There are regulatory "circuit breaker" conditions but I wonder if this was a more informal one. The internet hive mind wasn't in their risk models and I would wonder if there was risk of this event causing a cascading blow-up effect.
Can anybody chime in whether these systems, today, tend to be on distributed infra versus e.g. mainframes or other big vertical compute pieces?
Example of machine: http://www.carpenterstimesystems.com/Amano-TS3000i-OATS-Time...
This might all be historical now.
The order gateways' clocks are in sync, up to precision epsilon. So if you send two orders serially from same data center (to ignore effects of public Internet latencies and routing), and if they happen to be processed by different gateways, you still will see same ordering of your orders in matching engine, bar some cosmic ray events.
It is all commodity hardware running some flavor of Linux.
However this was couple years ago, they may be using ASICs and FPGAs now. The matching of orders for a single asset must still be done in single process otherwise you cannot guarantee price-time priority.
As in there does not exist the stock to cover those positions, which means institutions are going to be in a race to avoid bankruptcy and have to outbid each other to acquire the remaining stock held by all these retail investors who know what they have. This is why WSB posters are saying $5000 is a conceivable price.
This is what would cause regulatory intervention, and certainly sets up the means/motive/opportunity for interrupting trading. Like the Fed steps in and buys it out or something. This looks like an LTCM level event.