State of the REST API 2017

As we approach the State of the Word 2017 at WordCamp US, I think this is a good time to look back on the state of the REST API project and core focus over the last year.

2017 has been an interesting year for the REST API with highs and lows, and periods of intense development and slowdowns.


Immediately after WordCamp US 2016, WordPress 4.7 was released, which merged the second stage (endpoints) of the REST API. This was a major milestone for the REST API, and marked the culmination of our efforts over the previous 4 years.

In the months after 4.7’s release, we followed up by fixing bugs in the REST API, including two security bugs. One of these security bugs was a very serious privilege escalation issue, which was unfortunately caused by an unrelated low-level change in WordPress that the API hadn’t guarded against. A huge thanks to Securi, the security team, and to the hosts for working hard on mitigating, fixing, and deploying this issue. As the first large security release involving the API team, there was definitely much for us to learn, and we definitely learnt much.

With 4.7 representing a major culmination of the API team effort, most of the “core” REST API team (myself, Daniel, Rachel, and Joe) who lead the project pre-merge ramped down contribution. After years of intense effort on the project, we simply needed a break. As is the nature of open source projects, contribution is voluntary, and a massive thanks has to go to everyone involved for such a long time, especially to my co-lead Rachel. Thanks also to those who picked up the baton on the core work, including (but not limited to) Adam Silverstein, James Nylen, and K. Adam White.

The API has continued to improve in an iterative way throughout the year, with bug fixes and improvements from many members of the community. These have helped the API become more refined, stable, and most importantly, useful.

Core Focus

Organisationally, as we shifted from an independent feature project to part of core, the API also transitioned from a project to a “core focus”. This is a new concept and structure in WordPress, representing a large shift from the previous, release-driven product cycles.

With this change, our official goal was set as “getting first party wp-admin usage of the new endpoints, and hopefully [replacing] all of the core places where we still use admin-ajax”. Progress towards this goal throughout the year has been slow.

Part of the reason for this slowness has been a major drop in contribution. With our shift from GitHub to Trac, the number of drive-by contributions has fallen, with contributions coming more from regular core contributors. Additionally, with most of the core API team taking a break and moving on, the organisational and regular contribution has dropped massively. The combination of this drop in contribution to the API along with the ramp up in contribution on other focuses (Gutenberg in particular) means we are by far and away the slowest moving core focus, which has the flow-on effect of making us less attractive to contribution.

The scope of our official goal is also massive. As WordPress has grown organically from a static HTML admin to a more interactive interface, admin-ajax endpoints have grown likewise, with specialised endpoints added as and when needed. Our audit of the actions in WordPress showed 92 separate endpoints across 14 different categories, spanning every section of the admin. The organic growth of these has meant that a lot of the frontend code is tied specifically to the admin-ajax response, and vice versa, including passing generated HTML. In the process of investigating with these endpoints, experimentation showed that switching to use the API would essentially require rewriting the feature in order to use it.

Fundamentally, I think the core focus’ goal is at odds with the strengths of the REST API team. Our focus should be providing support to other teams (like the Gutenberg, Customiser, and mobile teams) and empowering developers to build on the API, rather than rewriting parts of the admin. The goal of switching the admin to the REST API is valuable, but it should be part of efforts to improve the user experience, rather than simply refactoring (especially when refactoring would needlessly break existing plugin functionality).

However, progress towards the goal across the entirety of WordPress has been good. One of the largest areas of admin-ajax usage is in the edit screen, where many of the admin-ajax endpoints will be replaced entirely through Gutenberg’s use of the REST API. Likewise, the Media endpoints will likely become legacy as the media library changes for use with Gutenberg. A significant portion of the endpoints are around themes, which are covered by the Customiser team’s efforts. The accessibility team is in the process of reworking the settings pages, which as a side-effect, will allow us to remove the settings endpoints; the Live Settings experiment also shows that we have the ability to improve the user experience here as well. All of these efforts move away from admin-ajax while also improving the user experience.

Moving forward into 2018, the REST API team needs to shift focus to the issues with the API, and focus on helping other teams do what they do best.

Our Big Issues

There are three big issues which the REST API should be focussed on to move forward: authentication, functionality, and empowerment.


I’ve talked about authentication endlessly, and will keep talking about it until we solve it, because it is one of the largest problems we still have. Without a viable external authentication solution, over a third of the code in the REST API isn’t usable outside of the site. Imagine if Stripe’s API didn’t let you charge new credit cards, or if Twitter’s mobile apps only let you read tweets but not reply. This is the situation facing API users right now, and is the biggest reason the WordPress mobile app is not powered by the REST API.

Discussions at the Contributor Summit this year with the mobile team were fruitful, and we have a practical plan forward for rolling out select support for the official mobile apps while we work on rolling out general solutions. (As well as potentially enhancing the user experience significantly with things like magic login links.)

Efforts at the WordCamp Europe contributor day provided a massive push forward on OAuth 2 support, with the plugin now beginning to stabilise. A further focus here will allow us to build up the crucial momentum for development, and work with client developers.

OAuth 2 also provides a much cleaner way forward to solving the distributed API problems than our previous solutions. In the coming weeks, we’ll publish the first alpha versions of this new solution, which stays true to the original design goals of the first version of the broker while improving upon it in every way.


With the push for merging the REST API, the nature of deadlines meant we had to push many features from the API to double-down on our core functionality. However, this means that the REST API is incomplete. In particular, we’re missing support for key objects in WordPress, including menus, widgets, plugins, and themes. We’re also missing some crucial functionality around existing objects, like post previewing. This means that while it’s possible to build apps on top of the API, some of the core functionality users expect is missing from those apps.

This has flow-on effects to the other core focuses. Incomplete support for drafts in the API has caused problems in Gutenberg, and the lack of support for appearance functionality causes the burden to shift on to the Customiser team. These fall squarely within our responsibility, and we’re currently letting them down.


The core of what the REST API does is empower developers to build things better and faster. To empower developers, we need to take the time to improve our documentation and provide better tooling. We have two fantastic API client libraries in the Backbone and Node libraries, and we should continue to push these forward while also helping to develop the client library ecosystem.

Key to this is ensuring that we are where our users are. Our previous contribution process on GitHub allowed us to benefit from drive-by contributions from developers using the API, and the momentum of Gutenberg on GitHub likewise shows the power of being where developers are. We recently migrated the developer reference to GitHub, and we need to look at further ways to embrace the external developer community here.

Additionally, we need to engage in more outreach with our users. While the user experience side of WordPress engages in user testing and outreach, there’s no equivalent for the REST API. We should be getting more client developers involved in the process, including the mobile team and developers of significant apps like Calypso and MarsEdit.

Our Future

While 2017 has been a tough year for the API, as we move into 2018, I think the state of the REST API is strong. We have the skills and the vision to push the API forward with our focuses, and empower other developers, whether inside the core development community or external.

But in order to enact these changes, make progress, and rebuild a strong contributor base, we need to make the fundamental changes to our organisation and goals to push forward.

We need to embrace our strengths and focus on the areas of highest impact. We need to listen to and work with our users on improving their experience and fixing their problems. And most importantly, we need to get more users by making the REST API more useful to more people.

Patch WordPress via GitHub

A few days ago, I started tweeting about the Stack Overflow Developer Survey, where 74% of developers surveyed said they dread working with WordPress. I received a tonne of replies that I’m still working through, and I’ll post about that soon.

One reply that did come up a few times was contributing via GitHub. Matt announced in the State of the Word that you’d soon be able to contribute to WP via pull requests, however that hasn’t happened so far. I had a few discussions with some of the core team about this, but alas it never got anywhere.

However, after this discussion, I realised I could do something about it right now as a proof-of-concept. Trac exposes an XML-RPC interface, and GitHub exposes a REST API, so hooking the two up only requires a minimal amount of code.

So, introducing GitHub-to-Patch, a tiny utility to allow submitting PRs to WordPress.

Here’s how you submit a pull request for WordPress using this:

  1. Find the ticket on Trac you want to upload a patch to.
  2. Submit a pull request to the WordPress/WordPress repo, then close it to keep GitHub clean. (You can still continue to update it.)
  3. Head to the GitHub-to-Patch page.
  4. Select your pull request.
  5. Enter the ticket number.
  6. Enter your Trac/ username and password.
  7. Preview the patch you’re about to submit and verify the details.
  8. Done! You should also leave a comment about the patch you just added. πŸ™‚

If you update your PR and want to upload your changes, simply repeat the same process; Trac will automatically name the patches correctly to avoid overwriting previous ones.

Internally, the utility uses GitHub’s API to get a patch format of the pull request, then uses Trac’s XML-RPC API to upload. This requires your credentials, and because of cross-origin policy, also requires an intermediary server. πŸ™ I hope to fix this in the future, either by integrating the tool into Trac itself, or by using OAuth with In the meantime, if you don’t trust my server, you can install and run the tool from GitHub with minimal effort.

In the future, I’ll likely create a PR bot to automatically close PRs and point users to the tool, and to note when people have uploaded their PR as a patch.

Thanks to Eric Andrew Lewis for his pull request to the grunt-patch-wordpress repo that made me realise I could do this. πŸ™‚

The (Complex) State of Meta in the WordPress REST API

One of the other discussion points in our recent API meeting was the state of meta in the REST API. We recently made the somewhat-controversial decision to remove generic meta handling from the API. As we didn’t have time to get into the specifics in the meeting, I wanted to expand on exactly what we’re doing here, and our future plans.1

WordPress has four different types of meta: post meta, comment meta, term meta, and user meta. These broadly act the same, so for simplicity’s sake, I’ll be grouping them together as just “meta”.

Meta also falls into two broad groups: plugin data, and user input. The distinction here is that plugin meta is set by a plugin programmatically, whereas user input is set via the Custom Fields metabox. These are broad categorisations, but the general difference is that plugin meta tends to be “protected” (typically prefixed with an underscore), whereas user input meta is any sort of freeform name (and occasionally no name at all).

Solution for Plugin Data

Right now, there is a viable solution for plugins to handle meta through the REST API: register_rest_field(). This function allows registering extra fields on a resource (like a post) and handling them in your own code.

For example, let’s say we have a plugin that adds “featured emoji” to a post, which saves a string of emoji characters for a post. We already have a metabox for this in the admin, and now we want to expose it via the API. This is super easy:

register_rest_field( 'post', 'featured_emoji', array(
    'get_callback' => function ( $data ) {
        return get_post_meta( $data['id'], '_featured_emoji', true );
    'update_callback' => function ( $value, $post ) {
        // TODO: sanitize and validate this field better
        $value = sanitize_text_field( $value );

        update_post_meta( $post->ID, '_featured_emoji', wp_slash( $value ) );
    'schema' => array(
        'description' => __( 'Featured emoji for the post to add a little flavour.', 'femoji' ),
        'type' => 'string',
        'context' => array( 'view', 'edit' ),

Solution for Custom Fields

User input meta is also handled, using the generic meta API. This is the /wp/v2/posts/{id}/meta route in the API, which is the route that was recently pulled out of the API plugin itself.

This route is practically only useful for replicating the Custom Fields metabox in the post editor and is not generally useful for plugins and themes. In fact, the endpoints have feature parity with the Custom Fields metabox and the same rules around visibility: if it appears in the metabox, it appears in the API (and vice versa).

Why Separate Solutions?

You may be wondering why we can’t use the same solution for both groups of meta. There are a number of complex issues here, but the key issue is that we cannot reliably separate the two groups. Unlike custom types (post types, taxonomies), meta doesn’t have to be registered before use. This is super handy most of the time, but also means that meta is a bit of a minefield. This leads to surprising behaviour for API users: plugin meta is (mostly) not available via the /meta endpoint.

Protected Meta

The _ prefix is used throughout WordPress to indicate that a field is “protected”. Unfortunately, exactly what “protected” means is usually undefined, but the one thing it reliably indicates is that the key shouldn’t be exposed through the Custom Fields metabox. As the /meta endpoint is designed to mirror the metabox, we don’t expose protected meta via the endpoints. This means that this endpoint isn’t useful for many plugins.

You can, however, whitelist individual keys by filtering is_protected_meta. This allows exposing plugin data via this standard meta API; for example, to expose WooCommerce’s _price field:

add_filter( 'is_protected_meta', function ( $protected, $key, $type ) {
    if ( $type === 'post' && $key === '_price' ) {
        // Expose the `_price` meta value publicly
        return true;
    return $protected;
}, 10, 3 );

This can be somewhat confusing though, because protected meta is still not exposed if it falls into one of a few other categories. In addition, it will now appear in the Custom Fields metabox as well.

Complex Values & Serialized Data

One of the categories of meta we can’t expose is serialized data. This applies regardless of whether the meta field is marked as protected or not. This is potentially surprising to plugin authors who might be explicitly whitelisting their meta field for the API, and yet it still isn’t exposed. The key reason for this is that accepting serialized data is either lossy or unsafe.

To understand why serialized data is unsafe, we need to look at what serialized data actually is. At its core, serialization is a way to pack complex data into simple data, in this case a string. We need to include enough data to reverse the process to ensure the process is lossless. The PHP serialization format encodes the two pieces of data that a variable contains: the type, and the value. For simple scalar values, the scalar type itself is encoded: integers become i:val;, such as i:42;; strings become s:size:value; such as s:3:foo;, etc. Arrays are encoded in a more complex way, as they need to encode the type (array), size, keys, and values: this is encoded as a:size:{key;value} where key is a serialized scalar value and value is any serialized value. For example, array('foo' => 42) serializes to a:1:{s:3:"foo";i:42;}.

Objects are slightly more complex, because the “type” itself is complex and includes the class. The format is very similar to arrays (as objects are essentially just the property array), but with the a type replaced with O:classnamelength:classname as the type. This gives a value like O:16:"WP_HTTP_Response":3:{s:4:"data";N;s:7:"headers";a:0:{}s:6:"status";i:200;}.2

The object type is where the problems with serialized meta arise. When a serialized value is unserialized, these classes are instantiated, and the __wakeup() method on the class is executed if it exists. Because of this, allowing serialized data to be saved allows remote code execution by the client saving the data. For example, if an attacker finds a class (and you only need one) with a __wakeup method, they can execute that code by submitting serialized data. Alternatively, if a class assumes that one of its properties is safe to run eval on, or to pass into the database directly, this can be exploited too.

This may sound a bit daft, but this is not a theoretical bug. YAML supports deserializing data into Ruby objects with --- !ruby/hash:classname. This wasn’t generally seen as an issue until it was discovered that a specific ActionDispatch object in Ruby on Rails was running eval on one of its properties. As a result, every Rails site was vulnerable to arbitrary code execution, which is one of the worst classes of bugs.

Serialized objects are not inherently dangerous, but they massively increase the attack surface of the API. Exposing serialized objects as read-only is almost a potential privacy issue, as it leaks internal implementation details (class names). For these reasons, we made a calculated decision not to allow serialized data.

One potential solution to allowing complex data is to convert it to JSON-native data. The issue with this is that JSON-encoding data is lossy. PHP objects will be converted down to a generic JSON object, and associative arrays and object data cannot be distinguished. Additionally, PHP doesn’t distinguish between numerically-indexed arrays (JSON lists) and associative arrays (JSON objects). These issues mean that simply sending back the object you received will cause data loss.

For these reasons, we can’t support serialized data in the API via any endpoints, including meta and a future options endpoint.3


As a result of most meta not being registered, the permissions area is a bit sketchy. While the add_post_meta, edit_post_meta and delete_post_meta meta-capbilities exist, there’s no similar meta-capability for reading post meta. This is the key reason meta is only available while authenticated, as we need to instead fall back to edit_post.

This again is a result of user input meta and plugin data not being clearly defined. In a very early version of the API, user input meta was exposed by default, until it was noted that users often use these fields for internal notes and workflow. (Despite this, the_meta() template tag exists to output these fields on the frontend.)

In addition, plugins adding meta have no fine-grained controls over meta field access. While write capabilities can be controlled precisely, whether someone can read the meta fields depends on how they’re used and can be inconsistent.

Making It All Better

So, how do we fix all of this? A while ago we talked about loosening the rules, but it turned out this wasn’t viable without core changes to WordPress. During the hackday for A Day of REST a few weeks ago, one of the groups took on this issue and came up with a plan. Key to this plan is changing core to support better meta registration.4

These changes to core should improve meta usage not just in the REST API, but also across the board for the rest of core and plugins. This also helps to lay some of the groundwork and low-level infrastructure for the fields API in a future version of WordPress.5 Expanding this out allows better tooling around meta as well; for example, we may be able to clean up metadata for deactivated plugins if meta is registered consistently.

As tooling and infrastructure develops around meta fields (including the fields API), this may allow us to solve the complex data issues as well. Being able to explicitly say that a field contains a list of strings (e.g.) would allow us to safely expose the values, and avoid potential data loss from JSON serialisation.

These changes will take time to finalise and execute, and it will be a while until the ecosystem fully adopts these changes. In the meantime though, we’d like to ship a REST API. Without these changes, we don’t have the ability to automatically expose plugin meta, however plugins can already register their fields manually, and future changes would simply provide better tools for developers.

We believe it’s in the WordPress project’s best interest to ship what we have and continue to iterate as we make these changes. Holding back the rest of the API for completion’s sake benefits nobody.

Thank you to the Meta Team at A Day of REST for volunteering to tackle this complex issue, and for their comprehensive discussion and planning. Thanks also to Brian Krogsgard for proofreading, and to Daniel Bachhuber, Joe Hoyle, and Rachel Baker for being generally awesome.

  1. We also had to gloss over exactly how progressive enhancement works, so I fleshed this out in a recent post if you missed it. []
  2. Objects implementing the Serializable interface instead use C: instead of O:, but these are not supported by WordPress for historical reasons. []
  3. “What about XML-RPC?” you may ask. The XML-RPC API only allows reading serialized meta, which is a minor privacy issue as it may expose internal implementation but is not a security issue. However, since serialized meta can’t be saved via the XML-RPC API, attempting to write the data you just read for a field will cause it to be saved double-encoded, which means it’s lossy. []
  4. Did you know there’s a system in core to register meta? I didn’t before we tackled this problem in the API, and that’s a key part of the problem. []
  5. This “groundwork” consists of expanding the scope of register_meta to take arbitrary parameters similar to register_post_type, plus promoting register_meta and making sure people know it actually exists. []

Progressive Enhancement With the WordPress REST API

In a REST API discussion today, we discussed the future of the REST API. Something I touched upon briefly in that meeting is the concept of progressive enhancement with the REST API. Since this topic hasn’t been brought up much previously, I want to elaborate on how progressive enhancement works.

Progressive enhancement is our key solution to a couple of related problems: forward-compatibility with future features and versions of WordPress, and robust handling of data types in WordPress. Progressive enhancement also unblocks the REST API project and ensures there’s no need to wait until the REST API has parity with every feature of the WordPress admin.

For instance, custom post types can do basically whatever they want with their data, so we wanted a robust system for indicating feature support via the REST API. For example, post types which don’t have the editor support flag won’t have content registered, similar to how the admin doesn’t show the content editor for those post types. In addition, plugins can do even crazier stuff like conditionally changing post types. The system in the REST API can handle these cases with ease, providing clients the ability to adapt on-the-fly to the format of the data they’re editing or displaying.

We also recognise that the REST API needs the ability to adapt to future versions of WordPress, and we want to avoid as many breaking changes as possible. Building the abilities for feature detection enables forwards-compatibility via progressive enhancement, and gives clients a reliable paradigm to safely check whether a WordPress supports a feature before trying to use it.

The progressive enhancement concept builds heavily on the model already used by browsers for this purpose. If you want to build a site that uses geolocation (e.g.), you can easily detect support for that and build while waiting for browser support, even including polyfills. Feature detection with the REST API can allow the same technique, and allow polyfilling while waiting for the long-tail of sites to update.

The interplay with the complexity of custom post types is almost a bonus here. If I’m building a replica of the post editor, using feature detection to select which “metaboxes” to show is basically a necessity. In the case of meta, clients need to be robust enough to do this already, as plugins can remove plugin support for custom-fields from the built-in post types, and clients need to respond to this.

Progressive enhancement exists in the REST API already, and is easily usable and accessible by clients that want to ensure robustness.

Building With Progressive Enhancement Today

As an example, let’s say I’m building a simple editor today that uses the REST API. Imagine essentially a slimmed down version of Calypso or MarsEdit.

My editor allows me to write posts, save them as drafts, edit them again later, and publish them when I’m ready. After the post is published, I can update and save, and that affects the live post. I can’t do post previews, as there’s no autosave support built in.1

For now, I build my client without the autosave support, and instead bake autosave features into the editor itself. The WordPress admin already does this with localStorage saving for offline connections, and this system doesn’t require server-side support.

Progressively Enhancing In A Future Release

In a future release, we have the autosaving process nailed down, so we mark our extra feature plugin as done and merge it into core. The autosave endpoint then gets rolled out in the next WordPress major release.

In my client, I want to add the extra server-side autosave support on top of my local autosaving. To do this, I look to see if the feature is supported on the site. In this case, the “feature” I want is the POST endpoint on the /wp/v2/posts/{id}/autosave route, so I check the index to see if the route is there and supports the method.

I see that the site supports the method, so my client transparently starts using server-side autosaves.

This feature detection already exists in the REST API today. For instance, compare the results of and You can easily see which supports creating posts by inspecting for /wp/v2/posts and see that it supports POST.

Plugin Detection

REST API clients can also easily detect which plugins are available on a site. REST API endpoints are registered with two parts to their name: the namespace and the route. The namespace typically looks like name/v1 with a unique slug combined with the version; for the core plugin, this is wp/v2. This system works similarly to function namespacing in plugins currently, and we expect (and strongly recommend) that plugins and themes treat this as their unique slice of the API space.

Let’s say I want to check if WooCommerce is installed on a site. I simply fetch the index route and check the namespaces key to see if woocommerce/v3 is registered.

Again, plugin detection already exists in the REST API. Compare again and The demo site supports the core endpoints, as it has wp/v2 registered, whereas only has the oEmbed endpoints.

More Granular Detection

We can far more granular with detection too. Each route supplies information about the schema that the response follows (and request data when creating or updating resources). This is available either via an OPTIONS request to the route, or by fetching the index with ?context=help.

To detect fields, we simply need to check the schema for that field. If generic meta support is pushed back to a future release, enabling clients to interact with this would be easy. For argument’s sake, let’s say it’s added as the custom_fields property on the post resource.

To detect feature support, we simply need to do an OPTIONS request on /wp/v2/posts/42 (for post 42), then check that $ exists, and matches the format we’re expecting. We can then display a “custom fields” metabox-style interface in the editor for this.

Again, this level of feature detection already exists in the REST API today, and even more than that, we already recommend using this process for existing endpoints. When interacting with custom post types, you can detect whether the post type is hierarchical by checking for $ You can detect whether a post supports reordering by checking for $

This applies even when not working with custom post types: you can detect whether a post supports featured images and whether the site/theme supports them by checking that $ exists. This isn’t a theoretical concern, robust editors already need to do this, as themes have differing support for WordPress features, and these changes need to flow through clients. In addition, plugins have essentially unlimited flexibility, and clients need to recognise this and support it in order to maximise compatibility across the long-tail of WordPress installs and configurations.

Meta with register_rest_field

One thing that was glossed over is that despite us pulling support for generic meta, we still have opt-in support for meta handling at a lower level.

If I’m a plugin author that wants to add my own data to a post response, I can simply use code like:

register_rest_field( 'post', 'rm_data', array(
    'get_callback' => function ( $data ) {
        return get_post_meta( $data['id'], '_rm_custom_field', true );
    'update_callback' => function ( $value, $post ) {
        update_post_meta( $post->ID, '_rm_custom_field', sanitize_text_field( $value ) );
    'schema' => array(
        'description' => 'My custom field!',
        'type' => 'string',
        'context' => array( 'view', 'edit' ),

Since I’ve registered the schema data, this is automatically added to the schema for me. Clients can then detect my feature automatically by checking for $ The API here gives me feature detection for free. The proposal to enhance register_meta in core ( will enable even easier integration with the API.

Moving Forward

Right now, the REST API team, and the WordPress community, needs a clear path forward to get from the feature-plugin as it exists today to a sustainable long-term project. Being able to ship and move on is a key part of this, as well as providing the room to expand in the future.

We believe that the progressive enhancement approach is the best approach for continuing API development. Progressive enhancement is a paradigm the REST API project ​must​ adopt, if it’s an API we want to add to (without breaking backwards compatibility) over the next 10 years.

  1. I’m choosing autosave support here, however it’s very possible this will be completed and merged in the very near future. It’s a convenient example to use though. []

How I Would Solve Plugin Dependencies

One of the longest standing issues with the plugin system in WordPress is how to solve the issue of dependencies. Plugins and themes want to bring in libraries, other plugins, or parent themes, but right now, the solutions are somewhat terrible. I thought it was time to get my thoughts down on (virtual) paper.

What’s the problem?

Software is invariably never built in isolation (“no man is an island”), so they are naturally drawn to using external libraries. Extending an existing system is also extremely useful; we can see that from the plugin ecosystem in WordPress itself.

However, right now, there’s no good way to do these in a way that interoperates with other plugins and sites. There are various third-party solutions, but often these require code duplication or offer a substandard user experience.

The Jetpack Problem

This lack of proper dependencies is one of the key reasons behind the system of ever-growing codebases, and is exactly why Jetpack is a gigantic plugin rather than being split out. In an ecosystem with a proper dependency system, Jetpack would simply be the “core” of other plugins, being depended on for core functionality, and offering UI to tie it all together.

One of my personal key problems with Jetpack is that it duplicates the plugin functionality in WordPress (poorly, at times), and hence doesn’t work with standard tooling. Real dependencies would help to solve this. A future Jetpack with a plugin dependency system shouldn’t look any different to the current UI, but would use real plugins internally. This would ensure that the Jetpack core stays lightweight while still offering all the functionality.

Changing this to use a real dependency system would have benefits both for developers and users. The install process of Jetpack could be improved by allowing the core of the plugin to be downloaded first, letting the user set up and configure Jetpack while the rest of the plugin downloads in the background. Users and developers concerned about the size of the plugin could install only the parts they need, reducing file size and potential attack surface across the plugin.

User Experience

In the wider ecosystem, we can see other plugins running into the same issue. The largest plugins, including WooCommerce, EDD and Yoast SEO, have some form of an extension list to attempt to solve this, but invariably end up offering a poorer user experience, sending users off to other sites.

Without creating a full library to handle this for a plugin, invariably we end up with terrible UX. I’ve seen plugins do everything from pop up a message on install saying “search for X, and install it”, to straight up installing plugins and breaking a site completely. This run-time verification also breaks workflow for version-controlled sites, as plugin installation and upgrading is typically done independently of the site itself.

Products vs Services

On a more selfish note, plugins like the REST API would see increased adoption from plugin and theme developers if they could use a unified, simple system to require it. For developers who actually care about user experience, giving terrible messages to users or including a complex library just for dependencies isn’t something they want to handle.

This has partially stymied adoption of the API, as “product” developers (theme and plugin developers) don’t want to offer a substandard experience, Worse, it has skewed our development pattern towards “service” developers (agencies doing work for clients, and teams running SaaS platforms), who have the ability to run anything they like without running into these issues. This means that very real issues that we need to tackle in order to scale to the long-tail may be deprioritised in favour of those affecting services.

How do we solve it?

This is one of those ideas that I’ve had floating around in my head for a while, basically fully-formed, but with no time to execute. I’m writing this as a guide to how I see the problem being solved, with the hopes that someone has the ability to execute this the way it should be done. Imagine this as a blueprint for a successful project, albeit not the final design.

(Note that whenever I say a plugin, I actually mean plugins or themes, as behaviour should be the same for both.)

Internal Workings, ft. Composer

Any PHP developer who has worked outside of WordPress recently will know Composer. Composer, for those who aren’t aware, is a command-line tool for managing dependencies in PHP. Composer is also not a good solution to the dependency problem for WordPress plugins: it requires CLI access and knowledge, it has a somewhat clunky interface and user experience (edit a JSON file, then generate a lock file and a vendor directory, then maybe commit one or more of those), and it also requires PHP 5.3+ (a non-starter for core integration, currently).

However, one of the key parts of Composer is the dependency solver, which is a port of the libzypp solver. This is a “SAT solver”: it takes note of what’s available and of what something requires, then it works out whether it can install the software (it solves the satisfiability problem). This solver is the key to working out the dependency chain for openSUSE packages (where libzypp is originally from), and the same system is used by Composer. This system would be a fantastic base for a plugin dependency system.

Developer User Experience (DUX)

The experience for developers needs to be a familiar one. Plugin headers are a great place to start, but they quickly become untenable in their current state, as they’re not built for complexity (check any theme with more than a few tags to see what I mean). It’s possibly that with some tweaking they could be used, but this may be hard to achieve.

Ideally, we’d want the dependencies to be declarative, since this would help out a bunch of automated tooling. However, we can’t solve every problem at once. For bootstrapping this project off the ground, procedural code will work just fine.

I have a semi-working proof of concept that looks something like this:

The top three lines of code are all that’s required to check if your dependencies exist. We can automatically detect which plugin called the function, and parsing it out is relatively simple; we just then need to pass it to to see if we can get it working.

I’ve also written up some more complex usage patterns for the system for developers doing more advanced usage. (Note that the documents linked here relate to an early prototype I was working on, so not everything there matches this document; notably, allowing Composer dependencies isn’t something I’d suggest for right now.)

End-User Experience (EUX)

The end-user experience is key to gaining adoption. You need to offer an experience that users are familiar with, and that doesn’t require a bunch of manual steps. We are working on computers, after all, which are meant to automate the dumb tasks for us.

The EUX starts before the user even installs a plugin or theme. The information screen needs to show them what the plugin needs (the full dependency tree, not just direct ones), as well as any potential conflicts with existing plugins. Installing that plugin should then also ensure that the dependencies are also installed, failing if any of the dependencies fails to install correctly. All of this needs to occur before the plugin is actually run, ensuring that the plugin doesn’t have to worry about double-checking everything before it can actually do any code. (This tends to overcomplicate a codebase with no gain.)

Once a plugin and its dependencies are installed, they then need to be maintained. Plugins should receive regular updates as usual, but the end user needs to at least be warned if an update will break compatibility with another. To accommodate urgent, breaking changes, users must be allowed to update plugins even if it would cause incompatibility, and the dependency system should ensure that the other plugins are disabled as needed. (If autoupdates for plugins are added to core, this would still be a manual process.) Trust the user to do the right thing, but ensure they cannot break their own system.

On the other end, uninstalling a plugin should correspondingly offer to remove anything it depends on if not being used by anything else. This again should always be the user’s choice, as depended-on plugins may have use apart from just being a dependency.


Getting these plugin dependencies available is the hardest part of the equation. Developers need to be able to depend on (ha ha) the system being available to them, otherwise it’s not going to get adoption regardless of how great it is. This is true for any core feature (like a REST API), but especially so for something that needs to essentially be hidden from the user.

The end goal here is core integration. If the solution doesn’t end up in core at the end, the project has failed, as it’s not ubiquitous. If this happens, throw out what you need and try again, but it must be in core to be a viable solution for many users.

The best alternative, and best way to bootstrap in the meantime, is to aim for integration into Jetpack. Jetpack is one of the most widely used plugins, giving you a huge userbase straight out of the gate. This solution would also be incredibly valuable to Jetpack in making it more modular, and allowing it to shed some of the weight it currently has. Obviously, no one except the Jetpack team has a say over this, but it’s a good way to get your foot in the door. (Plus, it gets the Jetpack team potential extra lock-in benefits, as everyone would need to require Jetpack, albeit temporary.)

There’s precedent in WordPress’ past for this too. Sidebar widgets were originally developed as a plugin by Automattic, then eventually integrated into WordPress core. Widgets used to bootstrap their development process, and in a modern WordPress, would likely piggy-back on Jetpack as well.

Potential Issues

One key potential issue I see is dependency versions. By allowing plugins to require certain versions, it’s possible to end up in situations where unrelated plugins cannot both be installed due to a mutual incompatibility with a library. This could be caused by a plugin requiring too specific a version (“only version 1.2.5, please!”) or an actual incompatibility between major branches. In order to balance these concerns, it may be wise to only allow requiring major versions, with the responsibility on plugin developers to stick to this system.

We also need to be careful to avoid situations like DLL Hell, where mutual incompatibilities between plugins cause installs and upgrades to be impossible without breaking something else. Encouraging plugins to maintain full compatibility is a top priority, which removing the ability to depend on specific versions may help with.

Distribution will be the biggest issue. It may be tempting to bundle with another large plugin (Yoast SEO, WooCommerce, etc), but you risk fragmentation by allowing bundling with more than one plugin, and no one’s going to want to be left without it if it’s that good. We can already see this problem with some of the libraries out there now, where mutually incompatible versions are used by different plugins.


I’m desperately hoping this post serves as inspiration for someone to create a proper solution to this. I don’t care if it gets solved the way I’ve thought of, there are plenty of other ways to skin this particular cat, and none of them is the “right” way.

(I started on a solution, but truly don’t have the time to dedicate to this. However, I’m willing to offer every piece of code I wrote for the prototype right now to kickstart this.)

What we need is something better than the current solutions. And not just better, but radically better.

Will you be the one to create it?

You’re Using Transients Wrong

The Transients API is an incredibly useful API in WordPress, and unfortunately
one of the most misunderstood. I’ve seen almost everyone misunderstand what
transients are used for and how they work, so I’m here to dispel those myths.

For those not familiar with transients, here’s a quick rundown of how they work.
You use a very simple API in WordPress that acts basically as a key-value store,
with an expiration. After the expiration time, the entry will be invalidated and
the transient will be deleted from the transient store. Transients essentially
operate the same as options, but with an additional expiration field.

By default in WordPress, transients are actually powered by the same backend as
options. Internally, when you set a transient (say foo), it gets transparently
translated to two options: one for the transient data (_transient_foo) and an
additional one for the expiration (_transient_timeout_foo). Once requested,
this will then be stored in the internal object cache and subsequent accesses in
the same request will reuse the value, in much the same way options are cached.
One of the most powerful parts of the transient API is that it uses the object
cache, allowing a full key-value store to be used in the backend. However the
default implementation, and how the object cache can change this, is where two
major incorrect assumptions come from.

Object Caching and the Database

The first incorrect assumption that developers make is to assume the database
will always be the canonical store of transient data. One big issue here is
attempting to directly manipulate transient data via the option API; after all,
transients are just a special type of option, right?

In the real world however, anything past your basic site will use an object
cache backend. Popular choices here include APC (including the new APCu) and
Memcache, which both cache objects in memory, not the database. With these
backends, using the option API will return invalid or no data, as the data
is never stored in the database.1

I’ve seen this used in real world plugins to determine if a transient is about
to expire by directly reading _transient_timeout_foo. This will break and
cause the transient to always be counted as expired with a non-default cache.
Before you think about how to do this in a cross-backend compatible way: you
can’t. Some backends simply can’t do this, and until WordPress decides to
provide an API for this, you can’t predict internal behaviour of the backends.


The second incorrect assumption that most developers make is that the expiration
date is when the transient will expire. In fact, the inline documentation even
states that the parameter specifies the “time until expiration in seconds”.
This assumption is correct for the built-in data store: WordPress only
invalidates transients when attempting to read them (which has lead to
garbage collection problems in the past). However, this is not guaranteed
for other backends.

As I noted previously, transients use the object cache for non-default
implementations. The really important part to note here is that the object cache
is a cache, and absolutely not a data store. What this means is that the
expiration is a maximum age, not a minimum or set point.

One place this can happen easily is with Memcache set in Least Recently Used
(LRU) mode. In this mode, Memcache will automatically discard entries that
haven’t been accessed recently when it needs room for new entries. This means
less frequently accessed data (such as that used by cron data) can be discarded
before it expires.

What the transient API does guarantee is that the data will not exist past the
expiration time. If I set a transient to expire in 24 hours, and then attempt to
access it in 25 hours time, I know that it will have expired. On the other hand,
I could access it in 5 seconds in a different request and find that it has
already expired.

Real world issues are common with the misunderstanding of expiration times. For
WordPress 3.7, it was proposed to wipe all transients on upgrade for
performance reasons. Although this eventually was changed to just expired
transients, it revealed that many developers expect that data will exist until
the expiration. As a concrete example of this,
WooCommerce Subscriptions originally used transients for
payment-related locking. Eventually, Brent (the lead developer) found that these
locks were being silently dropped and users could in fact be double-billed in
some cases. This is not a theoretical issue, but a real-world instance of the
expiration age issue. The solution to this particular issue was to swap it out
for options, which are guaranteed to not be dropped.

When Should I Use Transients?

“This all sounds pretty doom and gloom, Ryan, but surely transients have a valid
use?”, you say. Correct, astute reader, they’re a powerful tool in the right
circumstances and a much simpler API than others.

Transients are perfect for caching any sort of data that should be persisted
across requests. By default, WordPress’ built-in object cache uses global state
in the request to cache any data, making it useless for caching persistent data.
Transients fill the gap here, by using the object cache if available and falling
back to database storage if you have a non-persistent cache.

One application of this persistence caching that fits perfectly is fragment
caching. Fragment caching applies full page caching techniques (like object
caching) to individual components of your page, such as a sidebar or a specific
post’s content. Mark Jaquith’s popular implemention previously
eschewed transients due to the lack of garbage collection combined with
key-based caching, however this is not
a concern with the upcoming WordPress 3.7.

Another useful application of transient storage is for caching long-running
tasks. Tasks like update checking involve remote server calls, which can be
costly both in terms of time and bandwidth, so caching these makes sense.
WordPress internally caches the result from update checking, ensuring that
excess calls to the internal update check procedures don’t cause excessive load
on the server. While the object caching API would work here, the
default implementation would never cache the result persistently.


Transients are awesome, but there are some important things to watch out for:

  • Transients are a type of cache, not data storage
  • Transients aren’t always stored in the database, nor as options
  • Transients have a maximum age, not a guaranteed expiration
  • Transients can disappear at any time, and you cannot predict when this will

Now, go out and start caching your transient data!

  1. The reason I say invalid or no data here is
    because it’s possible for a transient to be stored in the database before
    enabling an object cache, so that would be read directly. []

Introducing WP API

As many of you are aware, I was accepted into Google’s Summer of Code program this year to work on a JSON REST API for WordPress. WordPress already has internal APIs for manipulating data via the admin-ajax.php handler in addition to the XML-RPC API. However, XML can be a huge pain to both safely create and parse, and the existing admin API is locked down to authenticated users and is also tailored to the admin interface. The goal of this project is to create a general data API that speaks the common language of the web and uses easily parsable data.

I’d now like to introduce the official repository and issue tracker. There’s also the SVN repository which is kept in sync.

For the next few months, my schedule will be busy implementing the API. Each week from now through the final submission has an individual plan, presented below.

May 27: Acceptance of Project, ensure up-to-speed on existing code
June 3: Work on design documents (response types/collections) and ensure agreement with mentors and interested parties (#264)
June 10: Complete core post type serialisation/deserialisation (basic reading/writing of raw data complete) (#265)
June 17: Work on collection pagination and metadata infrastructure (the full collection of posts can now be accessed and is correctly paginated, allowing for browsing via the API) (#266)
June 24: Creation of main collection views (main post archive, per date, search) (#267)
July 1: Further work on indexes and browsability (#268)
July 8: Create (independent) REST API unit tests for all endpoints covered so far (#269)
July 15: Creation of a Backbone.js example client for testing (#270)
July 22: Spare week to act as a buffer, since some tasks may take longer than expected
July 29: Midterm evaluation!
August 5: Creation/porting of existing generic post type API with page-specific data (#271)
August 12: Creation of attachment-related API (uploading and management) (#272)
August 19: Creation of revision API, and extending the post API to expose revisions (#273, #274)
August 26: Creation of term and taxonomy API (#275)
September 2: Finalisation of term and taxonomy API, and updating of test clients (#276)
September 9: Final testing with example clients (especially with various proxies and in live environments) and security review (#277)
September 15: Spare week for buffer
September 22: Final checking for bugs and preparation for final submission

At the end of each week throughout development, I’ll post a weekly update and tag a new release version, in a manner similar to the release process of MP6. The first release of the API will be posted shortly.

For those looking to keep track of development, I’ll be posting about the API here, which you can follow via the feed. A GSoC P2 is on its way and will be the official place to post comments and feedback (I’ll be crossposting back to here once that’s up). In the mean time, I’ll be posting on this blog and accepting comments here, which is a great way to ask questions and post feedback.

A Vagrant and the Puppet Master: Part 1

In my development workflow, my tools are the thing I deliberate over most. As anyone who follows me on Twitter can attest to, I’m a huge fan of Git and Sublime Text, and conversely I hate Subversion and PhpStorm. I genuinely believe that my tools can make or break how I work and I’m always looking to improve this, constantly searching out for new tools.

By far and away, the tool that has changed how I work the most in the past year is Vagrant with the Puppet configuration tool. For those who don’t know, Vagrant is a tool to create and manage virtual machines, while Puppet is a tool to configure and manage server configuration. The two work extremely well together as a tool for developing in a clean, reproducible environment. Plus, it also provides an easy way to replicate server configuration between your development and production servers.

So, how does it work? Let’s walk through how to set up a development server, plus using that configuration in production!

The first step to getting started is to work out what operating system you want to use. Personally, I’m a fan of Ubuntu Server (12.04 LTS, Precise Pangolin, to be specific), so I’ll be using that in examples, but you can use whatever you like. Some official boxes are available for Ubuntu, while others are available on (you can also build your own, but until you’re familiar with Vagrant, I’d recommend using a premade box).

To start off with, you’ll want to download your chosen box to avoid having to redownload it every time you recreate your box.

$ vagrant box precise32

Next up, you want to create your Vagrant configuration and get ready to boot it up. This will create a Vagrantfile in your current directory, so set yourself up a new directory where all your Vagrant-related stuff will live.

$ mkdir example-site/
$ cd example-site/
$ vagrant init precise32

Now, let’s boot up your new virtual machine and make sure it works. The up command will create a virtual machine from your base box if you don’t already have one, boot it, then provision it for usage. We’ll come back to that part in a bit.

$ vagrant up

To get access to your new (running) virtual machine, you’ll want SSH access. If you’re on Linux/Mac, vagrant ssh will work perfectly, but it’s a little harder on Windows. Vagrant tries to detect if your system supports command-line SSH, but doesn’t detect Cygwin environments. For cross-platform parity, I set up an alias called vsh that points to vagrant ssh on my Mac, and the SSH command on Windows, which looks something like:

# Mac/Linux
$ alias vsh='vagrant ssh'

# Windows
$ alias vsh='ssh -p 2222 -i "~/.vagrant.d/insecure_private_key" vagrant@'

We’re done testing our basic setup, so we can shut our VM down and destroy it, since we’ll want to boot from scratch next time.

$ vagrant destroy

We’ve now verified that the virtual machine works nicely, so let’s bust open your Vagrantfile and get tweaking it. Networking is the first thing we’ll need to get set up, so that we can access our server. Vagrant automatically forwards port 22 from the VM to port 2222 locally so that we can connect, but we also need port 80 for nginx, and we might need more later. We can either set up separate forwarded ports, or enable private networking (my preferred option). Uncomment the private networking line to enable it: :private_network, ip: ""

This IP address can be whatever you want, but you need to make sure it’s not covered by your existing network’s subnet. This is usually fine unless you have a custom subnet, in which case 10.x.x.x might be a better choice.

You’ll also want to set up a hostname for this. In your /etc/hosts file (on Windows, C:WindowsSystem32driversetchosts), point vagrant.local to this IP, along with any subdomains you may want. (Note: I’ve seen people use other names here like Keep in mind that these may end up being actual domain names some day with ICANN’s new TLD policy, whereas .local is reserved for exactly this use.) vagrant.local

We’ve now got a working Vagrant setup. In part 2, we’ll take a look at setting up Puppet to get your software working automatically.