使用 opencensus 跟踪失败的 FastAPI 请求

Question

我使用 opencensus-python 跟踪对生产中运行的 python FastAPI 应用程序的请求，并使用 opencensus 导出器将信息导出到 Azure AppInsights。我遵循了 Azure Monitor 文档，并得到了 this issues post 的帮助，它将所有必要的部分放入有用的中间件类中。

后来才意识到导致应用程序崩溃的请求（即未处理的 5xx 类型错误）永远不会被跟踪，因为执行请求逻辑的调用在任何跟踪发生之前就失败了。 Azure Monitor 文档仅讨论通过日志跟踪异常，但这与请求的跟踪是分开的，除非我遗漏了某些内容。我当然不想失去失败的请求，这些对于跟踪非常重要！我习惯于使用应用程序洞察中的“失败”选项卡来监控任何失败的请求。

我认为跟踪这些请求的方法是使用 try/catch 显式处理任何内部异常并导出跟踪，手动将结果代码设置为 500。但我发现很奇怪，似乎没有关于此的文档opencensus 或 Azure。

我现在遇到的问题是：这个中间件函数预计会传回一个“响应”对象，然后 fastapi 将其用作可调用对象（不知道为什么） - 但在我在底层处理（即在

await call_next(request)

）我没有任何返回响应。我尝试返回 None 但这只会导致进一步的异常（ None 不可调用）。

这是我的中间件类版本 - 它与我链接的问题帖子非常相似，但我正在尝试/捕获

await call_next(request)

，而不是让它徒手失败。向下滚动到最后 5 行代码即可看到。

import logging

from fastapi import Request
from opencensus.trace import (
    attributes_helper,
    execution_context,
    samplers,
)
from opencensus.ext.azure.trace_exporter import AzureExporter
from opencensus.trace import span as span_module
from opencensus.trace import tracer as tracer_module
from opencensus.trace import utils
from opencensus.trace.propagation import trace_context_http_header_format
from opencensus.ext.azure.log_exporter import AzureLogHandler
from starlette.types import ASGIApp

from src.settings import settings

HTTP_HOST = attributes_helper.COMMON_ATTRIBUTES["HTTP_HOST"]
HTTP_METHOD = attributes_helper.COMMON_ATTRIBUTES["HTTP_METHOD"]
HTTP_PATH = attributes_helper.COMMON_ATTRIBUTES["HTTP_PATH"]
HTTP_ROUTE = attributes_helper.COMMON_ATTRIBUTES["HTTP_ROUTE"]
HTTP_URL = attributes_helper.COMMON_ATTRIBUTES["HTTP_URL"]
HTTP_STATUS_CODE = attributes_helper.COMMON_ATTRIBUTES["HTTP_STATUS_CODE"]

module_logger = logging.getLogger(__name__)
module_logger.addHandler(AzureLogHandler(
    connection_string=settings.appinsights_connection_string
))

class AppInsightsMiddleware:
    """
    Middleware class to handle tracing of fastapi requests and exporting the data to AppInsights. 
    
    Most of the code here is copied from a github issue: https://github.com/census-instrumentation/opencensus-python/issues/1020
    """
    def __init__(
        self,
        app: ASGIApp,
        excludelist_paths=None,
        excludelist_hostnames=None,
        sampler=None,
        exporter=None,
        propagator=None,
    ) -> None:
        self.app = app
        self.excludelist_paths = excludelist_paths
        self.excludelist_hostnames = excludelist_hostnames
        self.sampler = sampler or samplers.AlwaysOnSampler()
        self.propagator = (
            propagator or trace_context_http_header_format.TraceContextPropagator()
        )
        self.exporter = exporter or AzureExporter(
            connection_string=settings.appinsights_connection_string
        )

    async def __call__(self, request: Request, call_next):

        # Do not trace if the url is in the exclude list
        if utils.disable_tracing_url(str(request.url), self.excludelist_paths):
            return await call_next(request)

        try:
            span_context = self.propagator.from_headers(request.headers)

            tracer = tracer_module.Tracer(
                span_context=span_context,
                sampler=self.sampler,
                exporter=self.exporter,
                propagator=self.propagator,
            )
        except Exception:
            module_logger.error("Failed to trace request", exc_info=True)
            return await call_next(request)

        try:
            span = tracer.start_span()
            span.span_kind = span_module.SpanKind.SERVER
            span.name = "[{}]{}".format(request.method, request.url)
            tracer.add_attribute_to_current_span(HTTP_HOST, request.url.hostname)
            tracer.add_attribute_to_current_span(HTTP_METHOD, request.method)
            tracer.add_attribute_to_current_span(HTTP_PATH, request.url.path)
            tracer.add_attribute_to_current_span(HTTP_URL, str(request.url))
            execution_context.set_opencensus_attr(
                "excludelist_hostnames", self.excludelist_hostnames
            )
        except Exception:  # pragma: NO COVER
            module_logger.error("Failed to trace request", exc_info=True)

        try:
            response = await call_next(request)
            tracer.add_attribute_to_current_span(HTTP_STATUS_CODE, response.status_code)
            tracer.end_span()
            return response
        # Explicitly handle any internal exception here, and set status code to 500 
        except Exception as exception:
            module_logger.exception(exception)
            tracer.add_attribute_to_current_span(HTTP_STATUS_CODE, 500)
            tracer.end_span()
            return None

然后我在

main.py

中注册这个中间件类，如下所示：

app.middleware("http")(AppInsightsMiddleware(app, sampler=samplers.AlwaysOnSampler()))

Answer 1

显式处理处理API请求时可能发生的任何异常。这允许您完成跟踪请求，将状态代码设置为 500。然后您可以重新抛出异常以确保应用程序引发预期的异常。

        try:
            response = await call_next(request)
            tracer.add_attribute_to_current_span(HTTP_STATUS_CODE, response.status_code)
            tracer.end_span()
            return response
        # Explicitly handle any internal exception here, and set status code to 500 
        except Exception as exception:
            module_logger.exception(exception)
            tracer.add_attribute_to_current_span(HTTP_STATUS_CODE, 500)
            tracer.end_span()
            raise exception