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We are going to write a concurrent program using Clojure, which is going to extract keywords from a huge amount of incoming mail which will be cross-checked with a database.
One of my teammates has suggested to use Erlang to write this program.
Here I want to note something that I am new to functional programming so I am in a little doubt whether clojure is a good choice for writing this program, or Erlang is more suitable.
Do you really mean concurrent or distributed?
If you mean concurrent (multi-threaded, multi-core etc.), then I'd say Clojure is the natural solution.
Clojure's STM model is perfectly designed for multi-core concurrency since it is very efficient at storing and managing shared state between threads. If you want to understand more, well worth looking at this excellent video.
Clojure STM allows safe mutation of data by concurrent threads. Erlang sidesteps this problem by making everything immutable, which is fine in itself but doesn't help when you genuinely need shared mutable state. If you want shared mutable state in Erlang, you have to implement it with a set of message interactions which is neither efficient nor convenient (that's the price of a nothing shared model....)
You will get inherently better performance with Clojure if you are in a concurrent setting in a large machine, since Clojure doesn't rely on message passing and hence communication between threads can be much more efficient.
If you mean distributed (i.e. many different machines sharing work over a network which are effectively running as isolated processes) then I'd say Erlang is the more natural solution:
Erlang's immutable, nothing-shared, message passing style forces you to write code in a way that can be distributed. So idiomatic Erlang automatically can be distributed across multiple machines and run in a distributed, fault-tolerant setting.
Erlang is therefore very well optimised for this use case, so would be the natural choice and would certainly be the quickest to get working.
Clojure could do it as well, but you will need to do much more work yourself (i.e. you'd either need to implement or choose some form of distributed computing framework) - Clojure does not currently come with such a framework by default.
In the long term, I hope that Clojure develops a distributed computing framework that matches Erlang - then you can have the best of both worlds!
The two languages and runtimes take different approaches to concurrency:
Erlang structures programs as many lightweight processes communicating between one another. In this case, you will probably have a master process sending jobs and data to many workers and more processes to handle the resulting data.
Clojure favors a design where several threads share data and state using common data structures. It sounds particularly suitable for cases where many threads access the same data (read-only) and share little mutable state.
You need to analyze your application to determine which model suits you best. This may also depend on the external tools you use -- for example, the ability of the database to handle concurrent requests.
Another practical consideration is that clojure runs on the JVM where many open source libraries are available.
Clojure is Lisp running on the Java JVM. Erlang is designed from the ground up to be highly fault tolerant and concurrent.
I believe the task is doable with either of these languages and many others as well. Your experience will depend on how well you understand the problem and how well you know the language. If you are new to both, I'd say the problem will be challenging no matter which one you choose.
Have you thought about something like Lucene/Solr? It's great software for indexing and searching documents. I don't know what "cross checking" means for your context, but this might be a good solution to consider.
My approach would be to write a simple test in each language and test the performance of each one. Both languages are somewhat different to C style languages and if you aren't used to them (and you don't have a team that is used to them) you may end up with a maintenance nightmare.
I'd also look at using something like Groovy 1.8. Groovy now includes GPars to enable parallel computing. String and file manipulation in Groovy is very easy indeed.
It depends what you mean by huge.
Strings in erlang are painful..
but:
If huge means tens of distributed machines, than go with erlang and write workers in text friendly languages (python?, perl?). You will have distributed layer on the top with highly concurrent local workers. Each worker would be represented by erlang process. If you need more performance, rewrite your worker into C. In Erlang it is super easy to talk to another languages.
If huge still means one strong machine go with JVM. It is not huge then.
If huge is hundreds of machines, I think you will need something stronger google-like (bigtable, map/reduce) probably on C++ stack. Erlang still OK, however you will need good devs to code it.
I understand it should cover threading primitives (mutex, semaphore, condition variables etc.) plus design patterns (such as those specified in POSA2). But what's more? Every project has its own multithreading scenarios and one may have not dealt with those that the job is expecting?. So how does one build their knowledge and prove that they have the ability?
Regardless of specifics solid, detailed and very deep knowledge is required. One should understand how the bottlenecks form, how to deal with scalability problems, how to diagnose cases where synchronization is required but is erroneously omitted.
If for example you had a job experience with multithreading and I ran an interview to assess you I'd ask detailed questions on typical scenarios that arise when developing multithreaded programs. I wouldn't expect you knew many technologies or some specific technology, but I'd expect you to have mastered the technology you claim you're familiar with in great detail and to understand which fundamental problems it solves and how.
I would expect the candidate has knowledge and experience of the issues that arise when multiple threads access shared resources. What problems can be caused by concurrent access and what problems the solutions (such as locking etc) present.
At the very least understanding of how to write and read asych code on the platform of choice.
After this it will be understanding the specifics if the platform - e.g. such how to access the primary window in windows system while many things needs up the display at the same time.
Fundamentally is about understanding what trade-offs are needed and when.
May I present a different view. I think you should understand the basics,
but really never give up on a job based on a flyer description. I have
not met a programming concept that could not be figured out in half a day.
So, basically, read a tutorial before the interview, do not try to misrepresent
your actual experience with threading, but make sure they know the things you
had more hands on experience, and see if there is a mutual interest in you working
for the company. They may like you even if you know nothing about threading if they are confident that you can pick it up at full speed.
I'm currently writing a large multi threaded C++ program (> 50K LOC).
As such I've been motivated to read up alot on various techniques for handling multi-threaded code. One theory I've found to be quite cool is:
http://en.wikipedia.org/wiki/Communicating_sequential_processes
And it's invented by a slightly famous guy, who's made other non-trivial contributions to concurrent programming.
However, is CSP used in practice? Can anyone point to any large application written in a CSP style?
Thanks!
CSP, as a process calculus, is fundamentally a theoretical thing that enables us to formalize and study some aspects of a parallel program.
If you instead want a theory that enables you to build distributed programs, then you should take a look to parallel structured programming.
Parallel structural programming is the base of the current HPC (high-performance computing) research and provides to you a methodology about how to approach and design parallel programs (essentially, flowcharts of communicating computing nodes) and runtime systems to implements them.
A central idea in parallel structured programming is that of algorithmic skeleton, developed initially by Murray Cole. A skeleton is a thing like a parallel design pattern with a cost model associated and (usually) a run-time system that supports it. A skeleton models, study and supports a class of parallel algorithms that have a certain "shape".
As a notable example, mapreduce (made popular by Google) is just a kind of skeleton that address data parallelism, where a computation can be described by a map phase (apply a function f to all elements that compose the input data), and a reduce phase (take all the transformed items and "combine" them using an associative operator +).
I found the idea of parallel structured programming both theoretical sound and practical useful, so I'll suggest to give a look to it.
A word about multi-threading: since skeletons addresses massive parallelism, usually they are implemented in distributed memory instead of shared. Intel has developed a tool, TBB, which address multi-threading and (partially) follows the parallel structured programming framework. It is a C++ library, so probably you can just start using it in your projects.
Yes and no. The basic idea of CSP is used quite a bit. For example, thread-safe queues in one form or another are frequently used as the primary (often only) communication mechanism to build a pipeline out of individual processes (threads).
Hoare being Hoare, however, there's quite a bit more to his original theory than that. He invented a notation for talking about the processes, defined a specific set of signals that can be sent between the processes, and so on. The notation has since been refined in various ways, quite a bit of work put into proving various aspects, and so on.
Application of that relatively formal model of CSP (as opposed to just the general idea) is much less common. It's been used in a few systems where high reliability was considered extremely important, but few programmers appear interested in learning (yet another) formal design notation.
When I've designed systems like this, I've generally used an approach that's less rigorous, but (at least to me) rather easier to understand: a fairly simple diagram, with boxes representing the processes, and arrows representing the lines of communication. I doubt I could really offer much in the way of a proof about most of the designs (and I'll admit I haven't designed a really huge system this way), but it's worked reasonably well nonetheless.
Take a look at the website for a company called Verum. Their ASD technology is based on CSP and is used by companies like Philips Healthcare, Ericsson and NXP semiconductors to build software for all kinds of high-tech equipment and applications.
So to answer your question: Yes, CSP is used on large software projects in real-life.
Full disclosure: I do freelance work for Verum
Answering a very old question, yet it seems important that one
There is Go where CSPs are a fundamental part of the language. In the FAQ to Go, the authors write:
Concurrency and multi-threaded programming have a reputation for difficulty. We believe this is due partly to complex designs such as pthreads and partly to overemphasis on low-level details such as mutexes, condition variables, and memory barriers. Higher-level interfaces enable much simpler code, even if there are still mutexes and such under the covers.
One of the most successful models for providing high-level linguistic support for concurrency comes from Hoare's Communicating Sequential Processes, or CSP. Occam and Erlang are two well known languages that stem from CSP. Go's concurrency primitives derive from a different part of the family tree whose main contribution is the powerful notion of channels as first class objects. Experience with several earlier languages has shown that the CSP model fits well into a procedural language framework.
Projects implemented in Go are:
Docker
Google's download server
Many more
This style is ubiquitous on Unix where many tools are designed to process from standard in to standard out. I don't have any first hand knowledge of large systems that are build that way, but I've seen many small once-off systems that are
for instance this simple command line uses (at least) 3 processes.
cat list-1 list-2 list-3 | sort | uniq > final.list
This system is only moderately sized, but I wrote a protocol processor that strips away and interprets successive layers of protocol in a message that used a style very similar to this. It was an event driven system using something akin to cooperative threading, but I could've used multithreading fairly easily with a couple of added tweaks.
The program is proprietary (unfortunately) so I can't show off the source code.
In my opinion, this style is useful for some things, but usually best mixed with some other techniques. Often there is a core part of your program that represents a processing bottleneck, and applying various concurrency increasing techniques there is likely to yield the biggest gains.
Microsoft had a technology called ActiveMovie (if I remember correctly) that did sequential processing on audio and video streams. Data got passed from one filter to another to go from input to output format (and source/sink). Maybe that's a practical example??
The Wikipedia article looks to me like a lot of funny symbols used to represent somewhat pedestrian concepts. For very large or extensible programs, the formalism can be very important to check how the (sub)processes are allowed to interact.
For a 50,000 line class program, you're probably better off architecting it as you see fit.
In general, following ideas such as these is a good idea in terms of performance. Persistent threads that process data in stages will tend not to contend, and exploit data locality well. Also, it is easy to throttle the threads to avoid data piling up as a fast stage feeds a slow stage: just block the fast one if its output buffer grows too big.
A little bit off-topic but for my thesis I used a tool framework called TERRA/LUNA which aims for software development for Embedded Control Systems but is used heavily for all sorts of software development at my institute (so only academical use here).
TERRA is a graphical CSP and software architecture editor and LUNA is both the name for a C++ library for CSP based constructs and the plugin you'll find in TERRA to generate C++ code from your CSP models.
It becomes very handy in combination with FDR3 (a CSP refinement checker) to detect any sort of (dead/life/etc) lock or even profiling.
a theoretical question. After reading Armstrongs 'programming erlang' book I was wondering the following:
It will take some time to learn Erlang. Let alone master it. It really is fundamentally different in a lot of respects.
So my question: Is it possible to write 'like erlang' or with some 'erlang like framework', which given that you take care not to create functions with sideffects, you can create scaleable reliable apps as well as in Erlang? Maybe with the same msgs sending, loads of 'mini processes' paradigm.
The advantage would be to not throw all your accumulated C/C++ knowledge over the fence.
Any thoughts about this would be welcome
Yes, it is possible, but...
Probably the best answer for this question is given by Robert Virding’s First Rule:
“Any sufficiently complicated
concurrent program in another language
contains an ad hoc,
informally-specified, bug-ridden, slow
implementation of half of Erlang.”
Very good rule is use the right tool for the task. Erlang excels in concurrency and reliability. C/C++ was not designed with these properties in mind.
If you don't want to throw away your C/C++ knowledge and experience and your project allows this kind of division, good approach is to create a mixed solution. Write concurrent, communication and error handling code in Erlang, then add C/C++ parts, which will do CPU and IO bound stuff.
You clearly can - the Erlang/OTP system is largely written in C (and Erlang). The question is 'why would you want to?'
In 'ye olde days' people used to write their own operating system - but why would you want to?
If you elect to use an operating system your unwritten software has certain properties - it can persist to hard disk, it can speak to a network, it can draw on screens, it can run from the command line, it can be invoked in batch mode, etc, etc...
The Erlang/OTP system is 1.5M lines of code which has been demonstrated to give 99.9999999% uptime in large systems (the UK phone system) - that's 31ms downtime a year.
With Erlang/OTP your unwritten software has high reliability, it can hot-swap itself, your unwritten application can failover when a physical computer dies.
Why would you want to rewrite that functionality?
I would break this into 2 questions
Can you write concurrent, scalable C++ applications
Yes. It's certainly possible to create the low level constructs needed in order to achieve this.
Would you want to write concurrent, scalable, C++ applications
Perhaps. But if I was going for a highly concurrent application, I would choose a language that was either designed to fill that void or easily lent itself to doing so (Erlang, F# and possibly C#).
C++ was not designed to build highly concurrent applications. But it can certainly be tweaked into doing so. The cost might be higher than you expect though once you factor in memory management.
Yes, but you will be doing some extra work.
Regarding side effects, consider how the .net/plinq team is approaching. Plinq won't be able to enforce you hand it stuff with no side effects, but it will assume you do so and play by its rules so we get to use a simpler api. Even if the language doesn't have built-in support for it, it will still simplify things as you can break the operations more easily.
What I can do in one Turing complete language I can do in any other Turing complete language.
So I interpret your question to read, is it as easy to write a reliable and scalable application in C++ as it is in Erlang?
The answer to that is highly subjective. For me it is easier to write it in C++ for the following reasons:
I have already done it in C++ (at least three times).
I don't know Erlang.
I have read a great deal about Stackless Python, which feels to me like a highly concurrent message based cooperative multitasking system in python, but of course python is written on top of C.
Having said that. If you already know both languages, and you have the problem well defined, you can then make the best choice based on all the information you have at hand.
the main 'problem' with C (or C++) for writing reliable and easy to extend programs is that in C you can do anything. so, the first step would be to write a simple framework that restricts just a bit. most good programmers do that anyway.
in this case, the restrictions would be mostly to make it easy to define a 'process' within whatever level of isolation you want. fork() has a reputation of being slow, and threads also need significant time to spawn, so you might want to use a cooperative multitasking, which can be far more efficient, and you could even make it preemptive (i think that's what Erlang does). to get multi-core efficiency, set a pool of threads and make all of them complete to run the tasks.
another important part would be to create an appropriate library of immutable data structures, so that using them (instead of the standard lib) your functions would be (mostly) side-effect-free.
then it's just a matter of setting a good API for message passing and futures... not easy, but at least it doesn't seem like changing the language itself.
When I hire developers for general mid-to-senior web app development positions, I generally expect them to understand core concurrent programming concepts such as liveness vs. safety, race conditions, thread synchronization and deadlocks. I'm not sure whether to consider topics like fork/join, wait/notify, lock ordering, memory model basics (just the basics) and so forth to be part of what every reasonably seasoned developer ought to know, or whether these are topics that are more for semi-specialists (i.e. developers who have made a conscious decision to know more than the average developer about concurrent programming).
I'd be curious to hear your thoughts.
I tend to think that at this point in time concurrent programming at any serious level of depth is still a specialist skill. Many will claim to know about it through study, but many will also make an almighty mess of it when they come to apply it.
In addition to the considerations listed, I would also look at resource implications and the various overheads of using processes, threads and fibers. In some contexts, e.g. mobile devices, excessive multithreading can have serious performance implications. This can lead to portability issues with multithreaded code.
I guess if I was interviewing a candidate in this situation, I would work with a real world example rather than hitting on more general topics which can be quoted back verbatim from a text book. I say this having done a fair bit of multithreaded work myself and remembering how badly I screwed up the first couple of times. Many can talk the talk... ;)
I know all these topics, but I studied them. I also know many competent senior programmers that don't know these. So unless you expect these programmers to be using those concepts actively, there is no reason to turn down a perfectly good candidate because they don't understand every aspect of concurrency
The real question is:
In what ways does it matter to the code they will be developing?
You should know which concepts the development position you're hiring for needs to know to be able to work on the projects that they will be responsible for.
As with anything in the programming world.. The devil is in the details, and you can't know everything. Would you expect them to know Perl if you were hiring for a Java position?
Also, concurrency, at this stage, while well described in generalized theory, is heavily implementation and platform dependent. Concurrency in Perl on an AIX box is not the same game as concurrency in a C++ Winforms app. They can have all the theory in the world under their belts, but if it's required for the job, then they should have intimate knowledge of the platform they are expected to use it on as well.
I interview folks for concurrency-related positions frequently and I look for three general aspects:
General understanding of core concepts like the ones you list (language-independent)
Specific understanding of Java concurrency libraries and primitives (specific to the work they'd be doing)
Ability to design the solution to a concurrent problem in a reasonable way.
I consider #1 a requirement (for my positions). I consider #2 a nice to have. If they understand it and can describe it in terms of pthreads or whatever other library, it's no biggie to learn the latest Java concurrency libraries (the concepts are the hard part). And #3 tends to separate the hires from the maybe-hires.
Per your question, I wouldn't consider fork/join to be known by almost anyone, esp someone applying for a web app developer position. I would look for developers to have experience with some (but not all) of those topics. Most developers I've interviewed have not used the Java 5+ concurrency libs at all but they can typically describe things like data race or deadlock.