Older blog entries for dan (starting at number 143)

Sinatra and the class/instance distinction

The Sinatra microframework is described as enabling “Classy Web Development”, and it turns out this is more literally true than I previously thought.

The Rack Specification says

A Rack application is an Ruby object (not a class) that responds to call. It takes exactly one argument, the environment and returns an Array of exactly three values: The status, the headers, and the body.

(emphasis mine). When you write a Sinatra app, though, it seems to want a class: whether you call MyApp.run! directly (we assume throughout this post that MyApp is a Sinatra::Base subclass) or use a config.ru or any other way to start the app running, there is a conspicuous lack of MyApp.new anywhere around. Yet the Rack spec says an app is an instance.

At first I thought I was being silly or didn’t understand how Rack works or had in general just misunderstood something, but it turns out not. Some ferretting through Sinatra source code is needed to see how it does this, but the bottom line is that MyApp has a class method MyApp.call which rack invokes, and this delegates to (after first, if necessary, instantiating) a singleton instance of MyApp stored in the prototype field. I am not at all sure why they did this. It may just be a hangover from Sinatra’s heritage and this stuff came along for the ride when Sinatra::Base was factored out of the Sinatra::Application classic app support. Or they may have a perfectly good reason (this is the hypothesis I am leaning towards and I suspect that “Rack middleware pipelines” is that reason). For my purposes currently it’s probably sufficient to know that they do it without needing to know why, and that I should stop trying to write Sinatra::Base subclasses which takes extra parameters to new.

:; irb -r sinatra/base
ruby-1.9.2-p0 > class MyApp  nil 
ruby-1.9.2-p0 > MyApp.respond_to?(:call)
 => true 
ruby-1.9.2-p0 > begin; MyApp.call({}); rescue Exception => e ;nil;end
 => nil 
ruby-1.9.2-p0 > MyApp.prototype.class
 => Sinatra::ShowExceptions 

Ta, and with emphasis, da! (The begin/end around MyApp.call is because for the purpose of this example I am too lazy to craft a legitimate rack environment argument and just want to demonstrate that prototype is created. And we should not be surprised that the prototype’s class is not the same class as we created, because there is middleware chained to it. In summary, this example may be more confusing in its incidentals than it is illuminating in its essentials. Oh well)

Syndicated 2011-05-04 12:39:05 from diary at Telent Netowrks

Preforking multi-process Sinatra serving (with Sequel)

Picture the scene. I have a largish Ruby web application (actually, a combination of several apps, all based on Sinatra, sharing a model layer, and tied together with Rack::URLMap), and I want a better way of reloading it on my development laptop when the files comprising it change.

At the same time, I have a largish Ruby web application (etc etc) and I wanted a better way of running several instances of it on the same machine on different ports, because running a one-request-at-a-time web server in production is not especially prudent if you can’t guarantee that (a) it will always generate a response very very quickly, and (b) there is no way that slow clients can stall it. So, I needed something like the thin command, but with more hooks to do stuff at worker startup time that I need to do but won’t bore you with.

And in the what-the-hell-why-not department I see no good reason that I shouldn’t be using the same code in development as is running in production and plenty of good reasons that I should. And a program that basically fork()s three times (for user-specified values of three) can’t be that hard to write, can it?

Version 0 of “thin-prefork” kind of escaped onto github and contains the germ of a good idea plus two big problems and an exceedingly boring name.

What’s good about it? It consists of a parent process and some workers that are started by fork(). There is a protocol for the master to send control messages to the workers over a socket (start, stop, reload, and basically whatever else you decide), and you subclass the Worker to implement these commands. This was found to be necessary, because version -1 used signals between parent and child, and it was found eventually and empirically that EventMachine (or thin, or something else somewhere in the stack) likes to install signal handlers that overwrote the ones I was depending on. And at that point I had two commands which each needed a signal and in accordance with the Zero-One-Infinity Rule I could easily foresee a future in which I would run out of spare Unix signals.

What’s not so good? Reloading – ironically, the whole reason we set out to write the thing. Reloading is implemented by having the master send a control message to the children, and the children then reload themselves (using Projectr or however else you want to). But when you have 300MB x n children to reload you’d much rather do the reload once in the parent and then kill and respawn the kids than you would have each of the kids go off and do it themselves – that way lies Thrash City, which is a better place for skateboarders than servers. (This would also be a bad thing for sharing pages between parent and child, but I am informed by someone who sounded convincingly knowledgeable that the garbage collector in MRI writes into pretty much every page anyway thus spitting all over COW, so this turns out not to be a concern at present. But someday, maybe – and in the meantime it’s still kinda ugly)

What’s also not so good is that the interaction between “baked in” stuff that needs to happen for some actions – like “quit” – and user-specified customizations is kind of fuzzy and it’s not presently at all obvious if, for example, a worker subclass command should call super: if you want to do somewthing before quitting, then obviously you should then hand off to the superclass to actually exit, but if you want to define a reload handler then you don’t want to call a non-existent superclass method when you’re done. But how do you know it doesn’t exist? Your worker might be based off another customisation that does want to do something important at reload time. So it’s back to the drawing board to work out the protocol there, though rereading what I’ve just written it sounds like I should make a distinction between notifiers and command implementations - “tell me when X is happening because I need to do something” vs “this is the code you should run to implement X”.

And why does the post title namecheck Sequel? Because my experience with other platforms is that holding database handles open across a fork() call can be somewhat fraught and I wanted somewhere to document everything I know about how Sequel handles this

Syndicated 2011-05-03 15:11:12 from diary at Telent Netowrks

Introducing Projectr

Why might you want to know the names of all the files in your project? One might turn the question around and ask why would you possibly would not want to, but maybe that’s not a constructive dialogue. So let’s list some use cases

  • to load them into your application
  • to load them into irb for debugging or for help in constructing test cases
  • to process them through rdoc
  • to put them in a gem
  • to print them (don’t laugh, I did that the other day when I was having trouble deciding how to refactor a bunch of stuff)

As far as I can see from my survey of the Ruby world, the current practices for each of these use cases are pretty ad hoc. Maybe you write a file full of require or require_relative statements (as the RBP blog author likes to do), maybe you use a glob, maybe you write a MANIFEST file, but there seems to be a significant lack of DRYness about it all. This led me to think there is a gap in the market for

  1. a language for describing the files that a project comprises
  2. some tools to interrogate a project description written in this form and find out what’s in it
  3. some code to load them into a running interpreter – and for bonus points, when files have previously been loaded into said image but since changed on disk, to reload them. This could be used in irb sessions, or could form the basis of a development-oriented web server that reloads changed files without needing to be stopped and started all the time

Note that item 3 above gives us something that “file containing list of require statements” doesn’t, because it allows us to reload files that we’ve already seen instead of just saying “meh, seen it already”. If you’re using a comparatively low-powered machine then reloading your entire app in irb every time you change a method definition is unnecessarily and obviously slow. If you’re also using Bundler (which I rather like now i’s settled down a bit, and will write more about in a future entry) then the additional bundle exec is not just slow, it’s SLow with a capital S and a capital L and a pulsating ever-growing O that rules from the centre of the underworld.

Here’s one I made earlier

Projectr::Project.new :test do
  # directories may be named by symbols or strings
  directory :example do
    #as may files
    file "file1"
    file :file2
    directory "subdir" do 
      file :subdir_file
    end
  end
end

h=Projectr::Project[:test]
h.load!   # it loads all the files
# and again
h.load!   # nothing happens this time
# touch example/file1.rb
h.load!   # loads only the changed file

At the time of writing this, the github version does about that much, but is quite clearly still version 0. Stuff I am still thinking about:

  • Load-order dependencies. Lisp programmers may recognise that Projectr was inspired by using (and indeed implementing a version of) defsystem (or more recently here) but Projectr is almost minimally featured compared to any of the Lisp-based defsystem facilities. Many of those features I don’t have any strong evidence that the Ruby world would find use for, but load-order dependencies allow us to say for example that if file A defines a DSL and files B and C use that DSL, changing A should make the computer reload B and C as well
  • It seems clear to me that defining a project and loading it are two separate operations – you may wish instead to define it and then generate a Gemspec, for example – but there’s still a lot of verbiage in the common case that you do want to load it, and I haven’t really found file layout and naming conventions that I feel good about
  • likewise, what happens when we redefine the project itself (as would happen if we want to add a file to it, for example) is slightly up for grabs. Should the project definition file be considered a part of the project?

I will doubtless form my own opinions on all of these issues in time and with more experience of using this tool in practice, but feedback on them and on the general approach is warmly welcomed.

Fork, clone, spindle, mutilate

Syndicated 2011-05-02 09:39:18 from diary at Telent Netowrks

I did it my way

short unoriginal observation on ruby blogging engines: quicker to write your own than evaluate all the other poorly documented ones
… this observation only holds if you skimp on the documentation of course. which is where we came in

If you can see this, you can see my blog design all changed again. This time it’s a Ruby Sinatra application (whence the name my-way) running on thin-prefork, which keeps the article texts in git and uses RedCloth plus some ugly regexps to turn them into HTML. The Markdown vs Textile decision is not an especially interesting one in the first place, but gets a lot easier still when you have something like 9 years worth of previous articles in Textile format.

Publishing is achieved by pushing to a git repository on the live machine (a Bytemark vm). A post-update hook in the remote repository is responsible for checking out the updated commit (git doesn’t like pushing to non-bare repositories) and sending SIGHUP to the running instance of my-way which causes it to reindex files.

dan@bytemark:~$ cat /home/git/my-way.git/hooks/post-update        
#!/bin/sh
GIT_WORK_TREE=/home/dan/src/git/my-way git checkout -f
kill -1 `cat /tmp/my-way.pid`

The version of my-way on github lags the actual version slightly, because I need to separate the engine from the articles and from the config data (there are things like adsense subscriber id, flickr api keys, etc) before I push the latter to a public service. Will clean it up in the next few days.

And my apologies to RSS feed subscribers. I’ve finally dropped the /diary prefix on the URL for this blog, and the old RSS feed didn’t use GUIDs and I’m too lazy to make the new one do so either, so the upshot is you just got the ten most recent articles in your feed again. Sorry.

Syndicated 2011-05-01 10:50:35 from diary at Telent Netowrks

What I miss most about Lisp

It’s been three months since I wrote anything longer than one line in Lisp, and over a year since I wrote more than a screenful of the stuff.

What I miss most is not CLOS or the REPL or even macros (per se, anyway). It’s

  • the distinction between READ and EVAL: a sane syntax for constructing complex data structures that look like code, but without actually having the data structures in question interpreted
  • and backtraces with the values of function parameters in them. When you’re doing the same thing 1000 times to different database rows or objects in a collection, and one of them has a nil in it somewhere, it would be really nice to know which one.

And maybe the REPL (although irb does most of that). And kinda sorta Defsystem, but I seem to be in the process of reimplementing that

But I hope soon to get Rubinius installed, just because I still have the irrational opinion that a grown-up programming language ought to be able to implement itself (and I have a thing for native code) so project 1 there is to see if I can hack the backtrace thingy at least into it.

Syndicated 2011-04-30 22:23:33 from diary at Telent Netowrks

TDD, BDD, executable specification

The new system at $WORK finally went live about a week ago, hurrah.

The upgrade itself took a few hours longer than I'd have liked, and (short shameful confession time), some (but probably not all) of this could have been caught by better test coverage. Which set me on a path towards SimpleCov (I'm using Ruby 1.9, rcov doesn't work), which led me to start looking at the uncovered parts, which set me to thinking. Which, as we all know, is dangerous.

TDD advocates (and pro-testing people in general) say "Don't test accessors". There are two reasons to say this that seem to my mind like good reasons: Ron says it because he wants you to write tests that do something else (something useful) that happens to involve calling those accessors. J B Rainsberger says it because "get/set methods just can't break, and if they can't break, then why test them?"

The problem comes when you adopt the mindset typified by BDD that "the test examples are actually your executable specification", because in that case how do you specify that the object has an accessor? This is not an unreasonable demand. Suppose we have objects whose purpose is to store structured data that will be used by client code - for example, User has an age property. Jbrains - which must surely be the Best Nickname Ever for a Java guy - says there's no useful test you can write for this (or not unless you don't trust your platform or something, but that way madness lies). But even if we are going to write one: a test that stores one or a few example values can easily be faked by the bloody-minded implementor ("the setter is called with the argument 42, then the getter is called and should return 42? I know! def age; 42; end") and a test that stores all possible values and tests they can be retrieved will take forever to write/read/run. Really the best notation in which to specify the behaviour of that property is the same notation which, when run by the interpreter, will implement the said behavior -

    attr_accessor :age
It's not just accessors either. Everything on the continuum between declarations of constants (SECONDS_PER_DAY=86400, are you really going to write a test for that?) and simple mathematical formulae
    class Triangle
      def area
        self.base * self.height / 2.0
      end
    end
are most readably expressed to humans as, well, the continuous functions that they are, not the three or four example data points that we might write examples to test for. For any finite number of test cases, you can write a giant case...when statement that passes all of them and still doesn't work in the general case.

Yes, we could and often should write a couple of tests just to make sure we haven't done anything boneheaded in implementing the function, but they're not spec. They're just examples.

But here's the rub: where or how do we put that code to make it obvious that it's specification that happens also to be a valid implementation - and not just implementation that may or may not meet a spec expressed in some other place/form? If we're laying out our app in conventional Ruby style, it can't go in the spec/ directory because that doesn't actually get run as part of the application, and it shouldn't go in lib/ or models/ or wherever else unless we are prepared to make our clients rootle through all that code looking for whatever "this is specification not just an implementation detail" flag we decide to adopt) when they want to use our interface.

I'm going to make a suggestion which is either radical or bone-headed: we should smush the rspec-stuff together with the app code: embed examples (which may in some cases be specification and in other cases be "smoke tests") in the same files as the implementation (which may itself sometimes be specification and other times be the result of our fallible human attempts to derive implementation from spec), and then we can have some kind of annotations to say which is which, and then we can have some kind of rdoc-on-'roids literate programming tool (To Be Implemented) go through the whole lot and produce two separate documents. One for people who want to use our code and want to know what it should do, and the other for the people who have to hack on it and need to know how it does. Or doesn't. And then maybe we can have code coverage metrics that actually mean something.

Syndicated 2011-04-07 14:26:57 from diary at telent netowrks

Corner cases

As you see in the image, right, my notebook recently took a dive onto a laminate floor and ended up a trifle dog-eared. Amazingly, the hardware crash didn't provoke a software crash, but not wishing to take any chances with running it while bending the motherboard, I shut it down myself. Then I dismantled it and dismantled its predecessor , and swapped parts about between them to create one combined functional machine . In the interests of extending battery life I left the fingerprint reader disconnected and removed the original HDD, leaving only the second (and faster) 2.5" disk in the optical media bay.

The new old laptop (henceforth to be known as Igor) worked for a couple of days until I left it running on battery overnight, and when I reapplied power in the morning got a rather nasty-looking "Non-system disk or disk error" message. The usual tedious mucking about with rescue images on USB keys revealed that the disk was still there and fully working, but the BIOS was determined not to see it until I remembered: PATA has two disks on the same channel. Sure enough, after removing the 'slave' jumper on the disk drive I suddenly had hda where previously I had a hole where hdb used to be.

Getting back to a state where it would actually boot, though, that was the trick. Something in Debian's initramfs generation does some kind of magic to detect that the root fs is stored in a logical volume of a volume group of an LVM physical volume on a LUKS device-mapper layer over the physical device, but something in the Debian installer's rescue mode wouldn't let me run anything grub-related (it tried: it failed with unhelpful error messages) after mounting the disk on /target, and nothing in the Debian installer would let me reinstall / without also clobbering /home, which I wasn't really interested in restoring from two-day-old backup. The reason for this rant, though, is not to vent (I did that on Twitter last week) but to document somewhere that if you're booting off a disk that has changed its name or address since this magic stuff happened, you can override it with appropriate kernel command line parameters: in my case it was

cryptopts=source=/dev/hda2,target=eamcs,lvm=eamcs-root
where source is the raw disk, target is the name of the mapping that the DM encryption code will set up for the decrypted PV (if you don't understand that clause then take heart because I'm not sure I do either, but if you set things up the debian way it's probably your hostname), and lvm is the appropriate LV name inside that PV.

Then it booted! There were some interesting warnings and it asked me for my passphrase an extra few times for luck, and then I edited /etc/crypttab and /etc/fstab and purged and reinstalled the grub-pc and initramfs-tools packages. Which may or may not have been strictly necessary but by that stage seemed prudent.

Syndicated 2011-03-07 14:25:12 from diary at telent netowrks

The Neighbour-Net Proxy Protocol

Borrowing a riff from Charlie Stross' "books I will not write" meme, I present the first in a series of indeterminate length entitled "Software I will not implement".

This is the result of a couple of days thinking about how to do a distributed Facebook (or at least, the interesting bits thereof) originally inspired by Eben Moglen's Freedom in the Cloud talk last year, and my subsequent disappointment to see that our most publically well-known hope Diaspora were all gungho about their implementation but publically completely silent about the protocols. In my opinion a monoculture is not the way to a robust ecology.

So (per the opening para of this post) why aren't I implementing it? Purely and simply, a severe deficiency of Copious Free Time. I am posting what I've got publically in the hope that it triggers some good ideas in others. In the (perhaps unlikely) event that anyone reading this thinks it's an awesomely good idea and does have the time to drive it forwards, take it or fork it and let me know and I will of course be deliriously happy to flipflop on this position and shelve something else instead if I can contribute.

https://github.com/telent/nnpp # READ ME; READ ME NOT; READ ME; READ ME NOT;

Syndicated 2011-03-02 14:01:46 from diary at telent netowrks

Backing up a workgroup server with rsnapshot

Lately I have been changing the way that $WORK backs up their office server. It should be pretty simple, right? Removeable USB drive they can take home at night, rsnapshot, win.

Almost. The problem is user-proofing.

The disk should not be mounted when not in use

Really, I don't want end-users unplugging a mounted drive. So automounting on drive plugging is a non-starter.

The disk has to be mounted before doing the backup

You'd think you could use cmd_preexec in rsnapshot.conf for this. No. If cmd_preexec fails (e.g. because the disk is not plugged in), rsnapshot prints a warning (which nobody will read) and then goes on and does the backup anyway.

There is a neat option no_create_root which you'd think was designed for this: "rsnapshot will not automatically create the snapshot_root directory. This is particularly useful if you are backing up to removable media, such as a FireWire or USB drive". However, it does this test before running cmd_preexec, which is entirely the wrong order of events for our purposes

The disk must be idle before being unplugged

Well, there's not really any physical way to stop people unplugging it during a backup, but we could at least give ourselves a sporting chance by telling them when the backup's happening.

There are several parts to this: first we spend an inordinate amount of time fighting with zenity to make it do something vaguely sane before giving up and creating a workaround that lets it do what it wants instead. The result looks a lot like rsync-wait.sh in which, if your external HDD is not called Fred, feel free to amend the text messages. If you're one of the vanishingly small number of people who has to use GNOME but isn't on a Linux kernel or for some other reason don't have inotify, you'll have to replace inotifywait with e.g sleep 10

We want this to be run for every user, because they all have physical access to the server and any of them might be charged with taking the disk home. So we need a way of launching this script on login and stopping it on logout - surprisingly the latter is harder. If you write an X client program in a real language it will probably eventually notice (or just die with SIGPIPE) when its X server socket closes, but a shell script doesn't necessarily have a persistent X connection open - so if we start the script from Xsession.d or similar and if you log out and in again the process won't die, and you end up with two copies running at once. Double bubble trouble.

To save you the afternoon that this took me to figure out, the simplest answer is "start it from xterm". So, create a .desktop file and drop it in /etc/xdg/autostart directory

dan@carnaby:~$ cat /etc/xdg/autostart/rsync-wait.desktop 
[Desktop Entry]
Encoding=UTF-8
Name=Backup Disk notifier
Comment=System tray icon for notifying rsnapshot running
Exec=/usr/bin/xterm -iconic -e /usr/local/bin/rsync-wait.sh
Terminal=false
Type=Application
NotShowIn=KDE;
StartupNotify=false
Categories=GTK;Monitor;System;

So far, this is working. I think.

Syndicated 2011-02-10 21:52:00 from diary at telent netowrks

How to create a diskless elastichosts node

Elastichosts is a PAYG (or monthly contract) "cloud" virtual server provider based on the Linux kvm technology. At $WORK we use it to provide a horizontally scalable app service, and we need to be able to add new app servers in less time than it takes to copy a complete working Debian system. Also we want to be running the same version of the same software on every server (think "security updates") and we don't want to be paying for another 3GB of Debian that we don't really need on each box. So, we need that stuff to be shared.

Elastichosts don't directly support kvm snapshots (or they didn't when I asked them about it) which leaves us looking for alternative ways to do the same thing. This blog entry describes one such approach: we use a read-only CD image for the root filesystem and then mount /usr and /home over NFS and a ramdisk (populated at boot) on /var. It's all done using standard Debian tools and Debian setup as of the "squeeze" 6.0 release.

The finished thing is on github at https://github.com/telent/squeeze-cd-nfsroot/ . To use, basically you clone the repo into /usr/local/client, edit the files, and run make. Slightly less basically, you almost certainly need to know what edits to make to which files, and you may also want to know how it works anyway. So read on ...

(Yes, you should be able to clone it elsewhere because I shouldn't have hardcoded that directory name into the Makefile. This may be fixed in a future version if I ever find the need to install it somewhere else myself. Or see the 'conclusion' section if you want to fix it yourself)

How the client boots

  1. the client boots off a CD (ISO9660) image created by initramfs-tools which is configured to look for an nfsroot directory. This directory is created on the server by a Makefile rule that copies the server's root dir and the replaces, renames and changes a bunch of stuff in /etc

  1. it then mounts a ramdisk on /tmp and another on /var. There is an initscript populate_var which creates all the empty directories that daemons will expect when they start up. Note that these directories are entirely ephemeral, which means for example that syslog must be configured to log remotely

  1. it mounts /usr and /home (readonly) directly from the server. This means that most of the packages on the server are available immediately on the clients - unless they include config files in /etc, in which case they aren't until you rerun the Makefile that creates the nfsroot (after, possibly, adjusting the config appropriately for the client)

A short guide to customising the system

These files are copied to the client - you may want to review their contents

  • template/etc/fstab needs to have the right hostname for your NFS server
  • template/etc/initramfs-tools/initramfs.conf - check DEVICE and NFSROOT settings
  • template/etc/network/interfaces may need tweaking
  • template/etc/resolv.conf is set up for our network, not yours
  • template/etc/init.d/populate_var might need directories added or chown invocations removed, depending on what packages you have installed
  • template/etc/rsyslog.conf needs editing for the syslog server's IP address

And also

  • insserv calls in Makefile may need adjusting if you have other services on the server that you don't want to also run on the client

And on the server

  • you'll need to be exporting the nfsroot/ directory as NFS, ditto /home and /usr. My /etc/exports looks something like this
    /usr/local/client/nfsroot 10.0.0.0/24(ro,no_root_squash,no_subtree_check)
    /home 10.0.0.0/24(ro,no_root_squash,no_subtree_check)
    /usr 10.0.0.0/24(ro,no_root_squash,no_subtree_check)
    

  • you need to run a dhcp server (I use "dnsmasq", which also provides DNS service)

  • If you want the clients able to syslog, you need to configure the syslog server to accept syslog messages from them

How we build the files

The nfsroot

Creating the nfsroot is done by the Makefile rootfs target

It starts by rsyncing the real root into nfsroot/ with a whole bunch of exclusions, then copies files from template/ over the copied files to cater for the bits that need a different configuration on the client than they do on the server, then does some other fiddling around. Most notable:

  • we have to copy libcrypto and libz into /usr because the dhcp client needs those libraries and it runs before /usr is mounted (see http://bugs.debian.org/592361 - though according to that page this bug is now fixed)

  • we blat the generated files etc/udev/rules.d/*persistent* which are correct for the server but not for the client.

  • Debian will run better with no /etc/hostname than it will with the wrong one

  • Debian squeeze uses a slightly exciting parallelising dependency-based system for running init scripts, so we can't just copy files into init.d, we need to run insserv to make it see then. (As a long-time Unix user who doesn't pay enough attention when these kinds of changes are made, this took ages to work out). Similarly to disable daemons that run only on the server, we use insserv -r.

  • a couple of files need to be writable, so we replace them with symlinks
    • /etc/network/run is pointed to /lib/init/rw
    • /etc/mtab is pointed to /proc/mounts

  • We create our own etc/resolv.conf. Our elastichosts clients generally have a public (dynamically allocated) IP address assigned to eth0 and a vlan attached to eth1. DHCP gets exciting here: the client boots off eth1 and gets the address of that interface using boot-time kernel code, then runs the user-space dhclient tool to get an eth0 address, and we'd rather one rely on the conjunction of all that to get /etc/resolv.conf right

  • populate_var pretty much does what it says on the tin but might need more directories adding/removing depending on what you have installed

The initramfs

The Makefile ramfs.img target makes an initramfs image which knows how to mount root on nfs. This particular magic is built into Debian and the only particular point of note here is that we use nfsroot/etc/initramfs-tools as the config directory so we know we're generating a config for the client without treading on the server's usual initramfs config (which it might need when it boots itself). In our setup the only file that's actually changed is template/etc/initramfs-tools/initramfs.conf which has settings for BOOT, DEVICE, NFSROOT that probably differ from what the server wants for itself

Creating the cd image

This is pretty straightforward too. The Makefile boot_cd.iso target runs mkisofs to generate a CD image using the initramfs image and other files taken from isolinux.

Uploading it

We had to slightly patch the elastichost-upload script to add the ability to create shared images as well as exclusive ones. This is controlled by the api key claim:type, which the elastichosts API docs describe as follows: "either 'exclusive' (the default) or 'shared' to allow multiple servers to access a drive simultaneously"

The patched version is in the git repo, accompanied by the patch

Once you've uploaded the first one you can uncomment the DRIVE_UUID param at the top of Makefile so that subsequent attempts update the same drive instead of creating a new one every time.

Conclusion

There you have it. It's certainly a bit rough and ready right now and requires editing a few too many files to be completely turnkey, but hopefully it will save someone somewhere some time. If you have bug fixes, send me patches (or fork it on github and send me pull requests); if you have suggestions, my inbox is open; if you know you need something like this but can't understand what I'm writing about, my consulting rates are reasonable ;-)

Syndicated 2011-01-22 22:35:28 from diary at telent netowrks

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