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Downloading a file regularly - how hard can it be?

adblockplus.org

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Re: Downloading a file regularly - how hard can it be?

#21

Post by fleitz »

I love random numbers for distribution. I had a similar problem with a set of distributed clients that needed to download email, but only one client downloading at a time. The email servers also had an issue where a large number of emails in the inbox would cause the server to slow down exponentially. (eg. it didn't matter how many MB of email were in the inbox but it did matter if there were more than about 1000 emails)

The downloaders would download the list of inboxes to be fetched, randomize them and then lock the inbox when they started downloading, then the downloader would randomly pick a size cutoff for the max email size it would download, 10K, 1 MB, unlimited with a n inversely proportional maximum email count so that about 100MB could be downloaded at anytime.

We even had an issue with one server behind an old cisco router that barf'd on window scaling, so a few machines in the pool had window scaling disabled and that account would naturally migrate to those servers with window scaling disabled.

It worked wonders for distributing the load and keeping the Inbox counts to a minimum.

Re: Downloading a file regularly - how hard can it be?

#22

Post by fromhet »

I know it's overkill for a browser extension, but wouldnt this be easily solved by having built-in bittorrent for updates?

The publisher would always be seeding the latest version, and the clients would connect maybe every other day. It would lower the preassure on the publishers servers and make sure everyone could always have the latest version.

With theese fancy magnet links, the publisher would only have to send the magnet and the actual file a couple of times, and then the peer to peer swarm would do the rest.

Re: Downloading a file regularly - how hard can it be?

#24

Post by SpiderX »

K2h wrote: post #18

call me oldschool, but having a huge peak demand is the perfect application for distributed source, like torrent. I know it is much more complicated to introduce P2P and way more risky if it gets poisoned, but it seems to me this underlying problem of huge peak demand was solved 10 years ago.

Yes, except this is a browser plugin, and no web browsers support bittorrents, so the download is not going to happen unless the plugin user installs a bittorrent updater engine, which probably isn't going to happen.

Re: Downloading a file regularly - how hard can it be?

#25

Post by masklinn »

sophacles wrote: post #3

A common solution to this problem, is to make a 2 stage process, where step 1 is a request of "should I download?", where there are 2 possible replies: "no, check again in N time" and "yes, here is a token". Step 2 is then presenting the token to the api point for download, and getting the file. On the server side, you don't even need specific instance tracking, just a simple decision based on current resource usage,…

> A common solution to this problem, is to make a 2 stage process, where step 1 is a request of "should I download?", where there are 2 possible replies: "no, check again in N time" and "yes, here is a token". Step 2 is then presenting the token to the api point for download, and getting the file.

You don't even need two steps, just have one step with previously known data. That's how HTTP conditional requests (Last-Modified/If-Modified-Since and ETag/If-None-Match) work: the client states "I want this file, I already have one from such moment with such metadata", and the server replies either "you're good" (304) or "here's your file (200).

Issue is, that only works when the file changes rarely enough, or you need additional server logic to reply that the file is still good when it's not.

> Now that I think of it: it seems that this would be a nice nginx plugin, with a simple client side library to handle it for reference. Anyone want to collaborate on this over the weekend?

I'd be very surprised if nginx didn't support conditional requests already.

edit: according to [0] and [1] — which may be outdated — Nginx provides built-in support for last-modified on static files, it does not provide ETag support (the developer believes this is not useful for static files — which is usually correct[2]) but [1] has apparently written a module to do so [3]. The module being 4 years old, it might be way out of date.

[0] http://serverfault.com/questions/211637/what-headers-to-add-...

[1] https://mikewest.org/2008/11/generating-etags-for-static-con...

[2] There are two situations in which it is not (keep in mind that this is for static content, dynamic is very different): if somebody willfully touches a file, it will change its Last-Modified but not its checksum, triggering a new send without ETag but not with it; and ETags can be coherent across servers (even in CDNs), the chances of last-modified being exactly the same on all your servers is far smaller.

On the other hand, no etag is better than a shitty etag, and both Apache and IIS generate dreadful etags — which may hinder more than help — by default.

[3] https://github.com/mikewest/nginx-static-etags/

Re: Downloading a file regularly - how hard can it be?

#26

Post by masklinn »

SpiderX wrote: post #24
K2h wrote: post #18

call me oldschool, but having a huge peak demand is the perfect application for distributed source, like torrent. I know it is much more complicated to introduce P2P and way more risky if it gets poisoned, but it seems to me this underlying problem of huge peak demand was solved 10 years ago.

Yes, except this is a browser plugin, and no web browsers support bittorrents, so the download is not going to happen unless the plugin user installs a bittorrent updater engine, which probably isn't going to happen.

> and no web browsers support bittorrents

Actually, Opera has native support for torrents downloading.

And because they basically have complete and absolute freedom, it should be possible to build a torrent-downloading Firefox extension if that does not already exist (it probably does).

Re: Downloading a file regularly - how hard can it be?

#27

Post by drzaiusapelord »

moe wrote: post #15
forgotusername wrote: post #10

Earlier quoted context omitted.

While this workaround has merit, it doesn't actually solve the underlying problem. I guess even Amazon will eventually pick up the phone and ask you to stop sending them weekly bandwidth spikes when the figures involved get large enough (I've personally seen this with another well known PaaS provider).

I guess even Amazon will eventually pick up the phone Why would they? You'd have to push dozens of GBit/s to even appear on their radar. The only time they'll call you is when they can't charge your CC anymore (a sustained 1 GBit/s will set you back $1000/day at their current rate).

Yep, they'll happily bill you. I imagine the budget adblock plus has is pretty small and, you know, can't be subsidized with ads.

Re: Downloading a file regularly - how hard can it be?

#28

Post by sophacles »

masklinn wrote: post #25
sophacles wrote: post #3

A common solution to this problem, is to make a 2 stage process, where step 1 is a request of "should I download?", where there are 2 possible replies: "no, check again in N time" and "yes, here is a token". Step 2 is then presenting the token to the api point for download, and getting the file. On the server side, you don't even need specific instance tracking, just a simple decision based on current resource usage,…

> A common solution to this problem, is to make a 2 stage process, where step 1 is a request of "should I download?", where there are 2 possible replies: "no, check again in N time" and "yes, here is a token". Step 2 is then presenting the token to the api point for download, and getting the file. You don't even need two steps, just have one step with previously known data. That's how HTTP conditional requests (Last-…

Yes, this work for cache updating, and it is fantastic for that purpose. It does not solve the actual stated problem, which is that periodic checks in an attempt to smooth server loading away from peaks don't usually drift towards extremely bursty behavior. When the file does change, you still get a large number of clients trying to download the new content all at once. The solution I was suggesting is similar to what you are talking about, but also has the feature of smoothing the load curves.

Issue is, that only works when the file changes rarely enough, or you need additional server logic to reply that the file is still good when it's not.

My algorithm is that logic -- albeit implemented with client side collusion rather than pure server side trickery (this allows better control should the client ignore the etags).

Re: Downloading a file regularly - how hard can it be?

#29

Post by masklinn »

sophacles wrote: post #28
masklinn wrote: post #25

Earlier quoted context omitted.

> A common solution to this problem, is to make a 2 stage process, where step 1 is a request of "should I download?", where there are 2 possible replies: "no, check again in N time" and "yes, here is a token". Step 2 is then presenting the token to the api point for download, and getting the file. You don't even need two steps, just have one step with previously known data. That's how HTTP conditional requests (Last-…

Yes, this work for cache updating, and it is fantastic for that purpose. It does not solve the actual stated problem, which is that periodic checks in an attempt to smooth server loading away from peaks don't usually drift towards extremely bursty behavior. When the file does change, you still get a large number of clients trying to download the new content all at once. The solution I was suggesting is similar to wha…

> The solution I was suggesting is similar to what you are talking about, but also has the feature of smoothing the load curves.

It has no more feature of smoothing the load curve than using Cache-Control with the right max-age.

> My algorithm is that logic

It is no more that logic than doing what I outlined with proprietary behaviors.

> this allows better control should the client ignore the etags

by making the whole client use a custom communication channel? I'd expect ensuring the client correctly speaks HTTP would be easier than implementing a custom client from scratch.

Re: Downloading a file regularly - how hard can it be?

#30

Post by sophacles »

masklinn wrote: post #29
sophacles wrote: post #28

Earlier quoted context omitted.

Yes, this work for cache updating, and it is fantastic for that purpose. It does not solve the actual stated problem, which is that periodic checks in an attempt to smooth server loading away from peaks don't usually drift towards extremely bursty behavior. When the file does change, you still get a large number of clients trying to download the new content all at once. The solution I was suggesting is similar to wha…

> The solution I was suggesting is similar to what you are talking about, but also has the feature of smoothing the load curves. It has no more feature of smoothing the load curve than using Cache-Control with the right max-age. > My algorithm is that logic It is no more that logic than doing what I outlined with proprietary behaviors. > this allows better control should the client ignore the etags by making the whol…

You still seem to be missing the point. Cache-Control as implemented commonly, and by your description, will instantly serve every request the new file as soon as a new file is available. It takes into account exactly one variable: file age.

The algorithm I describe takes into account variables which affect current system loading, and returns a "no, try again later", even when the file is actually different, because the server is trying to conserve some resource (usually in such cases it is bandwidth). Like I said, this can be done with etags, but a more explicit form of control is nicer. Which brings us to this:

> this allows better control should the client ignore the etags

by making the whole client use a custom communication channel? I'd expect ensuring the client correctly speaks HTTP would be easier than implementing a custom client from scratch.

A client speaking proper http would be perfect for this. So point your http client to:

domain.com/getlatest

if there is a token available, respond with a:

307 domain.com/reallatest?token=foo

If no token is available and no if-modified headers are sent, reply with:

503 + Retry-After N

if there is not a token available, and the requestor supplied approrpiate if modified headers respond with a:

304 + cache control for some scheduled time in the future (which the client can ignore or not)

Of course that last condition is strictly optional and not really required, since then it would be abusing cache control, rather than the using 503 as intended.

(also note, a request to domain.com/reallatest with an invalid token or no token could result in a 302 to /getlatest or a 403, or some other form of denial, depending on the specifics of the application).

edit: Strictly speaking, the multiple url scheme above isn't even needed, just a smart responder associated with the 503 is needed, however the url redirect method above was there because there may be a larger application context around system, in which getlatest does more than just serve the file, or in which multiple urls would redirect to reallatest, both easily imaginable situations.

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