How do I write design docs in Google: Designing Quota Service For Dropbox

The sample design today is a quota service that keeps track of users’ quotas usages and determines if a new uploading request should be served. This quota service is used by a uploading feature or product like Google Drive, Dropbox, and S3.

The hardest part is recovering quota from failed uploading, especially when the failure is caused by crash of the uploading server. Left unaddressed, those quota will be lost forever.

One intuitive solution is to periodically heartbeat each ongoing uploading request to detect such failures. We will provide a much cleaner and graceful solution. Another hard failure case is when the uploading is finished but we failed to update quota usage on DB.

The rough flow is in the diagram top. If you are interested for the full architecture, the design doc is here.