[clevengermatt]

On the web being the duck typing equivalent. That's ad-hoc duck typing at the wire format level. S3-compatible APIs exist because companies reverse-engineered S3's surface and matched it. You have to replicate the paths, headers, and response shapes closely enough that the client libraries can't tell the difference, or they break. There's no spec that declares "I satisfy the S3 interface" and no way for a tool to verify compatibility without running requests.

OBI operates at the contract level instead. Two services with different wire formats can satisfy the same interface as long as their operation shapes match. The binding executors handle the wire differences. That's the duck typing analogy. Match the shape, not the implementation.

You're right that the drivers are partly economic. But technical standards and economics aren't isolated from each other. Terraform, Kubernetes, and OpenAPI itself are technical solutions that enabled economic behavior that wasn't viable before them. Lowering the cost of interop changes what's economically rational to pursue.

On the xkcd, the post addresses this. OBI structurally can't replace OpenAPI, gRPC, or MCP. An OBI without sources and bindings pointing to them is an unbound contract, not an actionable interface. The dependency runs one way. Those specs are inputs, not competitors.

[yencabulator]

None of the non-AWS copies of S3 implement exactly the S3 protocol. And even if they did, the next update -- coming at any arbitrary time -- would invalidate the full compatibility.

[clevengermatt]

Fair correction. Wire-level copies don't even fully match, and upstream changes can invalidate whatever match exists. That's the weakness of wire-level duck typing. Contract-level compatibility doesn't fully solve the upstream changes problem (nothing does), but when you regenerate the OBI from an updated source spec, a compatibility check tells you structurally what changed. Wire-level clones find out by breaking.

The flip side is worth noting too. If you own the API and want to preserve contract compatibility, OBI lets you change your implementation (paths, payload shapes, even protocols) behind the same contract. Transforms bridge shape differences. Binding swaps change protocols. Consumers targeting the interface don't need to care.

[riwsky]

Ad-hoc duck typing—“if it looks like a duck[…]”—is the only kind that exists! The point of the term “duck typing” is that it doesn’t require explicit declaration of the contract by implementers, it’s not synonymous with polymorphism or with interfaces in general. Haskell type classes are not duck typed, nor are Swift protocols; Go interfaces are.

OBI is a boilerplate generator for the adapter pattern for service communication; its contracts are just another set of their own paths, payload shapes, and protocols. The distinction between “protocol” and “contract” in this context is nonsense.

[clevengermatt]

Pushing back on the terminology point. Duck typing is defined by how compatibility is determined, not by the absence of declaration. In Swift and Haskell, declaration is what makes a type conform. In Go, structure does, whether you add an optional assertion or not. OBI works the same way. Shapes determine compatibility. Roles and satisfies are optional declarations for explicitness, not a different mechanism.

Duck typing also requires being able to see a duck. Go has methods you can introspect. Raw REST doesn't. OpenAPI and gRPC reflection give you a structural surface, and OBI unifies those into one comparable form above the transport. What you're calling ad-hoc duck typing at the network boundary is humans reading docs and guessing whether the shapes match. That's a weaker version of the same idea, without the mechanical verification that makes duck typing useful. On "just another set of paths and protocols," OBI operation keys are transport-neutral. `tasks.create` has no route baked in. Bindings say how to reach it: REST at POST /tasks, gRPC at TaskService.Create, MCP as a tool. Same operation, multiple bindings, one identifier.

Thanks for the sharpening!

[riwsky]

You’re still not getting it. Duck typing comes from the phrase “if it looks like a duck, and quacks like a duck, then it must be a duck”. The Wikipedia page contrasts it with nominative typing that requires a declaration, and calls out that duck typing does not need the adapter pattern. What you are calling duck typing is just “interfaces”.

Also “tasks.create” is morally a route. Eg grpc has web transports and autogenerated CLIs too; even without those, there’s no particular reason why the OBI “client layer” couldn’t just be, say, gRPC under the hood, with the OBI specs just being used to codegen adapters from the source protocol to gRPC. It would then immediately have a much more widely understood and supported client layer, with better performance, while remaining as simple to implement as the adapters to this new custom OBI runtime protocol.

[clevengermatt]

You're right that what OBI does is structural typing or just interfaces, not duck typing. Transforms and bindings do use the adapter pattern. The critique lands and I can admit it. The duck typing framing was rhetorically appealing but imprecise.

On the gRPC-as-client-layer point, I want to make sure I understand. You're suggesting that between the OBI client SDK and the source service, there should be a gRPC layer, with generated gRPC adapters fronting each source? Or something else?

Worth noting either way: the spec doesn't dictate implementation. It defines operation contracts, binding sources, and the executor role. How an executor actually reaches a service is left open, so gRPC-based routing is a valid strategy within the spec regardless.

Thanks for sticking with this. This is exactly the kind of discussion I was hoping for. I really do appreciate it.