Storing Documents in AppEngine's Datastore

One of the advantages of Google AppEngine's Datastore is that it is schema-less. You can simply store any Entity you wish, as long as you abide the indexes.
In this post, I will mimic the MongoDB document store, where you can store any kind of JSON document in your collection, as long as it has attributes for your collection's indexes.

Objectives

• I would like to access the document store using a generic REST-JSON API, which implements basic CRUD methods plus a query method
• On the consumer side, I  would like to map to POJOs, not a map
• I would like to be able to store new kinds of objects without interrupting the service
• I would like to be able to introduce new attributes on my entities without updating any of the existing entities

Source for the json-doc-store

The source code is hosted on BitBucket.

hg clone https://bitbucket.org/f94os/json-doc-store

Binding request from JSON to Map

The intermediate data type when using Spring and jackson JSON mapper is the Map class:

    @RequestMapping(value="{kind}", method= RequestMethod.POST)
    public ResponseEntity createDocument(
            @PathVariable String kind,
            @RequestParam String id,
            @RequestBody Map props
            ) { 
      // Spring and Jackson binds my JSON object to a Map 

// Bind to AppEngine Datastore Entities // persist in datastore }

Read a document from the datastore

The Spring Controller method to create entities looks like this:

Create by kind only (generate key)

    @RequestMapping(value="{kind}/{id}", method= RequestMethod.GET)
    public ResponseEntity> getDocument(
            @PathVariable String kind,
            @PathVariable Object id
            ) {
        
        // query for ancestors, including ancestor

        // bind from Entities to Map

        // Jackson will serialize from Map to JSON 

     }

Namespace and Kind

A relational database can be compared to using Namespaces in the Datastore, and a table can be compared to using the Entity Kind.

Admin database

MySQL uses a database with tables for users, user databases, user tables and so on. The same concept is done here, with an admin Namespace of json-doc-store, and two Kinds, _DBs and _Users.