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Design Considerations

Storage

There is a case for storing System Metadata will be stored in a separate dataset for the following reasons:

1. Only the CDAP system can update System Metadata.  
2. System Metadata may have different authorization as well as retention policies than Business Metadata
3. System Metadata can be updated at specific times only, where users can update Business Metadata at any given time

However, if stored as a separate datasetAs a result, the metadata system will have to manage two different datasets. The storage format of both datasets . APIs may need filters, etc - TODO: Details

Storing History - same pattern as Business Metadata

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(both keys and values) will be identical, they will only write to separate tables.

A higher level construct (TBD, but an extended BusinessMetadataStore or MetadataAdmin) will have to be extended to interact with two separate datasets.

History

We will re-use the same pattern that the Business Metadata Dataset uses to store history.

Runtime

For interacting with the System Metadata Dataset, we will introduce a SystemMetadataUpdater interface, which will be injected at various stages outlined below, to add, update or delete business metadata

System Metadata will be added when:

1. An app is deployed - We will add a SystemMetadataUpdater stage in the deployment pipeline that will update system metadata for the app, as well as all the programs in the app.
2. A new dataset instance is created - The LineageWriterDatasetFramework can be extended to update system metadata when a dataset is addedwill be passed a SystemMetadataUpdater, to add system metadata in the addDatasetInstance call.
3. A new stream is created -  StreamAdmins will be passed a SystemMetadataUpdater as well, to add system metadata in the create API.

System Metadata will be updated when:

1. A dataset instance's properties are updated - The LineageWriterDatasetFramework's updateInstance method will use the SystemMetadataUpdater to update the passed properties
2. A stream's config is updated updated - The StreamAdmin's updateConfig method will use the SystemMetadataUpdater to update the stream's system metadata

System Metadata will be deleted when:

1. An app is deleted - The ApplicationLifecycleService will use the SystemMetadataUpdater to delete system metadata for the application
2. A program is removed from an existing app - The DeletedProgramsHandlerStage will use the SystemMetadataUpdater to delete system metadata for the programs
3. A dataset instance is deleted - The LineageWriterDatasetFramework's deleteInstance method will use the SystemMetadataUpdater to delete system metadata for the dataset instance
4. A stream is deleted - The StreamAdmin's drop method will use the SystemMetadataUpdater to delete system metadata for the stream 

System Metadata Updates

Only the CDAP system can update system metadata for entities. This capability will not be exposed to users. However, given this design choice, users will need a capability in CDAP to discover all the system tags/properties. To start off with, this can be exposed via a simple API that lists all tags/properties. It can later be extended via full-text search capabilities when CDAP has a more comprehensive search capability that extends beyond IndexedTables and prefix lookups.

REST APIs

The add/update/delete APIs for system metadata will not be documented, or be accessible from the Router. Internally, the SystemMetadataUpdater will preferably interact with the transactional store for system metadata directly.

If REST APIs are absolutely necessary (TBD):

• The REST APIs for adding/updating/deleting system metadata will not be documented, and will not be exposed via the Router
• The SystemMetadataUpdater will use service discovery to discover the Metadata Service and make REST calls.

Views

Up until 3.2, users could not associate metadata with stream views. We will need to add this capability in 3.2. However, there would not be any parent-child relationship between a view, and its stream, as far as metadata is concerned. A view will be a separate entity from its stream, and will show up separately in search results. Metadata of a stream will not be automatically available as metadata of a view. 

Implementation

REST APIs

Questions