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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 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 is 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(new Buffer(link, 'hex'))
// 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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