<div dir="ltr">Thanks for the response, John.<div><br></div><div>I suppose that this kind of prompts the question of what type of jobs we'll be running.</div><div><br></div><div>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 each server handle its own stuff.</div>
<div><br></div><div><div class="gmail_extra">If we have a gatekeeper server, then everything relies on that. If that goes down, or a link between that and the server goes down, then nothing can get done, and the gatekeeper can potentially be a bottleneck.</div>
<div class="gmail_extra"><br></div><div class="gmail_extra">Maybe the servers elect a leader, but even then, you'll need a majority of servers to be up in order to pick something. You can't get down to one server.</div>
<div class="gmail_extra"><br></div><div class="gmail_extra">-Greg<br><br><div class="gmail_quote">On Mon, Feb 3, 2014 at 4:44 PM, John Tanner <span dir="ltr"><<a href="mailto:johntanner@gmail.com" target="_blank">johntanner@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div>I am far from an expert as well, but here are my thoughts regarding the first point (just brainstorming)..<i><span style="font-family:arial,sans-serif;font-size:13px"><br>
</span></i></div><i><span style="font-family:arial,sans-serif;font-size:13px"><div>
<i><span style="font-family:arial,sans-serif;font-size:13px"><br></span></i></div>Due to networking problems, server A cannot communicate with server B. A</span><br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">has priority for running a task. Since they cannot communicate, B never</span><br style="font-family:arial,sans-serif;font-size:13px">
<span style="font-family:arial,sans-serif;font-size:13px">learns that A completed the task. So B runs it too. => How much of a</span><br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">problem is it if a task runs multiple times?</span></i><br>
<br>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 <u>either</u> Server A or Server B, mutually exclusively.<div>
<br></div><div>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.</div>
<div><br></div><div>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.</div>
<div><br></div><div>This, however, poses interesting questions on how to determine and communicate which changes need to be propagated by any given cronjob.</div><div><br></div><div>Cheers,</div><div>John</div></div><div class="gmail_extra">
<br><br><div class="gmail_quote">On Mon, Feb 3, 2014 at 4:01 AM, <span dir="ltr"><<a href="mailto:foa-cron-request@lists.wikimedia.org" target="_blank">foa-cron-request@lists.wikimedia.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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Today's Topics:<br>
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1. Fault Tolerant vs Live (Gregory Manis)<br>
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<br>
Message: 1<br>
Date: Mon, 3 Feb 2014 02:15:30 -0500<br>
From: Gregory Manis <<a href="mailto:glm79@cornell.edu" target="_blank">glm79@cornell.edu</a>><br>
To: <a href="mailto:foa-cron@lists.wikimedia.org" target="_blank">foa-cron@lists.wikimedia.org</a><br>
Subject: [Foa-cron] Fault Tolerant vs Live<br>
Message-ID:<br>
<CAFe==+-yLy_siAFm2Ya7cATCOJYCvHA1_CfN0TAmzW4DGGZ=<a href="mailto:Uw@mail.gmail.com" target="_blank">Uw@mail.gmail.com</a>><br>
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<br>
I (GLM) edited in a few decisions to be made a couple days ago on our Wiki<br>
page, and JT put in some ideas. I went to edit in a response, but then<br>
realized that communicating via Wiki edits seems like a tremendously silly<br>
idea when we have a mailing list.<br>
<br>
The current requirements of the project include distribution of execution<br>
and the guarantee that if at least one server is up, a task will be ran.<br>
I'm far from an expert in terms of this (or anything for that matter), but<br>
there are a few (perhaps naive) concerns that I have with the requirements.<br>
<br>
>From my understanding of how networking works, there's no way to guarantee<br>
that a node is down. So I'm worried about the following scenarios:<br>
<br>
Due to networking problems, server A cannot communicate with server B. A<br>
has priority for running a task. Since they cannot communicate, B never<br>
learns that A completed the task. So B runs it too. => How much of a<br>
problem is it if a task runs multiple times?<br>
<br>
A large number of servers are down, and the one that is up has low priority<br>
for running a task. There's necessarily a delayed execution because the<br>
running server has to wait for all the others to time out. This can be<br>
somewhat mitigated by keeping a list of nodes that are up, but that can<br>
lead to an out of date list resulting in the previous problem. => How<br>
delayed can running time be? The response on the Wiki mentioned<br>
administrators adjusting times. Note that this likely involves making the<br>
crontabs non-standard.<br>
<br>
An updated crontab is created and propagates through to the servers. One<br>
server is completely disconnected though, so it doesn't receive the new<br>
table and keeps running old commands. => How bad is it if a deleted job<br>
runs? I'm actually not that worried about this; I think it's reasonable to<br>
expect the sysadmin to make sure all servers get the new cron table.<br>
<br>
I suppose the point I'm trying to make is that if you want it to be fault<br>
tolerant and live down to a single server, it seems like you can run into<br>
duplication or late tasks if the network isn't perfect. Is there anything<br>
I'm missing? I'm certainly not discounting the possibility that there's a<br>
solution (whether clever or simple) to these problems; I just don't see it.<br>
<br>
Thanks!<br>
<br>
-Greg<br>
<br>
P.S. Sorry if this email is totally incoherent; it's 2:15 AM right now<br>
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