Use fragments in Application Accelerator

This topic tells you how to use fragments in Application Accelerator.

Introduction

Despite their benefits, writing and maintaining accelerators can become repetitive and verbose as new accelerators are added. Some create a project different from the next with similar aspects, requiring some form of copy-paste.

To alleviate this concern, Application Accelerators support a feature named Composition that allows the re-use of parts of an accelerator, called fragments.

Introducing fragments

A fragment looks exactly the same as an accelerator:

• It is made of a set of files.
• It contains an accelerator.yaml descriptor with options, declarations, and an accelerator.axl file that describes its root transform.

There are differences however. Namely:

• Fragments are declared to the system differently. They are filed as fragment custom resources.
• They deal with files differently. Because fragments deal with their own files and files from the accelerator using them, they use dedicated conflict resolution strategies (more on this later).

Fragments may be thought of as “functions” in programming languages. After being defined and referenced, they are “called” at various points in the main accelerator. The composition feature is designed with ease of use and “common use first” in mind, so these “functions” are typically called with as little noise as possible. You can also call them complex or different values.

Composition relies on two building blocks that play hand in hand:

• The imports section at the top of an accelerator manifest.
• The, InvokeFragment transform, to be used alongside any other transform.

The imports section explained

To be usable in composition, a fragment MUST be imported into the dedicated section of an accelerator manifest:

accelerator:
  name: my-awesome-accelerator
  options:
    - name: flavor
      dataType: string
      defaultValue: Strawberry
  imports:
    - name: my-first-fragment
    - name: another-fragment

The effect of importing a fragment this way is twofold:

• It makes its files available to the engine (therefore importing a fragment is required).
• It exposes all of its options as options of the accelerator as if they were defined by the accelerator itself.

So in the earlier example, if the my-first-fragment fragment had the following accelerator.yaml file:

accelerator
  name: my-first-fragment
  options:
    - name: optionFromFragment
      dataType: boolean
      description: this option comes from the fragment
...

Then it is as if the my-awesome-accelerator accelerator defined it:

accelerator:
  name: my-awesome-accelerator
  options:
    - name: flavor
      dataType: string
      defaultValue: Strawberry
    - name: optionFromFragment
      dataType: boolean
      description: this option comes from the fragment
  imports:
    - name: my-first-fragment
    - name: another-fragment

All the metadata about options (type, default value, description, choices if applicable, etc.) come along when imported.

Because of this, users are prompted for a value for those options that come from fragments, as if they were options of the accelerator.

Using the InvokeFragment Transform

The second part at play in the composition is the InvokeFragment Transform.

As with any other transform, it may be used anywhere in the engine tree and receives files that are “visible” at that point. Those files, alongside those that make up the fragment, are made available to the fragment logic. If the fragment wants to choose between two versions of a file for a path, two new conflict resolution strategies are available: FavorForeign and FavorOwn.

The behavior of the InvokeFragment transform is very straightforward: after having validated options that the fragment expects (and maybe after having set default values for options that support one), it executes the whole transform of the fragment as if it was written in place of InvokeFragment.

See the InvokeFragment reference page for more explanations, examples, and configuration options. This topic now focuses on additional features of the imports section that are seldom used but still available to cover more complex use cases.

Back to the imports section

The complete definition of the imports section is as follows, with features in increasing order of “complexity”:

accelerator:
  name: ...
  options:
    - name: ...
    ...
  imports:
    - name: some-fragment

    - name: another-fragment
      expose:
        - name: "*"
      exposeTypes:
        - name: "*"

    - name: yet-another-fragment
      expose:
        - name: someOption

        - name: someOtherOption
          as: aDifferentName
      exposeType:
        - name: SomeType

        - name: SomeOtherType
          as: ADifferentName

As shown earlier, the imports section calls a list of fragments to import. By default, all their options and types become options/type of the accelerator. Those options appear after the options defined by the accelerator, in the order the fragments are imported in.

It is even possible for a fragment to import another fragment, the semantics being the same as when an accelerator imports a fragment. This is a way to break apart a fragment even further if needed.

When importing a fragment, you can select which options of the fragment to make available as options of the accelerator. This feature should only be used when a name clash arises in option names.

The semantics of the expose block are as follows:

• For every name/as pair, don’t use the original (name) of the option but instead, use the alias (as). Other metadata about the option is left unchanged.
• If the special name: "*" (which is NOT a legit option name usually) appears, all imported option names that are not remapped (the index at which the * appears in the YAML list is irrelevant) might be exposed with their original name.
• The default value for expose is [{name: "*"}], that is, by default exposes all options with their original name.
• As soon as a single remap rule appears, the default is overridden. For example, to override some names AND expose the others unchanged, the * must be explicitly re-added.
• To explicitly un-expose ALL options from an imported fragment, an empty array may be used and overrides the default: expose: [].

Similarly, you can also select which custom types of the fragment to make available as types of the accelerator. This feature should only be used when a name clash arises in types names.

The semantics of the exposeTypes block are as follows:

• For every name/as pair, don’t use the original (name) of the type but instead, use the alias (as). Options that used the original name are automatically “rewritten” to use the new name.
• If the special name: "*" appears, which is NOT usually a legit type name, all imported other type names that are not remapped are exposed with their original name. The index at which the * appears in the YAML list is irrelevant.
• The default value for exposeTypes is [{name: "*"}], that is, by default exposes all types with their original name.
• As soon as a single remap rule appears, the default is overridden. For example, to override some names AND expose the others unchanged, the * must be explicitly re-added.
• To explicitly un-expose ALL types from an imported fragment, an empty array may be used, which overrides the default: exposeTypes: [].

Using dependsOn in the imports section

Lastly, as a convenience for the conditional use of fragments, you can make an exposed imported option depend on another option, as in the following example:

  imports:
    - name: tap-initialize
      expose:
        - name: gitRepository
          as: gitRepository
          dependsOn:
            name: deploymentType
            value: workload
        - name: gitBranch
          as: gitBranch
          dependsOn:
            name: deploymentType
            value: workload

This plays well with the use of if, as in the following example:

engine {
  ...
  if (#deploymentType == 'workload') {
    InvokeFragment('tap-initialize')
  }
  ...

Discovering fragments using Tanzu CLI accelerator plug-in

Using the accelerator plug-in for Tanzu CLI, you can view a list of available fragments. Run:

tanzu accelerator fragment list

To see a list of available accelerator fragments. For example:

NAME                                 READY   REPOSITORY
app-sso-client                       true    source-image: registry.example.com/app-accelerator/fragments/app-sso-client@sha256:ed5cf5544477d52d4c7baf3a76f71a112987856e77558697112e46947ada9241
java-version                         true    source-image: registry.example.com/app-accelerator/fragments/java-version@sha256:df99a5ace9513dc8d083fb5547e2a24770dfb08ec111b6591e98497a329b969d
live-update                          true    source-image: registry.example.com/app-accelerator/fragments/live-update@sha256:c2eda015b0f811b0eeaa587b6f2c5410ac87d40701906a357cca0decb3f226a4
spring-boot-app-sso-auth-code-flow   true    source-image: registry.example.com/app-accelerator/fragments/spring-boot-app-sso-auth-code-flow@sha256:78604c96dd52697ea0397d3933b42f5f5c3659cbcdc0616ff2f57be558650499
tap-initialize                       true    source-image: registry.example.com/app-accelerator/fragments/tap-initialize@sha256:7a3ae8f9277ef633200622dbf9d0f5a07dea25351ac3dbf803ea2fa759e3baac
tap-workload                         true    source-image: registry.example.com/app-accelerator/fragments/tap-workload@sha256:8056ad9f05388883327d9bbe457e6a0122dc452709d179f683eceb6d848338d0

The tanzu accelerator fragment get <fragment-name> command shows all the options defined for the fragment and also any accelerators or other fragments that import this fragment. Run:

tanzu accelerator fragment get java-version

The following output is displayed:

name: java-version
namespace: accelerator-system
displayName: Select Java Version
ready: true
options:
- choices:
  - text: Java 8
    value: "1.8"
  - text: Java 11
    value: "11"
  - text: Java 17
    value: "17"
  defaultValue: "11"
  inputType: select
  label: Java version to use
  name: javaVersion
  required: true
artifact:
  message: Resolved revision: registry.example.com/app-accelerator/fragments/java-version@sha256:df99a5ace9513dc8d083fb5547e2a24770dfb08ec111b6591e98497a329b969d
  ready: true
  url: http://source-controller-manager-artifact-service.source-system.svc.cluster.local./imagerepository/accelerator-system/java-version-frag-97nwp/df99a5ace9513dc8d083fb5547e2a24770dfb08ec111b6591e98497a329b969d.tar.gz
imports:
  None
importedBy:
  accelerator/java-rest-service
  accelerator/java-server-side-ui
  accelerator/spring-cloud-serverless

This shows the options and importedBy with a list of three accelerators that import this fragment.

Correspondingly, the tanzu accelerator get <accelerator-name> shows the fragments that an accelerator imports. Run:

tanzu accelerator get java-rest-service

The following output is shown:

name: java-rest-service
namespace: accelerator-system
description: A Spring Boot Restful web application including OpenAPI v3 document generation and database persistence, based on a three-layer architecture.
displayName: Tanzu Java Restful Web App
iconUrl: data:image/png;base64,...abbreviated...
source:
  image: registry.example.com/app-accelerator/samples/java-rest-service@sha256:c098bb38b50d8bbead0a1b1e9be5118c4fdce3e260758533c38487b39ae0922d
  secret-ref: [{reg-creds}]
tags:
- java
- spring
- web
- jpa
- postgresql
- tanzu
ready: true
options:
- defaultValue: customer-profile
  inputType: text
  label: Module artifact name
  name: artifactId
  required: true
- defaultValue: com.example
  inputType: text
  label: Module group name
  name: groupId
  required: true
- defaultValue: com.example.customerprofile
  inputType: text
  label: Module root package
  name: packageName
  required: true
- defaultValue: customer-profile-database
  inputType: text
  label: Database Instance Name this Application will use (can be existing one in
    the cluster)
  name: databaseName
  required: true
- choices:
  - text: Maven (https://maven.apache.org/)
    value: maven
  - text: Gradle (https://gradle.org/)
    value: gradle
  defaultValue: maven
  inputType: select
  name: buildTool
  required: true
- choices:
  - text: Flyway (https://flywaydb.org/)
    value: flyway
  - text: Liquibase (https://docs.liquibase.com/)
    value: liquibase
  defaultValue: flyway
  inputType: select
  name: databaseMigrationTool
  required: true
- dataType: boolean
  defaultValue: false
  label: Expose OpenAPI endpoint?
  name: exposeOpenAPIEndpoint
- defaultValue: ""
  dependsOn:
    name: exposeOpenAPIEndpoint
  inputType: text
  label: System API Belongs To
  name: apiSystem
- defaultValue: ""
  dependsOn:
    name: exposeOpenAPIEndpoint
  inputType: text
  label: Owner of API
  name: apiOwner
- defaultValue: ""
  dependsOn:
    name: exposeOpenAPIEndpoint
  inputType: text
  label: API Description
  name: apiDescription
- choices:
  - text: Java 8
    value: "1.8"
  - text: Java 11
    value: "11"
  - text: Java 17
    value: "17"
  defaultValue: "11"
  inputType: select
  label: Java version to use
  name: javaVersion
  required: true
- dataType: boolean
  defaultValue: true
  dependsOn:
    name: buildTool
    value: maven
  inputType: checkbox
  label: Include TAP IDE Support for Java Workloads
  name: liveUpdateIDESupport
- defaultValue: dev.local
  dependsOn:
    name: liveUpdateIDESupport
  description: The prefix for the source image repository where source can be stored
    during development
  inputType: text
  label: The source image repository prefix to use when pushing the source
  name: sourceRepositoryPrefix
artifact:
  message: Resolved revision: registry.example.com/app-accelerator/samples/java-rest-service@sha256:c098bb38b50d8bbead0a1b1e9be5118c4fdce3e260758533c38487b39ae0922d
  ready: true
  url: http://source-controller-manager-artifact-service.source-system.svc.cluster.local./imagerepository/accelerator-system/java-rest-service-acc-wcn8x/c098bb38b50d8bbead0a1b1e9be5118c4fdce3e260758533c38487b39ae0922d.tar.gz
imports:
  java-version
  live-update
  tap-workload

The imports section at the end shows the fragments that this accelerator imports. The options section shows all options defined for this accelerator. This includes all options defined in the imported fragments, for example, the options for the Java version imported from the java-version fragment.