Disparate systems communicate with each other using protocols. A protocol is a set of rules that governs the transmission of data between two independent systems. If you're familiar with protocols, then you will know that most are composed of simple, plain text commands. In this article Mitchell shows us how to create a protocol and implement it as a server using Visual Basic and Winsock.
Building A Document Request Protocol Part 1/2 - The ProcessCommands routine (Page 4 of 7 )
The great thing about our Visual Basic application is that it doesn't matter what type of client is passing the requests to it; as long as the client sends the commands in our pre-defined SARP format, then our Visual Basic app (in combination with our Winsock control) will return the data that the client's after.
The ProcessCommands function processes the data received from the client. It is the main loop of our entire VB application. Its signature looks like this:
Private Sub ProcessCommands(ByVal strCmds As String)
As you can guess, the strCmds argument is the command captured from the client by the Winsock control. It is split into an array, like this:
Dim arrCmds() As String
arrCmds = Split(strCmds, " ")
From here, we can use a select case command to determine what type of command the client wants the results for:
Select Case UCase(arrCmds(0))
Case "LOGIN"
...
Case "LIST"
...
Case "ADD"
...
Case Else
...
If the user is trying to login, then the array will contain three indexes: "LOGIN", their user id, and their password. These details are passed to the DoLogin() function, which sets the value of the global Boolean variable blnLoggedIn to true if the users credentials are valid. In our example I've simply hard coded the values, but you could just as easily add a table to our Access database:
Private Function DoLogin(ByVal strUser As String, ByVal strPass As String)
If UCase(strUser) = "XUSER" And UCase(strPass) = "XPASS" Then
DoLogin = "102 Login OK"
blnLoggedIn = True
Else
DoLogin = "103 Login Failed"
blnLoggedIn = False
End If
End Function
You'll notice that throughout our VB app, most of the functions return a message in the form of [status] [message text]. This is a common message format among protocols, and comes in handy when the client needs only a status number, or only a status message.
From this point onwards, the user must be logged in to execute any of our other SARP commands. If they are not, then they receive a message telling them so:
wsSARP.SendData "-1 You are not logged in" & vbCrLf
lstDetails.AddItem "- Sent Data ""-1 You are not logged in """
When our select case statement comes across a "LIST" command, it does some array value checking to determine whether the client wants a list of categories or articles:
Case "LIST"
If blnLoggedIn Then
'Is the user requesting groups or articles?
If UBound(arrCmds) >= 1 Then
If arrCmds(1) = "CATEGORIES" Then
strResult = GetCategories()
wsSARP.SendData strResult & vbCrLf
lstDetails.AddItem "- Sent Data """ & strResult & """"
ElseIf arrCmds(1) = "ARTICLES" Then
If UBound(arrCmds) = 2 Then
strResult = GetArticles(CInt(arrCmds(2)))
wsSARP.SendData strResult & vbCrLf
lstDetails.AddItem "- Sent Data """ & strResult & """"
The GetCategories function retrieves a list of categories from the database and returns them as a single separated-value string:
Set objRS = GetRecordset("SELECT * FROM Categories ORDER BY name ASC")
The GetRecordset function is a custom function that accepts an SQL query. It returns the results of this query as an ADO recordset. We loop through the results, create the strCats variable, and then return it.
The GetRecordset function uses the constant string variable CONNECTION_STRING, which is defined under the global declarations section. Our connection string references a system DSN that points to our Access database.
If the clients SARP command is LIST ARTICLES [Category Id], then the GetArticles function is called. It returns the complete details of each article whose catId field matches the category Id passed in by the client:
Private Function GetArticles(intCatId As Integer) As String
...
Set objRS = GetRecordset("SELECT * FROM Articles WHERE catId = " & intCatId & " ORDER BY title ASC")
PHP 
Building A Document Request Protocol Part 2/2
