But I'm sure similar infrastructure architecture was applied to gmail.com as it was to google.com and youtube.com.

And similar concepts of maintaining by sysadmins are practiced.

Hah...

Press and hold the F5 key on your keyboard for 2 minutes while on gmail.com. You will get a "service unavailable" error. About 500 other people whose data happens to be cohosted with you will also get the same error, and all of you will be unable to send or receive email, even by IMAP, for about 10 mins while your particular corner of the data store is restarted and the data integrity checked.

That doesn't happen on Google.com

Ok, I definitely want to know how you discovered that... (and found one of those 500 people to verify?)

Not sure if this is still the case, but if you did this a couple of times, your account data would be permanently migrated to an instance with more CPU and RAM allocated - you'd also be in with all the other badly behaved accounts, so reliability goes down lots. The benefit was much quicker complex searches, and being able to bulk label or delete emails without it taking minutes or hours.

Don't believe me how slow it is on a regular instance? Try going to "All mail", selecting all of your emails, and applying a label to them all. In my experience, it can only label about 50 mails per second, so it can take hours to do them all. It will keep going if you quit the browser, but will stop if the gmail devs do a software update, which they seem to do on usually tuesdays, but never fridays or the weekends.

I find it hard to believe that Gmail will always serve certain users from the same machines, especially in this day and age, with “cattle, not pets” and ephemeral containers.

I’m sure they have machines that are only used to serve G Suite and Google One customers, and maybe some other VIPs, but regular heavy users? It sounds like an urban legend to me.

Gmail accounts will sometimes be automatically "hospitalized" -- assigned more than the usual amount of resources because for some reason they are chronically behind or growing without bound -- or "jailed" -- moved into isolation along with other bozo accounts, to keep from disturbing normal people's accounts. Not a legend.

The data has to be sharded somehow, though. You might not be hitting the same exact machine, especially for the frontend, but your data isn't just magically everywhere in "the cloud".

Not the same machines, but different groups or machines (pools, shards, farms, whatever). They'll certainly be able to move you around to balance the pools, or to decomission pools, or to put you in a pool with primary data closer to where you usually access from etc. Grouping by behavior makes sense too --- separating heavy and light users makes a lot of sense, you can serve a lot more light users from one pool, and the heavy users won't impact their service.

It isn't about the compute. This is about the databases. I don't see it as stretch that google is using a sharding strategy for their datastores.

That's not the "cattle vs. pet" as I understand it. The servers are identical, i.e. cattle. This is just a case of sticky sessions. It's a common pattern to help latency and keep resource usage down.

I get brief gmail outages once every several months. Bulk labelling is really fast. I wonder if this is why...

Interesting. I held F5 down for less than a minute and I got an "Unusual usage - account temporarily locked down" message. Disappeared after a few seconds pause though...