Study about PouchDB

A brief introduction of PouchDB
(Free study in Golden Week holiday)

Transcript

1. Kelvin Ko
   Study about PouchDB
   2021 GWࣗ༝ݚڀൃදձ
   

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2. About me
   • Kelvin @ko_kelvin
   
   • LINE Client Team (LFK Shop part)
   
   • Android Developer
   

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3. Why study about PouchDB?
   Motivation
   • Working on a free project in our ΋͘΋͘ձ #z_lfk_mokumoku
   
   • Building a Developer friendly Note-taking Web App
   https://www.stashany.com
   

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4. Current Tech Stack
   • Frontend:
   
   • React - Redux - Typescript
   
   • Backend:
   
   • Firebase
   
   • Google Auth
   
   • Realtime Database
   For real time sync across devices
   

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5. But…
   Firebase real time database limitation
   • No offline support in Web (actually it support offline usage, but limited functions)
   
   • No search function (Require third party indexing service)
   
   • No self-host solution (some users concern privacy)
   

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6. Looking for alternative
   To replace Firebase Realtime Database
   • Sync data across clients in real time
   
   • Support Offline access
   
   • Open Source
   

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7. What kind of database match the requirement?
   Characteristics
   • AP Database (Availability, Partition-Tolerance)
   
   • In Note App use case:
   
   • Always can write and read the notes any
   time even it is offline
   
   • Notes across clients (e.g. Desktop, mobile)
   will be in-sync (eventually consistent)
   when get back online
   CAP Theorem
   

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8. PouchDB
   What it is?
   • In-browser database
   
   • NoSQL Database
   
   • Document Database to store JSON object (similar to Firebase DB)
   
   • CouchDB compatiable
   
   • designed for Sync
   

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9. How it work?
   Sync
   • Init local DB
   
   • Init Remote DB
   
   • Sync Data between databases
   
   •
   How it can be so simple !?😮
   How it handle conflict ? 🤔
   

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10. How it work?
    Storing a document
    • document can be any JSON object with field “_id”
    

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11. How it work?
    Retrieving a document
    • get with “id”
    
    • a new field
    “_rev” (revision) is added
    to your document
    

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12. How it work?
    Updating document
    • Required to include
    “_rev” (revision) in the
    document to update
    
    • the “_rev” in update
    document should match
    the one in database,
    otherwise will raise conflict
    
    • For success update, new
    “_rev” will be assigned
    Fail: if provide _rev different from current db one
    Success: a new _rev will be assigned to document
    

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13. Why need revision “_rev”?
    Make Sync work!
    • Git-like Database
    
    • Every update of a “document” is like a commit
    • “_rev” is like the commit hash
    

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14. What if conflict occur?
    In case both clients make change offline
    • client 1 updated document, rev become
    “rev 3-a”
    
    • client 2 updated document, rev become
    “rev 3-b”
    
    • When both clients get back online and
    “sync”
    Offline client 1 Offline client 2
    What will happen?
    

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15. What if conflict occur?
    • CouchDB will choose an arbitrary winner based on a deterministic
    algorithm*
    • Both clients will see the same winner once they're back online
    1. The revision with the longest revision history list becomes the winning revision.
    
    2. If they are the same, the _rev values are compared in ASCII sort order,
    * Usually, we don’t need to care about the algorithm, just for your interest about the implementation
    

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16. Handling conflict
    Example
    • Get with option: {conflicts: true} to detect any conflict in this document
    
    • Get the conflict version with a specific “_rev”:
    Return
    Winner
    Loss version
    

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17. Handling conflict
    • In application layer
    
    • You can decide just use the winner version
    
    • Or handle the conflict: in note app case,
    
    • Display both version of notes to user and let user decide which to keep
    Note1-rev-A.md Note1-rev-B.md
    

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18. Some interesting possibilities
    (If still have time)
    

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19. Some interesting features
    Plugins
    • Pouchdb-adapter-memory: In-memory version of PouchDB
    
    • Replicates with a local PouchDB, acting as a cache
    
    • Peer Pouch: Allow PouchDB instances transfer data over WebRTC
    
    • Databases in different clients can sync with each other without Server
    
    • PouchDB Quick Search: Full text search engine for PouchDB
    

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20. That’s all for my study in GW
    Thank you for listening
    (If you know other Firebase database alternative, do let me know 😃 )
    

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