doc: on cluster build documentation (#744)

Signed-off-by: Zbynek Roubalik <zroubali@redhat.com>

docs/guides/developers_guide.md

@@ -14,6 +14,9 @@ For detailed documentation on the CLI and its commands, please see the [Commands
 
 For detailed description of `func.yaml` file that is used for the Function project configuration, please see the [Project Configuration with `func.yaml` document](func_yaml.md).
 
+## Building Functions on Cluster
+
+For detailed description on how to build a Function on Cluster with Tekton Pipelines, please see the [On Cluster Build Guide](on-cluster-build.yaml).
 
 ## Language Guides
 

docs/guides/on-cluster-build.md

@@ -0,0 +1,99 @@
+# Building Functions on Cluster with Tekton Pipelines
+
+This guide describes how you can build a Function on Cluster with Tekton Pipelines. The on cluster build is enabled by fetching Function source code from a remote Git repository.
+
+> Please note that the following approach requires administrator privileges on the cluster and the build is executed on a privileged container.
+
+## Prerequisites
+1. Install Tekton Pipelines on the cluster. Please refer to [Tekton Pipelines documentation](https://github.com/tektoncd/pipeline/blob/main/docs/install.md) or run the following command:
+```bash
+kubectl apply -f https://storage.googleapis.com/tekton-releases/pipeline/latest/release.yaml
+```
+2. Create a Knative Deployer Cluster Role
+```bash
+kubectl apply -f https://raw.githubusercontent.com/knative-sandbox/kn-plugin-func/main/pipelines/resources/knative-deployer-clusterrole.yaml
+```
+
+## Enabling a namespace to run Function related Tekton Pipelines
+In each namespace that you would like to run Pipelines and deploy a Function you need to create or install the following resources.
+1. Create a Service Account that has permissions to deploy on Knative:
+```bash
+export NAMESPACE=<INSERT_YOUR_NAMESPACE>
+kubectl create serviceaccount knative-deployer-account -n $NAMESPACE
+kubectl create clusterrolebinding $NAMESPACE:knative-deployer-binding \
+--clusterrole=kn-deployer --serviceaccount=$NAMESPACE:knative-deployer-account
+```
+2. Install the Git Clone Tekton Task to fetch the Function source code:
+```bash
+kubectl apply -f https://raw.githubusercontent.com/tektoncd/catalog/master/task/git-clone/0.4/git-clone.yaml
+```
+3. Install the Buildpacks Tekton Task to be able to build the Function image:
+```bash
+kubectl apply -f https://raw.githubusercontent.com/tektoncd/catalog/master/task/buildpacks/0.3/buildpacks.yaml
+```
+4. Install the `kn func` Deploy Tekton Task to be able to deploy the Function on in the Pipeline:
+```bash
+kubectl apply -f https://raw.githubusercontent.com/knative-sandbox/kn-plugin-func/main/pipelines/resources/tekton/task/func-deploy/0.1/func-deploy.yaml
+```
+
+## Building a Function on Cluster
+1. Create a Function and implement the business logic
+```bash
+kn func create my-function
+```
+2. Get a reference to the remote Git repository that will host your Function source code (eg. `https://github.com/my-repo/my-function.git`)
+3. Initialize a Git repository in your Function project and add a reference to the remote repo
+```bash
+cd my-function
+git init
+git branch -M main
+git remote add origin git@github.com:my-repo/my-function.git
+```
+4. Update the Function configuration in `func.yaml` to enable on cluster builds for the Git repository:
+```yaml
+build: git                                          # required, specify `git` build type
+git:
+  url: https://github.com/my-repo/my-function.git   # required, git repository with the function source code
+  revision: main                                    # optional, git revision to be used (branch, tag, commit)
+  # contextDir: myfunction                          # optional, needed only if the function is not located
+                                                    # in the repository root folder
+```
+5. Implement the bussines logic of your Function, then commit and push changes
+```bash
+git commit -a -m "implementing my-function"
+git push origin main
+```
+6. Deploy your Function
+```bash
+kn func deploy
+```
+If everything goes fine, you will prompted to provide credentials for the remote container registry that hosts the Function image. You should see output similar to the following:
+```bash
+$ kn func deploy
+🕕 Creating Pipeline resources
+Please provide credentials for image registry used by Pipeline.
+? Server: https://index.docker.io/v1/
+? Username: my-repo
+? Password: ********
+   Function deployed at URL: http://test-function.default.svc.cluster.local
+```
+
+7. To update your Function, commit and push new changes, then run `kn func deploy` again.
+
+## Uninstall and clean-up
+1. In each namespace where Pipelines and Functions were deployed, uninstall following resources:
+```bash
+export NAMESPACE=<INSERT_YOUR_NAMESPACE>
+kubectl delete serviceaccount knative-deployer-account -n $NAMESPACE
+kubectl delete clusterrolebinding $NAMESPACE:knative-deployer-binding
+kubectl delete task.tekton.dev git-clone
+kubectl delete task.tekton.dev buildpacks
+```
+2. Delete Knative Deployer Cluster Role
+```bash
+kubectl delete clusterrole kn-deployer
+```
+3. Uninstall Tekton Pipelines
+```bash
+kubectl delete -f https://storage.googleapis.com/tekton-releases/pipeline/latest/release.yaml
+```