REST API: Add persistent caching to Comments Count endpoint

[chrisbliss18]

As discussed in #40587, the Comments Count endpoint does not offer use any caching layers. For some sights with large amounts of comments, this can lead to repeated high-cost queries when making multiple requests to this endpoint in a short amount of time.

This PR adds persistent caching support to this endpoint. This caching is limited to just sites that add a persistent cache plugins. Since the biggest sites and sites on managed platforms are likely to use such a plugin, we'll see some immediate benefit from this change. Further discussion will determine the best path forward to ensure that all sites (with or without a persistent cache plugin) can benefit from caching.

Fixes #40587

Proposed changes:

• Add use of persistent caching using WP_Object_Cache to data handlers for the Comments Count REST API endpoint.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Discussion in originating issue.

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

1. Switch to this branch (performance/rest-api-comment-counts).
2. Install and activate a persistent caching plugin. For my testing, I used the Docket Cache plugin.
3. Use wp-cli to get the cached last_change value for the comment group, such as:
   jetpack docker wp cache get last_changed comment
   For my testing, I got the following value:
   0.33421300 1739176585
4. If the above command returns an error stating that the requested object is not found, navigate to a post or page that has comments and refresh it a couple of times. Then run the command in step 3 again. Do this until a non-error value is returned. If you are having trouble getting a non-error response, try adding a new comment.
5. Use wp-cli to check for cached comments count, such as:
   jetpack docker wp cache get 'wp_count_comments:2b85d95aa79fb82b7cdb876efe6bc9d7:0.33421300 1739176585' jetpack-json-api
   Note that the 0.33421300 1739176585 value at the end matches the value I got in step 3 above. Make sure to replace that portion of the key with the value you got in step 3. You should get an error saying that the object was not found. This is expected.
6. Use the Developer Console to make a WPCOM API v1.1 request to GET /sites/$site/comments/. Make sure to replace $site with your test site's Blog ID. Confirm that you get a proper response to the request.
7. Run the wp-cli command that you ran in step 5 again. This time, you should get a result indicating that the comments count value was cached. For my test site, the response looked like the following:

array (
  0 => 
  (object) array(
     'comment_approved' => '0',
     'num_comments' => '1',
  ),
  1 => 
  (object) array(
     'comment_approved' => '1',
     'num_comments' => '1',
  ),
)

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the performance/rest-api-comment-counts branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack performance/rest-api-comment-counts
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add a "[Type]" label (Bug, Enhancement, Janitorial, Task).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

---

The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

---

Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Choose a review path based on your changes:
   • A. Team Review: add the "[Status] Needs Team Review" label
     • For most changes, including minor cross-team impacts.
     • Example: Updating a team-specific component or a small change to a shared library.
   • B. Crew Review: add the "[Status] Needs Review" label
     • For significant changes to core functionality.
     • Example: Major updates to a shared library or complex features.
   • C. Both: Start with Team, then request Crew
     • For complex changes or when you need extra confidence.
     • Example: Refactor affecting multiple systems.
3. Get at least one approval before merging.

Still unsure? Reach out in #jetpack-developers for guidance!

---

Jetpack plugin:

The Jetpack plugin has different release cadences depending on the platform:

• WordPress.com Simple releases happen semi-continuously (PCYsg-Jjm-p2).
• WoA releases happen weekly.
• Releases to self-hosted sites happen monthly. The next release is scheduled for none scheduled (scheduled code freeze on undefined).

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

[github-actions]

Code Coverage Summary

No summary data is avilable for parent commit e629b29, cannot calculate coverage changes. 😴

If that commit is a feature branch rather than a trunk commit, this is expected. Otherwise, you might try re-running the Tests / Publish coverage data job on this PR once the corresponding job on the trunk commit has succeeded.

Full summary · PHP report · JS report

[fgiannar]

LGTM 👍

I tested with both a JN and Simple site and confirmed the comment counts are now cached.
I also confirmed that after adding a comment, the cached comment count is invalidated as expected.

Many thanks for working on this, Chris!