Adding documentation for build resource vs pack in a new file, and also

adding links from README and build resource.

Signed-off-by: Juan Bustamante <[email protected]>

README.md

@@ -20,6 +20,7 @@ kpack also provides a build type to execute a single Cloud Native Buildpack OCI
 
 - [Install kpack](docs/install.md)
 - Get started with [the tutorial](docs/tutorial.md)
+- Are you familiar with pack? check our [comparison section](docs/kpack-vs-pack.md)
 - Check out the documentation on kpack concepts:
     - [Stacks](docs/stack.md)
     - [Images](docs/image.md)

docs/build.md

@@ -6,6 +6,8 @@ Corresponding `kp` cli command docs [here](https://github.com/vmware-tanzu/kpack
 
 Unlike with the [Image resource](image.md), using Builds directly allows granular control of when builds execute. Each build resource is immutable and corresponds to a single build execution. You will need to create a new build for every build execution as builds will not rebuild on source code and buildpack updates. Additionally, you will need to manually specify the source, and the cache volume. 
 
+A Build resource is comparable to `pack build`. Are you familiar with pack? if so, you can check the [comparison section](kpack-vs-pack.md) 
+
 ### Configuration
 
 ```yaml

docs/kpack-vs-pack.md

@@ -0,0 +1,158 @@
+
+# kpack vs pack
+
+So, you are a [pack][_pack] user trying to learn about [kpack][_kpack] and get your [Cloud Native Buildpacks][_cnb] journey to the next level? then you are in the right place, on the next sections we are going to explain the similarities between [pack][_pack] and [kpack][_kpack].
+
+First of all, both [kpack][_kpack] and [pack][_pack] implement the [platform interface](https://github.com/buildpacks/spec/blob/main/platform.md) [specification](https://github.com/buildpacks/spec/blob/main/platform.md), but they do it for two non-overlapping contexts: while [pack][_pack] targets developers and local builds, [kpack][_kpack] manages containerization on day-2 and at scale and is a [Kubernetes](https://kubernetes.io/) native implementation.
+
+We will define some basic use case scenarios and see how we can get the output from both tools.
+
+## Assumptions
+
+In order to make our comparison very simple, lets make some assumptions:
+1. Our application source code is one of the [samples](https://github.com/buildpacks/samples/tree/main/apps) application
+2. We are going to use [Cloud Native Buildpacks](_cnb) [sample builder](https://hub.docker.com/r/cnbs/sample-builder) to build our application source code
+3. We need **write** access to a remote registry to publish our application image
+
+## Build scenario
+
+Let's define our most basic use case as follows:
+
+`As a [pack|kpack] user, I want to convert my application source code into an image and publish it into a remote registry`
+
+### Pack Implementation
+
+In order to build our application source code using [pack][_pack] we need to run a command similar to this:
+
+`pack build --publish --path apps/<APP> --builder cnbs/sample-builder:<bionic OR alpine> <app-image-name>`
+
+After building your '<app-image-name>' must be written into your remote registry.
+
+### Kpack Implementation
+
+How do we get a similar functionality to a `pack build` command using [kpack][_kpack]? the answer is the Build resource!
+
+Once you have [kpack][_kpack] up and running on a kubernetes cluster, you need to create a Build resource and apply it to your cluster. for our scenario it looks like this:
+
+```yaml
+apiVersion: kpack.io/v1alpha2
+kind: Build
+metadata:
+  name: sample-build # This can be any name
+spec:
+  tags:
+    - <app-image-name>
+  builder:
+    image: cnbs/sample-builder:<bionic OR alpine>
+  source:
+    git:
+      url: https://github.com/buildpacks/samples.git
+      revision: main
+    subPath: "apps/<APP>"
+```
+
+Once you create yaml file, the next step is just to apply the resource into your kubernetes cluster, for example using
+
+```bash
+kubectl apply -f <your-build-resource.yaml>
+```
+
+After building, your '<app-image-name>' must be also written into your remote registry.
+
+**Note** Probably you will need to create some [secrets](secrets.md) to give [kpack][_kpack] access to your remote registry, but this is also required on [pack][_pack], so please check the documentation depending on your registry provider
+
+## Re-build scenario
+
+`As a [pack|kpack] user, I want to rebuild my application source code after some change and publish a new image into a remote registry`
+
+### Pack Implementation
+
+In [pack][_pack], in order to re-build your application image, you just need to run the `pack build` command after saving your application source code changes
+
+### Kpack Implementation
+
+As we mentioned above, All fields on a build are immutable, this mean that every time we want to run a build we must create a new `Build` resource. One way to do this is using `generateName` field in our resource definition.
+
+From our previous resource definition, let's remove the `metadata.name` field, and replace it with a `metadata.generateName` this value will be used by the server, to generate a unique name ONLY IF the Name field has not been provided.
+
+```yaml
+apiVersion: kpack.io/v1alpha2
+kind: Build
+metadata:
+  generateName: sample-build- # this value will be a prefix 
+spec:
+  tags:
+    - <app-image-name>
+  builder:
+    image: cnbs/sample-builder:<bionic OR alpine>
+  source:
+    git:
+      url: https://github.com/buildpacks/samples.git
+      revision: main
+    subPath: "apps/<APP>"
+```
+Once you create yaml file, any time you want to create a build, just run
+
+```bash
+kubectl create -f <your-build-resource.yaml>
+```
+
+A new build resource will be created and a unique suffix will be added to the value provided, for example: `sample-build-2vsz5`
+
+Note: use `create` instead of `apply` when using `generateName`
+
+## Rebase scenario
+
+`As a [pack|kpack] user, I want to rebase my application image with a new run-image from the stack`
+
+### Pack Implementation
+
+[pack][_pack] offers the `pack rebase` command to accomplish this goal, for example:
+
+```bash
+pack rebase --publish <app-image-name>
+```
+
+### Kpack Implementation
+
+A standalone build can be triggered to be rebase if [kpack][_kpack] detects it is a "rebase-able" build.
+
+A build is considered "rebase-able" if the following conditions are met:
+
+- the field `spec.lastBuild.stackId` is equal to <same-stack-id-as-the-builder>
+- An annotation key `image.kpack.io/reason` is equal to `STACK`
+
+An example resource that also is configured to use the `generateName` field could be as follows:
+
+```yaml
+apiVersion: kpack.io/v1alpha2
+kind: Build
+metadata:
+  generateName: sample-build- # this value will be a prefix 
+  annotations:
+    image.kpack.io/reason: STACK
+spec:
+  lastBuild:
+    stackId: <same-stack-id-as-the-builder>
+  tags:
+    - <app-image-name>
+  builder:
+    image: cnbs/sample-builder:<bionic OR alpine>
+  source:
+    git:
+      url: https://github.com/buildpacks/samples.git
+      revision: main
+    subPath: "apps/<APP>"
+```
+Once you create yaml file, just run
+
+```bash
+kubectl create -f <your-build-resource.yaml>
+```
+
+[kpack][_kpack] will create a pod execution the rebase operation
+
+
+[_pack]:https://github.com/buildpacks/pack
+[_kpack]:https://github.com/pivotal/kpack
+[_cnb]:https://buildpacks.io