An Introduction to Erlang

As some of you may have guessed, I am a fan of Erlang. I think that it's a very interesting language with a tremendous amount of promise for the type of server side applications that I usually end up working on. I have talked a lot about various things here on Erlangish, so I thought it would finally be appropriate to spend a bit of time talking about the topic of the blog. For the most part I will be delving into the, somewhat obscure, history of Erlang. I will also spend a bit of time providing some instructions on how to get started with the language.

So What Is Erlang and OTP?

Erlang is a distributed, concurrent, soft real time functional programming language and runtime environment developed by Ericsson, the Telecoms Infrastructure supplier. It has built-in support for concurrency, distribution and fault tolerance. Since its open source release in 1999, Erlang has been adopted by many leading telecom and IT related companies. It is now successfully being used in other sectors including banking, finance, ecommerce and computer telephony.

OTP is a large collection of libraries for Erlang to do everything from compiling ASN.1 to providing an application embeddable http server. It also consists of implementations of several patterns that have proven useful in massively concurrent development over the years. Most production Erlang applications are actually Erlang/OTP applications. OTP is also open source and distributed with Erlang.

Although Erlang is a general purpose language, it has tried to fill a certain niche. This niche mostly consists of distributed, reliable, soft real-time concurrent systems. These types of applications are telecommunication systems, Servers and Database applications which require soft real-time behavior. Erlang excels at these types of systems because these are the types of systems that it was originally designed around. It contains a number of features that make it particularly useful in this arena. For example; it provides a simple and powerful model for error containment and fault tolerance; concurrency and message passing are a fundamental to the language, applications written in Erlang are often composed of hundreds or thousands of lightweight processes; Context switching between Erlang processes is typically several orders of magnitude cheaper than switching between threads in other languages; it's distribution mechanisms are transparent, programs need not be aware that they are distributed and The runtime system allows code in a running system to be updated without interrupting the program.

Given that there are things that Erlang is good at there are bound to be a few things that it's not so good at. The most common class of 'less suitable' problems is characterized by iterative performance being a prime requirement with constant-factors having a large effect on performance. Typical examples are image processing, signal processing and sorting large volumes of data.

Origins of Erlang

I am firmly convinced that Erlang's history is a key ingredient to its success. I am not aware of any other language whose early development was so straightforward and pragmatic. For most of its life Erlang was developed inside Ericsson, originally for internal use only. Later on it was made available to the external world and eventually open sourced. The timeline of its development goes something like this.

1982–1985 Ericsson identified some issues that existed with telecom languages at the time. To address these difficulties they started experiments with the programming of telecom applications using more then twenty different languages. These early experimenters came up with a few features that a useful system needed to supply. They realized that the target language must be a very high level symbolic language in order to achieve productivity gains. This new requirement vastly subseted the language space and resulted in a very short list of languages. The languages included Lisp, Prolog, Parlog, etc.

1985–1986 Further experiments within this subseted list where performed. New results were generated from this round of experiments as well. They found that the theoretically ideal language also needed to contain primitives for concurrency and error recovery, and the execution model needed to exclude back-tracking. This ruled out two more of the contending languages, Lisp and Prolog. This ideal language also needed to have a granularity of concurrency such that there would be a one to one relationship between one asynchronous telephony process and one process in the language. This ruled out Parlog. At the end of this round of experiments they where left with out any languages in the list they had started with. Being the pragmatic folks that they were, they decided to develop their own language with the desirable features of Lisp, Prolog and Parlog, but with superior concurrency and error recovery built into the language.

• 1987 The first experiments began with this nascent language which became Erlang.
• 1988 The ACS/Dunder project was started at Ericsson. This was a prototype implementation of PABX by developers external to the core Erlang developers.
• 1989 The ACS/Dunder project became a full fledged project with the reconstruction of about a tenth of the complete, production PABX called the MD-110 system. The preliminary results where very promising. In this early phase developer productivity was already ten times greater then during the development of the original system in the PLEX language. This reimplementation also pushed forward experiments directed at increasing the speed of Erlang.
• 1991 At this point the experiments directed at speeding up Erlang bore fruit. A fast implementation of Erlang was released to growing user community. Erlang was also presented at an international telecom conference that year.
• 1992 During this year the user base started growing significantly. Erlang was ported to VxWorks, PC, Macintosh and other platforms. Three new, complete applications were written in Erlang where presented at another conference. The first two production projects using Erlang are started.
• 1993 Distribution primitives where added to the language, which made it possible to run a homogeneous Erlang system on heterogeneous hardware. A corporate decision was made within Ericsson to begin selling and supporting Erlang externally. A new organizational structure was built up to maintain and support Erlang implementations and Erlang Tools. This resulted in the creation of Erlang Systems AB.
• 1996 OTP was formed into a separate product group within Erlang Systems AB. This represented the maturing of the OTP platform within Erlang. After nearly ten years of development the (non-Erlang) AXE/N project was closed down and pronounced a failure. This left a large whole in Eriksson's product line and development of a new replacement product was started to fill it, it was decided that this replacement product would be written in Erlang.
• 1998 After two years the AXE/N replacement project, now called the AXD301 was delivered. Around this same time the CEO of Ericsson Radio became the CEO of Ericsson as a whole. This person had banned Erlang at Ericsson Radio and though he never banned Erlang at Ericsson proper it became career suicide to propose Erlang new Erlang projects. This problem effectively killed opportunities for Erlang Systems AB to sell the language and support. The primary question potential customers asked was 'Who wants to use a language developed by Ericsson when Ericsson won't use the language itself?'. This just goes to show that corporate bureaucracy will be corporate bureaucracy autocracy no matter where you are. In any case, this turned into a bit of a blessing in disguise for the rest of us.
• 1998-99 In order to drive further development a decision was made within Erlang Systems AB to release Erlang as an Open Source project. This didn't imply an abandonment of Erlang by Ericsson or Erlang Systems AB. Erlang continued and continues to be supported by these two organizations. This decision was made wholly with the idea of spreading Erlang and removing the, somewhat negative, idea that it was a proprietary language. It has remained open source and supported to this day.

As you can see the Erlang didn't start out as Erlang at all. It started out as just a series of requirements backed up by experiments. A large number of experiments where done to find the language that matched those requirements. When no existing language was found Ericsson decided to create their own. Considering Ericsson's resources and the costs associated with development of their products

I think this was a very pragmatic decision. However, that conclusion is open to interpretation. In any case, after the initial development there was a constant back and forth dialog between the users and developers of the language as the language moved through its formative process. I think this fact alone is one of the reasons that Erlang is as good as it is today. Later on in its development as Ericsson grew less resourceful Erlang started to have political problems within the company. Even though Ericsson had several successful and profitable products in Erlang and other languages the de-facto ban occurred. Fortunately by this time Erlang could and did stand on its own. The ban actually turned out to be fortunate for the rest of us because it led, pretty directly, to Erlang's eventual Open Sourcing.

Joe Armstrong, one of the original Erlang Developers and a productive member of the community put together a number of tutorials that are very useful. Its worth going through these and playing with the code.

• Robust Server
• Web Server
• Client Server
• Wiki

There are a couple of good editors to use with Erlang. The gold standard is the Erlang Emacs mode distributed as part of the Erlang distro. A very updated version is now available from the Erlware folks. You can get it here If you go this route I suggest you also get Distel, an add-on written by Luke Gorrie. It's available from the the good folks at google code. There are instructions included with both of these to get you up and going. For those of you more inclined to the IDE world you may want to take a look at Erlide. This is a set of Eclipse plugins that add support for Erlang to Eclipse. Its still pretty beta, but it's very usable.

So How Do I Get Started?

Learning Erlang is a fairly quick process. For an experienced developer it shouldn't take more then a few days before they can write nontrivial programs, about a week or two to feel really comfortable and a month or so before feeling ready to take on something big by themselves. It helps a lot to have someone who knows how to use Erlang around for some hand-holding.

Start off by going through the quick start part of the FAQ. Then go through the Erlang Course. You can skip the history part if you would like. I have gone over it in more detail here. Once you have done the course play around with some of the examples. Then go read the long version of the getting started docs. This should put you on the road to being able to write some Erlang code. If you are one to worry about coding conventions then you may want to take a look at http://www.erlang.se/doc/programming_rules.shtml. This has quite a number of useful and well thought out programming rules. One of the things that makes Erlang really interesting is the OTP System. If you really want to get to know something about Erlang then it make sense to spend a bit of time learning OTP and http://www.erlang.org/doc/design_principles/part_frame.html is a very good place to start.

An Overview of Concurrency

Introduction

Over the last few weeks I have had several conversations with people about concurrency, more specifically the ways in which shared information is handled in concurrent languages. I have gotten the impression that there isn't really a good understanding of whats out there in the world of concurrency. That being the case it would be a good idea to just give a quick overview of some of the mechanisms that are gaining mind share in the world of concurrency.

Aside from some engineers that are currently so deep in their rut that they can't see sunlight its been accepted that the current mainstream approach to concurrency just wont work. The idea of using mutexes and locks to try to take advantage of the up and coming massively multi-core chips is really just laughable. We can't ignore this topic. As software engineers we don't really have a choice about supporting large numbers of CPUs, thats the direction that hardware is going its up to us to figure out how to make it work in software. Fortunately a bunch of really smart folks have been thinking about this problem for a really long time. A few of the things they have been working on are slowly making their way into the gestalt of the software world.

We are going to talk about three things. They are Software Transactional Memory (STM), Dataflow Programing specifically Futures and Promises and Message Passing Concurrency. There are others, but these currently have the most mind share and the best implementations. I have limited space and limited time so I am going to stick to these three topics. You may have noticed that up to this point I have only talked about concurrency. More specifically the communication between processes in concurrent languages. Thats intentional, I am not going to talk about parallelism at all. Thats a different subject and only faintly related to concurrency, but often conflated with it. So if your looking for that you are looking in the wrong place. On another note, I am going to use processes and threads pretty much interchangeably. I know that in certain languages the two terms have very different meanings, however, in many other languages they mean the same or one of the terms doesn't apply. What I am getting at is that if you open the scope of the discussion to the languages at large the meanings become ambiguous. When I use the term process or thread I am talking about a single concurrent activity that may or may not communicate with other concurrent activities in same way. Thats about the best I can do for you.

Traditional Shared Memory Communication

Shared Memory Communication is the GOTO of our time. Like GOTO of years past its the current mainstream concurrent communication technique and has been for a long, long time. Just like GOTO, there are so many gotchas and ways to shoot yourself in the head that its scary. It so bad that this approach to concurrency has tainted an entire generation of engineers with an deeply abiding fear of concurrency in general. This great fear has crippled the ability of our industry to adapt to the new reality of multi-core systems. The sooner shared memory dies the horrible death it deserves then the better for us all. Having said that, I must now admit that, just like GOTOs, shared memory has a small niche where it probably can't be replaced. If you work in that niche then you already know you need shared memory and if you don't you don't. Just a hint, implementing business logic in services is not that niche. OK, I am all done with my rant and I feel much better, now on to the show.

Shared memory typically refers to a large block of random access memory that can be accessed by several different concurrently running processes. This block of memory is protected by some type of guard that makes sure that the block of memory isn't being accessed by more then one process at any particular time. These guards usually take the form of Locks, Mutexes, Semaphores, etc. There are a bunch of problems with this shared memory approach. There is complexity in managing the serial access to the block of memory. There is complexity managing lock contention for a heavily used resources. There is a very real possibility of creating deadlocks in your code in a way that isn't easily visible to you as a developer. There is just all kinds of nastiness here. This type of concurrency is found in all the current mainstream programming and scripting languages, C, C++, Java, Perl, Python, etc. For whatever reason its ubiquitous and we have all been exposed to it that doesn't mean we have to accept it as the status quo.

Software Transactional Memory (STM)

The first non-traditional concurrency mechanism we are going to look at is Software Transactional Memory, or STM for short. The most popular embodiment of STM is currently available in the GHC implementation of Haskell. As for the description I will let wikipedia handle it.

Software transactional memory (STM) is a concurrency control mechanism analogous to database transactions for controlling access to shared memory in concurrent computing. It functions as an alternative to lock-based synchronization, and is typically implemented in a lock-free way. A transaction in this context is a piece of code that executes a series of reads and writes to shared memory. These reads and writes logically occur at a single instant in time; intermediate states are not visible to other (successful) transactions.

STM has a few benefits that aren't immediately obvious. First and foremost STM is optimistic. Every thread does what it needs to do without knowing or caring if another thread is working with the same data. At the end of a manipulation if everything is good and nothing has changed then the changes are committed. If problems or conflicts occur the change can be rolled back and retried. The cool part of this is that there isn't any waiting for resources. Threads can write to different parts of a structure without sharing any lock. The bad part about this is that you have to retry failed commits. There is also some, not insignificant, overhead involved with the transaction subsystem itself that causes a performance hit. Additionally in certain situations there may be a memory hit, ie if n processes are modifying the same amount of memory you would need O(n) of memory to support the transaction. This is a million times better then the mainstream shared memory approach and if its the only alternative available to you you should definitely use it. I still consider it shared memory at its core. Thats an argument that I have had many, many times.

Dataflow - Futures and Promises

Another approach to concurrency is the use of Futures and Promises. Its most visible implementation is in Mozart-Oz. Once again I will let wikipedia the description for me.

In computer science, futures and promises are closely related constructs used for synchronization in some concurrent programming languages. They both refer to an object that acts as a proxy for a result that is initially not known, usually because the computation of its value has not yet completed.

Lets lay down the difference between Futures and Promises before we get started. A future is a contract that a specific thread will, at some point in the future, provide a value to fill that contract. A promise is, well a promise, that at some point some thread will provide the promised value. This is very much a dataflow style of programming and is mostly found in those languages that support that style, like Mozart-Oz and Alice ML.

Futures and Promises are conceptually pretty simple. They make passing around data in concurrent systems pretty intuitive. They also serve as a good foundation on which to build up more complex structures like channels. Those languages that support Futures and Promises usually support advanced ideas like unification and in that context Futures and Promises work really well. However, although Futures and Promises remove the most egregious possibilities for dead-locking it is still possible in some cases.

In the end both of these approaches involve shared memory. They both do a reasonably good job at mitigating the insidious problems of using shared memory, but they just mitigate those problems, they don't eliminate them. The next mechanism takes a completely different approach to the problem. For that reason it does manage to eliminate most of the problems involved with shared memory concurrency. Of course, there is always a trade off and in this case the trade off is in additional memory usage and copying costs. I am getting ahead of myself let me begin at the beginning and then proceed to the end in a linear fashion.

Message Passing Concurrency

The third form of concurrency is built around message passing. Once again I will let wikipedia describe the system as it tends to be better at it then I am.

Concurrent components communicate by exchanging messages (exemplified by Erlang and Occam). The exchange of messages may be carried out asynchronously (sometimes referred to as "send and pray", although it is standard practice to resend messages that are not acknowledged as received), or may use a rendezvous style in which the sender blocks until the message is received. Message-passing concurrency tends to be far easier to reason about than shared-memory concurrency, and is typically considered a more robust, although slower, form of concurrent programming. A wide variety of mathematical theories for understanding and analyzing message-passing systems are available, including the Actor model, and various process calculi.

Message passing concurrency is about processes communicating by sending messages to one another. Semantically these messages are completely separate entities unrelated to whatever data they where built from. This means that when you are writing code that uses this form of concurrency you don't need to worry about shared state at all, you just need to worry about how the messages will flow through your system. Of course, you don't get this for free. In many cases, message passing concurrency is built by doing a deep copy of the message before sending and then sending the copy instead of the actual message. The problem here is that that copy can be quite expensive for sufficiently large structures. This additional memory usage may have negative implications for you system if you are in any way memory constrained or are sending a lot of large messages. In practice, this means that you must be aware of and manage the size and complexity of the messages that you are sending and receiving. Much like Futures and Promises the most egregious 'shoot yourself in the head' possibilities of deadlocking are removed its still possible to do. You must be aware of that in your design and implementation.

Conclusion

In the end any one of these approaches is so much better then the shared memory approach that it almost doesn't matter which one you choose for your next project. However, they each have very different philosophical approaches to concurrency that greatly affect how you go about designing systems. You should explore each one so that you are able to make a logical decision about which one to use for your next project. That said, opinions are like umm, I can't really complete that, but you get my drift. My opinion on the subject is the message passing concurrency is by far the best of the three, where best is defined by most conceptually simple and scalable. In the end the industry will decide which direction is right for that and head in that direction. We are still to early in the multi-core age to get any good impression of which will win out.

Event Based Stream Parsing for JavaScript Object Notation (JSON)

Overview

I have been playing around with the idea of a stream based, sax like, parsing API for JSON. In my mind it has a few very direct benefits. I suspect that it would simplify implementing a parser for an already simple syntax. It would also allow for parsing arbitrarily large documents. In my case I need to return information to the user as quickly as possible, I absolutely cannot wait for the entire document to be parsed. This would solve that problem. All that said I seriously doubt that I am the first one to think of this, though I can't find any references to anything similar out there. If any of you have any pointers let me know in the comments and I will reference them. I plan to implement this in erlang and at least one other language. I will post links as soon as that is done. Without further ado.

Abstract

The JavaScript Object Notation (JSON) is defined in RFC 4627. This document describes method of parsing JSON using an event driven api. It is expect that details of implementation will change from language to language. However, this document should present a uniform approach to stream parsing of JSON


Introduction

JavaScript Object Notation (JSON) is describe as a text format for serialization of structured data. More commonly it is used as a transport protocol for various network based applications. This document describes an event based parsing mechanism for JSON data that can be implemented in a simple and strait forward manner. Reading and understanding RFC 4627 is a prerequisite to reading and understanding this document.


Description

The stream oriented parser produces a series of events that describe the structure currently being parsed. These events are then consumed by the calling application in some manner. The actual mechanism of consumption will vary from language type to language type though a few different types of APIs are discussed in the appendix of this document.

JSON is composed of two types of data structures primitive and complex types. The events for the two types of structures remain the same, changing only in the description of the structure being described.

The events around primitive types are relatively simple and should be implemented as simply as possible. Individual API definers will choose how they want to implement it using the examples in the appendix as a guide.


Primitive types in JSON are strings, numbers, booleans and null. These are described within the event itself

      string = STRING_DATA(value)
      number = NUMBER_DATA(value)
      boolean = BOOLEAN_DATA(value)
      null = NULL_DATA(value)

Complex types in JSON are objects and arrays. These are represented by a series of events that describe the object. Because they are complex types their representation is much more complex then that of primitive types. However, it allows the object to be consumed as it occurred in the stream.

       object =  OBJECT_BEGIN
                     KEY(string_value)
                     VALUE_BEGIN
                      ... # recursive type description
                     VALUE_END
                      ... # arbitrary number of additional key/value pairs
                  OBJECT_END

       array =  ARRAY_BEGIN
                     VALUE_BEGIN
                       ... # recursive type description
                     VALUE_END
                ARRAY_END

Callback API Description for Erlang

This would be implemented as a behavior that defines these callbacks. Client code that wishes to receive these callbacks would implement these methods. This should allow a high degree of flexibility for the client.

        %% should guard data will be one of the primitive types
        %% null will be represented by the atom null
        data(Value, State) -> State2

        object_begin(State) -> State2

        object_end(State) -> State2

        key(Value, State) -> State2

        value_begin(State) ->  State2

        value_end(State) -> State2

        array_begin(State) -> State2

        array_end(State) -> State2

Erlware Progress Update

I have had very little time to post of late. The
other maintainers and I have been coding feverishly to get
the erlware system to version 1.0 by the end of January. This has
taken up a huge amount of my time, leaving me very little to
time to blog. It been so long now that I feel bad. So
I decided that I needed to spend a bit of time talking how
development in our new model is going.

All I can say is wow! This Open Development Model has
proven to be a huge boon to the project. Not only has it
encouraged user participation by a large margin, it has also
encouraged quite a bit more design process. Rather
spontaneously the maintainers have started to submit design
specs when they are preparing to do major changes. These design
specs are usually one pagers that give a high level overview of
the problem and describe the nature of the fix the developer is
implementing. It lets the community and the other maintainers
know and comment on whats upcoming. It also makes the developer
think more closely about the changes he is planning to make to
the system. All in all it doesn't take a large amount of time
and its helps increase the quality of our offering. Since each
design spec goes to the mailing list it fits right in with the
low level patch model. I like it.

Another major development is that we have moved our website to
the Open Development Model. Originally, we used dokuwiki for
our website. Dokuwiki is an great product, but its a wiki with
the attendant problems that a wiki has. In our case, we had kept
wiki access pretty limited. Only the core maintainers actually
had access to it. This made it hard for the community to fix
issues and expand docs. What we really wanted was to combine the
easy editing of the wiki with our, somewhat more careful, patch
based approach to changes. We spent some time thinking about the
problem and decided that it would be best if our site was
updatable, via git, just like the rest of our projects. However,
we really didn't want to hand edit html and do all the manual
work involved. We liked the readable usable wiki syntax. We
needed some mix of wiki with static source files. We went to
google with low expectations. We where pleasently surprised by
the number of good offerings in various languages. Eventually we
settled on webgen. This is a nice
little ruby framework for static site generation. It supports
various markups, including our choice Markdown. What
we ended up with is an open site with very nice wikish syntax
that is easy to extend and change via a model that our
contributors are familiar with.

Once a change is in our canonical git repo getting the changes
onto the sight is completly automated. A cronjob on our server
pulls the site from the repository and runs the generator. Then
runs rsync to push the changes into the area that they are
served from. This is a very recent change so we don't actually
know if it will accomplish our purpose of more contributions to
the site. One unexpected side benefit is that our site is now
really fast. All the work is happening at generation time so
the server just needs to serve static html files.

Erlware and the Git Development Model

Introduction

Recently the core commiters for Erlware, mostly Martin and I, have decided to migrate Erlware's source control system from the previous Google hosted subversion repository to self-hosted git. Linus' Tech Talk on Git convinced us that this was a good idea. This isn't just a source control change, but a methodology change. It changes the way code gets from it's source (the Erlware community) into Erlware itself. The main things we really want to get out of this migration is

1. Greater visibility into the code base for participants
2. Increased code quality
3. Faster, cleaner incremental development
   

It's yet to be seen whether we will achieve our third goal, but we have already achieved the first two, which is encouraging.

We have had a few false starts. That is just to be expected. When we first started, we took the easy default and just replaced the central subversion repository with a git repository. We then treated that git repository as if it were just another subversion repo. That makes for a nasty commit history and it doesn't leverage git's new development model very well at all. So I started searching for information about how to do a large project in git in a truly distributed fashion. Let me tell you, there just isn't that much information out there about organising a project around git. Its just not there! Some people are working on that, I hear. In any case, I finally pulled aside a friend (hey Scott!), who was familiar with git, and asked them how this whole distributed development thing is supposed to work. Much to my amazement he actually knew! He could even explain it to a dim bulb like me!

What follows describes how we are implementing git in Erlware. It is based on what Scott told me, lurking on the git mailing list and widely varied sources out there on the interweb.

The Development Model

The model we are using is actually dirt-simple. Each person has one or more personal git repositories. This is where they do their hacking and keep track of their commit history. There is one canonical repository. However, this canonical repository never has commits pushed to it directly. To get some change into the canonical repository (and into the project) you have to send a patch representing your change to the erlware-dev@googlegroups.com mailing list. This means that every single change is pushed out and viewed by everyone on the mailing list. If we foster the community well, people will give good feedback and code reviews. Hopefully, we can build a community that is as engaged as the community that surrounds git itself.

In any case, once the patch arrives on the list, gets commented upon, changed as required, etc, it will be applied to the canonical repository and become part of Erlware. Think about this for a second, every change to Erlware goes through the Erlware dev list as a patch. Every member of the community has a chance to comment, critique and discuss it. The direction of the project is obvious to anyone who has access to the mailing list, which is anyone that wants access. It makes it extraordinarily easy for anyone to contribute to the project without ever having to code. They can just subscribe to the list and provide their knowledge and insight on the code passing through to the implementers of the patch. Of course, the commiters have the final say into what actually makes it into the canonical repository. However, the community has a huge amount of leeway in making sure that the code is correct and of the highest quality.


You may think that having to submit a patch would surely slow down development on the project. However, you would be wrong. Each developer has his own personal repository with which he can do anything he wants. We encourage developers, and require commiters, to make their repository publicly available. So that developers and anyone working with that developer can have easy access to each other's code. Their development velocity can be anything they are comfortable with. When they finally have something that they think is ready for commit to the canonical repository, they can create patches and submit them. Of course, they will need to spend some time actually refactoring their changes into a nice set of small interrelated patches. This, though, is time well spent and will give them one final chance to refactor their code.

Overall, this should be a huge boost to our productivity as a project and our transparency as project leaders.

The Nuts and Bolts

Actually getting this entire thing set up isn't a trivial project. You need to have some public place to put your git repo, http://git.erlware.org/git or git://git.erlware.org/git in our case). You need to set up that repo with, at the very least, an http server fronting it. You should probably also set up the git-daemon. It makes cloning a git repository very, very fast. Much faster then cloning a repository over http. git-daemon doesn't really have any idea of permissions or users though. So you should set this up for read access only. You do that by making sure that the user git-daemon is running as doesn't have authority to write to any of the files in the repo.

Setting Up a Git Repo

These instructions work whether you are setting up your own repository or the canonical repository.

1) Create a directory for your repos. You can just use mkdir for this, though if you are going to have multiple submitters you probably want to set the sticky group bit on the directory.

$> mkdir repo_dir
$> chown me:group_that_every_commiter_is_in repo_dir
$> chmod g+s repo_dir
$> cd repo_dir
$> mkdir project_git_dir
$> cd project_git_dir
$> git --bare init

This is going to act as a public server, so we want to enable a bit of index generation. To do that we simply make the post_update hook executable. Git will do the right thing with that.

$> chmod a+x hooks/post_update

If you look in post update you will see the command 'git-update-server-info'. This command allows git to update the indices that it needs when cloning over a 'dumb' protocol like http.

Point your http server at the repo and you are done! If you want to do some fancy stuff like send an email on every commit then you need to get then copy the post_receive_email script from the contrib/hooks directory of the git source tar ball to hooks/post_receive in your git repo. Then make that file executable. You need to fiddle with the config a bit, but the instructions for how to do that are in the file itself so there isn't any need to repeat it here.

$> cp /contrib/hooks/post_receive_email ./hooks/post-receive
$> chmod a+x ./hooks/post-receive

Working as a Member of The Project

Working with a git project means that there are three commands that you are going to be using quite a lot. These are git-format-patch, git-send-email, and git-am. Getting good with these commands will let you interact well with Erlware and any other git based project.

git-format-patch takes some command line options to detail which commits you want to turn into patches and then writes the appropriate patches out a set of files, one patch per commit. Its dirt simple, though you will need to learn how to structure your commits in a reasonable way. This isn't hard, but it is a bit involved so I will save that process for another blog. In any case, you will learn how to do it pretty quick once you start supplying patches.

git-send-email lets you take the patches created by git-format-patch and send them to a specified email address. The process is very well documented in the man page linked above so there really isn't any need to go into much detail. One note though, if you are a gmail user and don't already have sendmail or procmail setup to use gmail, I suggest you use msmtp. There are detail instructions on how to do it on the GitTips page of the GitWiki. If you are as blunt as I am, following the instructions there will save you a huge amount of time.

Finally, git-am allows you to take patches from the mailing list and apply them to your local git repository. It understands both mbox format and raw patches generated by git-format-patch. This makes it pretty damn useful. You can set up a pull from your mail server to a local mbox and then just run git-am on it. You can also just pull the patches manually and run git-am on those. The whole process is really well thought out and very, very simple.

The only real problem that I have had so far is that git-send-email doesn't let you prepend any information to the subject line of the sent email. They all end up with [PATCH] . This would be fine for most projects but we run several projects under Erlware and it would be really nice if we could do something so that the subject looked something like [PROJECT][PATCH] . Anything that would let us indicate the project would be great. When and if I get some time I intend to remedy this and send a patch back to the git community.


Conclusion

That's it. No doubt I have missed a huge number of things. Hopefully, the commenters will be nitpicky and point them out so I can fix the problems. I am really excited about this new model and expect it to do some really awesome things for our project. If it doesn't do anything but spread knowledge about the codebase to all our commiters I will be overjoyed.

New faxien

So its been a little bit since I posted. We have been working hard and heavy on several changes to the erlware repo formats. One of the big time sinks in this project has been writing a bootstrapper/otp release launcher for faxien. Well let me break out the two before I go into details.

The Repository
A few months ago we made some changes to the repository to reduce the complexity and reduce the total number of release versions we had to keep around. Originally we organized the OTP apps in our repository around the erts version number, major, minor and patch. This worked but it meant we had a lot of copies of the same apps laying around. We had hoped that we would be able to organize it around the erts major, minor version instead. This would have saved a huge amount of space. so we made some changes in support of that. Well after we made those changes we found out that the patch version number wasn't as unimportant as we thought. In fact, among other things it seems that the OTP guys feel free to modify the wire protocol in patch versions. They also change the magic version numbers in the c lib so that they wont communicate with with an erts with a patch version different then what they where compiled for. What this means for us is that we had to go back to supporting major, minor and patch versions. We also changed the way release packages are stored in the repo. Not much, but enough to require some code changes. All this took more time then we would have liked.

The Faxien Bootstrapper
The other problem we had is around matching erts versions with releases that faxien pulls down. In the past we used whatever version or erts/erlang was available on the local box. This caused problems if faxien was built for a version of erts that was greater then the one present on the box. Of course, this would be a problem for any and every bit of erlang code that faxien pulls down as well. So we had to come up with a way to pull down an erts to run on as well as the code to run. This meant that we had to come up with some way to bootstrap the system. After much thought and quite a few experiments we decided to write a minimal bootstrapper in ocaml. What this means is that folks can download a small binary that will pull down the required erts version, faxien and all its dependencies. The bootstrapper will then launch faxien to complete the install process.

With this approach the user doesn't need to have erlang on their system at all. They just pull down 'faxien' and it pulls down everything thats required. Thats pretty cool. It also only pulls whats needed so instead of going out and getting 20 megs of erlang distribution they will get what they need and just what they need.

We put in a few other nifty features to make command line OTP releases easier and to make cross platform OTP launch scripts easer to write. However, I will talk about those in a different post.

Sinan 0.8.4 Alpha is Out

The latest version of Sinan is out. This version adds support for spawning a new Erlang node with shell for the current project. This makes debugging and exploratory programming much, much easier. It also spawns the node with an sname 'sinan_shell' so that the new shell will interact well with distel. This is a change that a lot of people have asked for and I am pleased to be able to integrate it.

I am still having issues with the analyze task that I am working through. However, I hope to have it fixed and working shortly.