docs: add info to implementing/tasks about sequential execution (#1950)

Signed-off-by: Meg McRoberts <meg.mcroberts@dynatrace.com>
Co-authored-by: Giovanni Liva <giovanni.liva@dynatrace.com>

docs/content/en/docs/implementing/tasks/_index.md

@@ -7,35 +7,45 @@ hidechildren: false # this flag hides all sub-pages in the sidebar-multicard.htm
 
 A
 [KeptnTaskDefinition](../../yaml-crd-ref/taskdefinition.md/)
-resource defines tasks that the Keptn Lifecycle Toolkit runs
+resource defines one or more "executables"
+(functions, programs, scripts, etc)
+that the Keptn Lifecycle Toolkit runs
 as part of the pre- and post-deployment phases of a
 [KeptnApp](../../yaml-crd-ref/app.md) or
 [KeptnWorkload](../../crd-ref/lifecycle/v1alpha3/#keptnworkload).
 
-A Keptn task executes as a runner in an application
+A
+[KeptnTask](../../crd-ref/lifecycle/v1alpha3/#keptntask)
+executes as a runner in an application
 [container](https://kubernetes.io/docs/concepts/containers/),
 which runs as part of a Kubernetes
 [job](https://kubernetes.io/docs/concepts/workloads/controllers/job/).
-A `KeptnTaskDefinition` includes a function
-that defines the action taken by that task.
+A `KeptnTaskDefinition` includes calls to executables to be run.
 
-To implement a Keptn task:
+To implement a `KeptnTask`:
 
 - Define a
   [KeptnTaskDefinition](../../yaml-crd-ref/taskdefinition.md)
   resource that defines the runner to use for the container
+  and the executables to be run
+pre- and post-deployment
 - Apply [basic-annotations](../integrate/#basic-annotations)
   to your workloads to integrate your task with Kubernetes
-- Add your task to the [KeptnApp](../../yaml-crd-ref/app.md)
-  resource that associates your `KeptnTaskDefinition`
-  with the pre- and post-deployment tasks that should run in it;
+- Annotate the appropriate
+  [KeptnApp](../../yaml-crd-ref/app.md)
+  resource to associate your `KeptnTaskDefinition`
+  with the pre- and post-deployment tasks that should run it;
   see
   [Pre- and post-deployment tasks and checks](../integrate/#pre--and-post-deployment-checks)
   for more information.
 
 This page provides information to help you create your tasks:
 
 - Code your task in an appropriate [runner](#runners-and-containers)
+- How to control the
+  [execution order](#executing-sequential-tasks)
+  of functions, programs, and scripts
+  since all `KeptnTask` resources at the same level run in parallel
 - Understand how to use [Context](#context)
   that contains a Kubernetes cluster, a user, a namespace,
   the application name, workload name, and version.
@@ -53,6 +63,9 @@ to define the task.
 The `spec` section of the `KeptnTaskDefinition`
 defines the runner to use for the container:
 
+KLT provides a general Kubernetes that you can configure
+to do almost anything you want:
+
 - The `container-runtime` runner provides
   a pure custom Kubernetes application container
   that you define to includes a runtime, an application
@@ -84,6 +97,105 @@ See the
 [KeptnTaskDefinition](../../yaml-crd-ref/taskdefinition.md)
 reference page for the synopsis and examples for each runner.
 
+## Executing sequential tasks
+
+All `KeptnTask` resources that are defined by
+`KeptnTaskDefinition` resources at the same level
+(either pre-deployment or post-deployment) execute in parallel.
+This is by design, because Keptn is not a pipeline engine.
+**Task sequences that are not part of the lifecycle workflow
+should not be handled by KLT**
+but should instead be handled by the pipeline engine tools being used
+such as Jenkins, Argo Workflows, Flux, and Tekton.
+
+If your lifecycle workflow includes
+a sequence of executables that need to be run in order,
+you can put them all in one `KeptnTaskDefinition` resource,
+which can execute a virtually unlimited number
+of programs, scripts, and functions,
+as long as they all need the same runner, such as Python.
+
+Another option is to encode all your steps in the language of your choice
+and build a container that Keptn executes.
+This is often the best solution if you need to execute complex sequences.
+
+If you use either the `deno-runtime` or `python-runtime` runner,
+you can specify the actions to take by coding the actual calls
+inline in the manifest,
+by calling scripts from a remote webserver,
+or by calling other `KeptnTaskDefinition` resources you have defined.
+This provides great flexibility in
+how you define your `KeptnTaskDefinition` resources,
+allowing you to define the ideal mix of executables that run sequentially
+and executables (or sets of executables) that run in parallel.
+
+As an example, let's say you need to run a set of integration tests,
+a set of performance tests, and a set of regression tests.
+
+- You can create one `KeptnTaskDefinition` that calls all the tasks,
+  in order, either by putting the actual calls
+  in the `KeptnTaskDefinition` resource (`inline` syntax)
+  or by calling scripts from a remote webserver (`httpRef` syntax), or by providing a container to run.
+
+- You can create separate `KeptnTaskDefinition` resources
+  for integration tests, performance tests, and regression tests.
+
+  - If you annotate the `KeptnApp` resource to call
+    each of these `KeptnTask` resources,
+    the three sets of tests run in parallel.
+
+  - You can create a "parent" `KeptnTaskDefinition` resource
+    that uses the `functionref` syntax
+    to call the `KeptnTaskDefinition` resources
+    for integration tests, performance tests, and regression tests.
+    If you annotate the `KeptnApp` resource
+    to run this parent `KeptnTask` resource,
+    all tests run sequentially.
+
+    This approach also allows you to run the test sequence if, for example,
+    the integration tests require the `deno-runtime` runner
+    but the performance and regression tests
+    require the `python-runtime` runner.
+    The parent `KeptnTaskDefinition` can run `KeptnTask` resources
+    that use different runners,
+    although the "parent" definition runtime is used
+    for the container that runs all the tests.
+    In other words, the parent `KeptnTaskDefinition` resources
+    is not a merge of other `KeptnTaskDefinition` resources
+    but rather the code/container of the parent runner.
+
+- If you need to test your deployment for different platforms
+  (such as Linux, MacOS, and Windows)
+  or for different software versions
+  (such as Java 11, Java 17, and Java 21),
+  parallel testing can improve performance.
+  In this case, you can construct different `KeptnTaskDefinition` resources
+  for each platform,
+  perhaps defining different input parameters
+  (such as different secrets or environment variables).
+
+- Define one `KeptnTaskDefinition` resource that runs
+  integration tests, then regression test, then performance tests
+  in order.
+
+  - You could use the `functionRef` syntax
+    and code the calling sequences for all tests
+    inside your `KeptnTaskDefinition` resource
+    or you could use the `httpRef` syntax
+    to call scripts from an external webserver.
+  - You could code separate `KeptnTaskDefinition` resources
+    for integration tests, regression tests, and performance tests.
+    These three test sets could then run in parallel.
+  - You could create a `KeptnTaskDefinition` resource
+    that uses the `functionRef` syntax to call the
+    `KeptnTaskDefinition` resources for each type of testing.
+    Executing that resource would execute the three types of tests
+    in sequential order.
+
+If you define a `container-runtime` runner container
+for your `KeptnTaskDefinition`,
+you can do anything allowed by the configuration you define for that container.
+
 ## Context
 
 A Kubernetes context is a set of access parameters