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P2P music streaming Ricardo Santos @ricardovice

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“spotifiera”, anyone?

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What is Spotify? • On-demand music streaming service • Available in 7 European countries • Over 13 million tracks • Over 1 million paying users • Over 10 million total users • Legal • Really fast

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Business idea • Free ad-funded version • Paid subscription where users get: • No advertisements • Mobile access • Offline playback • API access

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“music itself is going to becomelike running water or electricity” David Bowie, 2002

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Accessibility • People should be able to enjoy music • Whenever they want • Wherever they are • Whatever they’re doing

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That’s cool and all… but let’s talk engineering!

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Latency sucks • High latency can be a problem, not only in First Person Shooters • Slow performance is one of the major reasons users abandon services • Users dont come back How to avoid it?

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Repeating songs? • Player caches tracks it has played • Default policy is to use 10% of free space (capped at 10 GB) • Caches are large (56% are over 5 GB) • LRU policy for cache eviction

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Streaming • Request first piece from Spotify servers • Meanwhile, search for peers with track • Download data in-order • When buffers are sufficient, switch to P2P • Towards end of a track, prefetch next one

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When to start playing? • Trade off between stutter & latency • Look at last 15 min of transfer rates • Model as Markov chain and simulate • Coupled with some heuristics

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How well does it work? • Current median latency to begin playing a track in Spotify is 265ms • Due to disk lookup, at times its actually faster to start playing a track from network than from disk • Below 1% of playbacks experienced stutter

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Spotify is fast, we get it, but you must have cool infrastructure, right?

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Production storage • Production storage is a cache with fast drives & lots of RAM • Serves the most popular content • A cache miss will generate a request to master storage, slightly higher latency • Production storage is available in several data centers to ensure closeness to the user (latency wise)

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Master storage • Works as a DHT, with some redundancy • Contains all available tracks but has slower drives and access • Tracks are kept in several formats, adding up to around 290TB

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Why P2P? • Easier to scale • Less servers • Less bandwidth • Better uptime • Less costs • Cool!

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P2P overview • Not a piracy network, all tracks are added by Spotify • Used on all desktop clients • All nodes are equal (no super nodes) • A track is downloaded from several peers

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P2P custom protocol • Ask for most urgent pieces first • If a peer is slow, re-request from new peers • When buffers run low, download from central servers • If loading from servers, estimate at what point P2P will catch up

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P2P resource usage • To ensure user experience we: • Cap number of neighbors • Cap number of simultaneous uploads • If buffers are very low, stop uploading • Cap cache size • Mobile clients don’t participate in P2P

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P2P finding peers • Partial central tracker (BitTorrent-style) • Broadcast query in small neighborhood (Gnutella-style) • Two mechanisms results in higher availability • Limited broadcast for local (LAN) peer discovery (cherry on top...)

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Security • P2P network needs to be safe and trusted • All peers should be trusted Spotify clients • Our client needs to be able to read metadata and play music • But we have to prevent reverse engineering from doing the same Security trough obscurity: we dont openlydiscuss the details…

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…but here’s a few tips • Closed environment • Integrity of downloaded files is checked • Data transfers are encrypted • Usernames are not exposed in P2P network, all peers assigned pseudonym • Software obfuscation, makes life difficult for reverse engineers

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Software obfuscation according to xkcd.com

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So, whats the outcome? • At over 10 million users the responses are: • 55.4% from client cache • 35.8% from the P2P network • 8.8% from the servers

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Not enough info? If you would like to know more: • Get in touch with us • Checkout Gunnar Kreitzs slides and academic papers on the subject: http://kreitz.se/spotify-p2p10/

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Oh btw,we have alotta funas well! :Dspotify.com/jobsjobs@spotify.com