Data communications over telegraph circuits 1915
The components themselves have improved too. Where we once had to fight to get to three nines (99.9% uptime) that can now be viewed as a baseline.
If that was all there was to the analysis, they did it wrong.
It is vitally important to take a look at what parts are important, and what their parameters are.
real world example :
I am currently involved in telephony; and the databases for subscriptions etc. Telephones have a "high-nine" reliability requirement; but they are reasonably simple, and the tables to support them are few and simple too. Interfaces are pretty standard, and good full-systems failover solutions exist. You can therefore get to 4-5 nines using a pretty straightforward path.
That is, until you consider the updates and update consistensies to databases. They have interlocking relationships that make duplication very difficult.
Data communications over telegraph circuits 1916
about the samme time we were looking at the previous example we were doing ha-cmp and we spent some time...
Data communications over telegraph circuits 1918
Morten Reistad These are excellent points and many technical people don't understand them. Every computer program should have appropriate (depending on criticality) failure exits, even for things that are never supposed to...
But, CHANGING a phone subscription is not a five-nine task. OPERATING IT is. Therefore move changes out, and make the transaction log handling a separate subsystem; and updating the production facilities a separate task with a lot lower resiliency than the phones, similar to the "web stuff" that take subsctiptions. Then the phones work even if the database master is blown to smithereens, or the web site is down.
Also, tasks can be queued if the recipient is unavailable. As long as uptime is adequate (i.e. 2-3 nines) all is fine.
This cost-benefit design is neglected all over the place.
I had a stint as ISP technical manager, and tried to tell this to all customers. Of 200, one listened. They are now the country's largest website.