High Availability - What does Crossover mean in this context?

Question

I'm working on a Mesos framework to run some jobs and it seems like a great opportunity to learn about making a highly available system. To that end, I'm doing some reading on distributed systems and I made the mistake of visiting wikipedia.

The passage in question is talking about a principle of HA engineering:

Reliable crossover. In multithreaded systems, the crossover point itself tends to become a single point of failure. High availability engineering must provide for reliable crossover.

My google-fu teaches me three things:

1) audio crossover devices split a single input into multiple outputs

2) genetic algorithms use crossover to combine solutions

3) buzzwordy white papers all copied from this wikipedia article :/

My question: What does a 'crossover point' mean in this context, and why is it single point of failure?

Answer 1

Reliable crossover in this context means:

The ability to switch from a node X (which is broken somehow) to a Node Y without losing data.

Non-reliable HA-database example:
Copy the database every 5 minutes to a passive node. => Here you can lose up to 5 minutes of data. => Here the copy action is the single point of failure.

Reliable HA-database example:
Setting up data replication where (per example) your insert statement only returns as "executed OK" when the transaction is copied to the secondary server. (yes: data replication is more complex than this, this is a simplified example in the context of the question)