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.

If input resource is declared as below, then resource will be copied to /workspace/task_resource_name directory from depended task PVC directory /pvc/previous_task/resource_name.

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

Resource definition in task can have custom target directory. If targetPath is mentioned in task input resource as below then resource will be copied to /workspace/outputstuff directory from depended task PVC directory /pvc/previous_task/resource_name.
```
kind: Task
metadata:
  name: get-gcs-task
  namespace: default
spec:
  resources:
    inputs:
      - name: gcs-workspace
        type: storage
        targetPath: /workspace/outputstuff
```

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 output resource is declared 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:
  resources:
    outputs:
      - name: gcs-workspace
        type: storage
```

Same as input, if the output resource is declared 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:
  resources:
    outputs:
      - name: gcs-workspace
        type: storage
        targetPath: /workspace/outputstuff

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

Reserved directories

/workspace

/workspace - This directory is where resources and workspaces are mounted.

/tekton

The /tekton/ directory is reserved on containers for internal usage.

Here is an example of a directory layout for a simple Task with 2 script steps:

/tekton
|-- creds
|-- downward
|   |-- ..2021_09_16_18_31_06.270542700
|   |   `-- ready
|   |-- ..data -> ..2021_09_16_18_31_06.270542700
|   `-- ready -> ..data/ready
|-- home
|-- results
|-- scripts
|   |-- script-0-t4jd8
|   `-- script-1-4pjwp
|-- steps
|   |-- 0 -> /tekton/steps/step-unnamed-0
|   |-- 1 -> /tekton/steps/step-foo
|   |-- step-foo
|   `-- step-unnamed-0
|       `-- exitCode
|-- termination
`-- tools
    |-- 0
    `-- entrypoint

Path	Description
/tekton	Directory used for Tekton specific functionality
/tekton/creds	Location of Tekton mounted secrets. See Authentication at Run Time for more details.
/tekton/downward	Location of data mounted via the Downward API.
/tekton/home	(deprecated - see #2013) Default home directory for user containers.
/tekton/results	Where results are written to (path available to `Task` authors via `$(results.name.path)`)
/tekton/scripts	Contains user provided scripts specified in the TaskSpec.
/tekton/steps	Where the `step` exitCodes are written to (path available to `Task` authors via `$(steps.<stepName>.exitCode.path)`)
/tekton/termination	where the eventual termination log message is written to Sequencing step containers
/tekton/tools	Contains tools like the entrypoint binary, post_files for coordinating step starts

The following directories are covered by the Tekton API Compatibility policy, and can be relied on for stability:

/tekton/results
/tekton/steps

All other files/directories are internal implementation details of Tekton - users should not rely on specific paths or behaviors as it may change in the future.

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-human-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

Task Results are returned to the TaskRun controller via the container's termination message. At time of writing this has a capped maximum size of "4096 bytes or 80 lines, whichever is smaller". This maximum size should not be considered the limit of a result's size. Tekton uses the termination message to return other data to the controller as well. The general advice should be that results are for very small pieces of data. The exact size is going to be a product of the platform's settings and the amount of other data Tekton needs to return for TaskRun book-keeping.

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.

Support for running in multi-tenant configuration

In order to support potential multi-tenant configurations the roles of the controller are split into two:

`tekton-pipelines-controller-cluster-access`: those permissions needed cluster-wide by the controller.
`tekton-pipelines-controller-tenant-access`: those permissions needed on a namespace-by-namespace basis.

By default the roles are cluster-scoped for backwards-compatibility and ease-of-use. If you want to start running a multi-tenant service you are able to bind tekton-pipelines-controller-tenant-access using a RoleBinding instead of a ClusterRoleBinding, thereby limiting the access that the controller has to specific tenant namespaces.

Adding feature gated API fields

We've introduced a feature-flag called enable-api-fields to the config-feature-flags.yaml file deployed as part of our releases.

This field can be configured either to be alpha or stable. This field is documented as part of our install docs.

For developers adding new features to Pipelines' CRDs we've got a couple of helpful tools to make gating those features simpler and to provide a consistent testing experience.

Guarding Features with Feature Gates

Writing new features is made trickier when you need to support both the existing stable behaviour as well as your new alpha behaviour.

In reconciler code you can guard your new features with an if statement such as the following:

alphaAPIEnabled := config.FromContextOrDefaults(ctx).FeatureFlags.EnableAPIFields == "alpha"
if alphaAPIEnabled {
  // new feature code goes here
} else {
  // existing stable code goes here
}

Notice that you'll need a context object to be passed into your function for this to work. When writing new features keep in mind that you might need to include this in your new function signatures.

Guarding Validations with Feature Gates

Just because your application code might be correctly observing the feature gate flag doesn't mean you're done yet! When a user submits a Tekton resource it's validated by Pipelines' webhook. Here too you'll need to ensure your new features aren't accidentally accepted when the feature gate suggests they shouldn't be. We've got a helper function, ValidateEnabledAPIFields, to make validating the current feature gate easier. Use it like this:

requiredVersion := config.AlphaAPIFields
// errs is an instance of *apis.FieldError, a common type in our validation code
errs = errs.Also(ValidateEnabledAPIFields(ctx, "your feature name", requiredVersion))

If the user's cluster isn't configured with the required feature gate it'll return an error like this:

<your feature> requires "enable-api-fields" feature gate to be "alpha" but it is "stable"

Unit Testing with Feature Gates

Any new code you write that uses the ctx context variable is trivially unit tested with different feature gate settings. You should make sure to unit test your code both with and without a feature gate enabled to make sure it's properly guarded. See the following for an example of a unit test that sets the feature gate to test behaviour:

featureFlags, err := config.NewFeatureFlagsFromMap(map[string]string{
        "enable-api-fields": "alpha",
})
if err != nil {
	t.Fatalf("unexpected error initializing feature flags: %v", err)
}
cfg := &config.Config{
        FeatureFlags: featureFlags,
}
ctx := config.ToContext(context.Background(), cfg)
if err := ts.TestThing(ctx); err != nil {
	t.Errorf("unexpected error with alpha feature gate enabled: %v", err)
}

Example YAMLs

Writing new YAML examples that require a feature gate to be set is easy. New YAML example files typically go in a directory called something like examples/v1beta1/taskruns in the root of the repo. To create a YAML that should only be exercised when the enable-api-fields flag is alpha just put it in an alpha subdirectory so the structure looks like:

examples/v1beta1/taskruns/alpha/your-example.yaml

This should work for both taskruns and pipelineruns.

Note: To execute alpha examples with the integration test runner you must manually set the enable-api-fields feature flag to alpha in your testing cluster before kicking off the tests.

When you set this flag to stable in your cluster it will prevent alpha examples from being created by the test runner. When you set the flag to alpha all examples are run, since we want to exercise backwards-compatibility of the examples under alpha conditions.

Integration Tests

For integration tests we provide the requireAnyGate function which should be passed to the setup function used by tests:

c, namespace := setup(ctx, t, requireAnyGate(map[string]string{"enable-api-fields": "alpha"}))

This will Skip your integration test if the feature gate is not set to alpha with a clear message explaining why it was skipped.

Note: As with running example YAMLs you have to manually set the enable-api-fields flag to alpha in your test cluster to see your alpha integration tests run. When the flag in your cluster is alpha all integration tests are executed, both stable and alpha. Setting the feature flag to stable will exclude alpha tests.

What and Why of `/tekton/steps`

/tekton/steps/ is an implicit volume mounted on a pod and created for storing the step specific information/metadata. There is one more subdirectory created under /tekton/steps/ for each step in a task.

Let's take an example of a task with three steps, each exiting with non-zero exit code:

kind: TaskRun
apiVersion: tekton.dev/v1beta1
metadata:
  generateName: test-taskrun-
spec:
  taskSpec:
    steps:
      - image: alpine
        name: step0
        onError: continue
        script: |
          echo "This is step 0"
          ls -1R /tekton/steps/
          exit 1
      - image: alpine
        onError: continue
        script: |
          echo "This is step 1"
          ls -1R /tekton/steps/
          exit 2
      - image: alpine
        name: step2
        onError: continue
        script: |
          echo "This is step 2"
          ls -1R /tekton/steps/
          exit 3

The container step-step0 for the first step step0 shows three subdirectories (one for each step) under /tekton/steps/ and all three of them are empty.

kubectl logs pod/test-taskrun-2rb9k-pod-bphct -c step-step0
+ echo 'This is step 0'
+ ls -1R /tekton/steps/
This is step 0
/tekton/steps/:
0
1
2
step-step0
step-step2
step-unnamed-1

/tekton/steps/step-step0:
/tekton/steps/step-step2:
/tekton/steps/step-unnamed-1:
+ exit 1

The container step-unnamed-1 for the second step which has no name shows three subdirectories (one for each step) under /tekton/steps/ along with the exitCode file under the first step directory which has finished executing:

kubectl logs pod/test-taskrun-2rb9k-pod-bphct -c step-unnamed-1
This is step 1
+ echo 'This is step 1'
+ ls -1R /tekton/steps/
/tekton/steps/:
0
1
2
step-step0
step-step2
step-unnamed-1

/tekton/steps/step-step0:
exitCode

/tekton/steps/step-step2:

/tekton/steps/step-unnamed-1:
+ exit 2

The container step-step2 for the third step step2 shows three subdirectories (one for each step) under /tekton/steps/ along with the exitCode file under the first and second step directory since both are done executing:

kubectl logs pod/test-taskrun-2rb9k-pod-bphct -c step-step2
This is step 2
+ echo 'This is step 2'
+ ls -1R /tekton/steps/
/tekton/steps/:
0
1
2
step-step0
step-step2
step-unnamed-1

/tekton/steps/step-step0:
exitCode

/tekton/steps/step-step2:

/tekton/steps/step-unnamed-1:
exitCode
+ exit 3

The entrypoint is modified to include an additional two flags representing the step specific directory and a symbolic link:

step_metadata_dir - the dir specified in this flag is created to hold a step specific metadata
step_metadata_dir_link - the dir specified in this flag is created as a symbolic link to step_metadata_dir

step_metadata_dir is set to /tekton/steps/step-step0 and step_metadata_dir_link is set to /tekton/steps/0 for the entrypoint of the first step in the above example task.

Notice an additional entries 0, 1, and 2 showing under /tekton/steps/. These are symbolic links created which are linked with their respective step directories, step-step0, step-unnamed-1, and step-step2. These symbolic links are created to provide simplified access to the step metadata directories i.e., instead of referring to a directory with the step name, access it via the step index. The step index becomes complex and hard to keep track of in a task with a long list of steps, for example, a task with 20 steps. Creating the step metadata directory using a step name and creating a symbolic link using the step index gives the user flexibility, and an option to choose whatever works best for them.

How to access the exit code of a step from any subsequent step in a task

The entrypoint now allows exiting with an error and continue running rest of the steps in a task i.e., it is possible for a step to exit with a non-zero exit code. Now, it is possible to design a task with a step which can take an action depending on the exit code of any prior steps. The user can access the exit code of a step by reading the file pointed by the path variable $(steps.step-<step-name>.exitCode.path) or $(steps.step-unnamed-<step-index>.exitCode.path). For example:

$(steps.step-my-awesome-step.exitCode.path) where the step name is my-awesome-step.
$(steps.step-unnamed-0.exitCode.path) where the first step in a task has no name.

The exit code of a step is stored in a file named exitCode under a directory /tekton/steps/step-<step-name>/ or /tekton/steps/step-unnamed-<step-index>/ which is reserved for any other step specific information in the future.

If you would like to use the tekton internal path, you can access the exit code by reading the file (which is not recommended though since the path might change in the future):

cat /tekton/steps/step-<step-name>/exitCode

And, access the step exit code without a step name:

cat /tekton/steps/step-unnamed-<step-index>/exitCode

Or, you can access the step metadata directory via symlink, for example, use cat /tekton/steps/0/exitCode for the first step in a task.

TaskRun Use of Pod Termination Messages

Tekton Pipelines uses a Pod's termination message to pass data from a Step's container to the Pipelines controller. Examples of this data include: the time that execution of the user's step began, contents of task results, contents of pipeline resource results.

The contents and format of the termination message can change. At time of writing the message takes the form of a serialized JSON blob. Some of the data from the message is internal to Tekton Pipelines, used for book-keeping, and some is distributed across a number of fields of the TaskRun's status. For example, a TaskRun's status.taskResults is populated from the termination message.

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README.md

README.md

Developer docs

How are resources shared between tasks

How are inputs handled

How are outputs handled

Entrypoint rewriting and step ordering

Reserved directories

/workspace

/tekton

Handling of injected sidecars

How task results are defined and outputted by a task

Known issues

How task results can be used in pipeline's tasks

Support for running in multi-tenant configuration

Adding feature gated API fields

Guarding Features with Feature Gates

Guarding Validations with Feature Gates

Unit Testing with Feature Gates

Example YAMLs

Integration Tests

What and Why of `/tekton/steps`

How to access the exit code of a step from any subsequent step in a task

TaskRun Use of Pod Termination Messages

Files

README.md

Latest commit

History

README.md

File metadata and controls

Developer docs

How are resources shared between tasks

How are inputs handled

How are outputs handled

Entrypoint rewriting and step ordering

Reserved directories

/workspace

/tekton

Handling of injected sidecars

How task results are defined and outputted by a task

Known issues

How task results can be used in pipeline's tasks

Support for running in multi-tenant configuration

Adding feature gated API fields

Guarding Features with Feature Gates

Guarding Validations with Feature Gates

Unit Testing with Feature Gates

Example YAMLs

Integration Tests

What and Why of /tekton/steps

How to access the exit code of a step from any subsequent step in a task

TaskRun Use of Pod Termination Messages

What and Why of `/tekton/steps`