When One Machine Is Not Enough
What You Give Up When the Network Splits
Last timeWhat the Network Actually Does
When machines cannot reach each other they must either answer with data that may be wrong or refuse to answer at all, and no cleverness produces a third option.
A network partition is the moment the subject stops being theoretical. The
machines are all healthy, the data is all intact, and the system has to do
something that will be wrong.
The situation
A worked example helps. A split lasting ten minutes, with a service taking
thirty five requests a second, means twenty one thousand requests arriving during
the window on the smaller side alone. Those twenty one thousand either get an
answer that might be wrong or get nothing, and somebody has to decide which
before the split happens, because there is no time to decide during it.
The two options, and only two
A machine that cannot reach the others has access to exactly one thing: what it
already holds. If it answers a request, it answers from that, which may be out of
date, and if it accepts a write, that write may conflict with one accepted on the
other side. If it declines to do either, it is unavailable to the clients that can
reach it, despite being perfectly healthy.
That is the whole of it. There is no third behaviour because there is no third
source of information. This was proved in 2002, and the proof is short precisely
because the statement is about what a machine can possibly know rather than about
any particular design.
The lesson stops here
5 more paragraphs to go
You have read the opening. The rest of the argument, the problems that check whether it landed, and the lines worth keeping at the end all come with a plan.
The first lesson of every course in the library reads the whole way through, free, so you can see exactly what the rest of them are.
See the planThe contentsThis is the reading half
Starting the course gives you your own copy of it. Every idea on every page has problems standing under it, marked with a reason rather than a tick, and any sentence you do not believe can be opened and argued with. None of that can happen on a page nobody owns.
The contents