FTP-001: A Modest Proposal



The FIDL Tuning Proposal (FTP) process is designed to provide a uniform and recorded path for making changes to the FIDL language, bindings, and tools.


There are several motivations for creating such an FTP system.

FIDL (the Fuchsia IPC system) is subject to a number of design constraints. These include performance, safety, and ergonomics. These are often at odds with each other, and the requirement to support IPC bindings in various target languages adds further tradeoffs. The FTP proposal system provides a way to litigate and to record decisions about these tradeoffs.

Recording decisions is valuable for several reasons. First, it provides a way to prevent revisiting the same decisions over and over when nothing has changed, while still allowing revisiting decisions when underlying assumptions actually have changed. Second, it provides new team members, or new clients of Fuchsia, some context into how FIDL has evolved and why certain decisions were made.

Finally, FIDL, as a sort of programming language, invites bikeshedding at a scale only Wadler's law can enable. This provides a place for such things to occur that isn't a several hundred person email list.


An FTP (FIDL Tuning Proposal) goes through several stages. These stages correspond to the Status: field of the heading of the template.

NB: The template is currently Google-internal.


One or more people get excited about a change! They make a copy of the tuning template, and start writing and designing. The proposal should address each of the section headings in the template, even if it is only to say “Not Applicable”.

At this stage they may start soliciting feedback on the draft from impacted parties.


At this stage, the FTP is formally circulated for commentary to the Fuchsia engineering organization. The authors of the proposal should solicit feedback from those especially likely to be impacted by the proposal.

For now, proposals should be left open for comment for at least one week, subject to reviewer discretion. It may be reasonable to be shorter for less controversial FTPs, and longer to wait for feedback from a particular person or group to come in.

Anyone may make a blocking comment on an FTP. Blocking comments do not prevent a particular accept-or-reject outcome from the review process, but reviewers are required to acknowledge the feedback given in the comment as part of the final FTP.


At this point the FTP, along with all outstanding commentary, is reviewed.

The proposal is reviewed by members of the Fuchsia FIDL team (unofficially know as luthiers), and anyone they see fit to include or to delegate to in the process. For example, they may include a particular language expert when making a decision about that language's bindings. If necessary, controversial decisions can be escalated like any other technical decision in Fuchsia.

The review can ultimately have three outcomes.

First, there may be outstanding questions or feedback required to make a decision. In this case the FTP is moved back to the Comment stage.

Second, the proposal may be Rejected, with reviewers providing a rationale as to why.

Third, it may be Accepted.


Rejected FTPs are valuable records of engineering decisions. When rejected, the rationale for rejected should be added to the FTP. The FTP will then be copied to the public record of all FTPs for posterity.

The given rationale should be actionable in the following two senses.

First, what would have to change about the world to have accepted this proposal?

Second, the rationale should address any blocking comments raised during the Comment period.


Accepted FTPs will also have a rationale section appended to them after review, and will receive a tracking bug.

The same constraints apply to the acceptance rationale as the rejection rationale. In particular, any blocking comments need to be addressed.

Then it's off to the races to implement the change.


At this stage, the proposal is landed. All the code has been changed. The tutorial has been updated. The bug is marked done. FIDL is in a more perfect tuning.

The final step of the process is landing a markdown-ified version of the FTP into the Fuchsia tree. This applies whether or not the proposal was accepted, as being able to point at already considered but rejected proposal is a substantial part of the value of this process.

Documentation and examples

This document (FTP-001) is the first such example of this process.

Ideally the template, plus the final version of this proposal, are sufficient documentation for the process.

Backwards compatibility





I believe this plan will have the modest benefit of providing a place for security review to happen. Currently all changes to FIDL are discussed via chat or code review. There's no paper trail, prior to the FTP process.


It feels easier to talk about success than about testing for this plan.

The immediate success criteria for this process will be whether the several outstanding ideas for changing FIDL go through the process without it being onerous.

One long term success metric would be whether old FTPs are regularly pointed at.

Drawbacks, alternatives, and unknowns

There's a small cost to serializing changes to FIDL through a slightly formal process. I believe that the cost is in fact small, in comparison to the engineering work needed to implement any change (especially as our ABIs harden and breaking changes get harder), and to the payoff of recording these decisions.

The biggest alternative I considered was a more open version. Currently, the comment and review process is currently only visible or open to Googlers. I believe that this is the correct decision for now, with an eye towards re-evaluating in the future.

I also wonder if there is a better way to capture commentary than a Google Doc, especially at the point of “freezing” the FTP into an accepted or rejected state.

I suspect we may want a version of this that captures decisions made about FIDL prior to the adoption of this process.

Finally, I wondered about how formal to be about acception or rejection criteria. I believe that this can evolve into something more formal over time, if needed, with the help of early FTP's decision rationales.

Prior art and references

Several open source programming languages have enhancement proposals or RFC mechanisms.

In particular, I looked a lot at the Python PEP process and the Rust RFC process while drafting this document.