Re: thoughts on rudimentary dependency handling

From: Laurent Bercot <>
Date: Tue, 06 Jan 2015 19:20:22 +0100

On 06/01/2015 18:46, Avery Payne wrote:
> 1. A service can ask another service to start.
> 2. A service can only signal itself to go down. It can never ask another
> service to go down.
> 3. A service can only mark itself with a ./down file. It can never mark
> another service with a ./down file.
> That's it. Numbers 2 and 3 are the only times I would go against what you
> are saying.

  So does number 1.
  When you ask another service to start, you change the global state. If
it is done automatically by any tool or service, this means the global
state changes without the admin having requested it. This is trying to
be smarter than the user, which is almost always a bad thing.

  Number 2 is in the same boat. If the admin wants you to be up,
then you can't just quit and decide that you will be down. You can't
change the global state behind the admin's back.

  Number 3 is different. It doesn't change the global state - unless there's
a serious incident. But it breaks resiliency against that incident: it
kills the guarantee the supervision tree offers you.

  I firmly believe that a tool, no matter what it is, should do what the
user wants, even if it's wrong or can't possibly work. If you cannot do
what the user wants, don't try to be smart; yell at the user, spam the
logs if necessary, and fail. But don't do anything the user has not
explicitly told you to do.

  Maybe a dependency manager needs to be smarter than that. In which case
I would call it a "global state manager". There would be the "current
state", which starts at "everything down" when the machine boots, and
the "wanted state", which starts at "everything up"; as long as those
states are not matched, the global state manager is running, implementing
retry policies and such, and may change the global state at any time,
bringing individual services up and down as it sees fit, with the
ultimate goal of matching the global current state with the global wanted
state. It could do stuff like exponential backoff so failing services
would not be "wanted up" all the time; but it would never, ever change
the global wanted state without an order to do so from the admin.

  If you want a dependency manager with online properties, I think this
is the way to do it.

Received on Tue Jan 06 2015 - 18:20:22 UTC

This archive was generated by hypermail 2.3.0 : Sun May 09 2021 - 19:44:18 UTC