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Readme flesh #2

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Add some fleshing of the README file so people can understand a bit more of it.

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Commits on Jan 27, 2011
  1. @jlouis
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-Agner is a rebar-friendly Erlang package index inspired by Clojars and Homebrew.
-Essentially, Agner is an index of Erlang packages with some extra capabilities such as versioning, downloads and so on.
+Agner is a rebar-friendly Erlang package index inspired by Clojars and
+Essentially, Agner is an index of Erlang packages with some extra
+capabilities such as versioning, downloads and so on.
+Agner is a shorthand for *A Giant Nebula of Erlang Repositories*. It
+also pays homage to the Danish statistician "Agner Krarup Erlang".
+By now, there is a large set of Erlang tools and libraries out there,
+all of them highly useful. The problem however is to provide an index
+of these packages, so other people
+ * Know of their existence
+ * Can easily use a package in their own projects
+Agner aims to provide such an index, by focusing on a number of
+* The index is loose in the sense that anyone can overlay the index
+ and add their own packages to the repository
+* The tool is as simple as possible, utilizing git (for the time
+ being) to maintain the indexes
+* Recognize the ideas of simplicity Joe Armstrong had in mind on
+ the erlang-questions@ mailing list the
+ [22-Jul-2010](
+This section introduces the terminology of Agner:
+* **index/indices:** Where Agner finds its package index. Usually this
+ is a github user with one or more packages among the users git
+ repositories.
+* **package:** A separate library or program indentified by the
+ index. It is a `.agner` repository underneath the index-user, so one
+ example would be `agner/gproc.agner` specifying a package for the
+ `gproc` library undernath the `agner`-user.
+* **project:** A software project, program or library, that contains
+ the actual source code for the program or library. In the example,
+ this is `esl/gproc` on github.
+* **release:** A release of a package signifying a point in time
+ where the package was deemed to be in a certain state. Is usually
+ used when a new version of the software is released to the
+ general public so you can refer to package X version Y
+* **flavour:** A moving target of a package with some specified
+ behaviour. It is used for tracking the development of a package
+ over time. Common flavours include the *@master* flavour, used to
+ track the development branch of a package and the *@release*
+ flavour, used to track the latest release of the package.
+Package organization
+When Agner is invoked, it will scan its *indices* for package
+lists. The default index is "agner", which is located at
+[]( The index is
+scanned by looking for *Agner repositories* which are normal (github)
+repositories suffixed with `.agner`. An example is the repository
+which contains the package details of the `getopt` package.
+It is important to nail down that there are three balls in the air:
+* The index user, who has a list of
+* `.agner` repositories, which points to
+* Erlang software projects
+By making a split between the repository containing the project and
+the repository containing the package, we make it easy to identify
+`.agner` repositories, and we enable a simple way to make the project
+live in another source control system, for instance Mercurial (hg). It
+is also way easier to keep the (small) `.agner` repositories in an
+index and in the long run, it provisions for local caching.
+Further indices can be added to Agner through the environment (TODO:
+flesh out how that is done). Indices are searched
+in the order of specification, allowing for overriding of a given
+index. This allows you to create local indices or special indices for
+your own use, or try something out on top of other indices.
+The multiple indices approach solves authorization questions by solving it
+"the git way". You put trust in the indices you add to Agner, so if
+you don't trust an index, you can simply refrain from adding it. The
+main "agner" index is intended to be the official source, but we
+recognize that individuals might have reasons to overlay another index
+on top. By having a loose index-construction, we hope to alleviate
+some of the problems with access rights.
Package names
-Package name is just either a package name such as <code>mochiweb</code>, or (in case of github indeces, it might also
-take a form of <code>account/package</code>, for example <code>yrashk/misultin</code>)
+Package name is just either a package name such as
+<code>mochiweb</code>, or (in case of github indeces, it might also
+take a form of <code>account/package</code>, for example
+<code>yrashk/misultin</code>). We use package names to identify a
+given package in Agner - but versions of the package is naturally not
+part of its name. This allows for packages to exist in multiple
+versions at a time.
Agner has two kinds of versions:
-* Release versions, normally something like <code>1.2.0</code>, represented using tags in .agner repos
-* Flavour versions, normally something like <code>@release</code>, represented using branches in .agner repos
+* Release versions, normally something like <code>1.2.0</code>,
+ represented using tags in `.agner` repos.
+* Flavour versions, normally something like <code>@release</code>,
+ represented using branches in `.agner` repos. Note the prefix of "@".
+The intention is that a *release* version marks a given point in time
+where a given version of the code base was released to the general
+public. When Erlang/OTP is released as OTP-R14B01 for instance, it
+signifies a *release* in Agner-terminology. On the other hand, a
+*flavour* signifies a moving target. Continuing the OTP-R14B01
+example from before, it would be natural to have a *@dev* flavour
+which tracks the Erlang/OTP branch called `dev`. The other important
+flavour is *@release* which will track the latest release.
+### How to create relases and flavours
+As hinted, a release version is a *tag* in a `.agner` repository. So
+to create a release, you alter the `.agner` repository to match your
+liking and then you tag it (with a standard `git tag` command
+invocation). Agner will now pick up the change.
+Likewise, for a flavour version, you *branch* the `.agner` repository
+and alter the branch so it does what your flavour intended to
+do. Flavours can be made for anything you would like to track over
+time. By default, the advice is to create two flavours, *@master* and
+*@release* tracking, respectively, the current development of a
+project and the latest stable release of that project.
+Keeping everything up-to-date is now outsourced to git and you can use
+usual git-commands to manipulate the `.agner` repository.
+### The contents of an .agner package
+The `.agner` package repository contains a file of Erlang-terms, called
+`agner.config`. This file looks like this:
+ {name, "etorrent"}.
+ {authors, ["Jesper Louis Andersen <>"]}.
+ {description, "Etorrent is a bittorrent client implementation in Erlang focusing on fault-tolerance"}.
+ {homepage, ""}.
+ {rebar_compatible, true}.
+ {license, "BSD2", "COPYING"}.
+ {erlang_versions, [otp_r14b, otp_r14b01, otp_r13b04]}.
+ {url, {git, "", {branch, "master"}}}.
+Or in a more generic way:
+ {name, ProjectName}.
+ {authors, [Author]}.
+ {description, ProjectDescription}.
+ {homepage, ProjectHomepage}.
+ {rebar_compatible, IsRebarCompatible}.
+ {license, LicenseType, LicenseFile}.
+ {erlang_versions, [OTPAtom]}.
+ {url, UrlSpec}.
+* `ProjectName :: string()` - is the project name. This is usually
+ named the same as the `.agner` package to minimize confusion.
+* `[Author] :: [string()]` - Can really be any string, but it is
+ usually the names of the project authors in a list including their
+ email-addresses for easy contact.
+* `ProjectDescription :: string()` - A description of the
+ project. Used for searching through projects.
+* `ProjectHomepage :: string()` - The URL of the homepage of the
+ project.
+* `IsRebarCompatible :: boolean()` - Set to `true` if this project
+ uses `rebar`.
+* `LicenseType :: string(), LicenseFile :: string()` - Two
+ strings. The first one specifies the general license type of the
+ project and the second string explains where the license is to be
+ found from the top level directory (usually file-names like
+ `COPYING` or `LICENSE` are used for this).
+* `[OTPAtom] :: [otp_rXXb | otp_rXXbYY]` - A list of what OTP versions
+ the project can be used with. the `XX` is a major release number in
+ Erlang/OTP (12,13,14,...) and `YY` is a minor release number (01,
+ 02, ...).
+* `UrlSpec :: {git, URL, GitSpec}` - Specifies where to fetch the
+ project. `GitSpec` has type `{tag, string()} | {branch, string()}`
+ and points to either a git *tag* or a *git* branch
+ respectively. Notice that you can't specify more than one target in
+ this file. To handle multiple versions, you use *releases* and
+ *flavours* by altering the `.agner` repository wherein this
+ configuration file lies.
agner list [-d/--descriptions]
- agner spec [package name] [-v/--version package_version]
+Will list all agner-packages. With the `-d` or `--descriptions`
+option, it will also print out the descriptions of the packages, for
+easy grepping to find relevant packages.
+ agner spec PACKAGE [-v/--version package_version]
+Will print a specification of a given package on stdout. If the
+optional version constraint is given (for example `agner spec gproc -v
+@release`) then the output is of that version. By default, the
+`@master` flavour is chosen.
+ agner fetch PACKAGE [DESTDIR] [-v/--version package_version]
+Fetch a given `PACKAGE` to either the current directory or,
+optionally, to the `DESTDIR` directory. The version constraint is as
+were the case for `agner spec`.
- agner fetch [package name] [destination directory] [-v/--version package_version]
+ agner versions PACKAGE
- agner versions [package name]
+List the versions of the given `PACKAGE`
-Agner-compatible rebar is available at [agner branch]( of [agner/rebar]( Or you can download ready-made rebar from [agner itself](
+Agner-compatible rebar is available at
+[agner branch]( of
+[agner/rebar]( Or you can download
+ready-made rebar from
+[agner itself]( We
+hope to get rebar integration in the upstream with time.
Using it with rebar is fairly simple, it uses rebar's deps feature:
@@ -50,4 +246,8 @@ You can also specify your own indices:
-Please read at the [wiki](
+Please read at the [wiki](
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