Protocol Extension

Summary

Extension to RPC protocol, hide details of remote call.

Contract:

  • When user calls invoke() method of Invoker object which's returned from refer() call, the protocol needs to correspondingly execute invoke() method of Invoker object passed from remote export() method associated with the same URL.
  • Moreover, it's protocol's responsibility to implement Invoker which's returned from refer(). Generally speaking, protocol sends remote request in the Invoker implementation, but needs not to care about the Invoker passed into export() since the framework will implement the logic and pass in the instance.

Notes:

  • Protocol does not need to care about the proxy of the business interface. The upper layer of the framework will convert Invoker into business interface.
  • It is not a requirement that the protocol must use TCP for network communication. It could be file-sharing, IPC, or others.

Extension Interface

  • com.alibaba.dubbo.rpc.Protocol
  • com.alibaba.dubbo.rpc.Exporter
  • com.alibaba.dubbo.rpc.Invoker
public interface Protocol {
    /**
     * Export remote service: <br>
     * 1. Should save address info for the request when the protocol receives it: RpcContext.getContext().setRemoteAddress();<br>
     * 2. export() must be implemented as idempotent, that is, it should not introduce side effect when the implementation gets called with the same Invoker for more than once.
     * 3. Invoker is passed by the framework, and the protocol should not care about it. <br>
     * 
     * @param <T> Service type
     * @param invoker Service invoker
     * @return exporter The reference of service exporter, used for cancelling service export.
     * @throws RpcException throw when there's any error during service export, e.g. the port is occupied
     */
    <T> Exporter<T> export(Invoker<T> invoker) throws RpcException;

    /**
     * Reference remote service: <br>
     * 1. When user calls `invoke()` method of `Invoker` object which's returned from `refer()` call, the protocol needs to correspondingly execute `invoke()` method of `Invoker` object passed from remote `export()` method associated with the same URL. <br>
     * 2. It's protocol's responsibility to implement `Invoker` which's returned from `refer()`. Generally speaking, protocol sends remote request in the `Invoker` implementation. <br>
     * 3. When there's check=false set in URL, the implementation must not throw exception but try to recover when connection fails.
     * 
     * @param <T> Service type
     * @param type Service type
     * @param url URL address for the remote service
     * @return invoker service's local proxy
     * @throws RpcException throw when there's any error while connecting to the service provider
     */
    <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException;

}

Extension Configuration

<!-- declare protocol, if id is not set, then use the value of name for id -->
<dubbo:protocol id="xxx1" name="xxx" />
<!-- reference protocol, if protocol's attribute is not set, then protocol configuration will be scanned automatically from ApplicationContext -->
<dubbo:service protocol="xxx1" />
<!-- default value for referenced protocol, it will be used if protocol attribute is not configured in <dubbo:service> --> 
<dubbo:provider protocol="xxx1" />

Existing Protocol

  • com.alibaba.dubbo.rpc.injvm.InjvmProtocol
  • com.alibaba.dubbo.rpc.dubbo.DubboProtocol
  • com.alibaba.dubbo.rpc.rmi.RmiProtocol
  • com.alibaba.dubbo.rpc.http.HttpProtocol
  • com.alibaba.dubbo.rpc.http.hessian.HessianProtocol

Extension Guide

Directory layout:

src
 |-main
    |-java
        |-com
            |-xxx
                |-XxxProtocol.java (Protocol implementation)
                |-XxxExporter.java (Exporter implementation)
                |-XxxInvoker.java (Invoker implementation)
    |-resources
        |-META-INF
            |-dubbo
                |-com.alibaba.dubbo.rpc.Protocol (plain text file with the content: xxx=com.xxx.XxxProtocol)

XxxProtocol.java:

package com.xxx;

import com.alibaba.dubbo.rpc.Protocol;

public class XxxProtocol implements Protocol {
    public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
        return new XxxExporter(invoker);
    }
    public <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException {
        return new XxxInvoker(type, url);
    }
}

XxxExporter.java:

package com.xxx;

import com.alibaba.dubbo.rpc.support.AbstractExporter;

public class XxxExporter<T> extends AbstractExporter<T> {
    public XxxExporter(Invoker<T> invoker) throws RemotingException{
        super(invoker);
        // ...
    }
    public void unexport() {
        super.unexport();
        // ...
    }
}

XxxInvoker.java:

package com.xxx;

import com.alibaba.dubbo.rpc.support.AbstractInvoker;

public class XxxInvoker<T> extends AbstractInvoker<T> {
    public XxxInvoker(Class<T> type, URL url) throws RemotingException{
        super(type, url);
    }
    protected abstract Object doInvoke(Invocation invocation) throws Throwable {
        // ...
    }
}

META-INF/dubbo/com.alibaba.dubbo.rpc.Protocol:

xxx=com.xxx.XxxProtocol

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