Architecture - Overview of the Core

# Overview of the Core

The main concept in the Library is a "request/response" model where an application issues a request for a URI (URL). The Library then tries to fulfill the request as efficient as possible either by requesting the URL at the origin server, a proxy server, a gateway, directly from the local file system, or a locally cached version. Data is delivered back to the application as soon as it gets ready which guarantees minimum access delay for the application. From version 3.0, the Library supports threads including its own platform independent thread model called "libwww threads". This allows multiple requests to be handled simultaneously without blocking the application while waiting on data.

## Requests and Responses

The "request/response" model is illustrated in the control/data diagram shown below. The diagram shows only the core modules - the other modules are "pasted in" later. Note, that the Library code is to the right of the thick vertical line (green), and the application to the left can be any type of application, for example a proxy or a client. The architecture of the Library does support clients and proxies in pretty much the same way as it makes little difference to the Library: a client has a user interface whereas a server has a network interface. It is a good idea to study the Line Mode Browser and the httpd as reference implementations using the Library to see this duality.

Another thing to note is that the Library from version 3.1 supports large scale data flow from the application to the network as well as from the network to the application. This has an important impact on the functionality that can be put into applications, for example allowing collaborative authoring possibilities via the Web. The architecture behind this is described in the section "Post Webs - an API for PUT and POST".

The thin lines (red) is control flow, the thick lines (blue) is data flow and the "lightning" (magenta) is control flow as a result of events handled by the Library. Let's see what happens when an application issues a request. The description is based on having an event loop - this can either be the one provided by the Library or an external event loop provided by the application. The section on libwww threads explains more on how this can be set up. The numbers refer to the figure above.

1. The event manager is waiting for an event from the application. This can for example be a user clicking the mouse on a link or types a number on the keyboard. When an event arrives, the event manager calls the user event handler provided by the application.
2. The user event handler creates a request object and uses one of the load methods.
3. The Request object creates a new Net object.
4. The Net object calls any call back functions registered to be called before the request is actually started. This can for example be mapping the URL to another destination, checking the cache, look for proxy servers and gateways etc.
5. if the request has to access the net then the Net object passes it to the protocol object
6. The after callback functions are called when the request is terminated. Types of operations you want to make here can for example be logging, history update etc. If the before call back functions implies that no net access is required then the protocol object is not used at all.
7. The event callback function is now called to actually get the document.
8. When data is arriving then the Format manager is contacted to build a stream stack.
9. The converted data is either handed from the network to the application or from the application to the network as it gets ready. If no data is ready, control is given back to the event manager.
10. If an error occurs then a dialog callback function is called to notify the user

This description is the "macro" description of how the core modules interact and in the rest of this document we shall see more of the details of what is going on inside the core modules and what objects are involved. Note that by using a threaded model, the Library can handle multiple requests simultaneously. An example on how to do this is described in the section "Libwww Threads".

Request Object
  The access manager is the main entry point for requesting a data object pointed to by a URI. It has a set of methods that allows the application to request different services, for example to get a URI, post a URI, or to search a URI.
Protocol Object
  The protocol manager is invoked by the access manager in order to access a document not found in memory or in cache. The manager consists of a set of protocol modules handling the access schemes HTTP, FTP, NNTP, Gopher, WAIS, Telnet, and access to the local file system. The protocol modules are registered dynamically (using static linking) and the User's Guide describes how modules can be registered. Each protocol module is responsible for establishing the connection to the remote server (or the local file-system) and extract information using a specific access method. When data arrives from the network, it is passed on to the format manager.
Format Manager
  The stream format manager takes care of the transportation of streams of data from the network to the application and vice versa. It also performs any parsing and data format conversion requested based on a set of registered format converters and a simple algorithm for selecting the best conversion. As the protocol modules, data format converters can be registered dynamically, and the current set of streams includes among others: MIME, SGML, HTML, and LaTeX.
Error Object
  This module manages an information stack which contains information of all errors occurred during the communication with a remote server or simply information about the current state. Using a stack for this kind of information provides the possibility of nested error messages where each message can be classified and filtered according to its impact on the current request, for example "Fatal", "Non-Fatal", "Warning" etc. The filtering can be used to decide which level of messages will be passed back to the user.
Net Object
  The net manager provides an interface for handling asynchronous sockets which is an integral part of the Library.
Event Manager
  The event manager is a "session layer" handling which thread should be the active thread. A thread can either be an internal libwww thread or an external thread, for example a Posix thread, and the event manager can itself be either the internal Library manager or an external event manager. Currently the internal event manager uses a select function call to decide which thread should be made the active one, however an external event manager can use another decision model. One of the design ideas behind the event manager is that it can be extended to a full session layer manager handling for example the control of a HTTP-NG connection. The event manager is described together with the internal thread model in the section "Libwww Threads".