我们先以最常用的Servlet接受请求为例来分析
在spring-cloud-sleuth的spring.factories文件中注入的很多类中包含了一个类：TraceWebServletAutoConfiguration，一看就知道，这是为Servlet环境量身定制的一个自动装配类
在这个类中，创建了一个Filter，这个Filter就是拦截web请求，完成Servlet请求链路的收集的利器
@Bean
@ConditionalOnMissingBean
public TracingFilter tracingFilter(HttpTracing tracing) {
return (TracingFilter) TracingFilter.create(tracing);
}
我们直接来看这个拦截器都是做了一些什么东西吧
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
HttpServletRequest httpRequest = (HttpServletRequest)request;
HttpServletResponse httpResponse = this.servlet.httpResponse(response);
TraceContext context = (TraceContext)request.getAttribute(TraceContext.class.getName());
if (context != null) {
Scope scope = this.currentTraceContext.maybeScope(context);
try {
chain.doFilter(request, response);
} finally {
scope.close();
}
} else {
Span span = this.handler.handleReceive(this.extractor, httpRequest);
request.setAttribute(SpanCustomizer.class.getName(), span.customizer());
request.setAttribute(TraceContext.class.getName(), span.context());
Throwable error = null;
Scope scope = this.currentTraceContext.newScope(span.context());
try {
chain.doFilter(httpRequest, httpResponse);
} catch (ServletException | RuntimeException | Error | IOException var19) {
error = var19;
throw var19;
} finally {
scope.close();
if (this.servlet.isAsync(httpRequest)) {
this.servlet.handleAsync(this.handler, httpRequest, span);
} else {
this.handler.handleSend(ADAPTER.adaptResponse(httpRequest, httpResponse), error, span);
}
}
}
}
Span的创建
第一步，尝试从request中获取TraceContext，TraceContext包含了本次请求的链路信息，假如这个请求是从上游系统过来的话，那么这里就会存在这个信息。
我们先重点看不存在上游系统时的分支，这个时候，第一步就应该去创建一个span。关于span和trace的概念上篇文章已经提到过了，这里就不再展开了。
public <C> Span handleReceive(TraceContext.Extractor<C> extractor, C carrier, Req request) {
Span span = nextSpan(extractor.extract(carrier), request);
span.kind(Span.Kind.SERVER);
return handleStart(request, span);
}
Span nextSpan(TraceContextOrSamplingFlags extracted, Req request) {
if (extracted.sampled() == null) { // Otherwise, try to make a new decision
extracted = extracted.sampled(sampler.trySample(adapter, request));
}
return extracted.context() != null
? tracer.joinSpan(extracted.context())
: tracer.nextSpan(extracted);
}
这个三目表达式的意思是看当前环境中是否存在span，如果存在则加入当前环境的span，否则继续进入创建span的逻辑
public Span nextSpan(TraceContextOrSamplingFlags extracted) {
TraceContext parent = extracted.context();
if (extracted.samplingFlags() != null) {
TraceContext implicitParent = currentTraceContext.get();
if (implicitParent == null) {
return toSpan(newContextBuilder(null, extracted.samplingFlags())
.extra(extracted.extra()).build());
}
// fall through, with an implicit parent, not an extracted one
parent = appendExtra(implicitParent, extracted.extra());
}
if (parent != null) {
TraceContext.Builder builder;
if (extracted.samplingFlags() != null) {
builder = newContextBuilder(parent, extracted.samplingFlags());
} else {
builder = newContextBuilder(parent, sampler);
}
return toSpan(builder.build());
}
TraceIdContext traceIdContext = extracted.traceIdContext();
if (extracted.traceIdContext() != null) {
Boolean sampled = traceIdContext.sampled();
if (sampled == null) sampled = sampler.isSampled(traceIdContext.traceId());
return toSpan(TraceContext.newBuilder()
.sampled(sampled)
.debug(traceIdContext.debug())
.traceIdHigh(traceIdContext.traceIdHigh()).traceId(traceIdContext.traceId())
.spanId(nextId())
.extra(extracted.extra()).build());
}
// TraceContextOrSamplingFlags is a union of 3 types, we've checked all three
throw new AssertionError("should not reach here");
}
首先会尝试获取trace，因为是第一次请求，所以这个时候trace也不存在所以会进入到toSpan方法
public Span toSpan(TraceContext context) {
if (context == null) throw new NullPointerException("context == null");
TraceContext decorated = propagationFactory.decorate(context);
if (!noop.get() && Boolean.TRUE.equals(decorated.sampled())) {
return RealSpan.create(decorated, recorder, errorParser);
}
return NoopSpan.create(decorated);
}
这里如果我们没有特殊指定的话会使用RealSpan来创建span，这个span的最终实现类是AutoValue_RealSpan
接着返回最开始的handleReceive方法
public <C> Span handleReceive(TraceContext.Extractor<C> extractor, C carrier, Req request) {
Span span = nextSpan(extractor.extract(carrier), request);
span.kind(Span.Kind.SERVER);
return handleStart(request, span);
}
span创建完毕后就会设置kind，这个kand代表了服务类型，这里就是设置了服务类型为服务端。
接下来就是去开启记录链路信息
Span handleStart(Req request, Span span) {
if (span.isNoop()) return span;
Scope ws = currentTraceContext.maybeScope(span.context());
try {
parser.request(adapter, request, span.customizer());
Endpoint.Builder remoteEndpoint = Endpoint.newBuilder();
if (parseRemoteEndpoint(request, remoteEndpoint)) {
span.remoteEndpoint(remoteEndpoint.build());
}
} finally {
ws.close();
}
return span.start();
}
开启过程中记录了几个信息
public <Req> void request(HttpAdapter<Req, ?> adapter, Req req, SpanCustomizer customizer) {
customizer.name(spanName(adapter, req));
String method = adapter.method(req);
if (method != null) customizer.tag("http.method", method);
String path = adapter.path(req);
if (path != null) customizer.tag("http.path", path);
}
public Span start() {
return start(clock.currentTimeMicroseconds());
}
synchronized MutableSpan start(long timestamp) {
span.timestamp(this.timestamp = timestamp);
return this;
}
接着在回到文章最开始提到的Filter方法中
在span和trace创建完成后，会把它们添加到request中
Scope的创建
然后是一个scope的创建，这个scope和日志组件说息息相关的。简单来说，它会把traceId、parentId、spanId打印到当前系统打印的每一行日志中
public Scope newScope(@Nullable TraceContext currentSpan) {
final String previousTraceId = MDC.get("traceId");
final String previousParentId = MDC.get("parentId");
final String previousSpanId = MDC.get("spanId");
final String spanExportable = MDC.get("spanExportable");
final String legacyPreviousTraceId = MDC.get(LEGACY_TRACE_ID_NAME);
final String legacyPreviousParentId = MDC.get(LEGACY_PARENT_ID_NAME);
final String legacyPreviousSpanId = MDC.get(LEGACY_SPAN_ID_NAME);
final String legacySpanExportable = MDC.get(LEGACY_EXPORTABLE_NAME);
if (currentSpan != null) {
String traceIdString = currentSpan.traceIdString();
MDC.put("traceId", traceIdString);
MDC.put(LEGACY_TRACE_ID_NAME, traceIdString);
String parentId = currentSpan.parentId() != null ?
HexCodec.toLowerHex(currentSpan.parentId()) :
null;
replace("parentId", parentId);
replace(LEGACY_PARENT_ID_NAME, parentId);
String spanId = HexCodec.toLowerHex(currentSpan.spanId());
MDC.put("spanId", spanId);
MDC.put(LEGACY_SPAN_ID_NAME, spanId);
String sampled = String.valueOf(currentSpan.sampled());
MDC.put("spanExportable", sampled);
MDC.put(LEGACY_EXPORTABLE_NAME, sampled);
log("Starting scope for span: {}", currentSpan);
if (currentSpan.parentId() != null) {
if (log.isTraceEnabled()) {
log.trace("With parent: {}", currentSpan.parentId());
}
}
}
else {
MDC.remove("traceId");
MDC.remove("parentId");
MDC.remove("spanId");
MDC.remove("spanExportable");
MDC.remove(LEGACY_TRACE_ID_NAME);
MDC.remove(LEGACY_PARENT_ID_NAME);
MDC.remove(LEGACY_SPAN_ID_NAME);
MDC.remove(LEGACY_EXPORTABLE_NAME);
}
Scope scope = this.delegate.newScope(currentSpan);
class ThreadContextCurrentTraceContextScope implements Scope {
@Override public void close() {
log("Closing scope for span: {}", currentSpan);
scope.close();
replace("traceId", previousTraceId);
replace("parentId", previousParentId);
replace("spanId", previousSpanId);
replace("spanExportable", spanExportable);
replace(LEGACY_TRACE_ID_NAME, legacyPreviousTraceId);
replace(LEGACY_PARENT_ID_NAME, legacyPreviousParentId);
replace(LEGACY_SPAN_ID_NAME, legacyPreviousSpanId);
replace(LEGACY_EXPORTABLE_NAME, legacySpanExportable);
}
}
return new ThreadContextCurrentTraceContextScope();
}
Span的上送
接下来当剩下的执行链执行完毕后，本次请求也就该结束了。在请求结束时，span就会被上送到Zipkin服务端中
public void handleSend(@Nullable Resp response, @Nullable Throwable error, Span span) {
handleFinish(response, error, span);
}
void handleFinish(@Nullable Resp response, @Nullable Throwable error, Span span) {
if (span.isNoop()) return;
try {
Scope ws = currentTraceContext.maybeScope(span.context());
try {
parser.response(adapter, response, error, span.customizer());
} finally {
ws.close(); // close the scope before finishing the span
}
} finally {
finishInNullScope(span);
}
}
首先在span中记录本次调用的相应信息
public <Resp> void response(HttpAdapter<?, Resp> adapter, @Nullable Resp res,
@Nullable Throwable error, SpanCustomizer customizer) {
int statusCode = 0;
if (res != null) {
statusCode = adapter.statusCodeAsInt(res);
String nameFromRoute = spanNameFromRoute(adapter, res, statusCode);
if (nameFromRoute != null) customizer.name(nameFromRoute);
String maybeStatus = maybeStatusAsString(statusCode, 299);
if (maybeStatus != null) customizer.tag("http.status_code", maybeStatus);
}
error(statusCode, error, customizer);
}
接着清空Scope
void finishInNullScope(Span span) {
Scope ws = currentTraceContext.maybeScope(null);
try {
span.finish();
} finally {
ws.close();
}
}
之后说span的上传
public void finish(TraceContext context) {
MutableSpan span = spanMap.remove(context);
if (span == null || noop.get()) return;
synchronized (span) {
span.finish(span.clock.currentTimeMicroseconds());
reporter.report(span.toSpan());
}
}
具体上传的实现是由Sender接口的实现类实现的，它的实现类默认情况下是这三个
屏幕快照 2019-11-18 下午10.31.01
而一个span内容则是这样的
屏幕快照 2019-11-13 下午9.45.27
RabbitMQ链路追踪
当看完SpringMVC链路追踪的实现方式之后，再去看其他的方式，我想肯定是非常简单的。这里我们以RabbitMQ为例：
首先查找spring-cloud-sleuth的spring.factories文件，看到关于消息中间件的追踪配置类是这个TraceMessagingAutoConfiguration
看这个类关于RabbitMQ的东西
@Configuration
@ConditionalOnProperty(value = "spring.sleuth.messaging.rabbit.enabled", matchIfMissing = true)
@ConditionalOnClass(RabbitTemplate.class)
protected static class SleuthRabbitConfiguration {
@Bean
@ConditionalOnMissingBean
SpringRabbitTracing springRabbitTracing(Tracing tracing,
SleuthMessagingProperties properties) {
return SpringRabbitTracing.newBuilder(tracing)
.remoteServiceName(properties.getMessaging().getRabbit().getRemoteServiceName())
.build();
}
@Bean
@ConditionalOnMissingBean
static SleuthRabbitBeanPostProcessor sleuthRabbitBeanPostProcessor(BeanFactory beanFactory) {
return new SleuthRabbitBeanPostProcessor(beanFactory);
}
}
这里其实大致就可以猜测出来了，肯定是使用了SleuthRabbitBeanPostProcessor在构造RabbitTemplate的使用做了一些改造，比如说加个拦截器啥的，然后当使用RabbitTemplate发送消息时自动添加Header等东西就完成了整个流程了