fix(web): stabilize dashboard snapshot hydration

apps/web/src/stores/dashboard.store.ts

@@ -120,6 +120,9 @@ const MAX_RECONNECT_ATTEMPTS = 10
 const BASE_RECONNECT_DELAY = 1000 // 1 second
 const MAX_RECONNECT_DELAY = 30000 // 30 seconds
 const HEARTBEAT_TIMEOUT = 45000 // 45 seconds
+const SNAPSHOT_FETCH_TIMEOUT = 8000 // 8 seconds
+const SNAPSHOT_FETCH_RETRY_DELAY = 750 // 0.75 second
+const SNAPSHOT_FETCH_ATTEMPTS = 2
 
 // =============================================================================
 // Store Implementation
@@ -129,6 +132,39 @@ const HEARTBEAT_TIMEOUT = 45000 // 45 seconds
 let eventSource: EventSource | null = null
 let reconnectTimeout: NodeJS.Timeout | null = null
 let heartbeatTimeout: NodeJS.Timeout | null = null
+let snapshotFetchInFlight: Promise<void> | null = null
+
+function normalizeApiUrl(apiBaseUrl: string): string {
+  return apiBaseUrl.replace(/\/+$/, '')
+}
+
+function errorMessage(error: unknown): string {
+  return error instanceof Error ? error.message : String(error)
+}
+
+function wait(ms: number): Promise<void> {
+  return new Promise((resolve) => setTimeout(resolve, ms))
+}
+
+async function fetchDashboardSnapshot(snapshotUrl: string): Promise<DashboardSnapshot> {
+  const controller = new AbortController()
+  const timeout = setTimeout(() => controller.abort(), SNAPSHOT_FETCH_TIMEOUT)
+
+  try {
+    const response = await fetch(snapshotUrl, {
+      cache: 'no-store',
+      signal: controller.signal,
+    })
+
+    if (!response.ok) {
+      throw new Error(`HTTP ${response.status}`)
+    }
+
+    return await response.json() as DashboardSnapshot
+  } finally {
+    clearTimeout(timeout)
+  }
+}
 
 export const useDashboardStore = create<DashboardState>()(
   subscribeWithSelector((set, get) => ({
@@ -312,6 +348,10 @@ export const useDashboardStore = create<DashboardState>()(
     },
 
     fetchSnapshot: async (apiBaseUrl: string) => {
+      if (snapshotFetchInFlight) {
+        return snapshotFetchInFlight
+      }
+
       // 統帥鐵律: 禁止任何 Fallback IP
       const resolvedApiBaseUrl = apiBaseUrl ||
         (typeof window !== 'undefined' ? process.env.NEXT_PUBLIC_API_URL : '')
@@ -321,20 +361,49 @@ export const useDashboardStore = create<DashboardState>()(
         return
       }
 
-      try {
-        console.log('[SSE] Fetching snapshot for hydration from:', resolvedApiBaseUrl)
-        const response = await fetch(`${resolvedApiBaseUrl}/api/v1/dashboard/snapshot`)
+      const snapshotUrl = `${normalizeApiUrl(resolvedApiBaseUrl)}/api/v1/dashboard/snapshot`
 
-        if (!response.ok) {
-          throw new Error(`HTTP ${response.status}`)
+      const run = (async () => {
+        let lastError: unknown = null
+
+        for (let attempt = 1; attempt <= SNAPSHOT_FETCH_ATTEMPTS; attempt += 1) {
+          try {
+            console.log('[SSE] Fetching snapshot for hydration from:', resolvedApiBaseUrl)
+            const snapshot = await fetchDashboardSnapshot(snapshotUrl)
+            get().applySnapshot(snapshot)
+            set({ error: null })
+            console.log('[SSE] Snapshot applied, hosts:', snapshot.hosts.length)
+            return
+          } catch (err) {
+            lastError = err
+            if (attempt < SNAPSHOT_FETCH_ATTEMPTS) {
+              console.warn(
+                `[SSE] Snapshot fetch attempt ${attempt} failed; retrying...`,
+                errorMessage(err)
+              )
+              await wait(SNAPSHOT_FETCH_RETRY_DELAY)
+            }
+          }
         }
 
-        const snapshot: DashboardSnapshot = await response.json()
-        get().applySnapshot(snapshot)
-        console.log('[SSE] Snapshot applied, hosts:', snapshot.hosts.length)
-      } catch (err) {
-        console.error('[SSE] Failed to fetch snapshot:', err)
-        set({ error: `Snapshot fetch failed: ${err}` })
+        const message = `Snapshot unavailable after ${SNAPSHOT_FETCH_ATTEMPTS} attempts: ${errorMessage(lastError)}`
+        if (get().snapshot) {
+          console.warn('[SSE] Snapshot refresh unavailable; retaining last snapshot:', errorMessage(lastError))
+          set({ error: null })
+          return
+        }
+
+        console.error('[SSE] Snapshot unavailable:', errorMessage(lastError))
+        set({ error: message })
+      })()
+
+      snapshotFetchInFlight = run
+      try {
+        await run
+      } finally {
+        if (snapshotFetchInFlight === run) {
+          snapshotFetchInFlight = null
+        }
       }
     },