so that only one version will ever be used. In our case, we could change the definition of third-party-dependency to be something like

- name: third-party-dependency
type: gcs
path: third-party-dependency-v1.1.0.tgz

The “Disable Everything and Pause” Kludge

The official solution for the “pinned version” scenario isn’t perfect. Clever users, like krishicks on GitHub, noticed that often your desire to pin a dependency is really just temporary. Furthermore it is possible that not all of your development team has an account on your Concourse instance such that they can reset the pipeline themselves, and you may unwittingly block them from doing their work this way. This struck the Concourse team as a valid concern and something that the product should support.

Furthermore, such clever users discovered an undocumented workflow for pinning the version of a resource without modifying their pipeline config:

  1. Visit the resource page and click the pause button to suspend checking.
  2. Disable all versions that were discovered after the one you wish to pin. Since jobs will use the latest available version that passes their constraints in get steps, then you have effectively pinned your resource to the desired version.

Now, if there have been many versions discovered since the breaking change occurred, this solution can require a lot of clicks. If your project has a lot of dependencies (e.g., you are developing Cloud Foundry!) then you may find yourself performing this onerous, many-click operation frequently. This is painful, and so the Concourse team wants to make it better.

First-Class Dynamic Version Pinning

Current state of the new pin functionality — to be included in the next major release

The Concourse team decided to add a pin icon to each version of the resource so that instead of performing all those clicks you can do just one — click the pin and then all other versions of the resource will be disabled (including new ones that are freshly discovered via checking). The road to this decision is documented in this GitHub issue.