implementing a download manager that supports resuming
- by Idan K
hi,
I intend on writing a small download manager in C++ that supports resuming (and multiple connections per download).
From the info I gathered so far, when sending the http request I need to add a header field with a key of "Range" and the value "bytes=startoff-endoff". Then the server returns a http response with the data between those offsets.
So roughly what I have in mind is to split the file to the number of allowed connections per file and send a http request per splitted part with the appropriate "Range". So if I have a 4mb file and 4 allowed connections, I'd split the file to 4 and have 4 http requests going, each with the appropriate "Range" field. Implementing the resume feature would involve remembering which offsets are already downloaded and simply not request those.
Is this the right way to do this?
What if the web server doesn't support resuming? (my guess is it will ignore the "Range" and just send the entire file)
When sending the http requests, should I specify in the range the entire splitted size? Or maybe ask smaller pieces, say 1024k per request?
When reading the data, should I write it immediately to the file or do some kind of buffering? I guess it could be wasteful to write small chunks.
Should I use a memory mapped file? If I remember correctly, it's recommended for frequent reads rather than writes (I could be wrong). Is it memory wise? What if I have several downloads simultaneously?
If I'm not using a memory mapped file, should I open the file per allowed connection? Or when needing to write to the file simply seek? (if I did use a memory mapped file this would be really easy, since I could simply have several pointers).
Note: I'll probably be using Qt, but this is a general question so I left code out of it.
