Why is an email I deleted from gmail 3 years ago still on their servers?

Question

As mentioned I deleted an email from my gmail account and trash 3 years ago, but kept a direct link to the original before doing so and have been monitoring it. It's been 3 years and that direct link still works. The email is nowhere to be found in my gmail account. This strikes me as odd and could possibly be used as a covert communication channel that would never get detected. Example send a bunch of emails between 2 accounts. Keep all the direct links to the emails. Delete all emails in both accounts. If anyone were to log into either account they would see nothing unless you had the direct links to the individual emails. Thoughts?

salawat · Accepted Answer

Basically, when you're talking OLTP write heavy applications, rule #1 is never really delete anything. Doing so becomes heinously expensive in terms of having to lock tables and update indices, so on, and so forth. This is a hard lock situation. I.e. No further transactions can be processed until such time as the deletion completes can be processed. Most systems therefore set a "deleted" flag, which causes the system to act as if the deleted thing is actually deleted, while actually just leaving the information alone.
This works great, as long as everywhere in the system actually honors it. That is not a given. If you were relying on actually being able to make that disappear, however, you're SOL.
Not sure what Google is doing in terms of data architecture/persistence mechanisms nowadays, but it sounds to me like they may just be using a deleted flag.