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Overview

A brief guide about Prow and its ecosystem

What is Prow?

Prow is a Kubernetes-based CI/CD system.

How can I use it?

If you’ve contributed to Kubernetes, chances are you’ve already used it. If you’ve ever used a TestGrid dashboard, you’ve definitely used it.

1 - Architecture

Prow is made up of a collection of microservices (aka “Prow components”) that work together in a service cluster, leveraging one or more build clusters to schedule Prow Jobs (or just “jobs”).

Prow in a Nutshell

Prow creates jobs based on various types of events, such as:

  • GitHub events (e.g., a new PR is created, or is merged, or a person comments “/retest” on a PR),

  • Pub/Sub messages,

  • time (these are created by Horologium and are called periodic jobs), and

  • retesting (triggered by Tide).

Jobs are created inside the Service Cluster as Kubernetes Custom Resources. The Prow Controller Manager takes triggered jobs and schedules them into a build cluster, where they run as Kubernetes pods. Crier then reports the results back to GitHub.

flowchart TD

    classDef yellow fill:#ff0
    classDef cyan fill:#0ff
    classDef pink fill:#f99

    subgraph Service Cluster["<span style='font-size: 40px;'><b>Service Cluster</b></span>"]
        Deck:::cyan
        Prowjobs:::yellow
        Crier:::cyan
        Tide:::cyan
        Horologium:::cyan
        Sinker:::cyan
        PCM[Prow Controller Manager]:::cyan
        Hook:::cyan
            subgraph Hook
                WebhookHandler["Webhook Handler"]
                PluginCat(["'cat' plugin"])
                PluginTrigger(["'trigger' plugin"])
            end
    end

    subgraph Build Cluster[<b>Build Cluster</b>]
        Pods[(Pods)]:::yellow
    end

    style Legend fill:#fff,stroke:#000,stroke-width:4px
    subgraph Legend["<span style='font-size: 20px;'><b>LEGEND</b></span>"]
        direction LR
        k8sResource[Kubernetes Resource]:::yellow
        prowComponent[Prow Component]:::cyan
        hookPlugin([Hook Plugin])
        Other
    end

    Prowjobs <-.-> Deck <-----> |Serve| prow.k8s.io
    GitHub ==> |Webhooks| WebhookHandler
    WebhookHandler --> |/meow| PluginCat
    WebhookHandler --> |/retest| PluginTrigger
    Prowjobs <-.-> Tide --> |Retest and Merge| GitHub
    Horologium ---> |Create| Prowjobs
    PluginCat --> |Comment| GitHub
    PluginTrigger --> |Create| Prowjobs
    Sinker ---> |Garbage collect| Prowjobs
    Sinker --> |Garbage collect| Pods
    PCM -.-> |List and update| Prowjobs
    PCM ---> |Report| Prowjobs
    PCM ==> |Create and Query| Pods
    Prowjobs <-.-> |Inform| Crier --> |Report| GitHub

Notes

Note that Prow can also work with Gerrit, albeit with less features. Specifically, neither Tide nor Hook work with Gerrit yet.