gzip yer uploads | Objectionable-C

NSURLSession opaquely handles HTTP negotiations about accepting compressed content and does the decompression (using formats like gzip and brotli) before you receive responses, which is great. Foundation, Apple’s OSes, and the web are doing work that we as developers don’t have to so our users consume less data and have a faster, less data-consuming experience. Hooray! I’ve always been impressed that this happens pretty much auto-magically without having to think about it.

However, I had a thought late in 2024 about transmitting data the other way: uploads. While looking I realized there’s not automatic upload compression built into NSURLSession. I’m sure there are good reasons for this, but I figured that some of the APIs I was using could accept compressed uploads. It turns out I was right, Mixpanel and Dropbox both accept compressed data.

Here’s how I’m doing the compression itself

#import <zlib.h>

// Function that gzips data
// Thanks Claude https://tijo.link/BGdVNx
static NSData *_gzipCompressData(NSData *const data, NSError **error) {
   if (!data.length) {
       if (error) {
           *error = [NSError errorWithDomain:@"CompressionErrorDomain" code:1
               userInfo:@{NSLocalizedDescriptionKey: @"Invalid input data"}];
       }
       return nil;
   }
   
   z_stream strm;
   strm.zalloc = Z_NULL;
   strm.zfree = Z_NULL;
   strm.opaque = Z_NULL;
   
   // Initialize deflate with gzip format
   if (deflateInit2(&strm, Z_DEFAULT_COMPRESSION, Z_DEFLATED, 15 + 16, 8, Z_DEFAULT_STRATEGY) != Z_OK) {
       if (error) {
           *error = [NSError errorWithDomain:@"CompressionErrorDomain" code:2
               userInfo:@{NSLocalizedDescriptionKey: @"Failed to initialize compression"}];
       }
       return nil;
   }
   
   // Set up input
   strm.avail_in = (uInt)data.length;
   strm.next_in = (Bytef *)data.bytes;
   
   // Prepare output buffer (compress can increase size)
   NSMutableData *compressedData = [NSMutableData dataWithLength:data.length * 1.1 + 12];
   strm.avail_out = (uInt)compressedData.length;
   strm.next_out = compressedData.mutableBytes;
   
   // Compress
   if (deflate(&strm, Z_FINISH) != Z_STREAM_END) {
       deflateEnd(&strm);
       if (error) {
           *error = [NSError errorWithDomain:@"CompressionErrorDomain" code:3
               userInfo:@{NSLocalizedDescriptionKey: @"Compression failed"}];
       }
       return nil;
   }
   
   // Cleanup and finalize
   [compressedData setLength:strm.total_out];
   deflateEnd(&strm);
   
   return compressedData;
}

(You could also use a library like this one if you don’t want to hand write the compression.)

Then, if using a data task:

NSData *compressedData = _gzipCompressData(originalData, &err);
NSURLRequest *request = ...;
[request setValue:@"gzip" forHTTPHeaderField:@"Content-Encoding"];
[request setHTTPBody:compressedData];
// Start a data task using request and an NSURLSession

or an upload task:

NSData *compressedData = _gzipCompressData(originalData, &err);
NSURLRequest *request = ...;
[request setValue:@"gzip" forHTTPHeaderField:@"Content-Encoding"];
[urlSession uploadTaskWithRequest:request fromData:compressedData];

With this you can shave a few percent off your upload sizes saving bandwidth and time!

Here are examples where I’m doing this in TJMixpanelLogger and TJDropbox.