Propagation
Context propagation for the Python SDK
Propagation is the mechanism that moves data between services and processes. Although not limited to tracing, it is what allows traces to build causal information about a system across services that are arbitrarily distributed across process and network boundaries.
Manual W3C Trace Context Propagation
The following generic example demonstrates how you can propagate trace context manually.
First, on the sending service, you’ll need to inject the current context:
from flask import Flask
import requests
from opentelemetry import trace, propagators, baggage
from opentelemetry.trace.propagation.tracecontext import TraceContextTextMapPropagator
from opentelemetry.baggage.propagation import W3CBaggagePropagator
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import ConsoleSpanExporter, BatchSpanProcessor
app = Flask(__name__)
trace.set_tracer_provider(TracerProvider())
trace.get_tracer_provider().add_span_processor(BatchSpanProcessor(ConsoleSpanExporter()))
tracer = trace.get_tracer(__name__)
@app.route('/')
def hello():
with tracer.start_as_current_span("api1_span") as span:
ctx = baggage.set_baggage("hello", "world")
headers = {}
W3CBaggagePropagator().inject(headers, ctx)
TraceContextTextMapPropagator().inject(headers, ctx)
print(headers)
response = requests.get('http://127.0.0.1:5001/', headers=headers)
return f"Hello from API 1! Response from API 2: {response.text}"
if __name__ == '__main__':
app.run(port=5002)
On the receiving service, you’ll need to extract context (for example, from
parsed HTTP headers) and then set them as the current trace context.
from flask import Flask, request
from opentelemetry import trace, baggage
from opentelemetry.trace.propagation.tracecontext import TraceContextTextMapPropagator
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import ConsoleSpanExporter, BatchSpanProcessor
from opentelemetry.baggage.propagation import W3CBaggagePropagator
app = Flask(__name__)
trace.set_tracer_provider(TracerProvider())
trace.get_tracer_provider().add_span_processor(BatchSpanProcessor(ConsoleSpanExporter()))
tracer = trace.get_tracer(__name__)
@app.route('/')
def hello():
# Example: Log headers received in the request in API 2
headers = dict(request.headers)
print(f"Received headers: {headers}")
carrier ={'traceparent': headers['Traceparent']}
ctx = TraceContextTextMapPropagator().extract(carrier=carrier)
print(f"Received context: {ctx}")
b2 ={'baggage': headers['Baggage']}
ctx2 = W3CBaggagePropagator().extract(b2, context=ctx)
print(f"Received context2: {ctx2}")
# Start a new span
with tracer.start_span("api2_span", context=ctx2):
# Use propagated context
print(baggage.get_baggage('hello', ctx2))
return "Hello from API 2!"
if __name__ == '__main__':
app.run(port=5001)
From there, when you have a deserialized active context, you can create spans that will be a part of the same trace from the other service.