Revision as of 18:57, 24 September 2020

WORK IN PROGRESS

Purpose

To minimize the disruptions to the consumers of the VPP API, while permitting the innovation for the VPP itself.

Motivation

Historically, API changes in VPP master branch were allowed at any point in time outside of a small window between the API freeze milestone and RC1 milestone. The API changes on the throttle branches were not permitted at all.

This model proved workable, however all the production use cases end up on throttle branches, with a lot of forklift activity when it is the time to upgrade to the next branch. The LTS releases were the first one to de-facto introduce the idea "It is okay to add a new API call, if no other APIs change at all".

This document outlines the structured process that harmonizes the behavior across all the VPP branches, and allows more flexibility for the consumer, while permitting the innovation in the VPP itself.

The Core Promise

"If a user is running a VPP version N and does not use any deprecated APIs, they should be able to simply upgrade the VPP to version N+1 and there should be no API breakage".

In-Progress, Production and Deprecated APIs

This proposal adds a classification of stability of an API call:

• "In-Progress": APIs in the process of the development, experimentation, and limited testing.
• "Production": tested as part of the "make test", considered stable for general usage.
• "Deprecated": used as a flag on Production APIs which are slated to be deprecated in the future release.

The "In-Progress" APIs or the APIs with the semantic version of 0.x.y are not subject to any stability checks, thus the developers are free to introduce them, modify their signatures, and as well remove them completely at will. The users should not use the in-progress APIs without the interactions with its maintainers, nor base the production code on those APIs. The goal of "in-progress" APIs to allow rapid iteration and modifications to ensure the API signature and function is stabilized. These API calls may be used for testing or experimentation and prototyping.

When the maintainer is satisfied with the quality of the APIs, and ensures that they are tested as part of the "Make test" runs, they can transition their status to "Production".

The "Production" APIs can *NOT* be changed in any way that modifies their representation on the wire and the signature (thus CRC). The only change that they may incur is to be marked as "Deprecated". These are the APIs that the downstream users can use for production purposes. They exist to fulfil a core promise of this process: The "Deprecated" APIs are the "Production" APIs that are about to be deleted. To ensure the above core promise is maintained, if the API call was marked as deprecated at any point between RC1 of release N and RC1 of release N+1, it MUST NOT be deleted until the RC1 milestone of the release N+2. The deprecated API SHOULD specify a replacement API - which MUST be a Production API, so as not to decrease the level of stability.

The time interval between a commit that marks an API as deprecated and a commit that deletes that API MUST be at least equal the time between the two subsequent releases (currently 4 months).

Doing so allows a for a good heads-up to those who are using the "one free upgrade" property to proactively catch and test the transition from the deprecated APIs using the master.

Marking an API as deprecated just 1 day before RC1 branch pull and then deleting that API one day after does *technically" satisfy "one free upgrade" promise, but is rather hostile to the users that are proactively tracking it.

Semantic API versioning

VPP APIs use semantic versioning according to semver.org, with the compatibility logic being applied at the moment the messages are marked as deprecated.

To discuss: i.e. if message_2 is being introduced which deprecates the message_1, then that same commit should increase the major version of the API.

The 0.x.x API versions, by virtue of being in-progress, are exempt from this treatment.

Tooling

See https://gerrit.fd.io/r/c/vpp/+/26881:

crcchecker.py is a tool to enforce the policy, with a few other bonus uses:

extras/scripts/crcchecker.py --check-patchset # returns -1 if backwards incompatible extras/scripts/crcchecker.py --dump-manifest extras/scripts/crcchecker.py --git-revision v20.01 <files> extras/scripts/crcchecker.py -- diff <oldfile> <newfile>

Notice that you can use this tool to get the list of API changes since a given past release.

The policy:

  1. Production APIs should never change.
     The definition of a "production API" is if the major version in
     the API file is > 0 that is not marked as "in-progress".
  2. APIs that are experimental / not released are not checked.
     An API message can be individually marked as in progress,
     by adding the following in the API definition:
        option status="in_progress"; (deprecated in favor of simpler notation "*option in_progress;*")
  3. An API can be deprecated in three-to-six steps (the steps
     with letters can be combined or split, depending on situation):
        Step 1a: A new "in-progress" API new_api_2 is added that
           is deemed to be a replacement.
        Step 1b: The existing API is marked as "replaced_by" this new API:
           option replaced_by="new_api_2";
        Step 2a: The new_api_2 is marked as production by deleting its in-progress status,
           provided that this API does have sufficient test coverage to deem it well tested.
        Step 2b: the existing API is marked as "deprecated":
           option deprecated="optional short message to humans reading it";
        Step 3: the deprecated API is deleted.

There is a time constraint that the minimum interval between the steps 2 and 3 must be at least 4 months. The proposal is to have step 2 around a couple of weeks before the F0 milestone for a release, as triggered by the release manager (and in the future by an automated means).

Use cases

Adding a new field to a production API

The simplest way to add a new field to a Porduction API message *foo_message* is to create a new In-Progress message *foo_message_v2*, and add the field to that one. Typically it will be an extension - so the API message handlers are trivially chained. If there are changes/adjustments that are needed, this new message can be freely altered without bothering the users of the Production API.

When the maintainer is happy with the quality of the implementation, and the foo_message_v2 is tested in "make test" to the same extent as the foo_message, they can make two commits: one, removing the in-progress status for foo_message_v2, and the second one - deprecating foo_message and pointing the foo_message_v2 as the replacement. Technically after the next throttle pull, they can delete the foo_message - the deprecation and the replacement will be already in the corresponding branch.

Rapid experimentation for a new feature

Add a message that is in-progress, and keep iterating with this message. This message is not subject to the change control process.

An in-progress API accidentally marked as "production"

This is expected to mainly apply during the initial period of 20.05->20.09, the proposal is to have it active for 4 weeks from Jun 17 till July 15th, with the following process.

If a developer finds that a given API or a set of APIs is not ready for production due to lack of tests and/or the general API stability, then they:

• Create a new gerrit change with *just* the marking of the API as in_progress, subject being: "API downgrade: ... which APIs and why are downgraded..."
• Add ayourtch@gmail.com as a reviewer (As a RM, for help in guiding the process / ensuring that there is progress and the commit is not forgotten)
• Send an email to vpp-dev mailing list with the subject being the same as the oneliner commit message, reference to the gerrit change, and the reasoning.
• Wait for the timeout period of two weeks for the feedback
• If no feedback received, assume the community agreement and commit the change to master branch.

This needs to be highlighted that this (or whatever is agreed upon) process is an *exception* - normally the transition is always in_progress => production => deprecated.

For the initial application during transitional period during the 20.05->20.09, a shorter than 2 weeks waiting period can be used, on an individual basis.

Real-world examples

https://gerrit.fd.io/r/c/vpp/+/25810 - rather than building an incompatible field change, a different, new in-progress API was made for rapid innovation.