Why do we want to consolidate templates and applications? In CDAP 3.0, an ApplicationTemplate is a way for somebody to write an Application that can be given some configuration to create an Adapter. The story is confusing; one would expect an ApplicationTemplate to create... Applications. Instead, we use the term Adapter because Application means something else already. In addition an ApplicationTemplate can only include a single workflow or a single worker, giving people different experiences for templates and applications.
Really, the goal of templates was to be able to write one piece of Application code that could be used to create multiple Applications. To do this requires that an Application can be configured at creation time instead of at compile time. For example, a user should be able to set the name of their dataset based on configuration instead of hardcoding it in the code. To support this, we plan on making it possible to get a configuration object from the ApplicationContext available in Application's configure() method. This allows somebody to pass in a config when creating an Application through the RESTful API, which can be used to configure an Application. The relevant programmatic API changes are shown below.
Artifact - A jar file containing classes that can be used by CDAP.
Application Class - A java class that implements the CDAP Application interface. Bundled in an artifact.
Application Config - Configuration given to CDAP to create an Application (can be empty).
Application - An instantiation of an Application Class, created by passing an Application Config to an Application Class
Plugin - An extension to an Artifact. Usually implements an interface used by Application Classes in the Artifact.
|old terminology||new terminology||description|
in 3.0 and 3.1, you create an Adapter by specifying an ApplicationTemplate and optionally some config
in 3.2, you create an Application by specifying an Artifact and optionally some config
Use Case Walkthrough
1. Create an Application that uses config
1.1 Deploying the Artifact
A developer writes a configurable Application Class that uses a Flow to read from a stream and write to a Table.
A jar named 'myapp-1.0.0.jar' is built which contains the Application Class. The jar is deployed via the RESTful API:
Version is determined from the Bundle-Version in the artifact Manifest. It can also be provided as a header. Artifact details are now visible through other RESTful API calls:
In addition, a call can be made to get all Application Classes:
1.2 Creating an Application
The user decides to create an application from the deployed artifact. From the calls above, the user gathers that input and output are both configurable. The user decides to create an Application that reads from the 'purchases' stream and writes to the 'events' table.
The Application now shows up in all the normal RESTful APIs, with all its programs, streams, and datasets.
1.3 Updating an Application
A bug is found in the code, a fix is provided, and a 'myapp-1.0.1.jar' release is made. The artifact is deployed:
Note: Artifacts are immutable unless they are snapshot versions. Deploying again to version 1.0.0 would cause a conflict error.
A call can be made to determine if there are any Applications using the older artifact:
Calls are made to stop running programs. Another call is then made to update the app:
The config section is optional. If none is given, the previous config will be used. If it is given, it will replace the old config (no merging is done).
1.4 Rolling Back an Application
Actually, version 1.0.1 has a bug that's even worse and needs to be rolled back. The same update call can be made:
1.5 Deploying an Artifact and Creating an App in one step
For backwards compatibility, the deploy app API will remain the same and will internally deploy an artifact and create the app in one call. An additional header will be supported specifying the Application Config.
2. Create an Application that uses plugins
2.1 Application Class changes
Now the user decides to update the MyApp Application Class to support pluggable ways of reading from a stream. This is done by introducing a 'StreamReader' interface in their project:
The user wants this StreamReader interface to be pluggable. There can be many implementations of StreamReader, and which implementation to use should be configurable. The Flowlet code changes to use the new StreamReader interface using the plugin java API:
The Application Class is changed to register a "streamreader" plugin based on configuration:
This becomes v2 of the Application Class. It is deployed via the same RESTful API:
The metadata about this artifact now includes additional information about the config:
2.2 Adding plugins
A default implementation of the streamreader plugin is created to implement the previous logic:
The plugin is bundled into a 'streamreaders-1.0.0.jar' artifact. It is added as an extension to the myapp artifact:
The plugin details can now be seen by querying for extensions to myapp:
2.3 Creating an Application that uses plugins
With this information a user is now able to create an Application that uses a Plugin:
3. System Artifacts
System artifacts are special artifacts that can be accessed in other namespaces. They cannot be deployed through the RESTful API unless a conf setting is set. Instead, they are placed in a directory on the CDAP master host. When CDAP starts up, the directory will be scanned and those artifacts will be added to the system. Example uses for system artifacts are the ETLBatch and ETLRealtime applications that we want to include out of the box.
System artifacts are included in results by default and are indicated with a special flag.
System artifacts can be excluded from results using a filter:
When a user wants to create an application from a system artifact, they make the same RESTful call as before, except adding a special flag to indicate it is a system artifact:
4. Deleting an Artifact
Non-snapshot artifacts will be immutable. Advanced users can delete an existing artifact, but the assumption will be that they know exactly what they are doing. Deleting an artifact may cause programs that are using it to fail.
5. CDAP Upgrade
The programmatic API changes are all backwards compatible, so existing apps will not need to be recompiled. They will, however, need to be added to the artifact repository as part of the upgrade tool (or force people to redeploy their existing apps).
Any existing adapters will need to be migrated. Ideally, the upgrade tool will create matching applications based on the adapter conf, but at a minimum we will simply delete existing adapters and templates.
6. Application Versioning
This was mentioned in stories 1 and 2, but versioning is now explicitly managed by CDAP.
Suppose a development team is working on a search application. There is a dev instance of CDAP running, and an initial version 0.1.0-SNAPSHOT of the artifact is deployed, and a corresponding application is created from it:
During development, every day, a new version of the artifact is built and deployed:
This replaces the version of the artifact that was there before. Any running programs will be using the old code, but any new programs started after the artifact is added will use the new code. Therefore, as part of the deployment process, application programs are restarted. After some time, the initial version of the application code is deemed ready for release. The project version is bumped to version 0.1.0, an artifact is built and deployed to CDAP:
This adds a newer version of the artifact. This version is not a snapshot version and is therefore immutable. Attempts to re-deploy it will fail. Deploying the artifact has no impact on existing applications. The 'search' application will continue to use version 0.1.0-SNAPSHOT of the artifact until it is updated to the new version:
If no config is given, the existing config will be used. Otherwise, if a config is given, it will entirely replace the existing config. Artifact version cannot be changed unless all running programs are stopped.
After some time, some bugs are found and version 0.1.1 is developed and released. The jar is built and deployed to CDAP:
The application is updated to use the new version of the artifact with the bug fixes:
After some more time, additional features are added and version 0.2.0 is built and released, and the application is changed to use the new version of the artifact:
If there is any schema evolution happening or any other backwards impacting changes, they must be handled correctly by the application logic. CDAP will not migrate data or have any guarantees of compatibility between artifact versions.
During the release, a serious bug is discovered and the application is rolled back to use the previous artifact version:
Again, no compatibility guarantees are made by CDAP. This operation may not be safe if the application logic does not make it safe, for example if there is data written in a new format that the old code cannot understand.
RESTful API changes
|GET||/v3/namespaces/<namespace-id>/apps?artifactName=<name>[&artifactVersion=<version>]||get all apps using the given artifact name and version|
|POST||/v3/namespaces/<namespace-id>/apps||application jar contents||Application-Config: <json of config>||same as deploy api today, except allows passing config as a header|
|PUT||/v3/namespaces/<namespace-id>/apps/<app-name>||application jar contents||Application-Config: <json of config>||same as deploy api today, except allows passing config as a header|
create an application from an existing artifact.
Note: Edits existing API, different behavior based on content-type
|POST||/v3/namespaces/<namespace-id>/apps/<app-name>/update||update an existing application. No programs can be running|
|GET||/v3/namespaces/<namespace-id>/artifacts/<artifact-name>||Get data about all artifact versions|
Artifact-Plugins: <json of plugins in the artifact>
Add a new artifact. Version header only needed if Bundle-Version is not in jar Manifest. If both present, header wins.
Artifact plugins can be explicitly given as a header.
This is to support the use case of 3rd party classes used as plugins, such as jdbc drivers
|GET||/v3/namespaces/<namespace-id>/artifacts/<artifact-name>/versions/<version>||Get details about the artifact, such as what plugins and applications are in the artifact and properties they support|
config properties can be nested now. For example:
Template APIs (will be removed)
|GET||/v3/templates/<template-name>||/v3/namespaces/<namespace-id>/artifacts/[cdap-etl-batch | cdap-etl-realtime]?scope=system|
|POST||/v3/namespaces/<namespace-id>/adapters/<adapter-name>/start||resume workflow schedule api for etl-batch, start worker api for etl-realtime|
|POST||/v3/namespaces/<namespace-id>/adapters/<adapter-name>/stop||pause workflow schedule api for etl-batch, stop worker api for etl-realtime|
|GET||/v3/namespaces/<namespace-id>/adapters/<adapter-name>/status||workflow schedule status api for etl-batch, worker status api for etl-realtime|
|GET||/v3/namespaces/<namespace-id>/adapters/<adapter-name>/runs||workflow runs api for etl-batch, worker runs api for etl-realtime|
|GET||/v3/namespaces/<namespace-id>/adapters/<adapter-name>/runs/<run-id>||workflow runs api for etl-batch, worker runs api for etl-realtime|