The Trouble with Shared State
Several years ago I was working on an app that allowed users to search a database for musical artists and load the artist’s music playlist into a web player. This was around the time Google Instant landed, which displays instant search results as you type your search query. AJAX-powered autocomplete was suddenly all the rage.
The only problem was that users often type faster than an API autocomplete search response can be returned, which caused some strange bugs. It would trigger race conditions, where newer suggestions would be replaced by outdated suggestions.
Why did that happen? Because each AJAX success handler was given access to directly update the suggestion list that was displayed to users. The slowest AJAX request would always win the user’s attention by blindly replacing results, even when those replaced results may have been newer.
To fix the problem, I created a suggestion manager — a single source of truth to manage the state of the query suggestions. It was aware of a currently pending AJAX request, and when the user typed something new, the pending AJAX request would be canceled before a new request was issued, so only a single response handler at a time would ever be able to trigger a UI state update.