An Abridged Guide to Event Sourcing
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Although event sourcing (and its sister pattern CQRS) has been gaining traction in recent years, it's still baffling for many engineers attempting to implement it for the first time. While there's plenty of material on the subject, most of it is too basic or theoretical for practical applications, and engineers often end up having to reinvent (or rediscover) suitable approaches and techniques. This talk focuses on practical aspects of building event-sourced systems, lessons learned from our experience building such systems at Wix. We'll walk through the design and implementation of a relatively simple event-sourced system, covering the event model, underlying persistence model, code layering/factoring and operational considerations. A talk given at Reversim Summit 2017 in Tel-Aviv, Israel.
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An Abridged Guide to Event Sourcing
- 1. An Abridged Guide to Event Sourcing Tomer Gabel Reversim Summit Tel-Aviv 2017 Image: Jack Zalium, “Abriged” via Flickr (CC-BY-ND 2.0)
- 2. Background • ADI is a site builder • It’s pretty nifty – Huge web application – … and it even works! • A cool app isn’t enough • Sites have to be stored somewhere!
- 3. Requirements • Features: – Store “site” blobs – Store version history – Soft-delete only • Not required: – Concurrent editing Image: ImgFlip
- 4. 1. WHY CRUD SUCKS crud Source: Oxford Dictionary /krəd/ noun informal 1. A substance which is considered unpleasant or disgusting, typically because of its dirtiness. 2. Nonsense.
- 5. Mutation Anxiety • So what’s wrong with CRUD? • Create • Read • Update • Delete • It’s all about mutable state
- 6. Mutation Anxiety Mutable state is bad. • Old data is lost • Hard to debug • Can’t fix retroactively • No built-in auditing Image: David Bleasdale, “Fahrenheit 451” via Flickr (CC-BY 2.0)
- 7. Mutation Anxiety “Who told you you’re allowed to destroy data?” -- Greg Young Image: Code on the Beach speaker profile (source)
- 8. Not To Scale • CRUD implies: – One source of truth – Reads, writes against the same store – Full consistency (ACID) • Difficult to scale! Image: Jason Baker, “Bank Vaults under Hotels in Toronto, Ontari” via Flickr (CC-BY 2.0)
- 9. What’s Holding Us Back? • Strong consistency – Assumed with RDBMS – Often not required! • Consistency is a product concern – “Does this have to be 100% fresh?” – “No? So how stale can this get?” Images: “Homemade Bread Freshly Baked” via MaxPixel (CC0, above); Tasha, “Homemade Croutons” via Flickr (CC-BY 2.0, below)
- 10. What’s Holding Us Back? • Storage cost – “How long must I hold on to data?” – “What do you mean forever?!” • Cost is an operational concern – “So how much data is there?” – “How much will it cost to retain?” Image: Calvin Fraites via Flickr (CC-BY-NC-ND 2.0)
- 11. 2. A BETTER WAY “A database is just a view over its transaction log.” -- Ancient Vulcan proverb
- 12. Event Sourcing • A very simple pattern • Each entity has its own event stream – Events are facts – Events are immutable – Events are forever Opened Account Deposited $100 Withdrawn $25 Deposited $12 Balance: $87
- 13. CQRS • Writes simply append events • But reads are projections: – Full snapshots – Partial/filter queries – Views and joins • Two separate concerns! – Different schemas – Different instances – Even different data stores! Time Site 1 Created Site 1 Updated Site 1 Updated Site 2 Created Site 1 Deleted Site ID Active 1 false 2 true “active sites”
- 14. Better how? • Tunable consistency – Full or eventual – Per use-case! • Decoupled reads/writes – Better scale – Much more flexible! • Built-in auditing – Easier to debug – Replayable!
- 15. 3. WALKTHROUGH Image: Pöllö via Wikimedia Commons (CC-BY 3.0)
- 16. An Event Model • Our business domain: a website • Our use cases: – “Let’s try this thing out” – “Adding more stuff” – “Oops! Revert” – “OK, I’m outta here” Created Updated Deleted Restored
- 17. Storing Events • What’s in an event? – The entity (i.e. site) ID – Some notion of time* – Event data (e.g. type) – Some metadata • We can model this! Column Type PK? Null? site_id binary(16) ✔ ✖ version int ✔ ✖ created_at timestamp ✖ ✖ payload blob ✖ ✖ * No, you can’t use timestamps for versioning. More on this later
- 18. Sketching the API • Commands are wishes • Wishes may be rejected: – Conflicting updates – Stale version – Invalid arguments • Or granted: – Result: new events! Command SLA Create Site Reasonable Get Latest Very fast Get Version Reasonable Update Very fast Delete Reasonable Restore Reasonable
- 19. Architecture 101 Front-end Event Store Site Service Site Materializer Command Appended Events Stored Events Snapsho t
- 20. Hold Your Horses • Still some concerns: –Conflicting updates –Performance –Operations • Let’s sort ‘em out Image: Woodennature, “Slow Down” via Wikimedia Commons (CC-BY 3.0)
- 21. Conflicting Updates • Concurrent operations can conflict – Rapid site activity – Multiple open tabs – Normal network issues • You have to deal with it Created Updated Updated Update Update V0 V1 V2 Time ?
- 22. Conflicting Updates • Strategies – Last-write wins – Optimistic locking – Smart resolution/merge • In our case – No concurrent editing – Simple optimistic locking Created Updated Updated Update Update V0 V1 V2 Time ?
- 23. Performance • Updates are easy • What about reads? – Load all events for site – Feed into materializer – Spit snapshot out • This can hurt. Image: Dominik Kowanda, “Autobahn” via Flickr (CC-BY-ND 2.0)
- 24. Performance • How many events? – Domain-specific (for ADI, easily 1000s) – But events never die • How big are they? – Again, domain-specific – Let’s assume 10-100KB • Naïve reads will fail. Image: madaise, “Jenga” via Flickr (CC-BY-ND 2.0)
- 25. Performance • Remember CQRS? – Reads are distinct from writes – We can use another store! • We’ll use snapshots – Immutable – Ephemeral – Tunable (space/performance) Time V0 V1 V2 V3 V4 V5 V6 V7 … S3 S6
- 26. Architecture Redux Front-end Event Store Site Service Site Materializer Stored Events Snapsho t Snapshot Store Base Snapshot Snapshot StrategyPersisted Snapshot
- 28. No Silver Bullet • Event sourcing is awesome • But it’s a trade-off – Learning curve – Increased storage – More knobs to turn • Make it wisely! Image: Ed Schipul, “silver bullet” via Flickr (CC-BY-SA 2.0)
- 29. Eventual Consistency • Consistency is a tradeoff – Performance – Complexity – Cost (tech, support) • Make it wisely! Image: Michael Coghlan, “Scales of Justice - Frankfurt Version” via Flickr (CC-BY-SA 2.0)
- 30. Forethought • Define target SLAs – Latency – Consistency • Place sanity limits – Stream size – Write throttling • Invest in tooling – Debug/replay – Schema evolution Image: U.S. Army Corps of Engineers, “USACE Base Camp Development Planning Course” via Flickr (CC-BY 2.0)
- 31. Further Reading Scaling Event Sourcing for Netflix Downloads Phillipa Avery & Robert Reta How shit works: Time Tomer Gabel
- 32. QUESTIONS? Thank you for listening tomer@tomergabel.com @tomerg http://engineering.wix.com Sample implementation: http://tinyurl.com/event-sourcing-sample This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.