doc: Add examples of zlib usage with http

doc/api/zlib.markdown

@@ -3,6 +3,16 @@
 You can access this module with:
 
     var zlib = require('zlib');
+
+This provides bindings to Gzip/Gunzip, Deflate/Inflate, and
+DeflateRaw/InflateRaw classes.  Each class takes the same options, and
+is a readable/writable Stream.
+
+### Examples
+
+Compressing or decompressing a file can be done by piping an
+fs.ReadStream into a zlib stream, then into an fs.WriteStream.
+
     var gzip = zlib.createGzip();
     var fs = require('fs');
     var inp = fs.createReadStream('input.txt');
@@ -10,9 +20,69 @@ You can access this module with:
 
     inp.pipe(gzip).pipe(out);
 
-This provides bindings to Gzip/Gunzip, Deflate/Inflate, and
-DeflateRaw/InflateRaw classes.  Each class takes the same options, and
-is a readable/writable Stream.
+To use this module in an HTTP client or server, use the
+[accept-encoding](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3)
+on requests, and the
+[content-encoding](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.11)
+header on responses.
+
+**Note: these examples are drastically simplified to show
+the basic concept.**  Zlib encoding can be expensive, and the results
+ought to be cached.  See <a href="#memory_Usage_Tuning">Memory Usage
+Tuning</a> below for more information on the speed/memory/compression
+tradeoffs involved in zlib usage.
+
+    // client request example
+    var zlib = require('zlib');
+    var http = require('http');
+    var fs = require('fs');
+    var request = http.get({ host: 'izs.me',
+                             path: '/',
+                             port: 80,
+                             headers: { 'accept-encoding': 'gzip,deflate' } });
+    request.on('response', function(response) {
+      var output = fs.createWriteStream('izs.me_index.html');
+
+      switch (response.headers['content-encoding']) {
+        // or, just use zlib.createUnzip() to handle both cases
+        case 'gzip':
+          response.pipe(zlib.createGunzip()).pipe(output);
+          break;
+        case 'deflate':
+          response.pipe(zlib.createInflate()).pipe(output);
+          break;
+        default:
+          response.pipe(output);
+          break;
+      }
+    });
+
+    // server example
+    // Running a gzip operation on every request is quite expensive.
+    // It would be much more efficient to cache the compressed buffer.
+    var zlib = require('zlib');
+    var http = require('http');
+    var fs = require('fs');
+    http.createServer(function(request, response) {
+      var raw = fs.createReadStream('index.html');
+      var acceptEncoding = request.headers['accept-encoding'];
+      if (!acceptEncoding) {
+        acceptEncoding = '';
+      }
+
+      // Note: this is not a conformant accept-encoding parser.
+      // See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
+      if (acceptEncoding.match(/\bdeflate\b/)) {
+        response.writeHead(200, { 'content-encoding': 'deflate' });
+        raw.pipe(zlib.createDeflate()).pipe(response);
+      } else if (acceptEncoding.match(/\bgzip\b/)) {
+        response.writeHead(200, { 'content-encoding': 'gzip' });
+        raw.pipe(zlib.createGzip()).pipe(response);
+      } else {
+        response.writeHead(200, {});
+        raw.pipe(response);
+      }
+    }).listen(1337);
 
 ### Constants
 
@@ -93,3 +163,13 @@ for small objects.
 
 This is in addition to a single internal output slab buffer of size
 `chunkSize`, which defaults to 16K.
+
+The speed of zlib compression is affected most dramatically by the
+`level` setting.  A higher level will result in better compression, but
+will take longer to complete.  A lower level will result in less
+compression, but will be much faster.
+
+In general, greater memory usage options will mean that node has to make
+fewer calls to zlib, since it'll be able to process more data in a
+single `write` operation.  So, this is another factor that affects the
+speed, at the cost of memory usage.