One API to POST instances, holdings and items

We have three separate APIs for bulk insert and bulk update:

• POST /instance-storage/batch/synchronous
• POST /holdings-storage/batch/synchronous
• POST /item-storage/batch/synchronous

When inserting an instance one often also inserts the holdings and items that belong to that instance. If there is a single endpoint that combines them into a single API the queries to fetch the instance for the holding and the query to fetch the holding for the item can be avoided.

POST /instance-holdings-item-storage/batch/synchronous
taking this body:

{
  "instances": [ { ... }, ... ],
  "holdings":  [ { ... }, ... ],
  "items":     [ { ... }, ... ]
}

Disadvantages:

• Difficult to implement because of complexity resulting in high development costs.
• Other optimizations have better cost/benefit ratio (improving the three batch APIs mentioned above).