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A walkthrough of some forwards-compatibility concerns encountered in the CloudEvents spec.
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Codelab: Versioning is Hard (aka the "SEF theorem")


You may have installed dependencies before that use semver, but have you built a software platform that is semantically versioned? What does this mean? Why do I claim this is hard?

A group I work with, CloudEvents, has started to see some of these issues when evaluating how to represent CloudEvents in Protocol Buffers. Even I shudder sometimes at the proto language. Proto generates libraries for both the binary Protocol Buffer binary format and JSON. Those libraries and the JSON format can sometimes feel clunky, but I've slowly seen wisdom in their ways.

Cartoon showing two boxes labeled "needles" and "poison tipped needles". Boxes are captioned "Clunkiness of JSON in proto-compatible APIs" and "Landmines in proto-incompatible APIs" respectively

This code lab shows one of the API pitfalls that Proto guarded us against. To be more approachable, we'll reproduce this bug without ever using or mentioning proto again. We'll show how the goals of forwards compatibility and extensibility are different and can even be at odds.



We will wear the hats of three different personas represented by three different directories:

  • Working Group (spec): The CloudEvents working group will release version 1.0 and 1.1 of the spec
  • Library Vendor (lib): An OSS contributor will write a library to help devs use CloudEvents according to spec.
  • App Author (app): Someone who uses CloudEvents by depending on lib/.

Additional data for this experiment (e.g. sample payloads) can be found in data/.

This repo has been annotated with tags. Each tag has a prefix of who authored the commit (e.g. spec-1.1). We'll use this to see how each persona's work affects the others.

Get started

This demo requires Go and assumes you have the GOPATH environment variable set. To get started, run the following commands:

mkdir -p ${GOPATH}/src/
git clone ${GOPATH}/src/
cd ${GOPATH}/src/
# This repository uses tags to help you walk through the steps and compare changes
git checkout app-1.0
go install

Congratulations! You have the whole world running at version 1.0. The spec 1.0 is still very simple1:

  • Any JSON object is a CloudEvent if it has an eventId string field
  • Extra fields may be present; these are "extensions" to the spec.

The library author has released their support

The app author uses CloudEvents 1.0 and also a "sampledRate" field because they downsample their data. Try the whole thing together!

versioningishard --data data/event-1.0.json
versioningishard --data data/event-1.1.json

You should see the following output:

> versioningishard --data data/event-1.0.json
I got event 123
(It was sampled at a rate of 1 in 10)

> versioningishard --data data/event-1.1.json
I got event abc
(It was sampled at a rate of 1 in 20)


Good news! The CloudEvents working group just released spec 1.1! Here's the version notes:

  • Adds a new optional optional attribute eventTime
  • Many developers have used sampledRate as an extension. It is now formally an optional attribute of the spec.

We can now upgrade our library to support the new spec:

git checkout lib-1.1

Now we can put on our app developer hats. Let's upgrade our dependency (it's only a minor change after all) and rerun the program. We'll see the following output:

go install
versioningishard --data data/event-1.0.json
versioningishard --data data/event-1.1.json

It looks like our output changed:

> versioningishard --data data/event-1.0.json
I got event 123
(It was not sampled)

> versioningishard --data data/event-1.1.json
I got event abc
(It was not sampled)

What just happened?! The code broke after a minor change! git show lib-1.1 does not show any obvious culprit.


Lets define three desirable features of a data format:

  • Structured: Using simple (C-style) structs, or "passive" data objects. These are transparent to developers, easily crafted in tests, terse in representation, and well suited to compiler checks or code-complete. These all happen by separating the definition of data from the scheme of the data (e.g. a struct def) which defines a contract compilers or tests can look up or enforce.
  • Extensible: Third-parties can offer features without changing the version of a product.
  • Forwards compatible: Official maintainers can add features without breaking existing dependencies.

Just like the CAP theorem defines trade-offs we must make in databases (CP or AP), we must choose at most two of these three attributes in most languages2:

  • Forwards compatible + Structured: This is one of the most common choices. If a library developer released a struct- based library, all new versions are forwards-compatible as long as new properties are not required3
  • Forwards compatible + Extensible: Our demo spec chose this route for JSON but our library accidentally dropped forwards- compatibility by using a struct. Our library MAY choose this route as well with some non-obvious work4. Any library that used the most common JSON idioms in Go would break users in the next upgrade.
  • Structured + Extensible: This is the option the library chose in this codelab. Standard features get typed support and extensions are possible, but extensions can only be promoted to standard features as a breaking change.

Back to proto

So what did any of this have to do with Protocol Buffers? Well, protobuffers force you to choose Structured as one of your three options. Next we can choose to either favor extensibility or forwards compatibility. The only workaround is that in version 1.1 of the spec, a promoted extension MUST still appear in the extensions map and MAY be included as a struct property. This workaround is impractical because there would be no motivation for users to read from the struct field (older senders will not set it) and thus little motiviation to start setting it.


1: This is the simplest JSON format but not the simplest JSON spec. Golang's encoding/json cannot yet support parsing unknown inline attributes (Issue 63213). To work around this, we use @duglin's jsonext tool. Thanks Doug!

2: Technically speaking, untyped languages make us give up structured support. Some untyped languages have varying support of some of the Structured goal, and some struggle. For example, TypeScript has the ability to define a general property bag with some well-known attributes, but the all values-types in the interface must be the same type. Similarly, there's no safety net if you accidentally misspell one of the well-known attributes.

3: To be totally fair, Golang has a function-style constructor in addition to the struct-style constructor. The function-style constructor lets consumers of the struct opt-out of some types of forwards compatibility.

4: TL;DR: the library would have to be built off of a map:

type CloudEvent map[string]interface{}

func (c CloudEvent) GetEventID() string {
  return c["eventId"]

func (c CloudEvent) SetEventID(id string) {
  c["eventId"] = id

// ...

Now the usage event["sampledRate"] is always safe.

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