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Tutorial: Python for OSGi Services


Introduction

OSGi Services are an excellent way to implement small, dynamic, component-based systems. With features as service versioning, support for real dynamics, service injection (e.g. the service component runtime aka declarative services), service dependency-handling, a flexible broker, and a clear separation between service contract and implementation make it a valuable technology particularly for the Internet of Things.

Traditionally, however, OSGi services have been both declared and implemented in Java. With the OSGi Remote Services specification, and ECF's implementation of that specification, it's now possible to create OSGi services implemented in Python and used/consumed in Java. This tutorial shows a simple example of creating an OSGi service implemented in Python, that uses Google Protocol Buffers for performant cross-language serialization, and that automatically inherits all of the aspects of an OSGi service (dynamics, service injection, versioning, etc.).

Declaring a Service

Here is a simple example Java interface declaring a single 'sayHello' method:

public interface IHello {

	HelloMsgContent sayHello(HelloMsgContent message) throws Exception;
}

This interface is declared here in this project.

The HelloMsgContent type was generated by the Protocol Buffers via a message declaration

syntax = "proto3";

package org.eclipse.ecf.examples.protobuf.hello;

message HelloMsgContent {
	string h = 1;
	string f = 2;
	string to = 3;
	string hellomsg = 4;
	repeated double x = 5;
}

This message declaration, when run through the protoc compiler, generates the Java HelloMsgContent source code, and also generates a Python version of the same class. The hellomsg.proto file is here.

With the IHello interface and the HelloMsgContent class, the service declaration is complete.

Python Implementation of IHello Service

To implement this IHello service in Python it's necessary to provide a Python implementation class. Here's an example:

from osgiservicebridge.bridge import Py4jServiceBridge, _wait_for_sec

from osgiservicebridge.protobuf import protobuf_remote_service, protobuf_remote_service_method
from hellomsg_pb2 import HelloMsgContent

@protobuf_remote_service(objectClass=['org.eclipse.ecf.examples.protobuf.hello.IHello'])
class HelloServiceImpl:
    
    @protobuf_remote_service_method(arg_type=HelloMsgContent,return_type=HelloMsgContent)
    def sayHello(self,pbarg):
        print("sayHello called with arg="+str(pbarg))
        resmsg = HelloMsgContent()
        resmsg.h = 'Another response from Python'
        resmsg.f = 'frompython'
        resmsg.to = 'tojava'
        resmsg.hellomsg = 'Greetings from Python!!'
        for x in range(0,5):
            resmsg.x.append(float(x))
        return resmsg
    
    
if __name__ == '__main__':
    bridge = Py4jServiceBridge()
    print("bridge created")
    bridge.connect()
    print("bridge connected")
    hsid = bridge.export(HelloServiceImpl())
    print("exported")
    _wait_for_sec(5)
    bridge.unexport(hsid)
    print("unexported")
    bridge.disconnect()
    print("disconnected...exiting")

Note a couple of things about the above Python code:

  1. A HelloServiceImpl class that implements the sayHello method
  2. The HelloServiceImpl class has a decorator:
@protobuf_remote_service(objectClass=['org.eclipse.ecf.examples.protobuf.hello.IHello'])
class HelloServiceImpl:

associates the HelloServiceImpl Python class with the IHello Java service interface.

  1. The sayHello method also has a decorator:
    @protobuf_remote_service_method(arg_type=HelloMsgContent,return_type=HelloMsgContent)
    def sayHello(self,pbarg):

indicates the types (Python class) of the pbarg type and the sayHello return type. In this example they are both HelloMsgContent, but they may be of any type as long as it is a protocol buffers Message type.

  1. The implementation of __main__ method creates and then connects a Py4jServiceBridge instance. This

Py4jServiceBridge is responsible for exporting the service from Python to Java.

The complete Python code is available here in the org.eclipse.ecf.examples.protobuf.hello project.

The declaration and implementation complete this service, and so now it may be consumed as an OSGi service.

Example Hello Consumer

The example service consumer is available in this project. This consumer uses has the IHello service instance dynamically injected by Declarative Services and then it uses the service via this code (in activate method):

			HelloMsgContent.Builder b1 = HelloMsgContent.newBuilder();
			b1.addX(1.1);
			b1.addX(1.2);
			b1.setF("fromjava");
			b1.setTo("topython");
			b1.setHellomsg("Hello message from java!");
			b1.setH("An additional message from java");
			HelloMsgContent request = b1.build();
			HelloMsgContent result = this.helloService.sayHello(request);
			System.out.println("Received result="+result);

The second to last line makes the call from Java to Python via the IHello service proxy.

The Java Gateway

The only other piece required is a bundle to start the Java-side gateway, so that when the Python code is run it can connect to the Java gateway. The Java Gateway is created and configured via this component in this project.

Running the Hello Example

First, start the Java side (with the org.eclipse.ecf.examples.protobuf.hello bundle, org.eclipse.ecf.examples.protobuf.hello.consumer, and org.eclipse.ecf.examples.protobuf.hello.provider bundles). If run within Eclipse you may use the protobufhello.javaconsumer.launch config (note that the target platform must be set first to this target: releng/org.eclipse.ecf.provider.py4j.releng.target/ecf-oxygen.target).

Then the run.py should be started from Python. Prior to starting the following Python libraries must be installed: protobuf version 3.2.0, py4j version 0.10.6, and the osgiservicebridge package located in this project.

Once the run.py is started and the Py4jServiceBridge is connected to the JavaGateway, the IHello proxy will be created, injected into the HelloConsumer class, and then the sayHello call will be made to Python->HelloServiceImpl->sayHello.

This IHello service is just an example. Any other service could be similarly declared, implemented in Python, and injected and consumed in Java.

Background and Related Articles

Tutorial:_Creating_Custom_Distribution_Providers

Getting Started with ECF's OSGi Remote Services Implementation

OSGi Remote Services and ECF

Asynchronous Proxies for Remote Services

Static File-based Discovery of Remote Service Endpoints

Download ECF Remote Services/RSA Implementation

How to Add Remote Services/RSA to Your Target Platform