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Developer docs

This document is aimed at helping maintainers/developers of project understand the complexity.

How are resources shared between tasks

PipelineRun uses PVC to share PipelineResources between tasks. PVC volume is mounted on path /pvc by PipelineRun.

  • If a resource in a task is declared as output then the TaskRun controller adds a step to copy each output resource to the directory path /pvc/task_name/resource_name.

  • If an input resource includes from condition then the TaskRun controller adds a step to copy from PVC directory path: /pvc/previous_task/resource_name.

If neither of these conditions are met, the PVC will not be created nor will the GCS storage / S3 buckets be used.

Another alternative is to use a GCS storage or S3 bucket to share the artifacts. This can be configured using a ConfigMap with the name config-artifact-bucket.

See the installation docs for configuration details.

Both options provide the same functionality to the pipeline. The choice is based on the infrastructure used, for example in some Kubernetes platforms, the creation of a persistent volume could be slower than uploading/downloading files to a bucket, or if the the cluster is running in multiple zones, the access to the persistent volume can fail.

How are inputs handled

Input resources, like source code (git) or artifacts, are dumped at path /workspace/task_resource_name. Resource definition in task can have custom target directory. If targetPath is mentioned in task input then the controllers are responsible for adding container definitions to create directories and also to fetch the versioned artifacts into that directory.

How are outputs handled

Output resources, like source code (git) or artifacts (storage resource), are expected in directory path /workspace/output/resource_name.

  • If resource has an output "action" like upload to blob storage, then the container step is added for this action.

  • If there is PVC volume present (TaskRun holds owner reference to PipelineRun) then copy step is added as well.

  • If the resource is declared only in output but not in input for task then the copy step includes resource being copied to PVC to path /pvc/task_name/resource_name from /workspace/output/resource_name like the following example.

    kind: Task
metadata:
  name: get-gcs-task
  namespace: default
spec:
  outputs:
    resources:
      - name: gcs-workspace
        type: storage

  • If the resource is declared only in output but not in input for task and the resource defined with TargetPath then the copy step includes resource being copied to PVC to path /pvc/task_name/resource_name from /workspace/outputstuff like the following example.

    kind: Task
metadata:
  name: get-gcs-task
  namespace: default
spec:
  outputs:
    resources:
      - name: gcs-workspace
        type: storage
        targetPath: /workspace/outputstuff

  • If the resource is declared both in input and output for task the then copy step includes resource being copied to PVC to path /pvc/task_name/resource_name from /workspace/random-space/ if input resource has custom target directory (random-space) declared like the following example.

    kind: Task
metadata:
  name: get-gcs-task
  namespace: default
spec:
  inputs:
    resources:
      - name: gcs-workspace
        type: storage
        targetPath: random-space
  outputs:
    resources:
      - name: gcs-workspace
        type: storage
    • If resource is declared both in input and output for task without custom target directory then copy step includes resource being copied to PVC to path /pvc/task_name/resource_name from /workspace/resource_name/ like the following example.
    kind: Task
metadata:
  name: get-gcs-task
  namespace: default
spec:
  inputs:
    resources:
      - name: gcs-workspace
        type: storage
  outputs:
    resources:
      - name: gcs-workspace
        type: storage

Entrypoint rewriting and step ordering

Entrypoint is injected into the Task Container(s), wraps the Task step to manage the execution order of the containers. The entrypoint binary has the following arguments:

  • wait_file - If specified, file to wait for
  • wait_file_content - If specified, wait until the file has non-zero size
  • post_file - If specified, file to write upon completion
  • entrypoint - The command to run in the image being wrapped

As part of the PodSpec created by TaskRun the entrypoint for each Task step is changed to the entrypoint binary with the mentioned arguments and a volume with the binary and file(s) is mounted.

If the image is a private registry, the service account should include an ImagePullSecret

Builder namespace on containers

The /tekton/ directory is reserved on containers for internal usage. Examples of how this directory is used:

  • Task Results are written to /tekton/results
  • Various tools like the entrypoint are placed in /tekton/tools
  • The termination log message is written to /tekton/termination
  • Sequencing step containers is done using both /tekton/downward/ready and numbered files in /tekton/tools

Handling of injected sidecars

Tekton has to take some special steps to support sidecars that are injected into TaskRun Pods. Without intervention sidecars will typically run for the entire lifetime of a Pod but in Tekton's case it's desirable for the sidecars to run only as long as Steps take to complete. There's also a need for Tekton to schedule the sidecars to start before a Task's Steps begin, just in case the Steps rely on a sidecars behavior, for example to join an Istio service mesh. To handle all of this, Tekton Pipelines implements the following lifecycle for sidecar containers:

First, the Downward API is used to project an annotation on the TaskRun's Pod into the entrypoint container as a file. The annotation starts as an empty string, so the file projected by the downward API has zero length. The entrypointer spins, waiting for that file to have non-zero size.

The sidecar containers start up. Once they're all in a ready state, the annotation is populated with string "READY", which in turn populates the Downward API projected file. The entrypoint binary recognizes that the projected file has a non-zero size and allows the Task's steps to begin.

On completion of all steps in a Task the TaskRun reconciler stops any sidecar containers. The Image field of any sidecar containers is swapped to the nop image. Kubernetes observes the change and relaunches the container with updated container image. The nop container image exits immediately because it does not provide the command that the sidecar is configured to run. The container is considered Terminated by Kubernetes and the TaskRun's Pod stops.

There are known issues with the existing implementation of sidecars:

  • When the nop image does provide the sidecar's command, the sidecar will continue to run even after nop has been swapped into the sidecar container's image field. See the issue tracking this bug for the issue tracking this bug. Until this issue is resolved the best way to avoid it is to avoid overriding the nop image when deploying the tekton controller, or ensuring that the overridden nop image contains as few commands as possible.

  • kubectl get pods will show a Completed pod when a sidecar exits successfully but an Error when the sidecar exits with an error. This is only apparent when using kubectl to get the pods of a TaskRun, not when describing the Pod using kubectl describe pod ... nor when looking at the TaskRun, but can be quite confusing.

How task results are defined and outputted by a task

Tasks can define results by adding a result on the task spec. This is an example:

apiVersion: tekton.dev/v1alpha1
kind: Task
metadata:
  name: print-date
  annotations:
    description: |
      A simple task that prints the date to make sure your cluster / Tekton is working properly.
spec:
  results:
    - name: "current-date"
      description: "The current date"
  steps:
    - name: print-date
      image: bash:latest
      args:
        - "-c"
        - |
          date > /tekton/results/current-date

The result is added to a file name with the specified result's name into the /tekton/results folder. This is then added to the task run status. Internally the results are a new argument -resultsto the entrypoint defined for the task. A user can defined more than one result for a single task.

For this task definition,

apiVersion: tekton.dev/v1alpha1
kind: Task
metadata:
  name: print-date
  annotations:
    description: |
      A simple task that prints the date to make sure your cluster / Tekton is working properly.
spec:
  results:
    - name: current-date-unix-timestamp
      description: The current date in unix timestamp format
    - name: current-date-human-readable
      description: The current date in humand readable format
  steps:
    - name: print-date-unix-timestamp
      image: bash:latest
      script: |
        #!/usr/bin/env bash
        date +%s | tee /tekton/results/current-date-unix-timestamp
    - name: print-date-humman-readable
      image: bash:latest
      script: |
        #!/usr/bin/env bash
        date | tee /tekton/results/current-date-human-readable

you end up with this task run status:

apiVersion: tekton.dev/v1alpha1
kind: TaskRun
...
status:
...
  taskResults:
  - name: current-date-human-readable
    value: |
      Wed Jan 22 19:47:26 UTC 2020
  - name: current-date-unix-timestamp
    value: |
      1579722445

Instead of hardcoding the path to the result file, the user can also use a variable. So /tekton/results/current-date-unix-timestamp can be replaced with: $(results.current-date-unix-timestamp.path). This is more flexible if the path to result files ever changes.

Known issues

How task results can be used in pipeline's tasks

Now that we have tasks that can return a result, the user can refer to a task result in a pipeline by using the syntax $(tasks.<task name>.results.<result name>). This will substitute the task result at the location of the variable.

apiVersion: tekton.dev/v1alpha1
kind: Pipeline
metadata:
  name: sum-and-multiply-pipeline
    #...
  tasks:
    - name: sum-inputs
    #...
    - name: multiply-inputs
    #...
- name: sum-and-multiply
      taskRef:
        name: sum
      params:
        - name: a
          value: "$(tasks.multiply-inputs.results.product)$(tasks.sum-inputs.results.sum)"
        - name: b
          value: "$(tasks.multiply-inputs.results.product)$(tasks.sum-inputs.results.sum)"

This results in:

tkn pipeline start sum-and-multiply-pipeline
? Value for param `a` of type `string`? (Default is `1`) 10
? Value for param `b` of type `string`? (Default is `1`) 15
Pipelinerun started: sum-and-multiply-pipeline-run-rgd9j

In order to track the pipelinerun progress run:
tkn pipelinerun logs sum-and-multiply-pipeline-run-rgd9j -f -n default
tkn pipelinerun logs sum-and-multiply-pipeline-run-rgd9j -f -n default
[multiply-inputs : product] 150

[sum-inputs : sum] 25

[sum-and-multiply : sum] 30050

As you can see, you can define multiple tasks in the same pipeline and use the result of more than one task inside another task parameter. The substitution is only done inside pipeline.spec.tasks[].params[]. For a complete example demonstrating Task Results in a Pipeline, see the pipelinerun example.