3) Fetch revision metadata and entity data separately

Date: 2022-05-24

Status

accepted

Context

In order to provide entity data in the REST API response body as well as revision metadata (revision ID and last-modified date) in the response headers, we need to fetch both the actual data and the metadata for a requested resource from the database. This opens up two possible approaches:

1. fetch both entity data and metadata with a single database request using the same lookup, e.g. EntityRevisionLookup
2. fetch the revision metadata in a first step and the entity data separately, possibly using a different lookup e.g. EntityLookup

While the first approach looks more efficient in terms of database access, it requires combining entity data and revision metadata in a single result object. In contrast, the second approach allows a more flexible handling of the lookup results, as metadata and entity data can be handled in separate results.

Decision

We decided to follow the second approach of fetching revision metadata separately from the entity data, for the following reasons:

• The MetadataRetriever, e.g. for items, can be re-used across several use cases.
• Separating revision metadata from entity data avoids complex result objects, which would have to combine item data, metadata as well as information for failure results (item not found, item is a redirect). Specific result objects would have to be created for every use case separately.

Consequences

We have separate data access services for retrieving entity data and revision metadata, which are used in a two-step process. The metadata retrieval result contains the revision id and timestamp as well as information about a potential redirect or not-found error. This can be used to handle various failure cases, so that the second step will only happen in the success case and simply returns entity data.