The doctrine for HA is that each AZ should be fully independent, and if you do that, your inter-AZ traffic is relatively minimal.

And I think the charges for inter-AZ transfer are to incentivize customers to do that.

Of course, to make them fully independent, you have to replicate everything, so you wind up buying several redundant copies of your system...

> Of course, to make them fully independent, you have to replicate everything, so you wind up buying several redundant copies of your system...

Yeah, and keeping around warm systems ready to failover in case of a zonal outage seems like a preposterous waste of resources.

The alternative... to keep around multiple replicas of your system in different zones, all ready to accept traffic and which do serve traffic, seems more practical and less wasteful.

This. Paying for n+2 capacity is really expensive when n=1, but pretty reasonable at n=5 or n=10. Until someone gouges you on data transfer …

It's not a waste when it serves a purpose. Availability is a big concern

Sure, from the perspective of availability, it makes sense to keep this around. The US military has redundancies in place to handle many kinds of adverse scenarios, which comes at a price, but is justifiable. The point I'm trying to make is that if availability is the _only_ value, it becomes hard to justify that if you're a scrappy for-profit corp looking at your bottom line.

If instead of availability being the only value, there would be a more value provided from actually using such resources, more folks would adopt cross-AZ architectures which would be a win-win for both the customer (get HA for lower or no cost and go down less often and succeed in the market) and thus the cloud provider (keep raking in the steady cloud revenue as the customer grows).

> so you wind up buying several redundant copies of your system

This is one of those gotcha's that company's hit. They see the public pricing page and think "wow that is much cheaper than one my internal IT department charges for X", and then when they go to actually implement they find that "best practice" says they basically have to more than double or even triple the cost to get a reliable system (more because not only do you have to duplicate all the infrastructure into a second AZ, you are getting charged for the replication traffic between them).

But in all fairness, if you actually implement that ”best practice” HA infrastructure, you will also be miles ahead of almost all internal IT departments.

Inter-AZ traffic is Metro Area Network ("MAN") traffic. There is a cost for running network between locations.

This explains the cost.

Price probably should be based on value, not on cost. Why do they charge for it? Because they decided it's a good way to make money and profit.

Something has to pay for the fat pipe between colos ( AZ's)

But those are probably far below $1/TB. Otherwise other colocation providers couldn't offer that even with peering.