alt Hacker NewsI split step 4 in their "high level, this is the general flow for data migrations".
4.0 Freeze old system
4.1 Cut over application traffic to the new system.
4.2 merge any diff that happened between snapshot 1. and cutover 4.1
4.3 go live
to me, the above reduces the pressure on downtime because the merge is significantly smaller between freeze and go live, than trying to go live with entire environment. If timed well, the diff could be minuscule.
What they are describing is basically, live mirror the resource. Okay, that is fancy nice. Love to be able to do that. Some of us have a mildly chewed bubble gum, a foot of duct tape, and a shoestring.
Yeah it depends on what the system is.
Lots of systems can tolerate a lot more downtime than the armchair VPs want them to have.
If people don't access to Instagram for 6 hours, the world won't end. Gmail or AWS S3 is a different story. Therefore Instagram should give their engineers a break and permit a migration with downtime. It makes the job a lot easier, requires fewer engineers and cost, and is much less likely to have bugs.