1 Quick Start
The Tracing Plane introduces two key abstractions: BaggageContexts and Execution-Flow Scoped Variables.
BaggageContext is a general-purpose request context, intended to be used within and across distributed services.
For example, a request in a microservices environment might involve multiple services, which make calls across the network to each other.
For each request, a
BaggageContext carries request metadata (things like request IDs, tags, etc.). Its goal is to be passed alongside the request while it executes.
It's very useful to pass around
BaggageContext objects at runtime. They are used by a range of different debugging and monitoring tools, the classic example being distributed tracing (like Zipkin and OpenTracing and Dapper). However, there are also other cool examples, like resource management and dynamic monitoring. We use the name tracing tools to refer to such tools.
2.2 Execution-Flow Scoped Variables
An execution-flow scoped variable is similar, in concept, to a thread-local variable. However, instead of being dynamically scoped to threads, EFS variables are scoped to end-to-end requests.
EFS variables follow requests inline as they execute. The
TraceID used by distributed tracing tools is an example of an execution-flow scoped variable.
Updates to an EFS variable occur locally to an EFS instance -- for example, if my request is doing several things concurrently, they might have different values for their respective EFS variables. This relates to the notion of causality -- EFS variables follow execution's causality. The
SpanID used by distributed tracing tools is an EFS variable that demonstrates this -- several concurrent execution branches could be executing simultaneously, each with a different value for
The Tracing Plane exposes EFS variables with an interface definition language, called BDL (Baggage Definition Language), and corresponding compiler. BDL is similar to protocol buffers, and generates accessors that interface with
BaggageContext instances and encapsulate all of the concurrency and propagation nuances that are easy to get wrong.
3 Project Information
3.1 Project Goals
It's actually very hard to get context propagation right and very hard to deploy new tracing tools in today's distributed systems:
- It's hard to instrument systems to pass around contexts, because it involves touching lots of little bits of code in many places. Instrumentation is hard -- only do it once.
- It's hard to agree on context formats across system components, especially if they use different languages or frameworks Agree on a general-purpose format -- bind specific tools to it later
- It's hard to get the behavior of contexts right -- for example, if a request has a high degree of fan-in, how do you reconcile mismatched IDs or context values? Encapsulate well-defined propagation behavior for data types
In general, we want a
BaggageContext that can carry any tracing tool's data in a consistent way.
The Tracing Plane is a layered design for context propagation in distributed systems. It involves a data serialization format, a protocol for interpreting data, an interface definition language (called BDL -- Baggage Definition Language), and compiler.
The tracing plane enables interoperability between systems and tracing applications. It provides a "narrow waist" for tracing, analogous to the role of TCP/IP in networking.
3.2 Similar Projects
There are some similar projects, that we list here to make it more concrete what the Tracing Plane is:
- Go's context package provides request-scoped contexts that you pass around in Go programs. Similarly, we propose
BaggageContextobjects be passed around in the same way (though our API is slightly different)
- Span contexts in Zipkin, OpenTracing and Dapper -- these pass around metadata (span and trace IDs) for distributed tracing. The goal of
BaggageContextis to provide a well-defined, concrete data format that these tracing tools would be able to use, to store their IDs.
- Instrumentation in OpenTracing is similar to instrumentation for the Tracing Plane. One difference is that
BaggageContextinstances are truly opaque at instrumentation time, and are passed across all execution boundaries (including, for example, in request responses); whereas OpenTracing spans are conceptually tightly bound to the task of distributed tracing.
3.3 Project Status
This is an active research project at Brown University by Jonathan Mace and Prof. Rodrigo Fonseca. This work is supported in part by NSF award 1452712, and from generous gifts from Facebook and Google.
The Tracing Plane is motivated by many years of collective experience in end-to-end tracing and numerous tracing-related research projects including X-Trace, Quanto, Retro, Pivot Tracing. You can also check out our research group's GitHub.
We currently provide a Java implementation of the Tracing Plane. However, the Tracing Plane is not tightly coupled to Java, and is designed with interoperability in mind - between languages, systems, platforms, etc.
Keep an eye out for our research paper about Baggage, coming soon.
4 Other Useful Links
Example Zipkin / OpenTracing Tracers that are backed by BDL: github.com/JonathanMace/tracingplane-opentracing