Spotify architecture - Pressing play
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Slides from a talk I gave at Scandinavian Developers Conference 2012 on the architecture of Spotify. The slides follows a story of playing a track and the steps to get there.
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Spotify architecture - Pressing play
- 1. Pressing play Niklas Gustavsson ngn@spotify.com @protocol7 Tuesday, April 17, 12
- 2. Who am I? • ngn@spotify.com • @protocol7 • Spotify backend dev based in Göteborg • Mainly from a JVM background, working on various stuff over the years • Apache Software Foundation member Tuesday, April 17, 12
- 3. What’s Spotify all about? • A big catalogue, tons of music • Available everywhere • Great user experience • More convenient than piracy • Fast, reliable, always available • Scalable for many, many users • Ad-supported or payed-for service Tuesday, April 17, 12
- 4. Pressing play Tuesday, April 17, 12
- 5. Where’s Spotify? • Let’s start the client, but where should it connect to? Tuesday, April 17, 12
- 6. Aside: SRV records • Example SRV _spotify-mac-client._tcp.spotify.com. 242 IN SRV 10 8 4070 C8.spotify.com. _spotify-mac-client._tcp.spotify.com. 242 IN SRV 10 16 4070 C4.spotify.com. name TTL class prio weight port host • GeoDNS used Tuesday, April 17, 12
- 7. What does that record really point to? • accesspoint • Handles authentication state, logging, routing, rate limiting and much more • Protocol between client and AP uses a single, encrypted multiplexed socket over TCP • Written in C++ Tuesday, April 17, 12
- 8. Tuesday, April 17, 12
- 9. Find something to play • Let’s search Tuesday, April 17, 12
- 10. Services • Probably close to 100 backend services, most small, handling a single task • UNIX philosophy • Many autonomous • Deployed on commodity servers • Always redundant Tuesday, April 17, 12
- 11. Services • Mostly written in Python, a few in Java and C • Storage optimized for each service, mostly PostgreSQL, Cassandra and Tokyo Cabinet • Many service uses in-memory caching using for example /dev/shm or memcached • Usually a small daemon, talking HTTP or Hermes • Got our own supervisor which keeps services running Tuesday, April 17, 12
- 12. Aside: Hermes • ZeroMQ for transport, protobuf for envelope and payload • HTTP-like verbs and caching • Request-reply and publish/subscribe • Very performant and introspectable Tuesday, April 17, 12
- 13. How does the accesspoint find search? • Everything has an SRV DNS record: • One record with same name for each service instance • Clients resolve to find servers providing that service • Lowest priority record is chosen with weighted shuffle • Clients retry other instances in case of failures Tuesday, April 17, 12
- 14. Read-only services • Stateless • Writes are hard • Simple to scale, just add more servers • Services can be restarted as needed • Indexes prefabricated, distributed to live servers Tuesday, April 17, 12
- 15. Read-write services • User generated content, e.g. playlists • Hard to ensure consistence of data across instances Solutions: • Eventual consistency: • Reads of just written data not guaranteed to be up-to-date • Locking, atomic operations • Creating globally unique keys, e.g. usernames • Transactions, e.g. billing Tuesday, April 17, 12
- 16. Sharding • Some services use Dynamo inspired DHTs • Each request has a key • Each service node is responsible for a range of hash keys • Data is distributed among service nodes • Redundancy is ensured by writing to replica node • Data must be transitioned when ring changes Tuesday, April 17, 12
- 17. DHT example Tuesday, April 17, 12
- 18. search • Java service • Lucene storage • New index published daily • Doesn’t store any metadata in itself, returns a list of identifiers • (Search suggestions are served from a separate service, optimized for speed) Tuesday, April 17, 12
- 19. Metadata services • Multiple read-only services • 60 Gb indices • Responds to metadata requests • Decorates metadata onto other service responses • We’re most likely moving away from this model Tuesday, April 17, 12
- 20. Tuesday, April 17, 12
- 21. Another aside: How does stuff get into Spotify? • >15 million tracks, we can’t maintain all that ourselves • Ingest audio, images and metadata from labels • Receive, transform, transcode, merge • All ends up in a metadata database from which indices are generated and distributed to services Tuesday, April 17, 12
- 22. Tuesday, April 17, 12
- 23. The Kent bug • Much of the metadata lacks identifiers which leaves us with heuristics. Tuesday, April 17, 12
- 24. Play Tuesday, April 17, 12
- 25. Audio encodings and files • Spotify supports multiple audio encodings • Ogg Vorbis 96 (-q2), 160 (-q5) and 320 000 (- q9) • MP3 320 000 (downloads) • For each track, a file for each encoding/bitrate is listed in the returned metadata • The client picks an appropriate choice Tuesday, April 17, 12
- 26. Get the audio data • The client now must fetch the actual audio data • Latency kills Tuesday, April 17, 12
- 27. Cache • Player caches tracks it has played • Caches are large (56% are over 5 GB) • Least Recently Used policy for cache eviction • 50% of data comes from local cache • Cached files are served in P2P overlay Tuesday, April 17, 12
- 28. Streaming • Request first piece from Spotify storage • Meanwhile, search peer-to-peer (P2P) for remainder • Switch back and forth between Spotify storage and peers as needed • Towards end of a track, start prefetching next one Tuesday, April 17, 12
- 29. P2P • All peers are equals (no supernodes) • A user only downloads data she needs • tracker service keeps peers for each track • P2P network becomes (weakly) clustered by interest • Oblivious to network architecture • Does not enforce fairness • Mobile clients does not participate in P2P h.p://www.csc.kth.se/~gkreitz/spo9fy/kreitz-‐spo9fy_kth11.pdf Tuesday, April 17, 12
- 30. Tuesday, April 17, 12
- 31. Tuesday, April 17, 12
- 32. Success! Tuesday, April 17, 12
- 33. YAA: Hadoop • We run analysis using Hadoop which feeds back into the previously described process, e.g. track popularity is used for weighing search results and toplists Tuesday, April 17, 12
- 34. Tuesday, April 17, 12
- 35. Development at Spotify • Uses almost exclusively open source software • Git, Debian, Munin, Zabbix, Puppet, Teamcity... • Developers use whatever development tools they are comfortable with • Scrum or Kanban in three week iterations • DevOps heavy. Freaking awesome ops • Monitor and measure all the things! Tuesday, April 17, 12
- 36. Development at Spotify • Development hubs in Stockholm, Göteborg and NYC • All in all, >220 people in tech • Very talented team • Hackdays and system owner days in each iteration • Hangs out on IRC • Growing and hiring Tuesday, April 17, 12
- 37. Languages at Spotify Tuesday, April 17, 12
- 39. Thank you Want to work at Spotify? http://www.spotify.com/jobs/ Tuesday, April 17, 12