Kickoff Data Liberation: Let's Build WordPress-first Data Migration Tools

[adamziel]

For project updates, see the tracking issue.

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Let's officially kickoff the Data Liberation efforts under the Playground umbrella and unlock powerful new use cases for WordPress.

Rationale

Why work on Data Liberation?

WordPress core really needs reliable data migration tools. There's just no reliable, free, open source solution for:

• Content import and export
• Site import and export
• Site transfer and bulk transfers, e.g. mass WordPress -> WordPress, or Tumblr -> WordPress
• Site-to-site synchronization

Yes, there's the WXR content export. However, it won't help you backup a photography blog full of media files, plugins, API integrations, and custom tables. There are paid products out there, but nothing in core.

At the same time, so many Playground use-cases are all about moving your data. Exporting your site as a zip archive, migrating between hosts with the Data Liberation browser extension, creating interactive tutorials and showcasing beautiful sites using the Playground block, previewing Pull Requests, building new themes, and editing documentation are just the tip of the iceberg.

Why the existing data migration tools fall short?

Moving data around seems easy, but it's a complex problem – consider migrating links.

Imagine you're moving a site from https://my-old-site.com to https://my-new-site.com/blog/. If you just moved the posts, all the links would still point to the old domain so you'll need an importer that can adjust all the URLs in your entire database. However, the typical tools like preg_replace or wp search_replace can only replace some URLs correctly. They won't reliably adjust deeply encoded data, such as this URL inside JSON inside an HTML comment inside a WXR export:

The only way to perform a reliable replacement here is to carefully parse each and every data format and replace the relevant parts of the URL at the bottom of it. That requires four parsers: an XML parser, an HTML parser, a JSON parser, a WHATWG URL parser. Most of those tools don't exist in PHP. PHP provides json_encode(), which isn't free of issues, and that's it. You can't even rely on DOMDocument to parse XML because of its limited availability and non-streaming nature.

Why build this in Playground?

Playground gives us a lot for free:

• Customer-centric environment. The need to move data around is so natural in Playground. So many people asked for reliable WXR imports, site exports, synchronization with git, and the ability to share their Playground. Playground allows us to get active users and customer feedback every step of the way.
• Free QA. Anyone can share a testing link and easily report any problems they found. Playground is the perfect environment to get ample, fast moving feedback.
• Space to mature the API. Playground doesn’t provide the same backward compatibility guarantees as WordPress core. It's easy to prototype a parser, find a use case where the design breaks down, and start over.
• Control over the runtime. Playground can lean on PHP extensions to validate our ideas, test them on a simulated slow hardware, and ship them to a tablet to see how they do when the app goes into background and the internet is flaky.

Playground enables methodically building spec-compliant software to create the solid foundation WordPress needs.

The way there

What needs to be built?

There's been a lot of gathering information, ideas, and tools. This writeup is based on 10 years worth of site transfer problems, WordPress synchronization plugins, chats with developers, analyzing existing codebases, past attempts at data importing, non-WordPress tools, discussions, and more.

WordPress needs parsers. Not just any parsers, they must be streaming, re-entrant, fast, standard compliant, and tested using a large body of possible inputs. The data synchronization tools must account for data conflicts, WordPress plugins, invalid inputs, and unexpected power outages. The errors must be non-fatal, retryable, and allow manual resolution by the user. No data loss, ever. The transfer target site should be usable as early as possible and show no broken links or images during the transfer. That's the gist of it.

A number of parsers have already been prototyped. There's even a draft of reliable URL rewriting library. Here's a bunch of early drafts of specific streaming use-cases:

• A URL parser
• A block markup parser
• An XML parser, also explored by @dmsnell and @jonsurrell
• A Zip archive parser
• A multihandle HTTP client without curl dependency
• A MySQL query parser started by @zieladam and now explored by @JanJakes
• A stream chaining API to connect all these pieces

On top of that, WordPress core now has an HTML parser, and @dmsnell have been exploring a UTF-8 decoder that would to enable fast and regex-less URL detection in long data streams.

There are still technical challenges to figure out, such as how to pause and resume the data streaming. As this work progresses, you'll start seeing incremental improvements in Playground. One possible roadmap is shipping a reliable content importer, then reliable site zip importer and exporter, then cloning a site, and then extends towards full-featured site transfers and synchronization.

How soon can it be shipped?

Three points:

• No dates.
• Let's keep building on top of prior work and ship meaningful user flows often.
• Let's not ship any stable public APIs until the design is mature.

For example, the Try WordPress extension can already give you a Playground site, even if you cannot migrate it to another WordPress site just yet.

Shipping matters. At the same time, taking the time required to build rigorous, reliable software is also important. An occasional early version of this or that parser may be shipped once its architecture seems alright, but the architecture and the stable API won't be rushed. That would jeopardize the entire project. This project aims for a solid design that will serve WordPress for years.

The progress will be communicated in the open, while maintaining feedback loops and using the work to ship new Playground features.

Plans, goals, details

Next steps

Let's start with building a tool to export and import a single WordPress post. Yes! Just one post. The tricky part is that all the URLs will have to be preserved.

From there, let's explore the breadth and depth of the problem, e.g.:

• Rewriting links
• Frontloading media files
• Preserving dependent data (post meta, custom tables, etc.)
• Exporting/importing a WXR file using the above
• Pausing and resuming a WXR export/import
• Exporting/importing a full WordPress site as a zip file

Ideally, each milestone will result in a small, readily reusable tool. For example "paste WordPress post, paste a new site URL, get your post migrated".

There's an ample body of existing work. Let's keep the existing codebases (e.g. WXR, site migration plugins) and discussions open in a browser window during this work. Let's involve the authors of these tools, ask them questions, ask them for reviews. Let's publish the progress and the challenges encountered on the way.

Design goals

• Fault tolerance – all the data tools should be able to start, stop, resume, tolerate errors, accept alternative data from the user, e.g. media files, posts etc.
• WordPress-first – let's build everything in PHP using WordPress naming conventions.
• Compatibility – Every WordPress version, PHP version (7.2+, CLI), and Playground runtime (web, CLI, browser extension, desktop app, CI etc.) should be supported.
• Dependency-free – No PHP extensions required. If this means we can't rely on cUrl, then let's build an HTTP client from scratch. Only minimal Composer dependencies allowed, and only when absolutely necessary.
• Simplicity – no advanced OOP patterns. Our role model is WP_HTML_Processor – a single class that can parse nearly all HTML. There's no "Node", "Element", "Attribute" classes etc. Let's aim for the same here.
• Extensibility – Playground should be able to benefit from, say, WASM markdown parser even if core WordPress cannot.
• Reusability – Each library should be framework-agnostic and usable outside of WordPress. We should be able to use them in WordPress core, WP-CLI, Blueprint steps, Drupal, Symfony bundles, non-WordPress tools like https://github.com/adamziel/playground-content-converters, and even in Next.js via PHP.wasm.

Prior art

Here's a few codebases that needs to be reviewed at minimum, and brought into this project at maximum:

• URL rewriter: https://github.com/adamziel/site-transfer-protocol
• URL detector : HTML API: Refactor make_clickable() wordpress-develop#7450
• WXR rewriter: https://github.com/adamziel/wxr-normalize/
• Stream Chain: StreamChain: An API for streams-processing data (e.g. HTTP → ZIP → XML → HTML) adamziel/wxr-normalize#1
• WIP: Create Unicode-aware comprehensive sluggify(). wordpress-develop#5466
• Experiment: Doodlings on how a Core URL parser could look. wordpress-develop#6666
• XML parser: Add WP_XML_Tag_Processor and WP_XML_Processor wordpress-develop#6713
• Streaming PHP parsers: https://github.com/WordPress/blueprints-library/tree/trunk/src/WordPress
• Zip64 support (in JS ZIP parser): Add github:artifact resource that unzips the doubly zipped files #1799
• Local Zip file reader in PHP (seeks to central directory, seeks back as needed): https://github.com/adamziel/wxr-normalize/blob/rewrite-remote-xml/zip-stream-reader-local.php
• Introduce custom UTF-8 decoding pipeline. wordpress-develop#6883
• Blocky formats – Markdown <-> Block markup WordPress plugin: https://github.com/dmsnell/blocky-formats
• Sandbox Site plugin that exports and imports WordPress to/from a zip file: https://github.com/WordPress/playground-tools/tree/trunk/packages/playground
• WordPress + Playground CLI setup to import, convert, and exporting data: https://github.com/adamziel/playground-content-converters
• Markdown -> Playground workflow and WordPress plugins: https://github.com/adamziel/playground-docs-workflow
• Edit Visually browser extension for bringing data in and out of Playground: Edit Visually browser extension – iteration 2 playground-tools#298
• Try WordPress browser extension that imports existing WordPress and non-WordPress sites to Playground: https://github.com/WordPress/try-wordpress/
• Humanmade WXR importer designed by @rmccue: https://github.com/humanmade/WordPress-Importer

Related resources

• Site transfer protocol
• Existing data migration plugins
• Solving rewriting site URLs in WordPress using the HTML API and URL parser data-liberation#74
• WordPress for Docs #1524
• Collaborative Editing gutenberg#65012

The project structure

The structure of the data-liberation package is an open exploration and will change multiple times. Here's what it aims to achieve.

Structural goals:

• Publish each library as a separate Composer package
• Publish each WordPress plugin separately (perhaps a single plugin would be the most useful?)
• No duplication of libraries between WordPress plugins
• Easy installation in Playground via Blueprints, e.g. no composer install required
• Compatibility with different Playground runtimes (web, CLI) and versions of WordPress and PHP

Logical parts

• First-party libraries, e.g. streaming parsers
• WordPress plugins where those libraries are used, e.g. content importers
• Third party libraries installed via Composer, e.g. a URL parser

Ideas:

• Use Composer dependency graph to automatically resolve dependencies between libraries and WordPress plugins
• or use WordPress "required plugins" feature to manage dependencies
• or use Blueprints to manage dependencies

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