I am trying to ilustrate the concept of distributed applications using Erlang. My system currently has one server and one ATM. I try to keep it as simple as possible.
For the moment my application runs locally. I am using gen_server for a client-server relationship between the banking server and the ATM. I also have a gen_fsm module to suggest different states my ATM has. In order to store any data i use the dict module (I don't want to make things more complicated using databases). To keep processes alive (the gen_server and gen_fsm) I am using a supervisor process. I've wrapped all modules as an application but for the moment it's all local. Any ideas would be highly appreciated.
I was thinking to start the same application on two different nodes and to illustrate the distributed concept to use some kind of failover/takeover mechanism but I have no ideas on what modules to use.
Is it mandatory to use target systems?(at some point i must do a hot upgrade to the application)
What's the correct order to do these things: first to upgrade and then distribute?
I would be very grateful if someone could give me some ideas on how to accomplish all those things.
I never tried it myself, but docs seem to point to:
Erlang Release Handling (11.3 Distributed Systems).
It's a really short paragraph showing the sync_nodes command, I suggest you to read the whole chapter because I noticed that often the concepts of concurrency and distribution are so persavive in Erlang that problems like yours have already been solved and included in OTP.
BTW, Erlang user guide also has a whole chapter dedicated to Distributed Applications that seems related to distributed applications config options, I think that the two should do the trick.
Hope this helps, if you need more help just ask!
Related
First, brief but profound thanks to all who have responded to my questions to date.
I'm climbing the Erlang OTP learning curve and enjoying every step. Now digging into Application design and integration. My question today is this:
Suppose I have a set of library modules: myLibrary. I want to employ these functions in an Erlang Application involving a set of application-specific modules: myApp. Mylibrary may well serve Applications beyond myApp.
I want, in turn, to deploy myApp over the web, over, on Webmachine, say. I also want need data persistence for myApp, could be Dets or Mnesia or Riak -- yet to be decided.
First, should I wrap myLibrary into the same Application as myApp? Or should they be separate Applications?
Second, how can I best integrate all these Apps into one functional Release? (I'm burning oil on the published docs re: this question, but finding them a bit confusing).
Thank you,
LRP
First of all, I'm thrilled that you are learning erlang and getting your questions answered by the community. I would like to suggest, however, that you reciprocate the good will by marking correct answers that we have provided as "accepted". Now that we have that out of the way, on to the actual answer...
myLibrary should be it its own repository if you believe it has potential for reuse. See edate as an example of such a library.
I agree that the business of packaging applications and their dependencies in erlang is, at the moment, a bit confusing. You may want to start with my answers here and here.
This set of three posts walks you through the process of integrating the holy trinity of webmachine, erlydtl, and riak. The material is a bit dated but as I recall the author touches on a number of points pertinent to your question.
This post is often given as a reference for using rebar and creating actual releases.
As of this writing I don't actually do real erlang releases in production. I use rebar to compile, test, and manage my dependencies and that's it - on my production servers I just clone my repository, run make, and away I go. But if you want the full-on release management experience, rebar can help you do that too.
So I've been tasked at work to write windows services to replace some old legacy VB6 WinForms apps currently running as services, consistently repeating tasks day-to-day. To give some general background, they have there own state machines built in to handle decision basing and not utilizing threading.
A lot of the senior developers here thought it would be worth a try to look into WorkFlow to replace the state machines rather than write my own business logic and try threading it programmaticly. So it's WF vs. the "Old College Try" I suppose.
My concern is that there aren't many books on the topic, and since it was implemented in .Net I've heard very little about it being used. I brought this up at work and another developer mentioned that it's because Biz Talk never really caught on and it was designed for that.
So is it broken? Do you think it will be supported long enough to not worry so much? I don't want an ill-functioning process injected into my services, my new babies at work, and then have WF's keel over. Leaving me with having to replace them with my own code in the event of an emergency; which does not seem like much of a grand scenario to me.
Any suggestions, recommendations would be super.
Workflow Foundation is used in Microsoft SharePoint, so I think they will continue supporting it.
There is an open source project called Stateless by Nicholas Blumhardt. It is quite flexible and very light weight. See my SO answer for details.
I chose this over Windows Workflow simply because I could define a state as State and thereby persist the state of my workflows back to the database using SubSonic. Configuration consists of one XML file. If I need to add tasks, I simply add nodes to the XML.
The each state can have a series of triggers that once satisfied will advance to appropriate state. This framework is a single assemble and fits nicely in your domain logic.
I've been checking out the Nitrogen Project which is supposed to be the most mature web development framework for Erlang.
Erlang, as a language, is extremely impressive. However, with regards to Nitrogen, what I am not too keen about is using Erlang's rather uncommon syntax (unless you're native in PROLOG) to build UIs.
What is your experience with it as opposed to other mainstream web frameworks such as Django or Rails?
I've done very little with Nitrogen so far, but I've been monitoring the mailing list for months, so I think I have something useful to say about it.
To your concern about the syntax of Erlang and the Nitrogen framework, I'd respond that that sounds like a pure case of unfamiliarity, rather than unsuitability. Objectively, HTML is not a beautiful language, and it has plenty of quirks. You're used to this now, so it doesn't seem so bad. Give Nitrogen/Erlang a chance and you may find that you get used to it soon enough, too.
To your question about comparison to other languages and frameworks, I'd say the biggest difference is that with Nitrogen, the entire web site is being served directly by the Erlang runtime. Ruby on Rails has such a mode, but it's intended only for testing. Many other frameworks don't even offer the option of running everything within a single long-running process.
Running the entire web application and its underlying infrastructure within a single long-running process has significant implications on how the site runs:
With Apache, each child gets killed off every N connections, where N=500 or so, and you can't say whether a given child will always handle all of a given client's requests. Because HTTP is stateless but web apps almost always require some client state, an Apache child must rebuild its view of client state as part of handling a new connection. By default, this means going back to disk for persistent data stored about that client. There are alternatives like memcached, but these aren't built into the core of a LAMP type stack. With Erlang, nothing is killed off periodically, and Erlang offers standard facilities like Mnesia which provide disk-backed in-memory DBs.
Incidentally, if you're familiar with nginx, it's built on the same principles as Erlang, and it's fast for the same reason. The main difference between nginx and an Erlang instance running a web server is that nginx isn't a programming environment, so it still has to delegate a lot of processing to outside code. That means it shares the same IPC and persistent state problems as Apache.
Because the runtime stays up continuously and is a fully-functional programming environment, you can probably build more parts of your system in Erlang than with a lashed-together LAMP type stack. This magnifies the above benefits. The various parts of your system can coordinate via message passing and Mnesia instead of heavyweight IPC and MySQL, and all the pieces stay up and running continually, leading to less time-consuming state reconstruction.
A dozen or so Apache children all accessing the persistent client state data store is a lock-based hairball. The frameworks all handle locking and such for you transparently, but what they can't hide is the time it takes to do all this correctly.
Erlang is an impure functional language, which implies but does not require data purity; it is also built with multiprocessing in mind, going clear down to the core of the runtime design. These two facts mean you're less likely to spend time waiting on locks in an Erlang based server than one naively built on one of the other frameworks. It is certainly possible to optimize away lock delays in the other systems, but is that really what you want to be doing? Do you want to be on the thousandth team that has to learn how to optimize its web stack after the service becomes popular, or would you rather leave it all up to the tooling so you can spend your time doing something no one else has done yet?
I, too, was once concerned about clunky Erlang syntax. I've built a couple of tools to alleviate its annoyances for everyday web programming, and perhaps you will find one or both of them helpful:
ErlyDTL is an Erlang implementation of the Django Template Language; it's not available in Nitrogen, but it is available in other frameworks, such as Zotonic, Erlang Web, BeepBeep, and Chicago Boss
Chicago Boss is a full-stack Erlang framework that does a lot of code generation so that you can access data fields with function calls instead of Erlang's rather verbose record syntax (e.g. Person:name() instead of Person#person.name)
Note that Nitrogen does not include a database layer, so it's not really comparable to Rails or Django. For a comprehensive comparison of the database-driven frameworks, check out my answer to this StackOverflow question:
https://stackoverflow.com/questions/1822518/current-state-of-erlang-web-development-frameworks-template-languages/2898271#2898271
I would check out Webmachine if I were you. It is quite simple, fast, and leaves the interface up to you.
Erlang Web should also be considered mature. It is an MVC framework, whereas Nitrogen is more event based. It's a matter of preference.
I haven't used the other tools mentioned here except Webmachine, which I think it's a wonderful tool, but it is not a web framework like the others. It is as HTTP processor, and is ideal for building a restful interfaces.
I would also suggest you give the Erlang syntax a chance. Erlang is one of my favourite languages to use.
How can i write a cloud-aware application? e.g. an application that takes benefit of being deployed on cloud. Is it same as an application that runs or a vps/dedicated server? if not then what are the differences? are there any design changes? What are the procedures that i need to take if i am to migrate an application to cloud-aware?
Also i am about to implement a web application idea which would need features like security, performance, caching, and more importantly free. I have been comparing some frameworks and found that django has least RAM/CPU usage and works great in prefork+threaded mode, but i have also read that django based sites stop to respond with huge load of connections. Other frameworks that i have seen/know are Zend, CakePHP, Lithium/Cake3, CodeIgnitor, Symfony, Ruby on Rails....
So i would leave this to your opinion as well, suggest me a good free framework based on my needs.
Finally thanks for reading the essay ;)
I feel a matrix moment coming on... "what is the cloud? The cloud is all around us, a prison for your program..." (what? the FAQ said bring your sense of humour...)
Ok so seriously, what is the cloud? It depends on the implementation but usual features include scalable computing resource and a charge per cpu-hour, storage area etc. So yes, it is a bit like developing on your VPS/a normal server.
As I understand it, Google App Engine allows you to consume as much as you want. The back-end resource management is done by Google and billed to you and you pay for what you use. I believe there's even a free threshold.
Amazon EC2 exposes an API that actually allows you to add virtual machine instances (someone correct me please if I'm wrong) having pre-configured them, deploy another instance of your web app, talk between private IP ranges if you wish (slicehost definitely allow this). As such, EC2 can allow you to act like a giant load balancer on the front-end passing work off to a whole number of VMs on the back end, or expose all that publicly, take your pick. I'm not sure on the exact detail because I didn't build the system but that's how I understand it.
I have a feeling (but I know least about Azure) that on Azure, resource management is done automatically, for you, by Microsoft, based on what your app uses.
So, in summary, the cloud is different things depending on which particular cloud you choose. EC2 seems to expose an API for managing resource, GAE and Azure appear to be environments which grow and shrink in the background based on your use.
Note: I am aware there are certain constraints developing in GAE, particularly with Java. In a minute, I'll edit in another thread where someone made an excellent comment on one of my posts to this effect.
Edit as promised, see this thread: Cloud Agnostic Architecture?
As for a choice of framework, it really doesn't matter as far as I'm concerned. If you are planning on deploying to one of these platforms you might want to check framework/language availability. I personally have just started Django and love it, having learnt python a while ago, so, in my totally unbiased opinion, use Django. Other developers will probably recommend other things, based on their preferences. What do you know? What are you most comfortable with? What do you like the most? I'd go with that. I chose Django purely because I'm not such a big fan of PHP, I like Python and I was comfortable with the framework when I initially played around with it.
Edit: So how do you write cloud-aware code? You design your software in such a way it fits on one of these architectures. Again, see the cloud-agnostic thread for some really good discussion on ways of doing this. For example, you might talk to some services on GAE which scale. That they are on GAE (example) doesn't really matter, you use loose coupling ideas. In essence, this is just a step up from the web service idea.
Also, another feature of the cloud I forgot to mention is the idea of CDN's being provided for you - some cloud implementations might move your data around the globe to make it more efficient to serve, or just because that's where they've got space. If that's an issue, don't use the cloud.
I cannot answer your question - I'm not experienced in such projects - but I can tell you one thing... both CakePHP and CodeIgniter are designed for PHP4 - in other words: for really old technology. And it seems nothing is going to change in their case. Symfony (especially 2.0 version which is still in heavy beta) is worth considering, but as I said on the very beginning - I can not support this with my own experience.
For designing applications for deployment for the cloud, the main thing to consider if recoverability. If your server is terminated, you may lose all of your data. If you're deploying on Amazon, I'd recommend putting all data that you need persisted onto an Elastic Block Storage (EBS) device. This would be data like user generated content/files, the database files and logs. I also use the EBS snapshot on a 5 day rotation so that's backed up itself. That said, I've had a cloud server up on AWS for over a year without any issues.
As for frameworks, I'm giving Grails a try at the minute and I'm quite enjoying it. Built to be syntactically similar to Rails but runs on the JVM. It means you can take advantage of all the Java goodness, like threading, concurrency and all the great libraries out there to build your web application.
I am considering Erlang as a potential for my upcoming project. I need a "Highly scalable, highly reliable" (duh, what project doesn't?) web server to accept HTTP requests, but not really serve up HTML. We have thousands of distributed clients (other systems, not users) that will be submitting binary data to central cluster of servers for offline processing. Responses would be very short, success, fail, error code, minimal data. We want to use HTTP since it is our best chance of traversing firewalls.
Given this limited information about the project, can you provide any weaknesses that might pop up using a technology like Erlang? For instance, I understand Erlang's text processing capabilities might leave something to be desired.
You comments are appreciated.
Thanks.
This sounds like a perfect candidate for a language like Erlang. The scaling properties of the language are very good, but if you're worried about the data processing abilities, you shouldn't be. It's a very powerful language, with many libraries available for developers. It's an old language, and it's been heavily used/tested in the past, so everything you want to do has probably already been done to some degree.
Make sure you use erlang version R11B5 or newer! Earlier versions of erlang did not provide the ability to timeout tcp sends. This results in stalled or malicious clients being able to execute a DoS attack on your application by refusing to recv data you send them, thus locking up the sending process.
See issue OTP-6684 from R11B5's release notes.
With Erlang the scalability and reliability is there but from your project definition you don't outline what type of text processing you will need.
I think Erlang's main limitation might be finding experienced developers in your area. Do some research on the availability of Erlang architects and coders.
If you are going to teach yourself or have your developers learn it on the job keep in mind that it is a very different way of coding and that while the core documentation is good a lot of people do wish there were more examples. Of course the very active community easily makes up for that.
I understand Erlang's text processing
capabilities might leave something to
be desired.
The starling project already provides basic unicode support and there is a EEP (Erlang Enhancement Proposal) currently in draft, but going in to bring it into the mainstream of Erlang/OTP support.
I encountered some problems with Redis read performance from Erlang. Here is my question. I tend to think the reason is Erlang-written module, which has troubles while processing tons of strings during communication with Redis.