Re: How to get Ada to “cross the chasm”?

["Dan'l Miller" <optikos@verizon.net>]

On Saturday, April 21, 2018 at 4:28:23 PM UTC-5, Paul Rubin wrote:
> Simon Wright writes:
> > What about the bank keeping track of your money?
> 
> Not realtime, or at least not hard realtime.  Nobody cares if checking
> your bank balance online takes a millisecond longer than it was supposed
> to.  They might care about the statistical distribution of the latency
> but they don't care at all about a single query.  (Hard) realtime means
> that disaster ensures if a deadline is missed even once.

https://en.wikipedia.org/wiki/Erlang_(unit)

Hard realtime has fixed-duration time deadlines that if missed make the late numerically-correct answer or late logically-correct event just as wrong as if the answer was numerically incorrect or illogical.  Elastic realtime often can be nearly as strict:  if the rate of successfully-computed retirement* of transactions cannot keep up with the erlangs of arrival rate of commencement of non-elidible transactions during extant-expected-peak (not theoretical peak) busy periods whose duration is longer than the elasticity time-duration, then some of those incoming transactions are going “fall on the cutting room floor” so to speak—the proverbial bit bucket.  If those transactions are Visa or MasterCard (or even Bitcoin nowadays) financial transactions, societal pandemonium will occur far more severely than one 90-year old grandma dying because her pacemaker skipped one beat.  If Visa and MasterCard cannot successfully account for a slew of transactions during extant-peak-busy times, suddenly people stop trusting electronic forms of money as a stand-in for paper & metal-coin money.  It wouldn't take too many steps to suddenly quickly arrive in points of freeze-up of the financial system to the point of, for example, General Motors Corporation going out of business because they cannot make payroll because they cannot take out their rolling 30-60-90-day rolling line-of-credit loans.  (General Motors Corporation was replaced by General Motors, Inc. for substantially this reason, eliding details to make it fit succinctly herein.)

* retirement here would (must) be one of the following:
1) sufficient funds, payment made;
2) insufficient funds, payment denied;
3) DoS/DDoS prevention, transaction balked overtly.
Successful retirement prohibits silent failure where retried transactions (e.g., due to timeout of the momentarily prior same-terminal/same-card transactions), especially where the tries and their retries (e.g., due to customer/merchant frustration) get confused or are non-idempotent. 

So maldesigned systems where erlangs of transaction-commencement exceed erlangs of successful retirement are another form of (slightly elastic**) realtime failure due to results arriving late to the point that they are completely useless/wrong.

** The elasticity time duration is the time-duration of a timeout of a credit-card customer an a terminal, because retries out of frustration add further traffic during the would-have-been-peak time period, raising the peak to disastrously higher and higher levels, assuring further deadline misses as retirement rate cannot keep up with transaction commencement rate.

This is how network processors define realtime regarding arrival rate of, say, Ethernet frames at, say, 10Gb/s.  The elasticity time duration is so short that elastic realtime very closely approximates hard realtime the way that engineering zero due to tolerance approximates true zero that the difference between definitions is purely academic and effectively moot.

> > What about those crazy high-frequency traders?
> 
> Their machines are not in data centers.

The heck they are not!  Arbitrage traders place their servers as close to telecom data centers (called central offices) as they can get!  At any price!  If they cannot get into the same telecom central office as the target financial market, they might instead even be in an outdoor cabinet along a railroad easement so that they can be at a repeater station along a main-trunk fiber-optic cable transmission line just outside their target financial exchange to shave off a few milliseconds of router switching time here and there.  Indeed, they try to eliminate layer-3 Internet routing entirely as much as possible, leasing expensive layer-0 (e.g., DWDM wavelength) or layer-1 (e.g., SONET-SDH) or layer-2 (PBB-TE Ethernet) transmission channel to have as direct least-routed least-touched-by-hardware-dataplane switch-fabrics as possible!

Arbitrage trading is a slightly different definition of “late”.  Instead of fixed length of time being the deadline, the deadline is being the fastest.  Second in the race misses the deadline, and is considered total failure, because the arbitrage opportunity was lost (along with hundreds of thousands or millions of dollars).