Low-Level Datastore API

I had to postpone the ACLs for a while and focus on a more important requirement. Until now my system offers data through REST architecture only for a single database schema, so it's time to make it more general and support any type of database structure based on user's preferences.

To achieve this I will use Java Low-Level Datastore API to work with the datastore, because it doesn't require a predefined schema and I can expose the service capabilities directly.

The datastore writes data in objects known as entities, and each entity has a key that identifies the entity. Entities can belong to the same entity group,which allows to perform a single transaction with multiple entities.

Let’s create an Entity representing the login information of an user.

import com.google.appengine.api.datastore.Entity;
import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.datastore.KeyFactory;

String user    = "test@example.com"
String message = "Hello world!";
Date   date    = new Date();

Entity LoginInfo = new Entity("Login", user);
       LoginInfo.setProperty("date", date);
       LoginInfo.setProperty("content", content);

Above we defined an Entity with a raw constructor. We are passing two strings: the kind(or the name of the schema) and the key(or the unique identifier). Entities are typeless, so we can specify any string as a type. In fact the number of kinds is limited only by the number of kinds that we need, and as long as we don’t lose track of them, we could have many different kinds without having to create a class for each one. 

The key name is what we’ll use to retrieve user later on when we need him again. Think of it as a Map or Dictionary Key. 

Once we have an Entity object, we need to define the properties. In this example I defined the current authentication date and a welcome message as properties. Note that, again, we can define as many properties as we want.

After we construct the entity, we instantiate the datastore service, and put the entity in the datastore:

DatastoreService datastore = 
             DatastoreServiceFactory.getDatastoreService();
datastore.put(LoginInfo); 

The low-level Java API provides a Query class for constructing queries for fetching and returning the entities that match the query from the datastore. Here is a simple example based on our code:

Query query = new Query("Login");
Iterator iterator = datastore.prepare(query)
                                     .asIterator();
while(iterator.hasNext()){
    Entity person = iterator.next();

}

This code creates a new query on the Login entity, which returns an interator to a list of Entity objects.
On the following days based on this approach I will rewrite the entire API to meet the requirement.