Exposing the data layer of your app using REST

The more we sepparate the concerns of our system, the more mainteinable it becomes.

It is very common to find applications written in such way that the data access mechanisms(SQL files, JDBC client code, ORM mappings...) are located just next to(coupled/interdependant) the service/bussiness logic. This often makes finding bug, making a change, etc.. harder.

Calculating a result and storing it, are different things. So why not sepparating those 2 responsibilities among different applications?

One would be responsible of making sure the results are calculated and the other will just provide data management support.
In my opinion the result of doing this is a system that is more understandable, maintainable and upgrade friendly.

In many companies, the data is often managed by database engineering teams which have: schedules, goals and even different managers than the development teams. In this type of organization, delays, missunderstandings, conflicts of interests and work de-synchronization are very common. So to make the most of a decoupled system, we not just need a good software approach, but also a process and team structure that are compatible with it(But this may be a topic for another post). This type of decoupling will not just make maintenance easy for the developers but also, it will probably encourage discussion about the process and the teams structure.

In my example I decided expose 2 persistent services via 1 url and persisting simultaniously in 2 types of databases(a sql and a no-sql DB).

This is the implementation of the no-sql adapter

 public class NoSqlAddressInsertAdapter implements CreateService {  
   private final MongoClient mongoClient;  
   @Inject  
   public NoSqlAddressInsertAdapter(MongoClient mongoClient) {  
     this.mongoClient = mongoClient;  
   }  
   @Override  
   public void create(Address address) {  
     DBCollection collection = mongoClient.getDB("radadata").getCollection("address");  
     collection.insert(toNoSqlAddress(address));  
   }  
   private AddressNoSql toNoSqlAddress(Address address) {  
     AddressNoSql addressNoSql = new AddressNoSql();  
     addressNoSql.append("firstline", address.getFirstLine());  
     addressNoSql.append("secondline", address.getSecondLine());  
     addressNoSql.append("postcode", address.getPostcode());  
     addressNoSql.append("persons", address.getPersons().stream().map(toNoSqlPersons()).collect(toList()));  
     return addressNoSql;  
   }  
   private Function<Person, PersonNoSql> toNoSqlPersons() {  
     return person -> {  
       PersonNoSql personNoSql = new PersonNoSql();  
       personNoSql.append("firstname", person.getFirstName());  
       personNoSql.append("secondname", person.getSecondName());  
       return personNoSql;  
     };  
   }  
 }  

This is the implementation of the sql-adapter

 public class SqlAddressInsertAdapter implements CreateService {  
   @Inject  
   public SqlAddressInsertAdapter() {  
   }  
   private static SessionFactory getSessionFactory() {  
     return HibernateUtil.getSessionFactory();  
   }  
   private Session session;  
   @Override  
   public void create(Address address) {  
     session = SqlAddressInsertAdapter.getSessionFactory().getCurrentSession();  
     session.beginTransaction();  
     Set<ORMPerson> ormPersons = address.getPersons().stream().map(toOrmPersons()).collect(toSet());  
     ORMAddress ormAddress = new ORMAddress();  
     ormAddress.setFirstLine(address.getFirstLine());  
     ormAddress.setSecondLine(address.getSecondLine());  
     ormAddress.setPostcode(address.getPostcode());  
     ormAddress.setOrmPersons(ormPersons);  
     session.save(ormAddress);  
     session.getTransaction().commit();  
   }  
   @Override  
   public void create(Person person) {  
     //  
   }  
   private Function<Person, ORMPerson> toOrmPersons() {  
     return person -> new ORMPerson(person.getFirstName(),person.getSecondName());  
   }  
 }  

Note that both adapters use their specific domain objects, one uses ORM(Those ORMClasses are hibernate entities) and the other doesn't.

This is a sample REST endpoint will allow access to those services simultaniously

 @Service  
 @Path("insertperson")  
 public class InsertAddressResource {  
   private final services.nosqlcrud.CreateService noSqlcreateService;  
   private final services.sqlcrud.CreateService sqlCreateService;  
   @Inject  
   public InsertAddressResource(services.nosqlcrud.CreateService noSqlcreateService,  
                  services.sqlcrud.CreateService sqlCreateService) {  
     this.noSqlcreateService = noSqlcreateService;  
     this.sqlCreateService = sqlCreateService;  
   }  
   @POST  
   @Consumes({"application/json"})  
   public void insert(Address address) {  
     noSqlcreateService.create(address);  
     sqlCreateService.create(address);  
   }  
   /*  
     A Sample Json to POST:  
     URL: http://localhost:9998/insertperson  
     Content Type: application/json  
     {  
      "firstline": "street bla bla",  
      "secondline": "town of bla bla",  
      "postcode": "ble ble ble",  
      "persons": [  
         {"firstname":"Armin","secondname":"Josef"},  
         {"firstname":"Johan","secondname":"Uhgler"}  
       ]  
     }  
   */  
 }  

This snippets of code are just part of a demo app I wrote some days ago to show how to expose the data layer via REST.
The rest of the project, can be found at: https://github.com/SFRJ/Rest-Approach-to-Data-Persistence-R.A.D.A-