Session: What is a Session and How Does it Work

After creating a RavenDB document store, we are ready to use the database server instance it is pointing at. For any operation we want to perform on RavenDB, we start by obtaining a new Session object from the document store. The Session object will contain everything we need to perform any operation necessary.

String companyId;

//store new object
try (IDocumentSession session = store.openSession()) {
    Company entity = new Company();
    entity.setName("Company");
    session.store(entity);
    session.saveChanges();

    // after calling saveChanges(), an id field if exists
    // is filled by the entity's id
    companyId = entity.getId();
}

try (IDocumentSession session = store.openSession()) {
    //load by id
    Company entity = session.load(Company.class, companyId);

    // do something with the loaded entity
}

The Client API, and using the Session object in particular, is very straightforward. Open the session, do some operations, and apply the changes to the RavenDB server. The usage of the second session is similar: open the session, get a document from the server, and do something with it.

Unit of Work

The Client API implements the Unit of Work pattern. That has several implications:

• In the context of a single session, a single document (identified by its ID) always resolves to the same instance.

Assert.assertSame(session.load(Company.class, companyId), session.load(Company.class, companyId));

• The session manages change tracking for all the entities that it has either loaded or stored.

try (IDocumentSession session = store.openSession()) {
    Company entity = session.load(Company.class, companyId);
    entity.setName("Another Company");

    // when a document is loaded by Id (or by query),
    // its changes are tracked (by default).
    // A call to saveChanges() sends all accumulated changes to the server
    session.saveChanges();
}

Batching

One of the most expensive operations in an application is making remote calls. The RavenDB Client API optimizes this for you by batching all write calls to the RavenDB server into a single call. This is the default behavior whenever using the Session object, you don't have to do anything to enable it. This also ensures that writes to the database are always executed in a single transaction, no matter how many operations you are actually executing.

Remarks

A very common problem with all ORMs and ORM-like APIs is the Select N+1 problem. This is relevant to any database API which is designed to work like an ORM, RavenDB Client API included. How RavenDB API attempts to mitigate this is not immediate, and should never be reached if RavenDB is being utilized correctly. Remote calls are expensive and the number of remote calls per "session" should be as close to "1" as possible. If the limit is reached, it is a sure sign of either a Select N+1 problem or other misuse of the RavenDB session.