Handling Large Files

CSE312 — Web Development

Buffering Uploads

TCP Reminder

• TCP creates a persistent connection
• Bytes are streamed over this connection
• Data can be sent and received until one side closes the connection
• With small GET requests
  • Read from the TCP socket once to read the entire request

Buffering File Uploads

• To read an HTTP request:
  • First, read data from the TCP socket

buffer := make([]byte, 2048)
bytesRead, err := reader.Read(buffer)

POST /form-path HTTP/1.1
Content-Length: 746
Content-Type: multipart/form-data; boundary=----WebKitFormBoundarycriD3u6M0UuPR1ia

------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="commenter"

Jesse
------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="upload"; filename="discord.png"
Content-Type: image/png

<bytes_of_the_file>
------WebKitFormBoundarycriD3u6M0UuPR1ia--

Buffering File Uploads

• The size of the buffer is the maximum number of bytes that will be read from the TCP socket
• bytesRead is how many bytes were actually read

buffer := make([]byte, 2048)
bytesRead, err := reader.Read(buffer)

POST /form-path HTTP/1.1
Content-Length: 746
Content-Type: multipart/form-data; boundary=----WebKitFormBoundarycriD3u6M0UuPR1ia

------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="commenter"

Jesse
------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="upload"; filename="discord.png"
Content-Type: image/png

<bytes_of_the_file>
------WebKitFormBoundarycriD3u6M0UuPR1ia--

Buffering File Uploads

• What if we receive a fairly large POST request?
  • Might not be able to read the entire request in one read from the socket

buffer := make([]byte, 2048)
bytesRead, err := reader.Read(buffer)

POST /form-path HTTP/1.1
Content-Length: 91320
Content-Type: multipart/form-data; boundary=----WebKitFormBoundarycriD3u6M0UuPR1ia

------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="commenter"

Jesse
------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="upload"; filename="flamingo.jpg"
Content-Type: image/jpeg

<bytes_of_the_file>
------WebKitFormBoundarycriD3u6M0UuPR1ia--

Buffering File Uploads

• What if a very large file is uploaded?
• We read at most 2048 bytes per read
• A bigger buffer doesn't help. The rest of the bytes may not have arrived yet

buffer := make([]byte, 2048)
bytesRead, err := reader.Read(buffer)

POST /form-path HTTP/1.1
Content-Length: 1884206
Content-Type: multipart/form-data; boundary=----WebKitFormBoundarycriD3u6M0UuPR1ia

------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="commenter"

Jesse
------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="upload"; filename="hq_image.png"
Content-Type: image/png

<bytes_of_the_file>
------WebKitFormBoundarycriD3u6M0UuPR1ia--

Buffering File Uploads

• We must read from the socket multiple times!

buffer := make([]byte, 2048)
bytesRead, err := reader.Read(buffer)

POST /form-path HTTP/1.1
Content-Length: 1884206
Content-Type: multipart/form-data; boundary=----WebKitFormBoundarycriD3u6M0UuPR1ia

------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="commenter"

Jesse
------WebKitFormBoundarycriD3u6M0UuPR1ia
Content-Disposition: form-data; name="upload"; filename="hq_image.png"
Content-Type: image/png

<bytes_of_the_file>
------WebKitFormBoundarycriD3u6M0UuPR1ia--

Buffers

• TCP socket libraries use buffers
• No matter your language/library you will have a method/function that reads bytes from the socket
  • Called when there are bytes that arrive over the socket
  • Returns some bytes of the request

Buffer Questions

• What happens when the user has a lot of data to send?
• What if the user has a slow connection?
• Does the socket server wait for all of the data to be received before calling your code?
• What if the data takes an hour to send?
• What if the data contains streaming video that never ends?

Buffer Answer

• The socket notifies your code when there is data to read - even if it's not the entire request
• The socket will have a buffer size, typically a few kB, and will read at most that many bytes in a single call
  • For GET requests the entire request is smaller than the buffer (Safe assumption in this course)

Buffers

• Now that we're handling file uploads, we must be aware of these buffers
• The server will need data that persists across multiple calls that read bytes from a socket
  • Create data structures that store the bytes read from a request
  • Combine the bytes from multiple calls to receive the entire file

HTTP Buffers

• When receiving a large HTTP request:
  • Read bytes from the socket
  • Parse the headers
  • Find the Content-Length header and store this value
  • Keep reading bytes from the socket until you have read Content-Length bytes after the first \r\n\r\n
  • Process the request
• Remember that the Content-Length does not include the bytes for the headers
  • The total number of bytes read for one request will be larger than the Content-Length

Common Buffering Bugs

• Read returns how many bytes it read. Only use that many

bytesRead, err := reader.Read(buffer)
body = append(body, buffer...)              // Wrong: Always appends 2048 bytes
body = append(body, buffer[:bytesRead]...)  // Right: Only the bytes that were read

• Read reuses the same buffer every time
  • Copy the bytes out before the next Read overwrites them (append copies)
  • Careful with slices of buffer (eg. the body from your first parse)
• If Read returns an error, the client disconnected mid-upload
  • Stop reading and close the connection. Don't wait forever for bytes that will never arrive
• Never convert the body to a string while buffering. Binary data will be corrupted

Assumptions

• Assumptions you may make on the HW:
• The first read will contain all the headers of the request
  • Use a buffer of at least 2048 bytes
  • You can safely parse the first bytes as the headers of the request (up to the first \r\n\r\n)
  • This allows you to read the Content-Length before you start buffering
• We will test with files much larger than your buffer size
  • Your buffering must work for any size (eg. A 1 GB video)
  • Do not read the entire body with one huge read: make([]byte, 1000000000)
  • Do not use functions that buffer for you (eg. io.ReadAll or io.ReadFull)

Serving Large Files

The Problem

• Your server hosts a 1 GB mp4
• A user opens the video and immediately skips to the middle
  • They should not have to download the first 500 MB
• A user watches 10 seconds and leaves
  • Your server should not send the other 990 MB
• Reading the whole file into memory for every request is also a problem
  • 20 viewers can be 20 GB of RAM

Range Requests

• HTTP lets the client ask for part of a file
• The client adds a Range header with the bytes it wants
• The server responds with 206 Partial Content and only those bytes
• Browsers do this automatically for <video> elements
  • Only if your server says it supports ranges
  • The browser requests the bytes it needs to start playing, then whichever part of the video the user skips to

Accept-Ranges

• Tell the browser that your server supports range requests
• Include Accept-Ranges: bytes in every response for an mp4
  • Including responses with the entire file
• Without this header, the browser may download the entire file and might not let the user skip ahead

HTTP/1.1 200 OK
Content-Type: video/mp4
Content-Length: 10000
Accept-Ranges: bytes
X-Content-Type-Options: nosniff

<all 10000 bytes of the file>

The Range Header

GET /public/videos/abc123.mp4 HTTP/1.1
Range: bytes=0-99

• Positions are byte offsets in the file, starting at 0
• Three forms:

Range	Meaning
bytes={first}-{last}	Bytes first through last, inclusive
bytes={first}-	Bytes first through the end of the file
bytes=-{n}	The last n bytes of the file

• A client can ask for several ranges at once (bytes=0-50,100-150)
  • Not required for HW. Browsers don't do this for video

206 Partial Content

HTTP/1.1 206 Partial Content
Content-Type: video/mp4
Content-Length: 100
Content-Range: bytes 0-99/10000
Accept-Ranges: bytes
X-Content-Type-Options: nosniff

<bytes 0 through 99 of the file>

• Content-Range: bytes {first}-{last}/{total}
  • first and last are the bytes you actually sent
  • total is the size of the entire file
• Content-Length is the number of bytes in this response, not the size of the file
  • last - first + 1

Off By One

• Ranges are inclusive on both ends
  • bytes=0-99 is 100 bytes, not 99
  • The last byte of a 10000 byte file is at position 9999
• Go slices are exclusive on the end

data[first : last+1]     // Bytes first through last, inclusive
length := last - first + 1

• Off-by-one errors are the most common bug in range requests
  • Your Content-Length won't match the bytes you send
  • The video may play for a while, then stop or corrupt

416 Range Not Satisfiable

• If first is past the end of the file, there are no bytes to send
• Respond with 416 Range Not Satisfiable
• Content-Range uses * for the range and still includes the total size

HTTP/1.1 416 Range Not Satisfiable
Content-Range: bytes */10000
Content-Length: 0

Limiting Response Size

• When a video first loads, browsers often request bytes=0-
  • Technically the entire file
• You may send fewer bytes than requested for an open-ended range
  • eg. At most 2 MB per response
  • Content-Range must describe the bytes you actually sent
• The browser will request the rest when it needs them

GET /public/videos/abc123.mp4 HTTP/1.1
Range: bytes=0-

HTTP/1.1 206 Partial Content
Content-Range: bytes 0-2097151/1073741824
Content-Length: 2097152

What the Browser Does

• Requests bytes=0- when the video loads
• Reads enough to start playing, then may close the connection in the middle of your response
• Requests a new range when the user skips ahead
• May request the end of the file
  • Some mp4s store their index at the end of the file
• Browser behavior varies between browsers and videos
  • Handle every form, even if your browser never sends it

Closed Connections

• The browser can close the connection while you are still sending
• Writing to a closed connection returns an error
  • eg. "broken pipe" or "connection reset by peer"
• Your server must not crash
  • Check the error from every write
  • Stop sending, close the connection, and return from the handler

Testing Range Requests

• Browser behavior is unpredictable. Use curl to send exactly the Range you want to test
• -D - prints the response headers, -o saves the body

curl -s -D - -o part.bin -H "Range: bytes=0-99" http://localhost:8080/public/videos/test.mp4

HTTP/1.1 206 Partial Content
Content-Range: bytes 0-99/10000
Content-Length: 100

• Check that part.bin is exactly 100 bytes
• Test every form, a range past the end, and a request with no Range header
• The Network tab in your browser's dev tools shows the Range and Content-Range of every video request

Further Reading

• HTTP Range Requests (MDN)
• RFC9110 - Range Requests