Why it matters

Firestore is GCP mobile-first DB. Understanding shapes app backend on GCP.

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The architecture

Documents: JSON-like. Collections: containers of docs.

Realtime: SDKs sync changes automatically.

Firestore featuresDocumentsJSON-likeRealtime syncSDK subscriptionsOffline supportcache + queueNative mode (new) vs Datastore mode (legacy); Native has realtime + better scale
Firestore modes.
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How it works end to end

Modes: Native (new; realtime + strongly consistent) vs Datastore mode (legacy compat).

Queries: limited (no joins). Structure data for queries.

Security rules: fine-grained access.

Pricing: reads + writes + storage.