*I suppose that this kind of prompts the question of what type of jobs
we'llbe running.*
I think the goal is to be able to handle any arbitrary job.
*If the jobs involve making changes to these servers themselves, then it
seems kind of arbitrary that we'd want to split up execution- just let
eachserver handle its own stuff. *
I am under the impression that we will want to keep execution to a single
machine per job, unless it fails, then another one takes the job over in
its entirety. Otherwise, if we're distributing single tasks between
multiple machines, things will get very complicated.
*If we have a gatekeeper server, then everything relies on that. If
thatgoes down, or a link between that and the server goes down, then
nothing can get done, and the gatekeeper can potentially be a bottleneck.
Maybe the servers elect a leader, but even then, you'll need a majority
ofservers to be up in order to pick something. You can't get down to one
server.*
Good point. Say we have a hypothetical build job that needs to get compiled
and moved to a particular folder to be accessed by its users. Some machine
(with several on standby in case it fails) after building, will eventually
need to execute the final step of moving the completed build to its proper
destination folder. Perhaps if a machine fails to respond to server
communications nearing this closing step of the job (making its work
public), we will still need a new elected leader to restart the job from
the beginning. If the first machine starts communicating again, they both
run and the first to finish causes the other to abort. Cleanup /
inconsistency becomes an issue in half-finished publishing steps, however.
John
On Mon, Feb 3, 2014 at 6:49 PM, <foa-cron-request(a)lists.wikimedia.org>wrote;wrote:
Send Foa-cron mailing list submissions to
foa-cron(a)lists.wikimedia.org
To subscribe or unsubscribe via the World Wide Web, visit
https://lists.wikimedia.org/mailman/listinfo/foa-cron
or, via email, send a message with subject or body 'help' to
foa-cron-request(a)lists.wikimedia.org
You can reach the person managing the list at
foa-cron-owner(a)lists.wikimedia.org
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Foa-cron digest..."
Today's Topics:
1. Re: Foa-cron Digest, Vol 2, Issue 1 (John Tanner)
2. Re: Fault Tolerant vs. Live (Gregory Manis)
----------------------------------------------------------------------
Message: 1
Date: Mon, 3 Feb 2014 13:44:08 -0800
From: John Tanner <johntanner(a)gmail.com>
To: foa-cron(a)lists.wikimedia.org
Subject: Re: [Foa-cron] Foa-cron Digest, Vol 2, Issue 1
Message-ID:
<
CAOw8P7CY7BdTnehpNG+dCLE64RjaJqd0bwKdEh1POByJT9pe0g(a)mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I am far from an expert as well, but here are my thoughts regarding the
first point (just brainstorming)..
* Due to networking problems, server A cannot communicate with server B.
Ahas priority for running a task. Since they cannot communicate, B never
learns that A completed the task. So B runs it too. => How much of aproblem
is it if a task runs multiple times?*
In order for it to be a problem that a task ends up running multiple times,
there must be some sort of communication between the servers involved. Only
once Server A says "I'm running the job" or "I'm done", and
Server B
acknowledges, do we have a known duplicate task. If Server B has not
finished the job, it aborts. If Server B has finished the job, an "I'm
done" message from Server A/B should result in changes be propagated by
*either* Server A or Server B, mutually exclusively.
The key is that the final set of changes brought about by a particular
server should only be synced after completion, and can only occur after
successful network communication (otherwise, how can it propagate to
anyone?). This seems to call for a necessary third server acting as a sort
of gatekeeper.
In the worst case, Server A and Server B have completed the identical task
in isolation, and nothing needs to change. One of them will not propagate
their effects (ie. generation of file, sending of an email, compilation of
source code) past the gatekeeper server, which will subsequently release
the effects to those who require it.
This, however, poses interesting questions on how to determine and
communicate which changes need to be propagated by any given cronjob.
Cheers,
John
On Mon, Feb 3, 2014 at 4:01 AM, <foa-cron-request(a)lists.wikimedia.org
wrote:
Send Foa-cron mailing list submissions to
foa-cron(a)lists.wikimedia.org
To subscribe or unsubscribe via the World Wide Web, visit
https://lists.wikimedia.org/mailman/listinfo/foa-cron
or, via email, send a message with subject or body 'help' to
foa-cron-request(a)lists.wikimedia.org
You can reach the person managing the list at
foa-cron-owner(a)lists.wikimedia.org
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Foa-cron digest..."
Today's Topics:
1. Fault Tolerant vs Live (Gregory Manis)
----------------------------------------------------------------------
Message: 1
Date: Mon, 3 Feb 2014 02:15:30 -0500
From: Gregory Manis <glm79(a)cornell.edu>
To: foa-cron(a)lists.wikimedia.org
Subject: [Foa-cron] Fault Tolerant vs Live
Message-ID:
<CAFe==+-yLy_siAFm2Ya7cATCOJYCvHA1_CfN0TAmzW4DGGZ=
Uw(a)mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I (GLM) edited in a few decisions to be made a couple days ago on our
Wiki
page, and JT put in some ideas. I went to edit in
a response, but then
realized that communicating via Wiki edits seems like a tremendously
silly
idea when we have a mailing list.
The current requirements of the project include distribution of execution
and the guarantee that if at least one server is up, a task will be ran.
I'm far from an expert in terms of this (or anything for that matter),
but
there are a few (perhaps naive) concerns that I
have with the
requirements.
>From my understanding of how networking works, there's no way to
guarantee
that a node is down. So I'm worried about the
following scenarios:
Due to networking problems, server A cannot communicate with server B. A
has priority for running a task. Since they cannot communicate, B never
learns that A completed the task. So B runs it too. => How much of a
problem is it if a task runs multiple times?
A large number of servers are down, and the one that is up has low
priority
for running a task. There's necessarily a
delayed execution because the
running server has to wait for all the others to time out. This can be
somewhat mitigated by keeping a list of nodes that are up, but that can
lead to an out of date list resulting in the previous problem. => How
delayed can running time be? The response on the Wiki mentioned
administrators adjusting times. Note that this likely involves making the
crontabs non-standard.
An updated crontab is created and propagates through to the servers. One
server is completely disconnected though, so it doesn't receive the new
table and keeps running old commands. => How bad is it if a deleted job
runs? I'm actually not that worried about this; I think it's reasonable
to
expect the sysadmin to make sure all servers get
the new cron table.
I suppose the point I'm trying to make is that if you want it to be fault
tolerant and live down to a single server, it seems like you can run into
duplication or late tasks if the network isn't perfect. Is there anything
I'm missing? I'm certainly not discounting the possibility that there's a
solution (whether clever or simple) to these problems; I just don't see
it.
>
> Thanks!
>
> -Greg
>
> P.S. Sorry if this email is totally incoherent; it's 2:15 AM right now
>