Architecture Design Records

Any significant design decision in PennyLane must first be discussed and agreed to by the core development team before implementation begins. This is done by drafting an Architectural Design Record (ADR), which details the context behind the proposal, potential solutions, and any consequences should the ADR be accepted.

This document describes the requirements and recommendations for PennyLane ADRs.

When to write an ADR

Not all contributions and modifications of the PennyLane source code requires an ADR. Bug fixes and minor modifications can usually proceed directly to a pull request. Further, straightforward modifications, where no major architectural or user interface decisions need to be made—such as adding a new template or optimizer—can usually proceed directly to a pull request.

On the other hand, the following changes typically require an ADR proposal:

  • Changes to the user interface

  • Changes to the device or plugin API

  • Changes that affect multiple core components of the library

  • New features that require decisions as to source code location and user interface

  • Structural changes to the source code, documentation, or tests

If you are unsure if a change requires an ADR proposal or not, please open a GitHub issue or post in the discussion forum to discuss it with the PennyLane development team.

What to include in a PennyLane ADR

The ADR is a Markdown or reStructuredText document that outlines:

  • The context behind this design discussion,

  • Analysis of potential options, pros/cons of each approach, and any implementation ‘gotchas’ that might arise, and

  • A summary of the decision and an outline of the required work package.

The ADR must have the following components:


The title should directly reflect the decision made by the ADR.

The title should be 50 characters or less, reflect the final decision, and be written in present tense imperative form. For example, “Move common plugin utilities to PennyLane core”, or “Add a high-level qnn module to PennyLane”.


This acts as a record of status updates for the ADR.

Statuses should be one of: ‘Proposed’, ‘Accepted’, ‘Rejected’, ‘Deprecated’, ‘Superseded’, or ‘Amended’. When the status changes, leave the old status and create a new entry for the new status, so that we have a log of when changes were made.


Background information, alongside any other general organizational or architectural needs, that allows readers to place the ADR in context.

The context section should provide sufficient information such that readers of the ADR understand the problem the ADR is addressing, and why the ADR is required. This section can include code/pseudo-code snippets, diagrams, and images if relevant.


The outcome of any research undertaken to make the decision.

This should include an overview of all possible alternatives, potential risks and downsides, assumptions that have been made, and all implications. As with the context section, this section can include code/pseudo-code snippets, diagrams, and images if relevant.


Outlines the final decision(s), in present-tense imperative form.

The decision section should also include:

  • A summary of the resulting work package. This is typically a bullet-point list, with each item corresponding to a task such as a pull request or follow-on ADR. Care should be taken such that the work package is sufficiently broken down; a series of smaller, self-contained pull requests should be favoured over fewer, larger pull requests.

  • Additional or further questions. An ADR may be accepted with some open questions, as long as their resolution is (a) not a short-term priority, or (b) must be informed by the upcoming implementation. These questions, and any missing information, should be recorded here.

In addition, the following optional sections may be included:

  • A review section, where notes can be made after the decision has been implemented for a record of the actual outcome and effect the decision has had.

  • A further reading section where links can be posted to relevant external documentation, articles, etc.