How to fix FAILED_PRECONDITION: The system is not in a state required for the operation in Firebase

When Firestore encounters a query that needs an index, it usually logs a direct link in the console:

You can create a composite index by going to:
https://console.firebase.google.com/v1/...

If you see this link, click it to auto-create the required index. On Android (React Native), check adb logcat output for the index link instead of the browser console.

If no link appears, proceed to the next step.

If the automatic link did not appear:

1. Go to [Firebase Console](https://console.firebase.google.com)
2. Select your project and navigate to Firestore Database
3. Click on the Indexes tab (next to Collections)
4. Click Create Index
5. Select your collection name
6. Add the fields you're querying on:
- For .where("status", "==", "active").orderBy("createdAt", "desc"), add:
- Field: status (Ascending)
- Field: createdAt (Descending)
7. Click Create Index

The index will take 1-5 minutes to build. Your queries will work once the index status shows "Enabled".

If you're seeing FAILED_PRECONDITION on transactions, implement retry logic:

import { getFirestore, runTransaction, doc } from "firebase/firestore";

const db = getFirestore();
const maxRetries = 3;
let retries = 0;

async function updateWithRetry() {
  while (retries < maxRetries) {
    try {
      await runTransaction(db, async (transaction) => {
        const userRef = doc(db, "users", "user123");
        const userDoc = await transaction.get(userRef);
        
        if (!userDoc.exists()) throw new Error("Document not found");
        
        transaction.update(userRef, {
          score: userDoc.data().score + 10,
        });
      });
      console.log("Transaction succeeded");
      break;
    } catch (error: any) {
      if (error.code === "failed-precondition" && retries < maxRetries - 1) {
        const delay = Math.pow(2, retries) * 100; // 100ms, 200ms, 400ms
        await new Promise((resolve) => setTimeout(resolve, delay));
        retries++;
      } else {
        throw error;
      }
    }
  }
}

This exponential backoff gives competing transactions time to complete before retrying.

FAILED_PRECONDITION can sometimes be a masked security rule denial. Check your rules:

1. Go to Firebase Console > Firestore > Rules tab
2. Ensure your rules allow the operation. Example:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      // Allow authenticated users to read/write their own documents
      allow read, write: if request.auth.uid == userId;
    }
    match /posts/{postId} {
      // Allow reads, require authentication for writes
      allow read: if true;
      allow write: if request.auth != null;
    }
  }
}

3. Click Publish to apply rule changes
4. Retry your operation

If you're encountering FAILED_PRECONDITION in Cloud Functions:

1. Start the Firebase Emulator Suite:

firebase emulators:start

2. Update your code to use the emulator:

import { connectFirestoreEmulator, getFirestore } from "firebase/firestore";

if (process.env.NODE_ENV === "development") {
  connectFirestoreEmulator(getFirestore(), "localhost", 8080);
}

3. Run your test against the emulator to see clearer error messages
4. Once tests pass locally, deploy to production:

firebase deploy --only functions,firestore

Collection group queries require explicit indexes:

1. Use collectionGroup() in your code:

const q = query(
  collectionGroup(db, "comments"),
  where("authorId", "==", userId),
  orderBy("createdAt", "desc")
);

2. Firestore will log an index creation link. Click it, or manually create it in:
- Firebase Console > Firestore > Indexes > Create Index
- Collection: Leave blank for collection group
- Fields: authorId (Ascending), createdAt (Descending)

3. Wait for the index to build (1-5 minutes), then retry the query.