Update README

README.md

@@ -3,3 +3,118 @@
 blog.bouzuya.net for Android.
 
 <https://play.google.com/store/apps/details?id=net.bouzuya.blog>
+
+## System Architecture Diagram
+
+```mermaid
+flowchart LR
+  subgraph android
+    frontend
+  end
+
+  subgraph google_cloud
+    subgraph cloud_run["Cloud Run"]
+      backend
+    end
+
+    scheduler["Cloud Scheduler"]
+
+    db["Firestore"]
+  end
+
+  subgraph users
+    user
+    bouzuya
+  end
+
+  bbn["blog.bouzuya.net API"]
+  expo["Expo Push API"]
+
+  backend --> bbn
+  backend --> db
+  backend --> expo
+  bouzuya .-> bbn
+  frontend --> backend
+  frontend --> bbn
+  frontend --> expo
+  scheduler --> backend
+  user --> frontend
+  expo .-> frontend
+```
+
+## UseCases
+
+### Fetch entries
+
+```mermaid
+sequenceDiagram
+  participant user
+  participant frontend
+  participant bbn as "blog.bouzuya.net API"
+
+  user ->> frontend : Run
+  Note over frontend : Register ExpoPushToken
+  frontend ->> bbn : Fetch entries
+  bbn -->> frontend : (entries)
+```
+
+### Register ExpoPushToken
+
+```mermaid
+sequenceDiagram
+  participant user
+  participant frontend
+  participant backend
+  participant db
+  participant expo as "Expo Push API"
+
+  user ->> frontend : Run
+  frontend ->> frontend : Get permissions
+  frontend ->> user : Request permissions
+  user -->> frontend : Grant permissions
+  frontend ->> expo : Create ExpoPushToken (expo-notifications)
+  expo -->> frontend : (ExpoPushToken)
+  frontend ->> backend : Register ExpoPushToken
+  backend ->> db : Write ExpoPushToken
+  backend -->> frontend : OK
+```
+
+### Send push notifications
+
+```mermaid
+sequenceDiagram
+  participant scheduler
+  participant backend
+  participant db
+  participant expo as "Expo Push API"
+  participant bbn as "blog.bouzuya.net API"
+
+  scheduler ->> backend : Run
+  backend ->> bbn : Fetch entries
+  bbn ->> backend : (entries)
+  backend ->> backend : Check updates
+  backend ->> db : Read ExpoPushTokens
+  db -->> backend : (ExpoPushTokens)
+  backend ->> expo : Send push notifications
+  expo -->> backend : (ExpoPushTickets)
+  backend ->> db : Write ExpoPushTickets
+  backend -->> scheduler : OK
+```
+
+### Delete invalid ExpoPushTokens
+
+```mermaid
+sequenceDiagram
+  participant scheduler
+  participant backend
+  participant db
+  participant expo as "Expo Push API"
+
+  scheduler ->> backend : Run (after 15+ mins)
+  backend ->> db : Read ExpoPushTickets
+  db -->> backend : (ExpoPushTickets)
+  backend ->> expo : Get push notification receipts
+  expo -->> backend : (ExpoPushReceipt)
+  backend ->> db : Delete invalid ExpoPushTokens
+  backend -->> scheduler : OK
+```