Fault Tolerant Shell

Paul Howe writes "Roaming around my school's computer science webserver I ran across what struck me as a great (and very prescient) idea: a fault tolerant scripting language. This makes a lot of sense when programming in environments that are almost fundamentally unstable i.e. distributed systems etc. I'm not sure how active this project is, but its clear that this is an idea whose time has come. Fault Tolerant Shell."

Comments

You're dealing with the problem too high up

[phaze3000]

IMO (as someone who works on clustered systems for a living) you're looking at this from the wrong point of view. A clustered shell is useful only if the system it is running on top of is inherently unstable.

The real benefit is in having a system which is sufficiently distributed that any program running on top of it can continue to do so despite any sort of underlying failure.

Re:You're dealing with the problem too high up

[unixbob]

Doesn't that depend on the definition of clustered though? Clustered systems can be things like beowulf clusters. But often a collection of standalone web servers behind a http load balancers is commonly referred to as a web cluster or array.

IMHO as someone who works in a complex web server / database server environment, there are many interdependancies brought by different software, different platforms and different applications. Whilst 100% uptime on all servers is a nice to have, it's a complex goal to achieve and requires not just expertise in the operating systems & web / database server software but an indepth understanding of the applications.

A system such as this fault tolerant shell is actually quite a neat idea. It allows for flexibility in system performance and availability, without requiring complex (and therefore possibly error prone or difficult to maintain) management jobs. An example would be server which replicates images using rsync. If one of the targets is busy serving web pages or running another application, ftsh would allow for that kind of unforeseen error to be catered for relatively easily.

Bad Idea

[teklob]

It's a well meaning idea, but it would cause more problems than it would solve. It would just encourage sloppy code; people would rationalize "I don't need to fix errors because it doesn't matter", which is a very bad habit to get into when programming, ignoring errors, or even warnings

Re:Bad Idea

[Tx]

I agree. Web browsers were designed to be fault tolerant, and just look at all the horrendously broken crap that passes for HTML out there. Dangerous stuff.

Re:Bad Idea

[cgenman]

It's a well meaning idea, but it would cause more problems than it would solve. It would just encourage sloppy code; people would rationalize "I don't need to fix errors because it doesn't matter", which is a very bad habit to get into when programming, ignoring errors, or even warnings

The same logic could be applied to any security system, from the automatic door lock on the front of your house to Airbags in your car. Spell checkers discourage people from learning to spell. Antibiotics prevent the growth of the immune system. Why have a lock on your trigger, if it will encourage you to leave it in a place where your kids can find it.

The fact of the matter is, if the code works, it's good code. This is a shell scripting language we're talking about here... Not exactly assembly. Programmers would be better off spending more time thinking about the higher structure of their applications and less time hunting down trivial mistakes.

Of course, I know that this isn't quite what the article is talking about, but it's the principle of the thing. Augmentation would be an improvement.

Re:Bad Idea

[Jerf]

Spell checkers discourage people from learning to spell.

Done correctly, spellcheckers can be the best spelling-learning tool there is.

"Correctly" here means the spell-checkers that give you red underlines when you've finished typing the word and it's wrong. Right-clicking lets you see suggestions, add it to your personal dict, etc.

"Incorrectly" is when you have to run the spell-checker manually at the "end" of typing. That's when people lean on it.

The reason, of course, is feedback; feedback is absolutely vital to learning and spell-checkers that highlight are the only thing I know of that cuts the feedback loop down to zero seconds. Compared to this, spelling tests in school where the teacher hands back the test three days from now are a complete waste of time. (This is one of many places where out of the box thinking with computers would greatly improve the education process but nobody has the guts to say, "We need to stop 'testing' spelling and start using proper spell-checkers, and come up with some way to encourage kids to use words they don't necessarily know how to spell instead of punishing them." The primary use of computers in education is to cut the feedback loop down to no time at all. But I digress...)

'gaim' is pretty close but it really ticks me off how it always spellchecks a word immediately, so if you're typing along and you're going to send the word "unfortunately", but you've only typed as far as "unfortun", it highlights it as a misspelled word. Bad program! Wait until I've left the word!

Wouldn't be much work in Tcl

[DavidNWelton]

... or probably Perl or Python, either.

It doesn't actually seem to grok the commands that are being run, so something like

proc try {times script} {
if { [catch [uplevel $script] err] } { cleanup ; retry }
}

is all that's needed (of course to do it right you'd need a bit more, but still...).

try {5 times} {
commands...
}

Although Tcl is a bit lower level, and would require you to do exec ls, you could of course wrap that too so that all commands in the $script block would just be 'exec'ed by default.

In any case, better to use a flexible tool that can be tweaked to do what you need then write highly specialized tools.

Re:Python

[Noryungi]

a auto-completing python interpreter and editor

Try the Wing IDE. It has most of the functions you wanted... But it's not free software.

Worst idea since spell checkers

[91degrees]

This will not improve people's skills. In fact, it willl make them more prone to mistakes, and more likely to get the result that they didn't expect. It's similat to computer spell checkers. Ever since people started relying on these, their spelling has gone way downhill simly because they don't bother thinking. Computer do all the spelling for them. They don;t need a spell checker. They need spelling lessons.

This si even worse. Computers will try to second guess what the user means, get get it wrong half tyhe time.

A qualified shell scripter will be not make these mistakes in the first place. Anyone who thinks they need this shell actually just need to learn to spell and to ytype accuratly.

Re:Worst idea since spell checkers

[FrostedWheat]

just need to learn to spell and to ytype accuratly. -- QED - Quite Easily Done

<Teal'c> Indeed </Teal'c>

It's got the concept backwards

[Moderation abuser]

While, yes, you manage distributed systems from the center, you don't *push* updates, changes, modifications because, it doesn't scale. You end up having to write stuff like this fault tolerant shell which is frankly backwards thinking.

Instead, you automate everything and *pull* updates, changes, scripts etc. That way if a system is up, it just works, if it's down, it'll get updated next time it's up.

I won't go into details but I'll point you at http://infrastructures.org/

This would be nice...

[Ritontor]

how many times have you hacked something together in perl that ended up being relied on for some pretty important stuff, only to find 6 months down the track that there's some condition (db connects fine, but fails halfway through script execution as an example) you didn't consider and the whole thing just collapses in a heap - a nasty to recover heap cause you didn't write much logging code either.

This would REALLY be useful when you're connecting to services external to yourself - network glitches cause more problems with my code than ANYTHING else, and it's a pain in the arse to write code to deal with it gracefully. i'd really really like to see a universal "try this for 5 minutes" wrapper, which, if it still failed, you'd only have one exit condition to worry about. hey, what the hell, maybe i'll spend a few days and write one myself.

Let's draw a line in the sand...

[humankind]

All the programmers who need the environment to compensate for their inadequacies, step on one side. All the programmers who want to learn from their mistakes and become better at their craft, get on the other side.

Most of us know where this line is located.

Re:Let's draw a line in the sand...

[Anonymous Coward]

java, c++, c#

-------------

asm, c ?

Re:Let's draw a line in the sand...

[LarsWestergren]

All the programmers who need the environment to compensate for their inadequacies, step on one side. All the programmers who want to learn from their mistakes and become better at their craft, get on the other side.

Most of us know where this line is located.

"In other news, at the local beach today a vicious fight broke out between geeks about where to draw a line. Sand was kicked, noses have been blooded, we have some unconfirmed reports of a wedgie. We will have more on this breaking news as it comes in."

Re:Let's draw a line in the sand...

[ebuck]

Too good to pass up...

Redmond city limits?

Nice, which brings me too....

[MrIrwin]

The idea of being to timeout and exception handle in scripts is a great idea......assuming you want to use scripts. I think most people end up resorting to Perl, Python or whatever for anything more complex. But perhaps with this facility Scripts would be more useful? But...now I come to a related topic. I build factory wide systems, systems which have eg. Automatic warehouses and whatever in the middle. I do a lot of stuff with VB6 not because it is fault tolerant but because it is 'fix tolerant'. During the comminssioning phases I can leave a program running in the debugger and, if it freaks out, I can debug, fix, test by iterating forwards and **backwards** in the the function that caused the hitch, and then continue to run were I left off. Many minor problems get fixed on the fly without users even realizing anything was amiss. In every other respect (syntax, structure, error trapping etc) VB6 is a disaster and not really suited at all to these types of progects, so the fact that I use it is a measure of how important this feature is. Like the fault tolerant shell, it is a 'non-pure' extension insofar as purists say it should not be neccessary, but in pratice it is a godsend. Anybody know an alternative for VB6 in this respect?

Building on their first example

[gazbo]

They are deleting a number of files on a number of different machines, then downloading an updated version. The implication is that the fault tolerance means a failure is not fatal.

So what happens if the files are crucial (let's use the toy example of kernel modules being updated): The modules get deleted, then the update fails because the remote host is down. Presumably the shell can't rollback the changes a la DBMS, as that would involve either hooks into the FS or every file util ever written.

Now I think it's a nice idea, but it could easily lead to such sloppy coding; if your shell automatically tries, backs off and cleans up, why would people bother doing it the 'correct' way and downloading the new files before removing the old ones?

Python already has that

[xlurker]

here you go

• import rlcompleter, readline and
• here

autocompletion in the editor is availible in vim here

login

[Rutje]

"Password fairly correct. Root login granted."

Re:login

[Linker3000]

Yup, imagine 'ole clippy:

Seems you're trying to guess the administrator password. Do you want me to:

* Let you in to save time
* Give you a couple of letters as a clue
* Not let you in until you get it right

this can essentially already be done in /bin/bash

[xlurker]

(the concept of fault-tolerant coding encourages sloppy coding. and it makes it harder to see what's actually happening in the script. but that's not what they actually mean.)

what they seem to essentially want is

• a try statement and error catching and
• a fortran like syntax for testing and arithmetic

I think the authors were a bit misguided. Instead of creating a whole new shell how about just extending a good existing shell with a new try statement a described.

it can even be done without extending the shell:

( cd /tmp/blabla
&&
rm -rf tmpdir
&&
wget http://some.thing/wome/where
) || echo something went wrong

as for the new syntax of .eq. .ne. .lt. .gt. .to.
certainly looks like fortran-hugging to me , yuck

as for integer arithmetic, that can be done with by either using backticks or the $[ ] expansion

% echo $[ 12 * 12 + 10 ]
% 154

Once again, Babbage was thinking ahead...

[andersen]

"On two occasions I have been asked [by members of Parliament!], 'Pray, Mr.
Babbage, if you put into the machine wrong figures, will the right answers
come out?' I am not able rightly to apprehend the kind of confusion of ideas
that could provoke such a question."
-- Charles Babbage

Missing the point

[SmallFurryCreature]

This is not about catching scripting errors. It does not fix your code. It is about catching errors in the enviroment that scripts are running in.

Shell scripts should be short and easy to write. I have seen plenty of them fail due to some resource or another being temporarily down. At first people are neat and then send an email to notify the admin. When this then results in a ton of emails everytime some dodo knocks out the DNS they turn it off and forget about it.

Every scripting language has their own special little niche. BASH for simple things, perl for heavy text manipulation, PHP for creating HTML output. This scripting language is pretty much like BASH but takes failure as given. The example shows clearly how it works. Instead of ending up with PERL like scripts to catch all the possible errors you add two lines and you got a wonderfull small script, wich is what shell scripts should be, that is none the less capable of recovering from an error. This script will simply retry when someone knocks out the DNS again.

This new language will not catch your errors. It will catch other peoples errors. Sure a really good programmer can do this himself. A really good programmer can also create his own libraries. Most find of us in admin jobs find it easier to use somebody elses code rather then constantly reinvent the wheel.

Re:Missing the point

[f0rt0r]

I think it still will promote bad programming/scripting practices. Many people ( including myself ) started with scripting before moving on to full-fledged programming. What they learned in scripting they carry forward with them into programming, and trust me, I learned to be very meticulous when it comes to interacting with things outside of my scripts control ( such as files ). Every I/O operation should be tested for success. Trying to open a file? Did it work? Ok, try writing to the file...did it work? Open a database connection...did it work? Let the user enter a number...did they enter a valid number? Error handling and input validation is something you just have to learn, like it or not. Something that holds your hand and lets you code while remaining oblivious to the realities of the scripting/programming environment is a bad thing IMHO.

On a side note for Perl, one thing I always hated were the examples that had something like "open( FH, "file/path" ) || die "Could not open file!" . $!; I mean, come one, you don't want your script to just quit if it encounters an error...how about putting in an example of error handling other than the script throwing up its hands and quitting! LOL.

Please excuse any grammatical/other typos above, I was on 4 hrs sleep when I wrote this. Thank You.

Re:Missing the point

[vidarh]

So what you are saying is that programming should be hard, and people should be expected to do it right, or it promotes bad practices.

Yet we are expected to excuse your grammatical and typos. Doesn't that just promote bad practices? Shouldn't we whack you over the head with a baseball bat just to make sure you won't post when you're not prepared to write flawless posts?

The more work you have to do to check errors, the more likely it is that however vigilant you might be, errors slip past. If you have to check the return values of a 100 commands, that is a 100 chances for forgetting to do the check or for doing the check the wrong way, or for handling the error incorrectly.

In this case, the shell offers a function that provides a more sensible default handling of errors: If you don't handle them, the shell won't continue executing by "accident" because you didn't catch an error, but will terminate. It also provides an optional feature that let you easily retry commands that are likely to fail sometimes and where the likely error handling would be to stop processing and retry without having to write the logic yourself.

Each time you have to write logic to handle exponential backoff and to retry according to specific patterns is one more chance of introducing errors.

No offense, but I would rather trust a SINGLE implementation that I can hammer the hell out of until I trust it and reuse again and again than trust you (or anyone else) to check the return code of every command and every function they call.

This shell does not remove the responsibility to for handling errors. It a) chooses a default behaviour that reduces the chance of catastrophic errors when an unhandled error occurs, and b) provides a mechanism for automatic recovery from a class of errors that occur frequently in a particular type of systems (distributed systems where network problems DO happen on a regular basis), and by that leave developers free to spend their time on more sensible things (I'd rather have my team doing testing than writing more code than they need to)

One of the few who get it apparently.

[SmallFurryCreature]

This is indeed little more then the wrapper that you describe. Yet most seem to comment on its non-claimed properties of fixing the programmers errors. Wich it really really doesn't. In fact it is worse since this one would happily keep trying to execute a command like "rm -Rf / home/me/tmp".

I have often had to write such wrappers myself. Sure even easier/better would have been if somebody added this to say BASH as an extension but perhaps that is not possible.

How often have you needed to write horrible bash code just to pull data from an unreliable source and ended up either with a script that worked totally blind "command && command && command &&" wich never reported if it failed for days on end or ended up with several pages just to catch all the damn network errors that could occur.

I will definitly be giving this little language a try in the near future. Just another tool for the smart sys-admin. (smart people write as little code as possible. Let others work for you)

OK, wise guys...

[JAPrufrock]

I'm working with Grid and ftsh as we speak. I'm a physicist, not a professional coder. I write reasonable code, but I'm no purist. With that said...

ftsh has great utility in the realm it's written for. Obviously, it's not a basis for installing kernels or doing password authentication. In a Grid (not just distributed) environment, things break for all sorts of reasons all the time. You're dealing with a Friendly Admin on another system, one who may well be unaffiliated with your institution, project or field of study. He doesn't have any particular reason to consult with you about system changes.

Now you find yourself writing a grid diagnostic or submitter or job manager. One does not need strongly typed compiled languages for this. Shell scripts are almost always more efficient to write, and the speed difference is unimportant. Right now, most Grid submitters are being written in bash or Python or some such. Bash sucks for exception handling of the sort we're talking about. Python does better with its try: statements, but there's room for improvement. ftsh is a good choice for a sublayer to these scripts. One writes some of the machinery that actually interacts with the Grid nodes and supervisors in this easy, clear and flexible form.

Now there are a lot of specific points to answer:

One needs a Windows port to be able to make the Grid software we write in Linux available to the poor drones who are stuck with Win boxes.

This is not a code spellchecker or coding environment. At all.

This is not a crutch for inadequate programmers. This is a collection of methods to deal with a specific set of recalcitrant problems.

As I was pointing out before, this is, after all, an unstable system. One is using diverse resources on diverse platforms in many countries at many institutions. I appreciate the comment made by unixbob about operating in heterogeneous environments.

This isn't a substitute for wget. One uses wget as an example because it's clear.

The "pull" model breaks down immediately when there is no unified environment, as is described on infrastructures.org. When you're not the admin, and your software has to be wiped out the minute your job is done, "push" is the only way to do it. This is the case with most Grid computing right now (that I know about)

All the woe and doom about the sloppy coding and letting the environment correct your deficiencies is... ill-thought-out. That's what a compiler is, folks. Should we all be coding in machine language? :) Use the right tool for the job and save time.

I do agree, however, that one should indeed hone one's craft. Sloppy coding in projects of importance is inexcusable (M$). There is no reason to stick to strict exception handling, however, in the applications being discussed by ftsh's developers (the same folks who brought you Condor). When code becomes 3/4 exception handling, even when the specific exceptions don't matter, there's a problem, IMHO. :)

"set -e" will go a long way to helping you

[divec]

The article says:

#!/bin/sh

cd /work/foo
rm -rf bar
cp -r /fresh/data .

Suppose that the /work filesystem is temporarily unavailable, perhaps due to an NFS failure. The cd command will fail and print a message on the console. The shell will ignore this error result -- it is primarily designed as a user interface tool -- and proceed to execute the rm and cp in the directory it happened to be before.

That shell script can be improved a lot by using " set -e " to exit on failure, as follows:
#!/bin/sh

set -e # exit on failure

cd /work/foo
rm -rf bar
cp -r /fresh/data .

This means that, if any command in the script fails, the script will exit immediately, instead of carrying on blindly.

The script's exit status will be non-zero, indicating failure. If it was called by another script, and that had "set -e", then that too will exit immediately. This is a little bit like exceptions in some other languages.

If you want a fault tolerant scripting language

[WetCat]

Erlang (http://www.erlang.org) has it.
You can have multiple linked interpreters and
even fault-tolerant database!
It is a scripting language.
From the FAQ:
1.1. In a nutshell, what is Erlang?
Erlang is a general-purpose programming language and runtime environment. Erlang has built-in support for concurrency, distribution and fault tolerance. Erlang is used in several large telecommunication systems from Ericsson. The most popular implementation of Erlang is available as open source from the open source erlang site.

What's with all the negativity?

[AxelTorvalds]

I know that if you need a ton of fault tolerance in your shell scripts that you should probably be using a different language but every time I look at any complex systems, not just a signle app but a system, there is always shell script glue. More importantly, I've never seen a shell script that checked the return codes of everything at best they look at a few key components and report on their success of failure. Exceptions would be nice.

I think perl is where it is because so many people use it as "super script." To me that says, a) we recode all the Bourne and csh and bash in perl or b) we look at why people do shell scripting in perl or other languages and add that to the shell. I couldn't tell you which is right. It's a neat idea though and I'm glad they made it.

A real example I can think of, I had a test machine that had some kind of ext3 corruption and so it mounted up in read-only mode when it booted. I spent time diagnosing an application error in our application because nothing caught that; these are redhat type startup scripts. I noticed that our app couldn't write logs and began to debug the system. More interestingly, a dozen or so start-up scripts failed to start up critical components and their failure wasn't noticed. If you can't write to the filesystem, you can't create a socket(AF_UNIX) and all sort's of things go tits up then. If that's how you debug it's only going to get more difficult as you add more and more complexity, you have to detect the lower level failures and report them. Perversely, this wouldn't have been noticed had a different partition been read-only. Turns out that a drive was going bad. Had it been a different partition, it would have been noticed at catastrophic system failure time when the drive died.

I've done a fair amount of embedded work and there is always a test for new guys, you can tell the new guy (new college grad, whatever) because he skips half or more of the error checking in his code. You know printf returns a value? Funnier still, if you develop something like a consumer app in embedded space, you'll eventually see things like printf fail. We know it never should, but with 20,000+ users in different environments and what not, things like that can and do fail and usually point to a greater problem, like a dead drive or something. Instead of logging/alerting something to the critical and unusual printf failure, the app fails in a different way because this printf failed. Heaven forbid that it was sprintf that failed and then you shove bad data in to a database or configuration file and not just fail the system but corrupt the data too. Inspite of all of that, even veterans will forget error checking at times, it's a common bug and so having higher level tools to help assist, like exception in the shell can only be a good thing.

FTSH is an exception system for shell programming

[ChaosDiscord]

What's with all of the people claiming that FTSH will ruin the world because it makes it easier to be a sloppy programmer. Did you freaking read the documentation?

To massively oversimplify, FTSH adds exceptions to shell scripting. Is that really so horrible? Is of line-after-line of "if [$? -eq 0] then" really an improvement? Welcome to the 1980's, we've discovered that programming languages should try and minimize the amount of time you spent typing the same thing over and over again. Human beings are bad at repetitive behavior, avoid repetition if you can.

Similarlly FTSH provides looping constructs to simplify the common case of "Try until it works, or until some timer or counter runs out." Less programmer time wasted coding Yet Another Loop, less opportunities for a stupid slip-up while coding that loop.

If you're so bothered by the possibility of people ignoring return codes it should please you to know that FTSH forces you to appreciate that return codes are very uncertain things. Did diff return 1 because the files are different, or because the linker failed to find a required library? Ultimately all you can say is that diff failed.

Christ, did C++ and Java get this sort of reaming early on? "How horrible, exceptions mean that you don't have to check return codes at every single level."