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Waterfall Tour

A ChromiumOS Waterfall hosts and lists groups of ChromiumOS builds.

ChromiumOS uses LUCI to run continuous builds, integration (commit queue) and tests. It uses chromite to centralize/abstract the process of building for various configurations.

There are a number of builder groups that each present a waterfall view of their status, but with chromiumos' use of ebuilds and manifests, many of the LUCI's paradigms are not appropriate. There is a column of changes, but these represent changes present in the repositories, but does not represent which ones are included in the product by the manifest references and the ebuilds used.


The LUCI scheduler watches the clock, git repositories and previous builds, telling builders to start building and testing new revisions. LUCI's Milo service serves the "waterfall" page that shows all the results. For the full and incremental builders every time a new revision is discovered, the client triggers builds. Each of these builders generally has a single bot, so it is usually busy when this happens. When the bot becomes idle the most recent trigger is chosen to represent all automatic builds to date and it is started. The rest of the pending builds are presumed to be covered enough, and are removed. The build itself for chromeos is a sequence of three steps:

  1. Update the builder recipes (Basically, the piece the Chromium infrastructure people maintain to run the builders)
  2. Update the chromite tools (Basically, the piece the ChromeOS build people maintain to drive a ChromiumOS build)
  3. Run the cbuildbot command with a configuration name to do the real work.

The third step involves many stages, and it is parsed and presented as individual steps by the recipe engine, but it is all coming from the one command. This command can be run on local checkouts, by developers, in the same ways as the bots do in order to predict and reproduce failures, tests, etc. This page is about the mechanics that get it run, not what it does once it is run.

While each of these steps is executing Milo is collecting the output, and presents the state on the web interface. There are other people and processes that also monitor the state the client exports, or the side effects of the build on git or google storage, to start further builds, send emails, etc.


Let's take a look at the waterfall page. For now, skip over the header (green, red, yellow, etc.) and the box at the top with lots of links and horizontal stripes (we'll come back to them later), and look down at the row of grey boxes with "changes" at the left. Those boxes show one column per builder. In ChromeOS we tend to name these by board/config name, and role. Each of these builders is associated with one or more bots, which are machines willing to run such builds. Each builder belongs to exactly one client (waterfall) but in some waterfalls they can be willing to serve more than one builder. The title in this row will link to a page that lists the recent builds completed for this configuration, and the builders associated with it. This is a useful view to see what recent runs have looked like, especially for comparing or contrasting.


Once a build request gets assigned to a builder it begins a "Build". Chromite/cbuildbot generally starts with clean up and an open sync to the state of the tree (tip of branch in the branch case). Chromite/cbuildbot will tell you what it is building in the sync stage, or provide references as links in the build steps. Each build will have a number, a bot, and will run until completion or fatal error. The waterfall view will present the exposed steps in reverse chronological order in the column it belongs in. There is a list of times (generally in Google Coordinated Time) in the leftmost column to record when observation are made, and the boxes change colors to represent the current state as they progress. When a bot is finished it may disconnect and locally clean itself up (e.g. by rebooting) before asking for its next slug of work.

Current Activity

In the row just above the titles for the grid there is a summary for each builder of what is currently going on, and what is upcoming (active builds, pending builds, delay to next time based decision). Beware that the ETA in this box is computed using a very simple predictive model that assume each build is roughly the same as the ones before. This goes very wrong when changing between builds that work, and builds which break a little ways in. Beware of these predictions in anything other than routine situations.

Last Build

The top row of the grid part is a summary of how the previous build finished. We often view this as the state of the builder. In a rapidly cycling build system it is a close representative of what the state of the tree is for the type of build represented. For a slow-cycling system, or build requests that don't follow a smooth tree ordering this is less true and relevant. For ChromeOS consider the type of builder before thinking about what the state (or color) of the builder means.

The Announcement

When present, we tend to use variations on a theme for the announcement box on top of the waterfall (and some other) pages. We use the announcement to abbreviate and present other interesting state information. The top bar and it's color represents the state of the tree, including the current message. Under that there are 2 panes.

Different types of ChromeOS builders

Almost all of these come from the workings of chromite, but are very useful to have an idea about to understand the big picture:

When the columns aren't right

Currently the set of columns and the allocations of bots to back them is managed by the Chrome Infrastructure team, please file an Infrastructure bug OS-Chrome to start the process of correcting it. Please describe the type of build, and the cbuildbot configuration it should be running.