Tutorial: Using REST and OSGi Standards for Micro Services

Introduction

It's currently popular to create REST-based networked services. This is understandable, as the ubiquity http/https, the simplicity of the REST approach, and the availability of open, cross-language object serialization formats like JSON and XML, along with the availability of quality distribution frameworks all make it easier than ever to define, implement, and deploy a service and it's associated API.

One thing that is relatively new, however, is that there are now open standards that deal with two levels of concerns

Transport/Distribution-Level Concerns: What protocol and serialization format are to be used? How to support cross-language type systems? How to be as bandwidth efficient and performant as possible? How to handle network failure? etc.
Service-Level Concerns: How to discover the service? How to version the service? How to handle service dynamics? How to secure the service? How to describe and document the service so that others may easily understand and use it? How to combine services and have service-level dependencies in a dynamic environment? etc.

Transport and Distribution Concerns

From the service implementer's perspective, the selection of a framework or implementation makes de-facto design choices about transport or distribution concerns. For example, some REST frameworks use XML, some JSON, some support both. However, once such a framework is chosen the service implemented and deployed, it can become very difficult, time-consuming, or even technically impossible to change to use other frameworks or distribution systems.

For REST-based services, however, standards are beginning to emerge that allow REST services to be defined and implemented without making a specific choice of distribution system or framework. One example in Java is JAX Remote Services or JAX-RS. JAX-RS defines/standardizes Java annotations for classes and methods that are to be exposed for remote access. JAX-RS implementations then use these annotations to 'export' the service (make it available for remote access) and handle the mapping between http/https requests, and the appropriate method invocation. For example, here's a small 'hello-world' service that uses JAX-RS annotations:

import javax.ws.rs.GET;
import javax.ws.rs.Produces;
import javax.ws.rs.Path;
 
// The Java class will be hosted at the URI path "/helloworld"
@Path("/helloworld")
public class HelloWorldResource implements HelloWorldService {
 
    // The Java method will process HTTP GET requests
    @GET
    // The Java method will produce content identified by the MIME Media
    // type "text/plain"
    @Produces("text/plain")
    public String getMessage() {
        // Return some textual content
        return "Hello World";
    }
}

One very useful aspect of the JAX-RS annotations is that multiple implementations may exist, and it is relatively easy to switch from one implementation to another.

Service-Level Concerns

Although frequently of secondary concern to service implementers, service-level concerns are typically of primary importance to service consumers. For example, before I can use any REST-based service, I have to know about (discover) the service and needed meta-data: the service location (i.e. it's URL), what methods exist and arguments are required, and what can be expected in response. Additionally, what version of the service is being accessed, what credentials/security are required, what are the run-time qualities of the remote service?

OSGi Remote Services

In recent releases the OSGi Alliance has defined a specification called Remote Services (chapter 100 in the enterprise spec). Along with the Remote Service Admin specification (chapter 122 in enterprise spec), Remote Services defines ways to express answers to service-level concerns in an implementation-independent way, similar to what JAX-RS does for transport-level concerns. For example, the Remote Service spec allows meta-data for service type, service version, service endpoint/URL identification, security, and quality of service properties to be communicated between service implementer and service consumer without referring to any Remote Services implementation. Further, Remote Services/RSA defines a way to dynamically discover and un-discover remote services, allowing for easy consumer support for networked and frequently unreliable services.

Several implementations of OSGi Remote Services exist, including open source ECF, CXF, Amdatu and commercial implementations.

OSGi RSA has the concept of a distribution system responsible for exporting a local service and making it available for remote access. According to the specification, many distributions systems may be used even for a single service. ECF's implementation is unique because it has an open API for creating/adding new distribution providers. Currently, ECF committers have implemented the following distribution providers

Jax-RS with OSGi Remote Services

Since ECF's RSA implementation implements the OSGi RS and RSA specifications, and Jersey implements the Jax-RS specification, it's possible create and export a remote service that deals with both the transport and service-level concerns in a way completely standardized, and not dependent upon either the OSGi RS/RSA implementation (e.g. ECF), nor on the distribution system provider implementation (Jersey).

For example, here is some example OSGi code for exporting a remote service using the Jax-RS/Jersey provider:

BundleContext bundleContext;
Dictionary<String, String> props = new Hashtable<String,String>();
// osgi rs property to signal to distribution system 
// that this is a remote service
props.put("service.exported.interfaces","*");
// specify the distribution provider with osgi rs property
props.put("service.exported.configs", "ecf.jaxrs.jersey.server");
// as per spec, <provider>.<prop> represents a property intended for use by this provider
props.put("ecf.jaxrs.jersey.server.alias", "/jersey");
// register (and export) HelloImpl as remote service described by Hello interface
bundleContext.registerService(HelloWorldService.class, new HelloWorldResource(), props);

HelloWorldResource implements a HelloWorldService interface which is defined

import javax.ws.rs.GET;
import javax.ws.rs.Produces;
import javax.ws.rs.Path;
 
// The Java class will be hosted at the URI path "/helloworld"
@Path("/helloworld")
public interface HelloWorldService {
 
    // The Java method will process HTTP GET requests
    @GET
    // The Java method will produce content identified by the MIME Media
    // type "text/plain"
    @Produces("text/plain")
    public String getMessage();
}

Note that the HelloWorldService has exactly the same Jax-RS annotations as the HelloWorldResource implementation class.

With the Jax-RS/Jersey distribution provider, the above registerService call will dynamically export the remote service (via Jersey) so that remote clients (Java/OSGi-based or not) can access getMessage by sending a GET to an URL like this:

curl http://localhost:8080/jersey/helloworld/1

Would respond with "Hello World"

The use of Jax-RS annotations to implement an ECF distribution provider, and the use of OSGi Remote Services has several important advantages for the service implementer and the service consumer:

An ability to flexibly handle both transport-level and service-level concerns in a way appropriate to the application
A clean separation between service contract (HelloWorldService interface), and service implementation (HelloWorldResource)
A clean separation between application concerns from transport or service-level concerns
Alternative implementations of Jax-RS and/or OSGi RS/RSA may be substituted at any time without application changes

Using the Remote Service

Since the HelloWorldService is exported via Jax-RS/Jersey provider, it may be accessed by any client (e.g. javascript, java, curl, etc) that can access via http calls. If, however, it is an OSGi client (or a non-OSGi Java Client) ECF Remote Service can automatically construct a proxy for the remote service, and make the proxy available as an OSGi Service on the client. For a description and example of doing this, see Exposing a Jax REST Service as an OSGi Service.

A More Complete Example

A more complex example exists in the Jax-RS/Jersey Provider repo. The remote service host/server is in the com.mycorp.examples.student.remoteservice.host bundle. The remote service consumer/client is in com.mycorp.examples.student.client bundle. Notice that neither of these bundles has references to ECF, Jersey, or even OSGi classes, but rather only to Jax-RS standard annotation types (javax.*) and model classes defined in the com.mycorp.examples.student bundle.