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Differential Sync and JSON Patch @ SpringOne2GX 2014

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Craig Walls and I presented a new approach to client-server communication at SpringOne2GX 2014.

Abstract:

The world of client-server has changed. The traditional application of REST is no longer the best fit. We're depolying applications into a world where users expect responsive UIs, on all their devices, even while disconnected. We're deploying into a world where connection latency, mobile radio usage and battery life have become primary concerns.

Differential Synchronization (DS) is an algorithm that syncs data across N parties, even in the face of dropped connections, offline devices, etc. It makes more efficient use of connections by batching and sending only changes, in both directions, from client to server and from server to client. We’ll look at how it can be used with JSON Patch to synchronize application data between clients and servers over HTTP Patch, WebSocket, and STOMP, and how it can be integrated into the Spring ecosystem.

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Differential Sync and JSON Patch @ SpringOne2GX 2014

  1. 1. Efficient Client Communication with Differential Synchronization and JSON Patch By Brian Cavalier and Craig Walls © 2014 SpringOne 2GX. All rights reserved. Do not distribute without permission.
  2. 2. What’s the problem with REST?
  3. 3. Nothing, REST is awesome
  4. 4. What’s the problem with how we typically use REST in practice?
  5. 5. Agenda • Motivation • Differential Synchronization (DS) • JSON Patch • DS + JSON Patch • DS w/Spring and JavaScript • The future
  6. 6. What’s the problem with how we typically use REST in practice?
  7. 7. Isn’t “typical REST” good enough? • “Typical REST”: 1 request per entity per operation type • create 2 entities, update 3, delete 1 = 6 requests • Expensive for mobile: connection latency, battery, data ($$$) • Doesn’t exploit return payload
  8. 8. Motivation: Goals • More efficient data transfer • More efficient use of mobile radios, networks and batteries • Take advantage of WebSocket & messaging • Data synchronization with multiple clients • Offline / disconnected
  9. 9. How can we get data from a Spring back-end all the way to the pixels more efficiently?
  10. 10. Differential Synchronization
  11. 11. Differential Sync • Algorithm for syncing N copies of a document • Potentially supports any kind of document/data • As long as diff & patch algorithms are available • Text, DOM, JSON • Nice properties • Efficient: Transfer only differences • Symmetrical: same algorithm at each node • Recoverable: disconnected/offline clients, lost messages • Published by Neil Fraser in 2009 • https://neil.fraser.name/writing/sync/
  12. 12. Differential Sync doc shadow doc diff diff patch patch live edits live edits
  13. 13. Differential Sync doc shadow shadow doc diff diff patch patch live edits live edits
  14. 14. Differential Sync • Cooperative synchronization loop • Distributed or local • Uses diff & patch algorithms
  15. 15. JSON Patch
  16. 16. JSON Patch • JSON-formatted Patch for structured documents • RFC 6902, plus related JSON Pointer RFC 6901 • https://tools.ietf.org/html/rfc6902 • https://tools.ietf.org/html/rfc6901 • Suitable for sending via HTTP Patch • Defines operations, format, algorithm, and mime type • application/json-patch+json • Can coexist w/handlers at same url via Content-Type routing • Does not define diff algorithm • Sensible requirement: patch(diff(a, b), a) === b
  17. 17. JSON Patch json! [{"value":"a"},{"value":"b"},{"value":"c"}]! + patch! [{"op":"add","path":"/3","value":{"value":"d"}},! {“op":"remove","path":"/1"}]! = new json! [{"value":"a"},{"value":"c"},{"value":"d"}]
  18. 18. JSON Patch json! {"name":{"first":"Brian","last":"Cavalier"},"occupation":"JavaScript Engineer"}! + patch! [{"op":"replace","path":"/occupation","value":"JavaScript Ranger"}]! = new json! {"name":{"first":"Brian","last":"Cavalier"},"occupation":"JavaScript Ranger"}
  19. 19. JSON Patch • Moves the operation type inside request payload • create 2 entities, update 3, delete 1 = 1 request • Moves the identifier inside request payload • Potentially patch many entity types in a single request • Patches are atomic • If any part of a patch fails, whole patch must fail (as per RFC) • Think: data integrity
  20. 20. “Typical REST” POST /todos {“description”: “Try JSON Patch”, “complete”: false} PUT /todos/1 {“description”: “…”, “complete”: true} PATCH /todos/2 {“complete”: true} DELETE /todos/3
  21. 21. JSON Patch PATCH /todos [ {“op”: “add”, “path”: “-“, “value”: { “description”: “Try JSON Patch”, “complete”: true}}, {“op”: “replace”, “path”: “/1”, “value”: { “description”: “…”, “complete”: true}}, {“op”: “replace”, “path”: “/2/complete”, “value”: true}}, {“op”: “remove”, “path”: “/3”}} ]
  22. 22. “Typical REST” POST /person { “name”: “Brian” } PUT /todos/1 { “description”: “…”, “complete”: true } PATCH /meetings/2 { “agenda”: “…” } DELETE /todos/3
  23. 23. Pie in the sky JSON Patch PATCH / [ {“op”: “add”, “path”: “/person/-“, “value”: { “name”: “Brian” }}, {“op”: “replace”, “path”: “/todos/1”, “value”: { “description”: “…”, “complete”: true }}, {“op”: “replace”, “path”: “/meetings/2/agenda”, “value”: “…” }}, {“op”: “remove”, “path”: “/todos/3” }} ]
  24. 24. JSON Patch • Reduce requests • “Typical” REST: • # Requests = Entity type x Operation type • JSON Patch + HTTP Patch • # Requests = 1 • Reduce payload size: Transfer only deltas
  25. 25. Demo
  26. 26. Introducing jiff.js • JavaScript library for JSON diff & patch • https://github.com/cujojs/jiff • Diffs arbitrary JSON or JavaScript object graphs • inc. objects containing arrays containing objects containing… • Patches arbitrary JSON or JavaScript object graphs atomically • Supports advanced features: inverse patches, rebase, contextual (“smart”) patching
  27. 27. jiff.js var jiff = require(‘jiff’); var rest = require(‘rest’); ! var changedData = jiff.clone(data); ! // … data changes via user interactions ! var patch = jiff.diff(data, changedData); ! rest({ method: ‘PATCH’, entity: patch });
  28. 28. Hmmmm …. Differential synchronization requires diff and patch algorithms JSON Patch defines a patch format and algorithm
  29. 29. Differential Sync + JSON Patch What if we put these two things together to synchronize structured data?
  30. 30. Differential Sync JSON Patch data model shadow data shadow model diff diff patch patch changes changes JSON Patch Spring Server Client (web browser, phone, etc.)
  31. 31. Demo
  32. 32. Differential Synchronization in Server-Side Spring
  33. 33. Applying JSON Patch to the Java-based Domain
  34. 34. Path to SpEL JSON Patch Path SpEL /0 [0] /complete .complete /1/description [1].description /clients/3/address/zip .clients[3].address.zip
  35. 35. Challenges with JSON Patch and Java • How do you “remove” or “move” a property? • How do you “move” a list item to a different index? • How do you “add” a list item to a specific index? • How do you avoid saving the entire list when patching a list? • How do you delete an item as the result of a “delete” op? • What if a “remove” is only intended for a particular view?
  36. 36. Introducing Spring Sync • GitHub: http://github.com/spring-projects/spring-sync • Maven/Gradle: org.springframework.sync:spring-sync:0.5.0.BUILD-SNAPSHOT • JsonPatch • Applies a JSON Patch to a Java object graph • DiffSync • Applies Differential Synchronization algorithm (leveraging JsonPatch) • DiffSyncController • Handles PATCH requests for “application/json-patch+json” • Returns a JSON Patch to update client • @EnableDifferentialSynchronization
  37. 37. Enabling Spring Sync @Configuration @EnableDifferentialSynchronization public class DiffSyncConfig extends DiffSyncConfigurerAdapter { ! @Autowired private PagingAndSortingRepository<Todo, Long> repo; @Override public void addPersistenceCallbacks( PersistenceCallbackRegistry registry) { ! registry.addPersistenceCallback( new JpaPersistenceCallback<Todo>(repo, Todo.class)); ! } }
  38. 38. Differential Synchronization in JavaScript
  39. 39. Differential Sync in JavaScript DOM User Interface patch patch JavaScript objects, arrays, etc Network patch patch Spring Server
  40. 40. Differential Sync in JavaScript • Synchronize from the Spring data model to the pixels • Decouple change frequency from communication frequency: • fast sync = responsive, but network/resource intense • slow sync = slower UI updates, but less network/resource intense • Current Status • Incubator JavaScript implementation based on cujojs/jiff.js
  41. 41. When you have a system based on patches, you can do some interesting things
  42. 42. Streaming changes w/WebSocket 42
  43. 43. Demo
  44. 44. Patch Algebra 44
  45. 45. Patch Algebra • Inverse patches • think Undo & Redo with no application specific logic • Merge and Rebase • Apply parallel changes from multiple parties without locking • jiff.js supports inverse, rebase
  46. 46. Demo
  47. 47. Challenges • DS in an Entity-oriented world • What constitutes a document? • Each participant must maintain a shadow copy of the document • Lists and arrays are tricky • Conflict resolution (not a big deal in practice?) • Hypothesis: Conflicts no more likely to occur than REST • DS in itself does not solve conflict resolution, but neither does REST
  48. 48. Today • Spring Sync • http://github.com/spring-projects/spring-sync • org.springframework.sync:spring-sync:0.5.0.BUILD-SNAPSHOT • JSON Patch + diff in JavaScript • https://github.com/cujojs/jiff • Experimental Spring support for Differential Sync and JSON Patch over HTTP Patch • https://github.com/royclarkson/spring-rest-todos
  49. 49. The Future • Continue evolution of Spring Sync and JavaScript DS implementations • Further Integrate with Spring ecosystem • Messaging & WebSocket • Smart patching • Offline/disconnected client support • Guidance (when to use it, how to tune it, etc.)

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