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SYNC.md

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Syncing Strategy for Lift

Overview

This document outlines the syncing strategy implemented in Lift. The app synchronizes data between a local database managed by SqlDelight and a remote database hosted on Firestore.

Sync Logic

Local Database (SqlDelight)

  • Entities: Exercise
  • Properties:
    • last_modified: Timestamp updated whenever a local change is made.
    • last_synced: Timestamp updated after successful sync with the remote database.

Remote Database (Firestore)

  • Entities: Exercise
  • Properties:
    • lastUpdated: Server-managed timestamp (@ServerTimestamp) updated on creation or modification.

Sync Process

The sync process consists of two main operations: pulling data from remote to local and pushing data from local to remote.

Pulling Data (Remote to Local)

  1. Fetch all remote exercises where lastUpdated > local lastSync.
  2. Upsert fetched exercises into the local database:
    • If local exercise does not exist.
    • If local last_modified < remote lastUpdated.
  3. Set both last_synced and last_modified to remote lastUpdated timestamp for each upserted exercise.

Pushing Data (Local to Remote)

  1. Select local exercises where last_synced is null or last_modified > last_synced.
  2. Push selected exercises to the remote database and update last_synced.

Conflict Resolution

In case of concurrent updates, remote changes are prioritized. This needs to be handled carefully to avoid overwriting unsynced local changes.

Sequence Diagram

sequenceDiagram
    participant LocalDB as Local Database (SqlDelight)
    participant RemoteDB as Remote Database (Firestore)
    participant App as Application

    App->>LocalDB: Query for Sync (last_synced, last_modified)
    LocalDB-->>App: Local Exercises to Sync

    loop Push Local Changes
        App->>RemoteDB: Update/Create Exercise
        RemoteDB-->>App: Acknowledge Update
        App->>LocalDB: Update last_synced
    end

    App->>RemoteDB: Request Exercises (lastUpdated > lastSync)
    RemoteDB-->>App: Remote Exercises to Sync

    loop Pull Remote Changes
        App->>LocalDB: Upsert Remote Exercise
        App->>LocalDB: Set last_modified and last_synced to remote lastUpdated
    end
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sequenceDiagram
    participant LocalDB as Local Database (SqlDelight)
    participant RemoteDB as Remote Database (Firestore)
    participant App as Application

    alt Local Modification or Creation
        App->>LocalDB: Create/Update Exercise
        LocalDB-->>App: Set last_modified, Keep last_synced null/unchanged
    end

    App->>LocalDB: Query for Sync (last_synced, last_modified)
    LocalDB-->>App: Local Exercises to Sync

    loop Push Local Changes
        App->>RemoteDB: Update/Create Exercise
        RemoteDB-->>App: Acknowledge Update
        App->>LocalDB: Update last_synced
    end

    App->>RemoteDB: Request Exercises (lastUpdated > lastSync)
    RemoteDB-->>App: Remote Exercises to Sync

    loop Pull Remote Changes
        App->>LocalDB: Upsert Remote Exercise
        alt If Local Exercise is New or Outdated
            App->>LocalDB: Set last_synced to remote lastUpdated
        else If Local Exercise is Unchanged
            App->>LocalDB: Keep last_modified Unchanged
        end
    end
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