本文出处shenyifengtk.github.io 转载请说明

概念

Span

Span 是分布式跟踪系统中一个重要且常用的概念. 可从 Google Dapper Paper 和 OpenTracing 学习更多与 Span 相关的知识.

SkyWalking 从 2017 年开始支持 OpenTracing 和 OpenTracing-Java API, 我们的 Span 概念与论文和 OpenTracing 类似. 我们也扩展了 Span.

Span 有三种类型

1.1 EntrySpan

EntrySpan 代表服务提供者, 也是服务器端的端点. 作为一个 APM 系统, 我们的目标是应用服务器. 所以几乎所有的服务和 MQ-消费者 都是 EntrySpan。可以理解一个进程处理第一个span就是EntrySpan，意思为entiry span 进入服务span。

1.2 LocalSpan

LocalSpan 表示普通的 Java 方法, 它与远程服务无关, 也不是 MQ 生产者/消费者, 也不是服务（例如 HTTP 服务）提供者/消费者。所有本地方法调用都是localSpan,包括异步线程调用，线程池提交任务都是。

1.3 ExitSpan

ExitSpan 代表一个服务客户端或MQ的生产者, 在 SkyWalking 的早期命名为 LeafSpan. 例如 通过 JDBC 访问DB, 读取 Redis/Memcached 被归类为 ExitSpan.

image.png

上下文载体 (ContextCarrier)

为了实现分布式跟踪, 需要绑定跨进程的追踪, 并且上下文应该在整个过程中随之传播. 这就是 ContextCarrier 的职责.

以下是有关如何在 A -> B 分布式调用中使用 ContextCarrier 的步骤.

1. 在客户端, 创建一个新的空的 ContextCarrier.
2. 通过 ContextManager#createExitSpan 创建一个 ExitSpan 或者使用 ContextManager#inject 来初始化 ContextCarrier.
3. 将 ContextCarrier 所有信息放到请求头 (如 HTTP HEAD), 附件(如 Dubbo RPC 框架), 或者消息 (如 Kafka) 中,详情可以看官方给出跨进程传输协议sw8
4. 通过服务调用, 将 ContextCarrier 传递到服务端.
5. 在服务端, 在对应组件的头部, 附件或消息中获取 ContextCarrier 所有内容.
6. 通过 ContestManager#createEntrySpan 创建 EntrySpan 或者使用 ContextManager#extract 将服务端和客户端的绑定.

让我们通过 Apache HttpComponent client 插件和 Tomcat 7 服务器插件演示, 步骤如下:

1. 客户端 Apache HttpComponent client 插件

span = ContextManager.createExitSpan("/span/operation/name", contextCarrier, "ip:port");
CarrierItem next = contextCarrier.items();
while (next.hasNext()) {
    next = next.next();
    httpRequest.setHeader(next.getHeadKey(), next.getHeadValue());
}

2. 服务端 Tomcat 7 服务器插件

ContextCarrier contextCarrier = new ContextCarrier();
CarrierItem next = contextCarrier.items();
while (next.hasNext()) {
    next = next.next();
    next.setHeadValue(request.getHeader(next.getHeadKey()));
}

span = ContextManager.createEntrySpan("/span/operation/name", contextCarrier);

上下文快照 (ContextSnapshot)

除了跨进程, 跨线程也是需要支持的, 例如异步线程（内存中的消息队列）和批处理在 Java 中很常见, 跨进程和跨线程十分相似, 因为都是需要传播上下文. 唯一的区别是, 不需要跨线程序列化.

以下是有关跨线程传播的三个步骤：

1. 使用 ContextManager#capture 方法获取 ContextSnapshot 对象.
2. 让子线程以任何方式, 通过方法参数或由现有参数携带来访问 ContextSnapshot
3. 在子线程中使用 ContextManager#continued。

跨进程Span传输原理

public class CarrierItem implements Iterator<CarrierItem> {
    private String headKey;
    private String headValue;
    private CarrierItem next;

    public CarrierItem(String headKey, String headValue) {
        this(headKey, headValue, null);
    }

    public CarrierItem(String headKey, String headValue, CarrierItem next) {
        this.headKey = headKey;
        this.headValue = headValue;
        this.next = next;
    }

    public String getHeadKey() {
        return headKey;
    }

    public String getHeadValue() {
        return headValue;
    }

    public void setHeadValue(String headValue) {
        this.headValue = headValue;
    }

    @Override
    public boolean hasNext() {
        return next != null;
    }

    @Override
    public CarrierItem next() {
        return next;
    }

    @Override
    public void remove() {

    }
}

CarrierItem 类似Map key value的数据接口，通过一个单向连接将K/V连接起来。
看下 ContextCarrier.items()方法如何创建CarrierItem

    public CarrierItem items() {
       //内置一个 sw8-x key
        SW8ExtensionCarrierItem sw8ExtensionCarrierItem = new SW8ExtensionCarrierItem(extensionContext, null); 
       //内置  sw8-correlation key
        SW8CorrelationCarrierItem sw8CorrelationCarrierItem = new SW8CorrelationCarrierItem(
            correlationContext, sw8ExtensionCarrierItem);
       //内置 sw8 key 
        SW8CarrierItem sw8CarrierItem = new SW8CarrierItem(this, sw8CorrelationCarrierItem);
        return new CarrierItemHead(sw8CarrierItem);
    }

创建一个链接CarrierItemHead->SW8CarrierItem ->SW8CorrelationCarrierItem->SW8ExtensionCarrierItem
在看下上面tomcat7 遍历CarrierItem，调用key从http header获取值设置到对象内置值，这样就可以做到将上一个进程header 值设置到下一个进程里，在调用

    ContextCarrier deserialize(String text, HeaderVersion version) {
        if (text == null) {
            return this;
        }
        if (HeaderVersion.v3.equals(version)) {
            String[] parts = text.split("-", 8);
            if (parts.length == 8) {
                try {
                    // parts[0] is sample flag, always trace if header exists.
                    this.traceId = Base64.decode2UTFString(parts[1]);
                    this.traceSegmentId = Base64.decode2UTFString(parts[2]);
                    this.spanId = Integer.parseInt(parts[3]);
                    this.parentService = Base64.decode2UTFString(parts[4]);
                    this.parentServiceInstance = Base64.decode2UTFString(parts[5]);
                    this.parentEndpoint = Base64.decode2UTFString(parts[6]);
                    this.addressUsedAtClient = Base64.decode2UTFString(parts[7]);
                } catch (IllegalArgumentException ignored) {

                }
            }
        }
        return this;
    }

这样刚刚new 出来ContextCarrier就可以从上一个调用者上继承所有的属性，新创建span就可以跟上一个span 关联起来了了。

开发插件

知识点

追踪的基本方法是拦截 Java 方法, 使用字节码操作技术（byte-buddy）和 AOP 概念. SkyWalking 包装了字节码操作技术并追踪上下文的传播, 所以你只需要定义拦截点（换句话说就是 Spring 的切面）。

ClassInstanceMethodsEnhancePluginDefine定义了构造方法 Contructor 拦截点和 instance method 实例方法拦截点，主要有三个方法需要被重写

     /**
     * 需要被拦截Class
     * @return
     */
    @Override
    protected ClassMatch enhanceClass() {
        return null;
    }

    /**
     * 构造器切点
     * @return
     */
    @Override
    public ConstructorInterceptPoint[] getConstructorsInterceptPoints() {
        return new ConstructorInterceptPoint[0];
    }

    /**
     * 方法切点
     * @return InstanceMethodsInterceptPoint 里面会声明拦截按个方法
     */
    @Override
    public InstanceMethodsInterceptPoint[] getInstanceMethodsInterceptPoints() {
        return new InstanceMethodsInterceptPoint[0];
    }

ClassMatch 以下有四种方法表示如何去匹配目标类:

• NameMatch.byName, 通过类的全限定名(Fully Qualified Class Name, 即 包名 + . + 类名).
• ClassAnnotationMatch.byClassAnnotationMatch, 根据目标类是否存在某些注解.
• MethodAnnotationMatchbyMethodAnnotationMatch, 根据目标类的方法是否存在某些注解.
• HierarchyMatch.byHierarchyMatch, 根据目标类的父类或接口

ClassStaticMethodsEnhancePluginDefine 定义了类方法 class 静态method 拦截点。

public abstract class ClassStaticMethodsEnhancePluginDefine extends ClassEnhancePluginDefine {

    /**
     * 构造器切点
     * @return null, means enhance no constructors.
     */
    @Override
    public ConstructorInterceptPoint[] getConstructorsInterceptPoints() {
        return null;
    }

    /**
     * 方法切点
     * @return null, means enhance no instance methods.
     */
    @Override
    public InstanceMethodsInterceptPoint[] getInstanceMethodsInterceptPoints() {
        return null;
    }
}

InstanceMethodsInterceptPoint 普通方法接口切点有哪些方法

public interface InstanceMethodsInterceptPoint {
    /**
     * class instance methods matcher.
     *  可以理解成功对Class 那些方法进行增强
     *  ElementMatcher 是bytebuddy 类库一个方法匹配器，里面封装了各种方法匹配
     * @return methods matcher
     */
    ElementMatcher<MethodDescription> getMethodsMatcher();

    /**
     * @return represents a class name, the class instance must instanceof InstanceMethodsAroundInterceptor.
     *  返回一个拦截器全类名，所有拦截器必须实现    InstanceMethodsAroundInterceptor 接口
     */
    String getMethodsInterceptor();

    /**
     *  是否要覆盖原方法入参
     * @return
     */
    boolean isOverrideArgs();
}

在看下拦截器有那些方法

/**
 * A interceptor, which intercept method's invocation. The target methods will be defined in {@link
 * ClassEnhancePluginDefine}'s subclass, most likely in {@link ClassInstanceMethodsEnhancePluginDefine}
 */
public interface InstanceMethodsAroundInterceptor {
    /**
     * called before target method invocation.
     * 前置通知
     * @param result change this result, if you want to truncate the method.
     */
    void beforeMethod(EnhancedInstance objInst, Method method, Object[] allArguments, Class<?>[] argumentsTypes,
        MethodInterceptResult result) throws Throwable;

    /**
     * called after target method invocation. Even method's invocation triggers an exception.
     * 后置通知
     * @param ret the method's original return value. May be null if the method triggers an exception.
     * @return the method's actual return value.
     */
    Object afterMethod(EnhancedInstance objInst, Method method, Object[] allArguments, Class<?>[] argumentsTypes,
        Object ret) throws Throwable;

    /**
     * called when occur exception.
     * 异常通知
     * @param t the exception occur.
     */
    void handleMethodException(EnhancedInstance objInst, Method method, Object[] allArguments,
        Class<?>[] argumentsTypes, Throwable t);
}

开发Skywalking实战

项目maven环境配置

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>tk.shenyifeng</groupId>
    <artifactId>skywalking-plugin</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <skywalking.version>8.10.0</skywalking.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.apache.skywalking</groupId>
            <artifactId>apm-agent-core</artifactId>
            <version>${skywalking.version}</version>
            <scope>provided</scope>
        </dependency>

        <dependency>
            <groupId>org.apache.skywalking</groupId>
            <artifactId>java-agent-util</artifactId>
            <version>${skywalking.version}</version>
            <scope>provided</scope>
        </dependency>
    </dependencies>


    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>8</source>
                    <target>8</target>
                </configuration>
            </plugin>

            <plugin>
                <artifactId>maven-shade-plugin</artifactId>
                <version>3.2.4</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <shadedArtifactAttached>false</shadedArtifactAttached>
                            <createDependencyReducedPom>true</createDependencyReducedPom>
                            <createSourcesJar>true</createSourcesJar>
                            <shadeSourcesContent>true</shadeSourcesContent>
                            <relocations>
                                <relocation>
                                    <pattern>net.bytebuddy</pattern>
                                    <shadedPattern>org.apache.skywalking.apm.dependencies.net.bytebuddy</shadedPattern>
                                </relocation>
                            </relocations>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

为了更有代表性一些，使用Skywalking官方开发的ES插件来做一个例子。为了兼容不同版本框架，Skywalking 官方使用witnessClasses，当前框架Jar存在这个Class就会任务是某个版本、同样witnessMethods当Class存在某个Method。

public class AdapterActionFutureInstrumentation extends ClassEnhancePluginDefine {

    @Override
    public ConstructorInterceptPoint[] getConstructorsInterceptPoints() {
        return new ConstructorInterceptPoint[0];
    }

    @Override
    public InstanceMethodsInterceptPoint[] getInstanceMethodsInterceptPoints() {
        return new InstanceMethodsInterceptPoint[] {
            new InstanceMethodsInterceptPoint() {
                @Override
                public ElementMatcher<MethodDescription> getMethodsMatcher() {
                    return named("actionGet"); //拦截方法
                }

                @Override
                public String getMethodsInterceptor() {  //拦截器全类名
                    return "org.apache.skywalking.apm.plugin.elasticsearch.v7.interceptor.AdapterActionFutureActionGetMethodsInterceptor";
                }

                @Override
                public boolean isOverrideArgs() {
                    return false;
                }
            }
        };
    }

    @Override
    public StaticMethodsInterceptPoint[] getStaticMethodsInterceptPoints() {
        return new StaticMethodsInterceptPoint[0];
    }

    @Override
    protected ClassMatch enhanceClass() { //增强Class
        return byName("org.elasticsearch.action.support.AdapterActionFuture");
    }

    @Override
    protected String[] witnessClasses() {//ES7 存在Class
        return new String[] {"org.elasticsearch.transport.TaskTransportChannel"};
    }

    @Override
    protected List<WitnessMethod> witnessMethods() { //ES7 SearchHits 存在方法
        return Collections.singletonList(new WitnessMethod(
            "org.elasticsearch.search.SearchHits",
          named("getTotalHits").and(takesArguments(0)).and(returns(named("org.apache.lucene.search.TotalHits")))
        ));
    }
}

创建一个给定类名的拦截器，实现InstanceMethodsAroundInterceptor接口。创建一个EntrySpan

public class TomcatInvokeInterceptor implements InstanceMethodsAroundInterceptor {

    private static boolean IS_SERVLET_GET_STATUS_METHOD_EXIST;
    private static final String SERVLET_RESPONSE_CLASS = "javax.servlet.http.HttpServletResponse";
    private static final String GET_STATUS_METHOD = "getStatus";

    static {
        IS_SERVLET_GET_STATUS_METHOD_EXIST = MethodUtil.isMethodExist(
            TomcatInvokeInterceptor.class.getClassLoader(), SERVLET_RESPONSE_CLASS, GET_STATUS_METHOD);
    }

    /**
     * * The {@link TraceSegment#ref} of current trace segment will reference to the trace segment id of the previous
     * level if the serialized context is not null.
     *
     * @param result change this result, if you want to truncate the method.
     */
    @Override
    public void beforeMethod(EnhancedInstance objInst, Method method, Object[] allArguments, Class<?>[] argumentsTypes,
                             MethodInterceptResult result) throws Throwable {
        Request request = (Request) allArguments[0];
        ContextCarrier contextCarrier = new ContextCarrier();

        CarrierItem next = contextCarrier.items();
       //如果 HTTP 请求头中有符合sw8 传输协议的请求头则 取出来设置到上下文ContextCarrier
        while (next.hasNext()) {
            next = next.next();
            next.setHeadValue(request.getHeader(next.getHeadKey()));
        }
        String operationName =  String.join(":", request.getMethod(), request.getRequestURI());
        AbstractSpan span = ContextManager.createEntrySpan(operationName, contextCarrier);//关联起来
        Tags.URL.set(span, request.getRequestURL().toString()); //添加 span 参数
        Tags.HTTP.METHOD.set(span, request.getMethod());
        span.setComponent(ComponentsDefine.TOMCAT);
        SpanLayer.asHttp(span);

        if (TomcatPluginConfig.Plugin.Tomcat.COLLECT_HTTP_PARAMS) {
            collectHttpParam(request, span);
        }
    }

    @Override
    public Object afterMethod(EnhancedInstance objInst, Method method, Object[] allArguments, Class<?>[] argumentsTypes,
                              Object ret) throws Throwable {
        Request request = (Request) allArguments[0];
        HttpServletResponse response = (HttpServletResponse) allArguments[1];

        AbstractSpan span = ContextManager.activeSpan();
        if (IS_SERVLET_GET_STATUS_METHOD_EXIST && response.getStatus() >= 400) {
            span.errorOccurred();
            Tags.HTTP_RESPONSE_STATUS_CODE.set(span, response.getStatus());
        }
        // Active HTTP parameter collection automatically in the profiling context.
        if (!TomcatPluginConfig.Plugin.Tomcat.COLLECT_HTTP_PARAMS && span.isProfiling()) {
            collectHttpParam(request, span);
        }
        ContextManager.getRuntimeContext().remove(Constants.FORWARD_REQUEST_FLAG);
        ContextManager.stopSpan();
        return ret;
    }

    @Override
    public void handleMethodException(EnhancedInstance objInst, Method method, Object[] allArguments,
                                      Class<?>[] argumentsTypes, Throwable t) {
        AbstractSpan span = ContextManager.activeSpan();
        span.log(t);
    }

    private void collectHttpParam(Request request, AbstractSpan span) {
        final Map<String, String[]> parameterMap = new HashMap<>();
        final org.apache.coyote.Request coyoteRequest = request.getCoyoteRequest();
        final Parameters parameters = coyoteRequest.getParameters();
        for (final Enumeration<String> names = parameters.getParameterNames(); names.hasMoreElements(); ) {
            final String name = names.nextElement();
            parameterMap.put(name, parameters.getParameterValues(name));
        }

        if (!parameterMap.isEmpty()) {
            String tagValue = CollectionUtil.toString(parameterMap);
            tagValue = TomcatPluginConfig.Plugin.Http.HTTP_PARAMS_LENGTH_THRESHOLD > 0 ?
                StringUtil.cut(tagValue, TomcatPluginConfig.Plugin.Http.HTTP_PARAMS_LENGTH_THRESHOLD) :
                tagValue;
            Tags.HTTP.PARAMS.set(span, tagValue);
        }
    }
}

开发完成拦截器后，一定要在类路径上添加skywalking-plugin.def文件，将开发后的全类名添加到配置。

xxxName = tk.shenyifeng.skywalking.plugin.RepladInstrumentation

如果jar 里面没有这个文件，插件不会被Skywalking加载的。
最后将打包的jar 放到Skywalking的plugin或者activations目录就可以了。

xml配置插件

<?xml version="1.0" encoding="UTF-8"?>
<enhanced>
    <class class_name="test.apache.skywalking.apm.testcase.customize.service.TestService1">
        <method method="staticMethod()" operation_name="/is_static_method" static="true"></method>
        <method method="staticMethod(java.lang.String,int.class,java.util.Map,java.util.List,[Ljava.lang.Object;)"
                operation_name="/is_static_method_args" static="true">
            <operation_name_suffix>arg[0]</operation_name_suffix>
            <operation_name_suffix>arg[1]</operation_name_suffix>
            <operation_name_suffix>arg[3].[0]</operation_name_suffix>
            <tag key="tag_1">arg[2].['k1']</tag>
            <tag key="tag_2">arg[4].[1]</tag>
            <log key="log_1">arg[4].[2]</log>
        </method>
        <method method="method()" static="false"></method>
        <method method="method(java.lang.String,int.class)" operation_name="/method_2" static="false">
            <operation_name_suffix>arg[0]</operation_name_suffix>
            <tag key="tag_1">arg[0]</tag>
            <log key="log_1">arg[1]</log>
        </method>
        <method
            method="method(test.apache.skywalking.apm.testcase.customize.model.Model0,java.lang.String,int.class)"
            operation_name="/method_3" static="false">
            <operation_name_suffix>arg[0].id</operation_name_suffix>
            <operation_name_suffix>arg[0].model1.name</operation_name_suffix>
            <operation_name_suffix>arg[0].model1.getId()</operation_name_suffix>
            <tag key="tag_os">arg[0].os.[1]</tag>
            <log key="log_map">arg[0].getM().['k1']</log>
        </method>
        <method method="retString(java.lang.String)" operation_name="/retString" static="false">
            <tag key="tag_ret">returnedObj</tag>
            <log key="log_map">returnedObj</log>
        </method>
        <method method="retModel0(test.apache.skywalking.apm.testcase.customize.model.Model0)"
          operation_name="/retModel0" static="false">
            <tag key="tag_ret">returnedObj.model1.id</tag>
            <log key="log_map">returnedObj.model1.getId()</log>
        </method>
    </class>
    
</enhanced>

通过xml配置可以省去编写Java代码，打包jar步骤。
xml规则

在配置文件agent.config中添加配置：

plugin.customize.enhance_file=customize_enhance.xml的绝对路径

引用资料
https://www.itmuch.com/skywalking/apm-customize-enhance-plugin/
https://skyapm.github.io/document-cn-translation-of-skywalking/zh/6.1.0/guides/Java-Plugin-Development-Guide.html