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-<!doctype article PUBLIC "-//Davenport//DTD DocBook V3.0//EN" [
-<!entity Evolution "<application>Evolution</application>">
-<!entity Camel "Camel">
-]>
-
-<article class="whitepaper" id="camel">
-
- <artheader>
- <title>The &Camel; Messaging Library</title>
-
- <authorgroup>
- <author>
- <firstname>Jeffrey</firstname>
- <surname>Stedfast</surname>
- <affiliation>
- <address>
- <email>fejj@helixcode.com</email>
- </address>
- </affiliation>
- </author>
-
- <author>
- <firstname>Michael</firstname>
- <surname>Zucchi</surname>
- <affiliation>
- <address>
- <email>notzed@helixcode.com</email>
- </address>
- </affiliation>
- </author>
-
- <author>
- <firstname>Dan</firstname>
- <surname>Winship</surname>
- <affiliation>
- <address>
- <email>danw@helixcode.com</email>
- </address>
- </affiliation>
- </author>
-
- <author>
- <firstname>Bertrand</firstname>
- <surname>Guiheneuf</surname>
- <affiliation>
- <address>
- <email>bertrand@helixcode.com</email>
- </address>
- </affiliation>
- </author>
- </authorgroup>
-
- <copyright>
- <year>2000, 2001</year>
- <holder>Ximian, Inc.</holder>
- </copyright>
-
- </artheader>
-
- <sect1 id="introduction">
- <title>Introduction</title>
-
- <para>
- &Camel; is a generic messaging library. It is being used as the
- back end for the mail component of &Evolution;. The name
- "&Camel;" is an acronym; it refers to the fact that the
- library is capable of going several days without food or water.
- It means : Camel's Acronym Makes Everyone Laugh.
- </para>
-
- <para>
- &Camel;'s initial design is heavily based on Sun's
- <trademark>JavaMail</trademark> API. It uses the Gtk+ object
- system, and many of its classes are direct analags of JavaMail
- classes. Its design has also been influenced by the features of
- IMAP, and the limitations of the standard UNIX mbox mail store,
- which set some of the boundaries on its requirements and
- extensibility.
- </para>
-
- <para>
- &Camel; sees all message repositories as stores containing
- folders. These folders in turn contain the messages the client
- actually accesses. The use of such a unified interface allows
- the client applications to be very extensible. &Camel; includes
- an external provider mechanism which allows applications to
- dynamically load and use protocols which were not available when
- the application was initially written.
- </para>
-
- <para>
- The abstract store/folder mechanism is a powerful and versatile
- way of accessing messages. No particular asumptions are made on
- the client side, thus allowing new ways of managing the
- messages. For example, the messages stored in the folders don't
- necessarily have to share some common physical location. The
- folder can be a purely virtual folder, containing only
- references to the actual messages. This is used by the "vFolder"
- provider, which allows you select messages meeting particular
- criteria and deal with them as a group.
- </para>
-
- <para>
- In addition to these possibilities, &Camel; has full MIME
- support. &Camel; MIME messages are lightweight objects
- representing the MIME skeleton of the actual message. The data
- contained in the subparts are never stored in memory except when
- they are actually needed. The application, when accessing the
- various MIME objects contained in the message (text parts,
- attachments, embedded binary objects ...) asks &Camel; for a
- stream that it can read data from. This scheme is particularly
- useful with the IMAP provider. IMAP has strong MIME support
- built-in, which allows &Camel; to download only the parts of
- messages that it actually needs: attachments need not be
- downloaded until they are viewed, and unnecessary
- "multipart/alternative" parts will never be read off the server.
- </para>
- </sect1>
-
- <sect1 id="overview">
- <title>Overview</title>
-
- <graphic format="gif" fileref="camel"></graphic>
-
- <para>
- To begin using &Camel;, an application first creates subclassed
- <classname>CamelSession</classname> object. This object is used
- to store application defaults, and to coordinate communication
- between providers and the application.
- </para>
-
- <para>
- A <classname>CamelProvider</classname> is a dynamically-loadable
- module that provides functionality associated with a specific
- service. Examples of providers are POP, IMAP and SMTP. Providers
- include subclasses of the various other &Camel; classes for
- accessing and manipulating messages.
- </para>
-
- <para>
- <classname>CamelService</classname> is an abstract class for
- describing a connection to a local or remote service. It
- currently has two subclasses: <classname>CamelStore</classname>,
- for services that store messages (such as IMAP servers and mbox
- files), and <classname>CamelTransport</classname>, for services
- that deliver messages (such as SMTP or a local MTA). A provider
- could also be both a store and a transport, as in the case of
- NNTP.
- </para>
-
- <para>
- A <classname>CamelStore</classname> contains some number of
- <classname>CamelFolder</classname> objects, which in turn
- contain messages. A <classname>CamelFolder</classname> provides
- a <classname>CamelFolderSummary</classname> object, which
- includes details about the subject, date, and sender of each
- message in the folder. The folder also includes the messages
- themselves, as subclasses of <classname>CamelMedium</classname>.
- </para>
-
- <para>
- Email messages are represented by the
- <classname>CamelMimeMessage</classname> class, a subclass of
- <classname>CamelMedium</classname>. This class includes
- operations for accessing RFC822 and MIME headers, accessing
- subparts of MIME messages, encoding and decoding Base64 and
- Quoted-Printable, etc.
- </para>
-
- <para>
- <classname>CamelTransport</classname> includes methods for
- delivering messages. While the abstract
- <function>CamelTransport::send</function> method takes a
- <classname>CamelMedium</classname>, its subclasses may only be
- able to deliver messages of specific
- <classname>CamelMedium</classname> subclasses. For instance,
- <classname>CamelSendmailTransport</classname> requires a
- <classname>CamelMimeMessage</classname>, because it needs a
- message that includes a "To:" header. A hypothetical
- <classname>CamelNNTPTransport</classname> would need a
- <classname>CamelNewsMessage</classname>, which would have a
- "Newsgroups:" header.
- </para>
-
- <para>
- The content of messages are referred to using
- <classname>CamelStream</classname> and its subclasses. In the
- case of an mbox-based store, the
- <classname>CamelStream</classname> would abstract the operation
- of reading the correct section of the mbox file. For IMAP,
- reading off the <classname>CamelStream</classname> might result
- in commands being issued to the remote IMAP server and data
- being read off a socket.
- </para>
-
- <para>
- The final major class in &Camel; is
- <classname>CamelException</classname>, which is used to
- propagate information about errors. Many methods take a
- <classname>CamelException</classname> as an argument, which the
- caller can then check if an error occurs. It includes both a
- numeric error code which can be interpreted by the program, and
- a text error message that can be displayed to the user.
- </para>
- </sect1>
-
- <sect1 id="classes">
- <title>Major Subcomponents</title>
-
- <sect2 id="store">
- <title>The Message Store</title>
-
- <para>
- A <classname>CamelStore</classname> inherits the ability to
- connect and authenticate to a service from its parent class,
- <classname>CamelService</classname>. It then adds the ability
- to retrieve folders. A store must contain at least one folder,
- which can be retrieved with
- <function>CamelStore::get_default_folder</function>. There are
- also methods to retrieve the "top-level" folder (for
- hieararchical stores), and to retrieve an arbitrary folder by
- name.
- </para>
-
- <para>
- All <classname>CamelFolder</classname>s must implement certain
- core operations, most notably generating a summary and
- retrieving and deleting messages. A
- <classname>CamelFolder</classname> must assign a permanently
- unique identifier to each message it contains. Messages can
- then be retrieved via
- <function>CamelFolder::get_message</function>.
- </para>
-
- <para>
- Folders must also implement the
- <function>get_parent_folder</function> and
- <function>list_subfolders</function> methods. For stores that
- don't allow multiple folders, they would return NULL and an
- empty list, respectively. Stores that do allow multiple
- folders will also define methods for creating and deleting
- folders, and for moving messages between them (assuming the
- folders are writable).
- </para>
-
- <para>
- Folders that support searching can define the
- <function>search_by_expression</function> method. For mbox
- folders, this is implemented by indexing the messages with the
- ibex library and using that to search them later. For IMAP
- folders, this uses the IMAP SEARCH command. Other folder types
- might not be able to implement this functionality, in which
- case users would not be able to do full-content searches on
- them.
- </para>
- </sect2>
-
- <sect2 id="messages">
- <title>Messages</title>
-
- <para>
- As mentioned before, messages are represented by subclasses of
- <classname>CamelMedium</classname>.
- <classname>CamelMedium</classname> itself is a subclass of
- <classname>CamelDataWrapper</classname>, a generic class for
- connecting a typed data source to a data sink.
- <classname>CamelMedium</classname> adds the concept of message
- headers versus message body.
- (<classname>CamelDataWrapper</classname> has one other
- important subclass, <classname>CamelMultipart</classname>,
- which is used to provide separate access to the multiple
- independent parts of a multipart MIME type.)
- <classname>CamelMedium</classname>'s subclasses provide more
- specialized handling of various headers:
- <classname>CamelMimePart</classname> adds special handling for
- the &ldquot;Content-*&rdquot; headers in MIME messages, and
- its subclass <classname>CamelMimeMessage</classname> adds
- handling for the RFC822 headers.
- </para>
-
- <graphic format="gif" fileref="mimemessage"></graphic>
-
- <para>
- Consider a message with two parts: a text part (in both plain
- text and HTML), and an attached image:
-
- <programlisting>
-
- From: Dan Winship &lt;danw@helixcode.com&gt;
- To: Matt Loper &lt;matt@helixcode.com&gt;
- Subject: the Camel white paper
- MIME-Version: 1.0
- Content-Type: multipart/mixed;
- boundary="jhTYrnsRrdhDFGa"
-
- This is a multi-part message in MIME format.
- --jhTYrnsRrdhDFGa
- Content-Type: multipart/alternative;
- boundary="sFSenbAFDSgDfg"
-
- --sFSenbAFDSgDfg
- Content-Type: text/plain
-
- Hey, Matt
-
- Check out this graphic...
-
- -- Dan
-
- --sFSenbAFDSgDfg
- Content-Type: text/html
-
- Hey, Matt&lt;br&gt;
- &lt;br&gt;
- Check out this graphic...&lt;br&gt;
- &lt;br&gt;
- -- Dan&lt;br&gt;
- &lt;br&gt;
- --sFSenbAFDSgDfg--
-
- --jhTYrnsRrdhDFGa
- Content-Type: image/png
- Content-Transfer-Encoding: base64
-
- F4JLw0ORrkRa8AwAMQJLAaI3UDIGsco9RAaB92...
- --jhTYrnsRrdhDFGa--
- </programlisting>
-
- <para>
- In &Camel;, this would be represented as follows:
- </para>
-
- <graphic fileref="samplemsg"></graphic>
- </sect2>
-
- <sect2 id="streams">
- <title>Streams</title>
-
- <para>
- Streams are a generic data transport layer. Two basic stream
- classes are <classname>CamelStreamFs</classname>, for
- reading and writing files, and
- <classname>CamelStreamMem</classname>, for reading from and
- writing to objects that are already in memory.
- </para>
-
- <para>
- Streams can also be filtered. So a CamelMimePart
- containing base64-encoded data can filter its output through
- CamelMimeFilterBasic. Other parts of the application that want
- to read its data will never need to even realize that the
- original data was encoded.
- </para>
- </sect2>
-
-</article>