Engineering decisions inside a custom plugin
Choosing where the data lives
The first design question is storage. Custom post types give you the editor, revisions, REST support and capabilities almost for free, which makes them right for things people write and publish. They are the wrong place for high-volume records. A booking log or a sync queue kept in post meta turns every report into a chain of joins. For that kind of data we create dedicated tables with dbDelta, store a schema version in an option and run upgrade routines when the version changes. Settings stay small and are not autoloaded unless every page needs them.
Hooks in, hooks out
A plugin should change WordPress only through actions and filters, at the latest point that still works and at a priority chosen on purpose. We also publish our own hooks. If your theme or another plugin needs to adjust a price, a label or an email, it can filter the value instead of editing our files. That is what update-safe means in practice: nobody ever has a reason to touch code that a future release will overwrite.
A security checklist applied to every handler
Plugin vulnerabilities usually come down to a handful of mistakes: a missing capability check on an AJAX or REST handler, a form without a nonce, output printed without escaping, or SQL built by joining strings. So each handler follows the same order. Verify the nonce, check current_user_can, validate and sanitize input, do the work through prepared queries or core APIs, escape on output. PHP_CodeSniffer with the WordPress Coding Standards rules runs on every commit, and a second developer reviews anything that accepts user input.
Cron jobs that finish
WP-Cron only fires when someone visits the site, and a job that tries to process ten thousand rows in one request will hit a timeout on modest hosting. We split long work into batches, record progress so a failed run resumes where it stopped, and use Action Scheduler when jobs need a queue and a visible history. For sites that depend on timing we ask the host to trigger cron from the server instead of from page views.
Shipping and supporting a commercial plugin
Selling a plugin adds work that buyers rarely budget for. Updates must reach customer sites through the standard Plugins screen, which means a license server answering WordPress update checks. Activation, deactivation and uninstall each need defined behavior, including what happens to stored data. Strings must be translatable. The code has to run on the range of PHP and WordPress versions your customers really use. We set that pipeline up once, typically with license keys sold through a store such as Easy Digital Downloads, and document it so releases become routine.