Question

I have a Delphi 6 application that uses an Indy TIdTCPClient instance to communicate with a web server. The reason I am not using an HTTP client directly is because the the server is an image streaming server that uses the same socket connection for receiving the command to start streaming as it does to start “pushing” images back to you. In other words, after you send it a typical HTTP POST request, it replies with an HTTP response, and immediately after that it starts sending out a stream of JPEG images.

I already know how to craft a proper POST request and send it using the TIdTCPClient WriteBuffer() method and then use the ReadBuffer() method to receive reply data. What I’d like to do instead is to send a POST request and then ask Indy to wait for a typical HTTP response including retrieving all the bytes in the response body if there is a Content-Length header variable. I of course want it to leave the JPEG frames intact that may have piled in after the HTTP response in the receive queue until I start requesting them (that is, I don’t want it including any of the JPEG frames in the HTTP response to my streaming request command until I ask for them using a successive read call).

Is there a method that I can call on a TIdTCPClient that will retrieve completely a typical HTTP response with body content, and nothing else? I thought about using SendCmd() and checking the LastCmdResult property (type: TIdRFCReply) for the response, but I can’t tell from the Indy documentation if it retrieves the response body content too if there is a Content-Length header variable as part of the response it returns, nor can I tell if it leaves the rest of the receive queue after the response intact.

What is the best way to accomplish this mixed mode interaction with an HTTP web server that pushes out a stream of JPEG frames right after you make the HTTP request to start streaming?

Also, if there is a clever way to have Indy split the frames using the JPEG frame WINBONDBOUDARY delimiting string, rather than accumulating blocks of data and parsing them out myself, please share that technique.

Answer 1

The correct way to read an HTTP response is to first read the CRLF-delimited response headers line-by-line until a blank line is encountered, aka a CRLF+CRLF sequence, then you can use those headers to decide how to read the remaining response data. The headers will tell you not only what kind of stream is being sent (via the Content-Type header), but also how the data is being framed (Content-Length, Transfer-Encoding: chunked, something specific to the particular Content-Type, etc).

To receive the headers, you can use the connection’s Capture() method, setting its ADelim parameter to a blank string.

How you read the remaining data afterwards depends on the actual formatting/framing of the stream. Without knowing exactly what kind of stream you are receiving, there is no way to advise you how best to read it, as there are several different types of streaming protocols used by HTTP servers, and most of them are not standardized. Provide that information, then I/we can show you how to implement it with Indy.

You cannot use SendCmd() as the HTTP protocol does not format its responses in a way that is compatible with that method.