What is an XPCA-Spec?

XPCA-Spec stands for XPCA (eXtensible Process Control Architecture) Specification. An XPCA-Spec is a design document providing information to the XPCA community, or describing a new feature for XPCA or its processes or environment. The XPCA-Spec should provide a concise technical specification of the feature and a rationale for the feature.

We intend XPCA-Specs to be the primary mechanisms for proposing new features, for collecting community input on an issue, and for documenting the design decisions that have gone into XPCA. The XPCA-Spec author is responsible for building consensus within the community and documenting dissenting opinions.

The model for XPCA-Specs borrows heavily from the Python Enhancement Proposal (PEP) process 1. We gratefully acknowledge the work performed by the Python community. Utilising their excellent resources is in line with the XPCA philosophy of “reuse over reinvention”, and we appreciate the opportunity this has given us to formalise our specification process with minimal effort.

The bulk of the text of this document is sourced from “PEP-1, What is a Python Enhancement Proposal” and is essentially a rewording of the original to fit the terminology used within XPCA.

XPCA-Spec Types

There are three kinds of XPCA-Spec:

A Standards Track XPCA-Spec describes a new feature or implementation for XPCA.

An Informational XPCA-Spec describes a XPCA design issue, or provides general guidelines or information to the XPCA community, but does not propose a new feature. Informational XPCA-Specs do not necessarily represent a XPCA community consensus or recommendation, so users and implementors are free to ignore Informational XPCA-Specs or follow their advice.

A Process XPCA-Spec describes a process surrounding XPCA, or proposes a change to (or an event in) a process. Process XPCA-Specs are like Standards Track XPCA-Specs but apply to areas other than the XPCA development itself. They may propose an implementation, but not to describe a concrete implementation of an XPCA feature; they often require community consensus; unlike Informational XPCA-Specs, they are more than recommendations, and users are typically not free to ignore them. Examples include procedures, guidelines, changes to the decision-making process, and changes to the tools or environment used in XPCA development. Any meta-XPCA-Spec is also considered a Process XPCA-Spec.

XPCA-Spec Work Flow

The XPCA-Spec editors assign XPCA-Spec numbers and change their status. Please send all XPCA-Spec-related email to xpca-specs@xpca.org (no cross-posting please). Also see XPCA-Spec Editor Responsibilities & Workflow below.

The XPCA-Spec process begins with a new idea for XPCA. It is highly recommended that a single XPCA-Spec contain a single key proposal or new idea. Small enhancements or patches often don’t need an XPCA-Spec and can be injected into the XPCA development work flow with a patch submission to the XPCA issue tracker 2. The more focussed the XPCA-Spec, the more successful it tends to be. The XPCA-Spec editor reserves the right to reject XPCA-Spec proposals if they appear too unfocussed or too broad. If in doubt, split your XPCA-Spec into several well-focussed ones.

Each XPCA-Spec must have a champion – someone who writes the XPCA-Spec using the style and format described below, shepherds the discussions in the appropriate forums, and attempts to build community consensus around the idea. The XPCA-Spec champion (a.k.a. Author) should first attempt to ascertain whether the idea is XPCA-Spec-able. Posting to the XPCA Google Group is the best way to go about this.

Vetting an idea publicly before going as far as writing an XPCA-Spec is meant to save the potential author time. Asking the XPCA community first if an idea is original helps prevent too much time being spent on something that is guaranteed to be rejected based on prior discussions (searching the internet does not always do the trick). It also helps to make sure the idea is applicable to the entire community and not just the author. Just because an idea sounds good to the author does not mean it will work for most people in most areas where XPCA is used.

Once the champion has asked the XPCA community as to whether an idea has any chance of acceptance, a draft XPCA-Spec should be presented to the XPCA Google Group. This gives the author a chance to flesh out the draft XPCA-Spec to make properly formatted, of high quality, and to address initial concerns about the proposal.

Following a discussion on XPCA Google Group, the proposal should be sent to the XPCA-Spec editors xpca-specs@xpca.org. This draft must be written in XPCA-Spec style as described below, else it will be sent back without further regard until proper formatting rules are followed.

If the XPCA-Spec editor approves, he will assign the XPCA-Spec a number, label it as Standards Track, Informational, or Process, give it status “Draft”, and create and check-in the initial draft of the XPCA-Spec. The XPCA-Spec editor will not unreasonably deny an XPCA-Spec. Reasons for denying XPCA-Spec status include duplication of effort, being technically unsound, not providing proper motivation or addressing backwards compatibility, or not in keeping with the XPCA philosophy.

As updates are necessary, the XPCA-Spec author can check in new versions if they have git push privileges, or can email new XPCA-Spec versions to the XPCA-Spec editor for committing.

Standards Track XPCA-Specs consist of two parts, a design document and a reference implementation. The XPCA-Spec should be reviewed and accepted before a reference implementation is begun, unless a reference implementation will aid people in studying the XPCA-Spec. Standards Track XPCA-Specs must include an implementation – in the form of code, a patch, or a URL to same – before it can be considered Final.

XPCA-Spec authors are responsible for collecting community feedback on an XPCA-Spec before submitting it for review. However, wherever possible, long open-ended discussions on public mailing lists should be avoided. Strategies to keep the discussions efficient include: setting up a separate SIG mailing list for the topic, having the XPCA-Spec author accept private comments in the early design phases, setting up a wiki page, etc. XPCA-Spec authors should use their discretion here.

Once the authors have completed an XPCA-Spec, they must inform the XPCA-Spec editor that it is ready for review. XPCA-Specs are reviewed by the editors, who may accept or reject an XPCA-Spec or send it back to the author(s) for revision. For an XPCA-Spec that is pre-determined to be acceptable (e.g., it is an obvious win as-is and/or its implementation has already been checked in) the editors may also initiate an XPCA-Spec review, first notifying the XPCA-Spec author(s) and giving them a chance to make revisions.

For an XPCA-Spec to be accepted it must meet certain minimum criteria. It must be a clear and complete description of the proposed enhancement. The enhancement must represent a net improvement. The proposed implementation, if applicable, must be solid and must not complicate XPCA unduly. Finally, a proposed enhancement must conform the general philosophy of the XPCA project.

Once an XPCA-Spec has been accepted, the reference implementation must be completed. When the reference implementation is complete and accepted by the editors, the status will be changed to “Final”.

An XPCA-Spec can also be assigned status “Deferred”. The XPCA-Spec author or editor can assign the XPCA-Spec this status when no progress is being made on the XPCA-Spec. Once an XPCA-Spec is deferred, the XPCA-Spec editor can re-assign it to draft status.

An XPCA-Spec can also be “Rejected”. Perhaps after all is said and done it was not a good idea. It is still important to have a record of this fact.

XPCA-Specs can also be superseded by a different XPCA-Spec, rendering the original obsolete. This is intended for Informational XPCA-Specs, where version 2 of an API can replace version 1.

The possible paths of the status of XPCA-Specs are as follows:

Some Informational and Process XPCA-Specs may also have a status of “Active” if they are never meant to be completed. E.g. XPCA-Spec 1 (this XPCA-Spec).

What belongs in a successful XPCA-Spec?

Each XPCA-Spec should have the following parts:

Preamble – RFC 822 style headers containing meta-data about the XPCA-Spec, including the XPCA-Spec number, a short descriptive title (limited to a maximum of 44 characters), the names, and optionally the contact info for each author, etc.

Abstract – a short (~200 word) description of the technical issue being addressed.

Copyright/public domain – Each XPCA-Spec must either be explicitly labelled as placed in the public domain (see this XPCA-Spec as an example) or licensed under the Open Publication License 3.

Specification – The technical specification should describe the syntax and semantics of any new feature. The specification should be detailed enough to allow competing, interoperable implementations for any current XPCA platform.

Motivation – The motivation is critical for XPCA-Specs that want to change or extend XPCA. It should clearly explain why the existing specifications are inadequate to address the problem that the XPCA-Spec solves. XPCA-Spec submissions without sufficient motivation may be rejected outright.

Rationale – The rationale fleshes out the specification by describing what motivated the design and why particular design decisions were made. It should describe alternate designs that were considered and related work, e.g. how the feature is supported in other process control architectures.

The rationale should provide evidence of consensus within the community and discuss important objections or concerns raised during discussion.

Backwards Compatibility – All XPCA-Specs that introduce backwards incompatibilities must include a section describing these incompatibilities and their severity. The XPCA-Spec must explain how the author proposes to deal with these incompatibilities. XPCA-Spec submissions without a sufficient backwards compatibility treatise may be rejected outright.

Reference Implementation – The reference implementation must be completed before any XPCA-Spec is given status “Final”, but it need not be completed before the XPCA-Spec is accepted. It is better to finish the specification and rationale first and reach consensus on it before writing code.

The final implementation must include test code and documentation appropriate for either inclusion in existing XPCA platforms or the standard reference implementation.

XPCA-Spec Formats and Templates

There are two XPCA-Spec formats available to authors: plaintext and markdown4. Both are UTF-8-encoded text files.

Plaintext XPCA-Specs are written with minimal structural markup that adheres to a rigid style. A future XPCA-Spec will contain instructions and a template to be used when writing plaintext XPCA-Specs.

Markdown XPCA-Specs allow for rich markup that is still quite easy to read, but results in much better-looking and more functional HTML. Another future XPCA-Spec will contain instructions and a template for Markdown XPCA-Specs.

Both styles of XPCA-Specs are converted to HTML for viewing on the web using the nanoc 5 static content generator. Parsing of plaintext XPCA-Specs is yet to be determined. Markdown XPCA-Specs are parsed and converted by the Maruku 6 gem called as a filter from the nanoc generator.

XPCA-Spec Header Preamble

Each XPCA-Spec must begin with an RFC 822 style header preamble. The headers must appear in the following order. Headers marked with ”” are optional and are described below. All other headers are required.

XPCA-Spec: <XPCA-Spec number> Title: <XPCA-Spec title> Version: <version string> Last-Modified: <date string> Author: <list of authors’ real names and optionally, email addrs> * Discussions-To: <email address> Status: <Draft | Active | Accepted | Deferred | Rejected | Withdrawn | Final | Superseded> Type: <Standards Track | Informational | Process>

Random J. User address@dom.ain if the email address is included, and just

Random J. User if the address is not given. For historical reasons the format “address@dom.ain (Random J. User)” may appear in a XPCA-Spec, however new XPCA-Specs must use the mandated format above, and it is acceptable to change to this format when XPCA-Specs are updated.

If there are multiple authors, each should be on a separate line following RFC 2822 continuation line conventions. Note that personal email addresses in XPCA-Specs will be obscured as a defense against spam harvesters.

Note: The Resolution header is required for Standards Track XPCA-Specs only. It contains a URL that should point to an email message or other web resource where the pronouncement about the XPCA-Spec is made.

While a XPCA-Spec is in private discussions (usually during the initial Draft phase), a Discussions-To header will indicate the mailing list or URL where the XPCA-Spec is being discussed. No Discussions-To header is necessary if the XPCA-Spec is being discussed privately with the author, or on the XPCA-list or XPCA-dev email mailing lists. Note that email addresses in the Discussions-To header will not be obscured.

The Type header specifies the type of XPCA-Spec: Standards Track, Informational, or Process.

The format of a XPCA-Spec is specified with a Content-Type header. The acceptable values are “text/plain” for plaintext XPCA-Specs and “text/x-markdown” for Markdown XPCA-Specs. Plaintext (“text/plain”) is the default if no Content-Type header is present.

The Created header records the date that the XPCA-Spec was assigned a number, while Post-History is used to record the dates of when new versions of the XPCA-Spec are posted to XPCA-list and/or XPCA-dev. Both headers should be in dd-mmm-yyyy format, e.g. 14-Aug-2001.

Standards Track XPCA-Specs must have a XPCA-Version header which indicates the version of XPCA that the feature will be released with. Informational and Process XPCA-Specs do not need a XPCA-Version header.

XPCA-Specs may have a Requires header, indicating the XPCA-Spec numbers that this XPCA-Spec depends on.

XPCA-Specs may also have a Superseded-By header indicating that a XPCA-Spec has been rendered obsolete by a later document; the value is the number of the XPCA-Spec that replaces the current document. The newer XPCA-Spec must have a Replaces header containing the number of the XPCA-Spec that it rendered obsolete.

Auxiliary Files

XPCA-Specs may include auxiliary files such as diagrams. Such files must be named XPCA-Spec-XXXX-Y.ext, where “XXXX” is the XPCA-Spec number, “Y” is a serial number (starting at 1), and “ext” is replaced by the actual file extension (e.g. “png”).

Reporting XPCA-Spec Bugs, or Submitting XPCA-Spec Updates

How you report a bug, or submit a XPCA-Spec update depends on several factors, such as the maturity of the XPCA-Spec, the preferences of the XPCA-Spec author, and the nature of your comments. For the early draft stages of the XPCA-Spec, it’s probably best to send your comments and changes directly to the XPCA-Spec author. For more mature, or finished XPCA-Specs you may want to submit corrections to the XPCA issue tracker so that your changes don’t get lost. If the XPCA-Spec author is a XPCA developer, assign the bug/patch to him, otherwise assign it to the XPCA-Spec editor.

When in doubt about where to send your changes, please check first with the XPCA-Spec author and/or XPCA-Spec editor.

XPCA-Spec authors who are also XPCA committers can update the XPCA-Specs themselves by using “git push” to submit their changes.

Transferring XPCA-Spec Ownership

It occasionally becomes necessary to transfer ownership of XPCA-Specs to a new champion. In general, we’d like to retain the original author as a co-author of the transferred XPCA-Spec, but that’s really up to the original author. A good reason to transfer ownership is because the original author no longer has the time or interest in updating it or following through with the XPCA-Spec process, or has fallen off the face of the ‘net (i.e. is unreachable or not responding to email). A bad reason to transfer ownership is because you don’t agree with the direction of the XPCA-Spec. We try to build consensus around a XPCA-Spec, but if that’s not possible, you can always submit a competing XPCA-Spec.

If you are interested in assuming ownership of a XPCA-Spec, send a message asking to take over, addressed to both the original author and the XPCA-Spec editor xpca-specs@xpca.org. If the original author doesn’t respond to email in a timely manner, the XPCA-Spec editor will make a unilateral decision (it’s not like such decisions can’t be reversed :).

XPCA-Spec Editor Responsibilities & Workflow

A XPCA-Spec editor must subscribe to the xpca-specs@xpca.org list. All XPCA-Spec-related correspondence should be sent (or CC’d) to xpca-specs@xpca.org (but please do not cross-post!).

For each new XPCA-Spec that comes in an editor does the following:

Read the XPCA-Spec to check if it is ready: sound and complete. The ideas must make technical sense, even if they don’t seem likely to be accepted. The title should accurately describe the content. Edit the XPCA-Spec for language (spelling, grammar, sentence structure, etc.), markup (for reST XPCA-Specs), code style (examples should match XPCA-Spec 8 & 7). If the XPCA-Spec isn’t ready, the editor will send it back to the author for revision, with specific instructions.

Once the XPCA-Spec is ready for the repository, the XPCA-Spec editor will:

Assign a XPCA-Spec number (almost always just the next available number, but sometimes it’s a special/joke number, like 666 or 3141).

List the XPCA-Spec in XPCA-Spec 0 (in two places: the categorized list, and the numeric list).

Add the XPCA-Spec to Git. For git work flow instructions, follow The XPCA Developers Guide

The git repo for the XPCA-Specs is:


Monitor XPCA.org to make sure the XPCA-Spec gets added to the site properly.

Send email back to the XPCA-Spec author with next steps (post to XPCA-list & -dev).

Updates to existing XPCA-Specs also come in to xpca-specs@xpca.org. Some XPCA-Spec authors may not have commit access to the repository, in which case we will do the commits for them.

Many XPCA-Specs are expected to be written and maintained by developers with write access to the XPCA reference implementation codebase. The XPCA-Spec editors will monitor the XPCA-checkins list for XPCA-Spec changes, and correct any structure, grammar, spelling, or markup mistakes we see.

The editors don’t pass judgement on XPCA-Specs. We merely do the administrative & editorial part. Except for times like this, there’s relatively low volume.

This document has been placed in the public domain.

  1. Index of Python Enhancement Proposals (PEPs), http://www.python.org/dev/peps/

  2. XPCA_org issue tracker, https://github.com/ferrisoxide/xpca_org/issues

  3. http://www.opencontent.org/openpub/

  4. http://en.wikipedia.org/wiki/Markdown

  5. nanoc Home Page, http://nanoc.stoneship.org/

  6. http://maruku.rubyforge.org/maruku.html