Dubbo2.6.3中的RpcContext和ContextFilter

本文介绍了Dubbo2.6.3中的RpcContext和ContextFilter，以及在使用调用链的跟踪工具时出现的问题。通过代码分析，发现Dubbo的attachment是通过ThreadLocal来传递信息，因此在节点丢失时会出现错误的调用关系。同时，本文也提供了相关代码。

缘起

起因，我们在使用调用链的跟踪工具的时候发现这么一个情况。

有下面一个三个模块的系统，A调用B，B调用C，调用都是Dubbo接口调用。

graph LR
  A --> B  --> C

通常来说现在大部分的调用链工具都是通过字节码增强，通过ThreadLocal来保存Trace的信息，然后通过调用协议的一些特性在各个节点之间传递。

比如在dubbo中，trace信息就是通过attachment来传递。

单在我们实施的时候出来一点意外，忘记给B模块没有安装agent。

就出现下面的一个错误的调用关系。

graph LR
  A --> C

根据往常的经验中，假如在系统中丢了节点，就会导致调用链断开，而不是错误的连接起来。有一份错误的数据带来的危害可能比没有数据更糟糕。

原因

直到看了dubbo的代码才发现问题。在dubbo的设计中attachment就是通过ThreadLocal来传递信息，于是就把Trace的也顺带的带上了。至于后续如何识别出来还需要另外想办法。

代码不多，全部贴上。

/*
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 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
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package com.alibaba.dubbo.rpc.filter;

import com.alibaba.dubbo.common.Constants;
import com.alibaba.dubbo.common.extension.Activate;
import com.alibaba.dubbo.rpc.Filter;
import com.alibaba.dubbo.rpc.Invocation;
import com.alibaba.dubbo.rpc.Invoker;
import com.alibaba.dubbo.rpc.Result;
import com.alibaba.dubbo.rpc.RpcContext;
import com.alibaba.dubbo.rpc.RpcException;
import com.alibaba.dubbo.rpc.RpcInvocation;
import com.alibaba.dubbo.rpc.RpcResult;

import java.util.HashMap;
import java.util.Map;

/**
 * ContextInvokerFilter
 */
@Activate(group = Constants.PROVIDER, order = -10000)
public class ContextFilter implements Filter {

    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        Map<String, String> attachments = invocation.getAttachments();
        if (attachments != null) {
            attachments = new HashMap<String, String>(attachments);
            attachments.remove(Constants.PATH_KEY);
            attachments.remove(Constants.GROUP_KEY);
            attachments.remove(Constants.VERSION_KEY);
            attachments.remove(Constants.DUBBO_VERSION_KEY);
            attachments.remove(Constants.TOKEN_KEY);
            attachments.remove(Constants.TIMEOUT_KEY);
            attachments.remove(Constants.ASYNC_KEY);// Remove async property to avoid being passed to the following invoke chain.
        }
        RpcContext.getContext()
                .setInvoker(invoker)
                .setInvocation(invocation)
//                .setAttachments(attachments)  // merged from dubbox
                .setLocalAddress(invoker.getUrl().getHost(),
                        invoker.getUrl().getPort());

        // mreged from dubbox
        // we may already added some attachments into RpcContext before this filter (e.g. in rest protocol)
        if (attachments != null) {
            if (RpcContext.getContext().getAttachments() != null) {
                RpcContext.getContext().getAttachments().putAll(attachments);
            } else {
                RpcContext.getContext().setAttachments(attachments);
            }
        }

        if (invocation instanceof RpcInvocation) {
            ((RpcInvocation) invocation).setInvoker(invoker);
        }
        try {
            RpcResult result = (RpcResult) invoker.invoke(invocation);
            // pass attachments to result
            result.addAttachments(RpcContext.getServerContext().getAttachments());
            return result;
        } finally {
            RpcContext.removeContext();
            RpcContext.getServerContext().clearAttachments();
        }
    }
}