Implement distributed tracing

note

This feature is currently only available in the Python SDK. Support for TypeScript is coming soon.

Sometimes, you need to trace a request across multiple services.

LangSmith supports distributed tracing out of the box, linking runs within a trace across services using context propagation headers (langsmith-trace and optional baggage for metadata/tags).

Example client-server setup:

• Trace starts on client
• Continues on server

# client.py
from langsmith.run_helpers import get_current_run_tree, traceable
import httpx

@traceable
async def my_client_function():
    headers = {}
    async with httpx.AsyncClient(base_url="...") as client:
        if run_tree := get_current_run_tree():
            # add langsmith-id to headers
            headers.update(run_tree.to_headers())
        return await client.post("/my-route", headers=headers)

Then the server (or other service) can continue the trace by passing the headers in as langsmith_extra:

# server.py
from langsmith import traceable
from langsmith.run_helpers import tracing_context
from fastapi import FastAPI, Request


@traceable
async def my_application():
    ...


app = FastAPI()  # Or Flask, Django, or any other framework


@app.post("/my-route")
async def fake_route(request: Request):
    # request.headers:  {"langsmith-trace": "..."}
    # as well as optional metadata/tags in `baggage`
    with tracing_context(parent=request.headers):
        return await my_application()

The example above uses the tracing_context context manager. You can also directly specify the parent run context in the langsmith_extra parameter of a method wrapped with @traceable.

from langsmith.run_helpers import traceable, trace
# ... same as above
@app.post("/my-route")
async def fake_route(request: Request):
    # request.headers:  {"langsmith-trace": "..."}
    my_application(langsmith_extra={"parent": request.headers})