|
The Hypertext Transfer Protocol (HTTP) is the most prevalent
application protocol used on the Internet today. It was originally used
for document retrieval, and has grown into a complex protocol which
supports file uploading, script execution, file management and
distributed web authoring through extensions such as WebDAV. The
SocketTools Hypertext Transfer Protocol control implements version 0.9,
1.0 and 1.1 of the protocol, including features such as support for
proxy servers, persistent connections, user-defined header fields and
chunked data.
File Transfers
Similar to the interface used with the File Transfer Protocol
control, you can use HTTP to upload and download files. In addition
to the standard method for downloading files, the control supports
two methods for uploading files, using either the PUT or the POST
command. When downloading a file from the server, you can either
store the contents in a local file, or you can copy the data into a
memory buffer that you allocate. Similarly, when uploading files, you
can either specify a local file to upload, or you can provide a
memory buffer that contains the file data to send to the server. High
level methods such as PutFile and GetFile can be used to transfer
files in a single method call. There are also methods such as
OpenFile and CreateFile which provide lower level file I/O
interfaces.
Script Execution
Another common use for HTTP is to execute scripts on the web server.
The application can pass additional data to the script, which is
similar in concept to how arguments are passed to a command that is
entered from the command prompt. This uses the standard POST command,
and the resulting output from the script is returned back to the
application where it can be displayed or processed. An application
can use the Command method to execute the script and then process the
output in code, or can use the higher level method PostData which
will execute the script and return the output from that script in a
single method call.
Uniform Resource Locators
Anyone who has used a web browser is familiar with the Uniform
Resource Locator (URL); it is the value that is entered as the
address of a website. URLs have a specific format which provides
information about the server, the port number and the name of
the resource that is being accessed:
http://[username : [password] @] remotehost
[:remoteport] / resource [? parameters ]
The first part of the URL identifies the protocol, also known as
the scheme, which will be used. With web servers, this will be
either http or https for secure connections. If a username and
password is required for authentication, then this will be included
in the URL before the name of the server. Next, there is the
name of the server to connect to, optionally followed by a port
number. If no port number is given, then the default port for the
protocol will be used. This is followed by the resource, which is
usually a path to a file or script on the server. Parameters to the
resource may also be specified, called the query string, which are
typically used as arguments to a script that is executed on the
server.
Understanding how a URL is constructed will help in understanding
how the different methods in the control work together. For example,
the server name and port number portion of the URL are the
values passed to the Connect method to establish the connection. The
user name and password values are assigned to the UserName and
Password properties to authenticate the client session. And the
resource name is passed to the GetData or GetFile methods to transfer
it to the local system.
The following properties, methods and events are available for use
by your application:
Initialize
Initialize the control and validate the runtime license key for the
current process. This method is normally not used if the control is
placed on a form in languages such as Visual Basic. However, if the
control is being created dynamically using a function similar to
CreateObject, then the application must call this method to
initialize the component before setting any properties or calling any
other methods in the control.
Connect
Establish a connection to the server. Once the connection has been
established, the other methods in the control may be used to access
the resources on the server.
Disconnect
Disconnect from the server and release any resources that have been
allocated for the client session. After this method is called, the
client session is no longer valid.
Uninitialize
Unload the Windows Sockets library and release any resources that
have been allocated for the current process. This is the last method
call that the application should make prior to terminating. This is
only necessary if the application has previously called the
Initialize method.
Using an interface similar to the File Transfer Protocol control,
this control provides several methods which can be used to transfer
files between the local and server. This group of methods is high
level, meaning that it is not necessary to actually write the code to
read and/or write the file data. The control automatically handles the
lower level file I/O and notifies your application of the status of the
transfer by periodically generating progress events.
GetData
This method transfers a file from the server to the local
system, storing the file data in memory. This can be useful if your
application needs to perform some operation based on the contents of
the file, but does not need to store the file locally.
GetFile
This method transfers a file from the server and stores it in a
file on the local system.
PutData
This method creates a file on the server containing the data
that you provide. This can be useful if your application wants to
upload dynamically created content without having to create a
temporary file on the local system.
PutFile
This method uploads a file from the local system to the server
using the PUT command. Not all servers support this command, and some
may require that the client authenticate prior to calling this
method.
PostFile
This method uploads a file from the local system to the server
using the POST command. This enables your application to upload a
file in the same way that a user would when using a form in a web
browser.
File Management
The control can also perform some basic file management methods as
well as send custom commands to the server. Some web servers also
provide more advanced document management methods using WebDAV, an
extension to HTTP for distributed document authoring.
GetFileSize
Return the size of a file on the server without actually downloading
the contents of the file. It is important to note that most servers
will only return file size information for actual documents stored on
the server, not for dynamically created content generated by scripts
or web pages which use server-side includes.
GetFileTime
Return the modification time for the specified file on the server.
This can be used by your application to determine if the file has
been changed since the time that you last uploaded or downloaded the
contents.
DeleteFile
Remove a file from the server. This operation requires that the
current user have the appropriate permissions to delete the file. Not
all servers support the use of this command, and it would typically
require that the client authenticate prior to calling this
method.
Command
This method enables the client to send any command directly to the
server. This is commonly used to issue custom commands to servers
that are configured to use extensions to the standard protocol.
The control also provides methods to execute scripts on the web
server and return the output from those scripts back to your
application. Your program can pass additional data to the script,
typically either as a query string or as form data, which is similar in
concept to how arguments are passed to a command that is entered from
the command prompt.
GetData
In addition to being used to simply return the contents of a file,
this method can also be used to execute a script on the server and
return the output of that script to your program. Arguments to the
script can be specified by passing them as a query string. For
example, consider the following resource name:
/cgi-bin/test.cgi?data1=value1&data2=value2
This would specify that the script /cgi-bin/test.cgi is to be
executed, and two arguments will be passed to that script:
data1=value and data2=value2. The ampersand is used to separate the
arguments, and they are grouped as pairs of values separated by an
equal sign. Note that the actual format and value of the query string
depends on how the script is written.
PostData
An alternative method of providing information to a script is to post
data to the script. Instead of the data being part of the resource
name itself, posted data is sent separately and is provided as input
to the script. This is the same method that is typically used when a
user clicks the Submit button on a web-based form. This method
requires the name of the script and the address of a buffer that
contains the data that will be posted. The resulting output from the
script is returned to the caller in the same way that the GetData
method works.
|