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+# DIY Dat
+
+This document shows how to write your own compatible `dat` client using node modules.
+
+The three essential node modules are called [hyperdrive](https://npmjs.org/hyperdrive), [discovery-swarm](https://npmjs.org/discovery-swarm) and [level](https://npmjs.org/level). Hyperdrive does file synchronization and versioning, discovery-swarm does peer discovery over local networks and the Internet, and level provides a local LevelDB for storing metadata. More details are available in [How Dat Works](how-dat-works.md). The [dat](https://npmjs.org/dat) module itself is just some code that combines these two modules and wraps them in a command-line API.
+
+Here's the minimal code needed to download data from a dat:
+
+```js
+var Swarm = require('discovery-swarm')
+var Hyperdrive = require('hyperdrive')
+var Level = require('level')
+
+// run this like: node thisfile.js 4c325f7874b4070blahblahetc
+// the dat link someone sent us, we want to download the data from it
+var link = new Buffer(process.argv[2], 'hex')
+
+// here are the default config dat uses:
+// used for MDNS and also as the dns 'app name', you prob shouldnt change this
+var DAT_DOMAIN = 'dat.local'
+// dat will try this first and pick the first open port if its taken
+var DEFAULT_LOCAL_PORT = 3282 
+// we run the servers below you can use if you want or run your own
+var DEFAULT_DISCOVERY = [
+  'discovery1.publicbits.org',
+  'discovery2.publicbits.org'
+]
+var DEFAULT_BOOTSTRAP = [
+  'bootstrap1.publicbits.org:6881',
+  'bootstrap2.publicbits.org:6881',
+  'bootstrap3.publicbits.org:6881',
+  'bootstrap4.publicbits.org:6881'
+]
+
+var db = Level('./dat.db')
+var drive = Hyperdrive(db)
+var swarm = Swarm({
+  id: drive.core.id,
+  dns: {server: DEFAULT_DISCOVERY, domain: DAT_DOMAIN},
+  dht: {bootstrap: DEFAULT_BOOTSTRAP},
+  stream: function () {
+    // this is how the swarm and hyperdrive interface
+    console.log('new peer stream')
+    return drive.createPeerStream()
+  }
+})
+
+swarm.once('listening', function () {
+  console.log('joining', link)
+  // join the swarm
+  swarm.join(link)
+  // tell hyperdrive to start downloading/uploading in ./data
+  var archive = drive.get(link, process.cwd() + '/data')
+  archive.ready(function (err) {
+    console.log('archive ready')
+    // a stream of all metadata. after retrieving each entry metadata will be cached locally
+    // but the first time it has to fetch it from the swarm
+    var metadata = archive.createEntryStream()
+    // start downloading all entries, or choose your own filter logic to download specific entries
+    // entries will be either files or directories
+    metadata.on('data', function (entry) {
+      var dl = archive.download(entry)
+      console.log('downloading', entry.name, dl)
+      
+      dl.on('ready', function () {
+        console.log('download started', entry.name, dl)
+      })
+    })
+  })
+})
+
+swarm.listen(DEFAULT_LOCAL_PORT)
+```
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