Extended Tour Documentation

Makefile

@@ -68,7 +68,7 @@ test:
 
 .PHONY: generate
 generate:
-	@$(REPO_ROOT)/hack/generate.sh $(REPO_ROOT)/pkg... $(REPO_ROOT)/cmds/ocm/... $(REPO_ROOT)/cmds/helminst/...
+	@$(REPO_ROOT)/hack/generate.sh $(REPO_ROOT)/pkg... $(REPO_ROOT)/cmds/ocm/... $(REPO_ROOT)/cmds/helminst/... $(REPO_ROOT)/examples/...
 
 .PHONY: generate-deepcopy
 generate-deepcopy: controller-gen

examples/lib/tour/01-getting-started/README.md

@@ -1,13 +1,270 @@
+<!-- DO NOT MODIFY                               -->
+<!-- this file is generated by mdref             -->
+<!-- from ../docsrc/01-getting-started/README.md -->
 # Basic Usage of OCM Repositories
 
 This [tour](example.go) illustrates the basic usage of the API to
 access component versions in an OCM repository.
 
+## Running the example
+
 You can just call the main program with some config file argument
 with the following content:
 
 ```yaml
 component: github.com/mandelsoft/examples/cred1
 repository: ghcr.io/mandelsoft/ocm
 version: 0.1.0
-```
+```
+
+## Walkthrough
+
+The basic entry point for using the OCM library is always
+an [OCM Context object](../../contexts.md). It bundles all
+configuration settings and type registrations, like
+access methods, repository types, etc, and
+configuration settings, like credentials,
+which should be used when working with the OCM
+ecosystem.
+
+Therefore, the first step is always to get access to such
+a context object. Our example uses the default context
+provided by the library, which covers the complete
+type registration contained in the executable.
+
+It can be accessed by a function of the ocm package.
+
+```go
+	ctx := ocm.DefaultContext()
+```
+
+The context acts as the central entry
+point to get access to OCM elements.
+First, we get a repository, to look for
+component versions. We use the OCM
+repository providing the standard OCM
+components hosted on `ghcr.io`.
+
+For every storage technology used to store
+OCM components, there is a serializable
+descriptor object, the *repository specification*.
+It describes the information required to access
+the repository and can be used to store the serialized
+form as part of other resources, for example
+Kubernetes resources or configuration settings.
+The available repository implementations can be found
+under `.../pkg/contexts/ocm/repositories`.
+
+```go
+	spec := ocireg.NewRepositorySpec("ghcr.io/open-component-model/ocm")
+```
+
+The context can now be used to map the descriptor
+into a repository object, which then provides access
+to the OCM elements stored in this repository.
+
+```go
+	repo, err := ctx.RepositoryForSpec(spec)
+	if err != nil {
+		return errors.Wrapf(err, "cannot setup repository")
+	}
+```
+
+Many objects must be closed, if they should not be used
+anymore, to release potentially allocated temporary resources.
+This is typically done by a `defer` statement placed after a
+successful object retrieval.
+
+```go
+	defer repo.Close()
+```
+
+Now we look for the versions of the component
+available in this repository.
+
+```go
+	versions, err := c.ListVersions()
+	if err != nil {
+		return errors.Wrapf(err, "cannot query version names")
+	}
+```
+
+OCM version names must follow the semver rules.
+Therefore, we can simply order the versions and print them.
+
+```go
+	err = semverutils.SortVersions(versions)
+	if err != nil {
+		return errors.Wrapf(err, "cannot sort versions")
+	}
+	fmt.Printf("versions for component ocm.software/ocmcli: %s\n", strings.Join(versions, ", "))
+```
+
+Now, we have a look at the latest version, it is
+the last one in the list.
+
+```go
+	cv, err := c.LookupVersion(versions[len(versions)-1])
+	if err != nil {
+		return errors.Wrapf(err, "cannot get latest version")
+	}
+	defer cv.Close()
+```
+
+<a id="describe-version"></a>
+
+The component version object provides access
+to the component descriptor
+
+```go
+	cd := cv.GetDescriptor()
+	fmt.Printf("resources of the latest version:\n")
+	fmt.Printf("  version:  %s\n", cv.GetVersion())
+	fmt.Printf("  provider: %s\n", cd.Provider.Name)
+```
+
+and the resources described by the component version.
+
+```go
+	for i, r := range cv.GetResources() {
+		fmt.Printf("  %2d: name:           %s\n", i+1, r.Meta().GetName())
+		fmt.Printf("      extra identity: %s\n", r.Meta().GetExtraIdentity())
+		fmt.Printf("      resource type:  %s\n", r.Meta().GetType())
+		acc, err := r.Access()
+		if err != nil {
+			fmt.Printf("      access:         error: %s\n", err)
+		} else {
+			fmt.Printf("      access:         %s\n", acc.Describe(ctx))
+		}
+	}
+```
+
+This results in the following output (the shown version might
+differ, because the code always describes the latest version):
+
+```
+resources of the latest version:
+  version:  0.6.0
+  provider: ocm.software
+   1: name:           ocmcli
+      extra identity: "architecture"="amd64","os"="linux"
+      resource type:  executable
+      access:         Local blob sha256:6672528b57fd77cefa4c5a3395431b6a5aa14dc3ddad3ffe52343a7a518c2cd3[]
+   2: name:           ocmcli
+      extra identity: "architecture"="arm64","os"="linux"
+      resource type:  executable
+      access:         Local blob sha256:9088cb8bbef1593b905d6bd3af6652165ff82cebd0d86540a7be9637324d036b[]
+   3: name:           ocmcli-image
+      extra identity: 
+      resource type:  ociImage
+      access:         OCI artifact ghcr.io/open-component-model/ocm/ocm.software/ocmcli/ocmcli-image:0.6.0
+```
+
+Resources have some metadata, like the resource identity and a resource type.
+And they describe how the content of the resource (as blob) can be accessed.
+This is done by an *access specification*, again a serializable descriptor,
+like the respository specification.
+
+The component version contains the executables for the OCM CLI
+for various platforms. The next step is to
+get the executable for the actual environment.
+The identity of a resource described by a component version
+consists of a set of properties. The property name must
+always be given.
+
+A convention is to use dedicated labels to indicate the operating system
+and the architecture for executables.
+
+```go
+	id := metav1.NewIdentity("ocmcli",
+		extraid.ExecutableOperatingSystem, runtime.GOOS,
+		extraid.ExecutableArchitecture, runtime.GOARCH,
+	)
+
+	res, err := cv.GetResource(id)
+	if err != nil {
+		return errors.Wrapf(err, "resource %s", id)
+	}
+```
+
+Now we want to retrieve the executable. There are two basic ways
+to do this.
+
+First, there is the direct way to gain access to the blob by using
+the basic model operations to get a reader for the resource blob.
+Therefore, in a first step we get the access method for the resource
+
+```go
+		var m ocm.AccessMethod
+		m, err = res.AccessMethod()
+		if err != nil {
+			return errors.Wrapf(err, "cannot get access method")
+		}
+		defer m.Close()
+```
+
+The method needs to be closed, because the method
+object may cache the technical blob representation
+generated accessing the underlying access technology.
+(for example, accessing an OCI image requires a sequence of
+backend accesses for the manifest, the layers, etc which will
+then be packaged into a tar archive returned as blob).
+This caching may not be required, if the backend directly
+returns a blob.
+
+Now we get access to the reader providing the blob content.
+The blob features a mime type, which can be used to understand
+the format of the blob. Here, we have a plain octet stream.
+
+```go
+		fmt.Printf("  found blob with mime type %s\n", m.MimeType())
+		reader, err = m.Reader()
+```
+
+Because this sequence is a common operation, there is a
+utility function handling this sequence. A shorter way to get
+a resource reader is as follows:
+
+```go
+		reader, err = utils.GetResourceReader(res)
+```
+
+Before we download the content we check the error and prepare
+closing the reader, again
+
+```go
+	if err != nil {
+		return errors.Wrapf(err, "cannot get resource reader")
+	}
+	defer reader.Close()
+```
+
+Now, we just read the content and copy it to the intended 
+output file.
+
+```go
+	file, err := os.OpenFile("/tmp/ocmcli", os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0766)
+	if err != nil {
+		return errors.Wrapf(err, "cannot open output file")
+	}
+	defer file.Close()
+
+	n, err := io.Copy(file, reader)
+	if err != nil {
+		return errors.Wrapf(err, "write executable")
+	}
+	fmt.Printf("%d bytes written\n", n)
+```
+
+Another way to download a resource is to use registered downloaders.
+`download.DownloadResource` is used to download resources with specific handlers for the
+selected resource and mime type combinations.
+The executable downloader is registered by default and automatically
+sets the X flag.
+
+```go
+	_, err = download.DownloadResource(ctx, res, "/tmp/ocmcli", download.WithPrinter(common.NewPrinter(os.Stdout)))
+	if err != nil {
+		return errors.Wrapf(err, "download failed")
+	}
+```