The Starlette application class (which is the parent of the FastAPI application class) hardcodes the ServerErrorMiddleware to appear as the very first middleware. This means that if there is an error in some downstream middleware, handling the request with ServerErrorMiddleware will not have an active trace.
Also, if you happen to do something like this:
app = FastAPI(...)
app.add_middleware(...)
app.add_middlware(....)
The additional middleware added after the first __init__ of FastAPI will also not have an active trace established.
There is a relatively easy way around this to ensure that the OpenTelemetry middleware is always first:
class _InstrumentedFastAPI(fastapi.FastAPI):
_tracer_provider = None
_excluded_urls = None
_server_request_hook: _ServerRequestHookT = None
_client_request_hook: _ClientRequestHookT = None
_client_response_hook: _ClientResponseHookT = None
def build_middleware_stack(self) -> ASGIApp:
original_mw_app = super().build_middleware_stack()
return OpenTelemetryMiddleware(
original_mw_app,
excluded_urls=_InstrumentedFastAPI._excluded_urls,
default_span_details=_get_route_details,
server_request_hook=_InstrumentedFastAPI._server_request_hook,
client_request_hook=_InstrumentedFastAPI._client_request_hook,
client_response_hook=_InstrumentedFastAPI._client_response_hook,
tracer_provider=_InstrumentedFastAPI._tracer_provider,
)FastAPI version - 0.70.0
Starlette version - 0.16.0
Python version - 3.9
The
Starletteapplication class (which is the parent of theFastAPIapplication class) hardcodes theServerErrorMiddlewareto appear as the very first middleware. This means that if there is an error in some downstream middleware, handling the request withServerErrorMiddlewarewill not have an active trace.Also, if you happen to do something like this:
The additional middleware added after the first
__init__ofFastAPIwill also not have an active trace established.There is a relatively easy way around this to ensure that the OpenTelemetry middleware is always first:
FastAPI version - 0.70.0
Starlette version - 0.16.0
Python version - 3.9