On 12/24/2015 02:30 PM, Clint Byrum wrote:
This is entirely philosophical, but we should think about when it is
appropriate to adopt which mode of operation.
There are basically two ways being discussed:
1) Fail fast.
2) Retry forever.
Fail fast pros- Immediate feedback for problems, no zombies to worry
about staying dormant and resurrecting because their configs accidentally
become right again. Much more determinism. Debugging is much simpler. To
summarize, it's up and working, or down and not.
Fail fast cons- Ripple effects. If you have a database or network blip
while services are starting, you must be aware of all of the downstream
dependencies and trigger them to start again, or have automation which
retries forever, giving up some of the benefits of fail-fast. Circular
dependencies require special workflow to unroll (Service1 aspect A relies
on aspect X of service2, service2 aspect X relies on aspect B of service1
which would start fine without service2). To summarize: this moves the
retry-forever problem to orchestration, and complicates some corner cases.
Retry forever pros- Circular dependencies are cake. Blips auto-recover.
Bring-up orchestration is simpler (start everything, wait..). To
summarize: this makes orchestration simpler.
Retry forever cons- Non-determinism. It's impossible to just look at the
thing from outside and know if it is ready to do useful work. May
actually be hiding intermittent problems, requiring more logging and
indicators in general to allow analysis.
I honestly think any distributed system needs both.
So do I. I was proposing only that we deal with unrecoverable
configuration errors on startup in a fail-fast way. I was not proposing
that we remove the existing functionality that retries requests in the
occasion where an already-up-and-running scheduler service experiences
(typically transient) I/O disruptions to a dependent service like the DB
or MQ.
<snip>
That said, the scheduler is, IMO, an _extremely_ complex piece of
OpenStack, with up and down stream dependencies on several levels (which
is why redesigning it gets debated so often on openstack-dev).
It's actually not all that complex. Or at least, it doesn't need to be :)
Best,
-jay
> Making
it fail fast would complicate the process of bringing and keeping an
OpenStack cloud up. There are probably some benefits I haven't thought
of, but the main benefit you stated would be that one would know when
their configuration tooling was wrong and giving their scheduler the
wrong database information, which is not, IMO, a hard problem (one can
read the config file after all). But I'm sure we could think of more if
we tried hard.
I hope I'm not too vague here.. I *want* fail-fast on everything.
However, I also don't think it can just be a blanket policy without
requiring everybody to deploy complex orchestration on top.
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