Architecture - Core Objects and Managers

# Core Objects and Managers

The Library Core contains a set of objects central to the Library. Each of the core modules as explained in section "Control and Data Flow"are relying on one or more of these objects. This section describes the relationship between the core modules, the core objects and the relationship between the core objects themselves.

The figure below is very similar to the one in section "Control and Data Flow", but it also introduces the core objects associated.

HTRequest
  The HTRequest object contains information necessary to handle a request issued by the application. It contains information about the method to be used (for example "GET" and "PUT"), user preferences (language, content type etc.) specific for this request, where the output of the data object should go etc. A HTRequest object exists until the request reaches a final state, either success or failure, after which it can be discarded. Normally, a HTRequest object is created by the application, but the Library is capable of creating them on its own under certain circumstances. An example is when the Library creates a "Post Web" as explained in section "Building a POST Web, an API for PUT and POST".
HTAnchor
  HTAnchor objects represent any document which may be the source or destination of hypertext links. The HTAnchor object contains all information about the object, whether it has been loaded, metainformation like language, media type etc., and any relations to other objects. The Library defines two anchor classes: a parent anchor and a child anchor. The former contains information about whole data objects and the latter contains about subparts of a data object. The HTAnchor object is a generic superclass of both parent anchors and child anchors. Section "Anchor Objects" describes anchors and their relations in more detail.
HTNet
  The HTNet is a network object which contains all information required to read and write from the network. It contains the current socket descriptor (or ANSI C file descriptor) used for reading and writing, which input buffer to use and where to put the data once they are read. It also contains timing information on how long it takes to connect to a remote host and how many times it has tried to connect. This information is used by the DNS Cache in order to optimize access on multi homed hosts. The HTNet object is also a key element in the libwww thread model where it is used to identify a "thread". The libwww thread model is explained in "Description of libwww Threads".
HTError
  The HTError object contains information about errors occured along the way when a request is handled by the Library. Errors can be nested and the object is independent of the natural langue used to pass information to the User. The definition of the messages may be handled by the application - or it can ignore it all together.
HTStream
  The stream object accepts sequences of characters. It is a destination of data which can be thought of much like an output stream in C++ or an ANSI C-file stream for writing data to a disk or another peripheral device. The broad definition makes streams very flexible and they are used as the main method to transport data from the application to the network and vise versa. The Library defines two stream classes: A generic stream class and a specialized stream class for structured data using SGML lexical tokens. The contents of the two classes is described in detail in section "Streams Objects".

The following figure illustrates the relations between the core objects themselves.

1. When an application issues a request the access manager binds the anchor corresponding to a URL together with a request object. The binding exists until the request reaches a final state after which the application can discard the request object. Normally the anchor object stays in memory during the whole life time of the application as the set of anchors represent the part of the Web that the application has been in touch with including metainformation etc.
2. The application can make a binding between the request object and the desired destination for the data when it arrives, typically from the network. The request object is by default bound to a presentation stream which presents a hypertext object to the user on the screen, but it can also be written to a file, represented as source text etc.
3. If the file cache is enabled a cache object is created and linked to the anchor object by the cache manager so that the access manager on any future requests can use the cached version (if not stale). As mentioned, the cache manager is yet to be fully designed, and the current approach may change.
4. If the data object is not found in the cache or in memory the protocol manager is called by the access manager. The protocol manager then executes a specific protocol module which creates a HTNet object and binds it to the request object. The HTNet object is maintained uniquely by the protocol module and is removed by the protocol module as soon as the communication with the remote server reaches a final state.
5. The request object also has a link to any error information related to it. At the end of the request this information is handled by the error manager and an error message may be generated and passed to the user.
6. When data starts arriving, typically from the network, it is directed down the stream chain which can either already exist or is created as data arrives (stream chains are described in the section "Stream Objects". In the case where the application is transmitting a data object to a remote server, there are two steam chains directed in opposite directions: one from the application to the network and one from the network to the application.
7. The end of the stream chain is the stream that the user may have defined when the request first was issued or it can be the default destination which is presenting the information on the screen. Between the first and the last stream in the stream chain there can be any number of other stream objects performing operations either directly on the data, or on the stream flow itself. A T-stream is an example of the latter where the stream flow is divided into two.
8. The application receives the data arriving from the network via the "HText" object (or any of the other stream interfaces as explained in section The HTML Parser in the User's Guide).