Production Request Tracing

SGlang exports request trace data based on the OpenTelemetry Collector. You can enable tracing by adding the --enable-trace and configure the OpenTelemetry Collector endpoint using --otlp-traces-endpoint when launching the server.

You can find example screenshots of the visualization in https://github.com/sgl-project/sglang/issues/8965.

Setup Guide

This section explains how to configure the request tracing and export the trace data.

1. Install the required packages and tools

   • install Docker and Docker Compose
   • install the dependencies
   # enter the SGLang root directory
   pip install -e "python[tracing]"
   
   # or manually install the dependencies using pip
   pip install opentelemetry-sdk opentelemetry-api opentelemetry-exporter-otlp opentelemetry-exporter-otlp-proto-grpc
   

2. launch opentelemetry collector and jaeger

   docker compose -f examples/monitoring/tracing_compose.yaml up -d
   

3. start your SGLang server with tracing enabled

   # set env variables
   export SGLANG_OTLP_EXPORTER_SCHEDULE_DELAY_MILLIS=500
   export SGLANG_OTLP_EXPORTER_MAX_EXPORT_BATCH_SIZE=64
   # start the prefill and decode server
   python -m sglang.launch_server --enable-trace --otlp-traces-endpoint 0.0.0.0:4317 <other option>
   # start the mini lb
   python -m sglang_router.launch_router --enable-trace --otlp-traces-endpoint 0.0.0.0:4317 <other option>
   

   Replace 0.0.0.0:4317 with the actual endpoint of the opentelemetry collector. If you launched the openTelemetry collector with tracing_compose.yaml, the default receiving port is 4317.

   To use the HTTP/protobuf span exporter, set the following environment variable and point to an HTTP endpoint, for example, http://0.0.0.0:4318/v1/traces.

   export OTEL_EXPORTER_OTLP_TRACES_PROTOCOL=http/protobuf
   

4. raise some requests

5. Observe whether trace data is being exported

   • Access port 16686 of Jaeger using a web browser to visualize the request traces.
   • The OpenTelemetry Collector also exports trace data in JSON format to /tmp/otel_trace.json. In a follow-up patch, we will provide a tool to convert this data into a Perfetto-compatible format, enabling visualization of requests in the Perfetto UI.

How to add Tracing for slices you’re interested in?

We have already inserted instrumentation points in the tokenizer and scheduler main threads. If you wish to trace additional request execution segments or perform finer-grained tracing, please use the APIs from the tracing package as described below.

1. initialization

   Every process involved in tracing during the initialization phase should execute:

   process_tracing_init(otlp_traces_endpoint, server_name)
   

   The otlp_traces_endpoint is obtained from the arguments, and you can set server_name freely, but it should remain consistent across all processes.

   Every thread involved in tracing during the initialization phase should execute:

   trace_set_thread_info("thread label", tp_rank, dp_rank)
   

   The “thread label” can be regarded as the name of the thread, used to distinguish different threads in the visualization view.

2. Mark the beginning and end of a request

   trace_req_start(rid, bootstrap_room)
   trace_req_finish(rid)
   

   These two APIs must be called within the same process, for example, in the tokenizer.

3. Add tracing for slice

   • Add slice tracing normally:

     trace_slice_start("slice A", rid)
     trace_slice_end("slice A", rid)
     
   • Use the “anonymous” flag to not specify a slice name at the start of the slice, allowing the slice name to be determined by trace_slice_end.

Note: Anonymous slices must not be nested.

trace_slice_start("", rid, anonymous = True)
trace_slice_end("slice A", rid)

• In trace_slice_end, use auto_next_anon to automatically create the next anonymous slice, which can reduce the number of instrumentation points needed.
  trace_slice_start("", rid, anonymous = True)
  trace_slice_end("slice A", rid, auto_next_anon = True)
  trace_slice_end("slice B", rid, auto_next_anon = True)
  trace_slice_end("slice C", rid, auto_next_anon = True)
  trace_slice_end("slice D", rid)
  
• The end of the last slice in a thread must be marked with thread_finish_flag=True; otherwise, the thread’s span will not be properly generated.
  trace_slice_end("slice D", rid, thread_finish_flag = True)
  

4. When the request execution flow transfers to another thread, the trace context needs to be explicitly propagated.

   • sender: Execute the following code before sending the request to another thread via ZMQ
     trace_context = trace_get_proc_propagate_context(rid)
     req.trace_context = trace_context
     
   • receiver: Execute the following code after receiving the request via ZMQ
     trace_set_proc_propagate_context(rid, req.trace_context)
     

5. When the request execution flow transfers to another node(PD disaggregation), the trace context needs to be explicitly propagated.

   • sender: Execute the following code before sending the request to node thread via http
     trace_context = trace_get_remote_propagate_context(bootstrap_room_list)
     headers = {"trace_context": trace_context}
     session.post(url, headers=headers)
     
   • receiver: Execute the following code after receiving the request via http
     trace_set_remote_propagate_context(request.headers['trace_context'])
     

How to Extend the Tracing Framework to Support Complex Tracing Scenarios

The currently provided tracing package still has potential for further development. If you wish to build more advanced features upon it, you must first understand its existing design principles.

The core of the tracing framework’s implementation lies in the design of the span structure and the trace context. To aggregate scattered slices and enable concurrent tracking of multiple requests, we have designed a two-level trace context structure and a four-level span structure: SglangTraceReqContext, SglangTraceThreadContext. Their relationship is as follows:

SglangTraceReqContext (req_id="req-123")
+-- SglangTraceThreadContext(thread_label="scheduler", tp_rank=0)
|
+-- SglangTraceThreadContext(thread_label="scheduler", tp_rank=1)

Each traced request maintains a global SglangTraceReqContext. For every thread processing the request, a corresponding SglangTraceThreadContext is recorded and composed within the SglangTraceReqContext. Within each thread, every currently traced slice (possibly nested) is stored in a list.

In addition to the above hierarchy, each slice also records its previous slice via Span.add_link(), which can be used to trace the execution flow.

When the request execution flow transfers to a new thread, the trace context needs to be explicitly propagated. In the framework, this is represented by SglangTracePropagateContext, which contains the context of the request span and the previous slice span.

We designed a four-level span structure, consisting of bootstrap_room_span, req_root_span, thread_span, and slice_span. Among them, req_root_span and thread_span correspond to SglangTraceReqContext and SglangTraceThreadContext, respectively, and slice_span is stored within the SglangTraceThreadContext. The bootstrap_room_span is designed to accommodate the separation of PD-disaggregation. On different nodes, we may want to add certain attributes to the req_root_span. However, if the req_root_span is shared across all nodes, the Prefill and Decode nodes would not be allowed to add attributes due to the constraints imposed by OpenTelemetry’s design.

bootstrap room span
+-- router req root span
|    +-- router thread span
|          +-- slice span
+-- prefill req root span
|    +-- tokenizer thread span
|    |     +-- slice span
|    +-- scheduler thread span
|          +-- slice span
+-- decode req root span
      +-- tokenizer thread span
      |    +-- slice span
      +-- scheduler thread span
           +-- slice span