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Firestore Batch Writes — Complete Guide

DodaTech Updated 2026-06-24 2 min read

In this tutorial, you'll learn about Firestore Batch Writes. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

The Problem

You write 600 documents in a loop (over the 500-batch limit), or one write fails but the others succeed — leaving your data in an inconsistent state.

Wrong Approach ❌

// Writing 600 documents one by one — slow and not atomic
for (i in 0 until 600) {
    db.collection("items").document("item$i")
        .set(Item("Item $i", i))
        .await() // 600 separate network writes!
}

Output: 600 network calls. If call 300 fails, first 299 are committed.

Right Approach ✅

class BatchRepository(private val db: FirebaseFirestore) {

    // Atomic batch write — max 500 operations
    suspend fun createItems(items: List<Item>) {
        val batches = items.chunked(499) // 499 to leave room for deletes

        batches.forEach { chunk ->
            val batch = db.batch()

            chunk.forEach { item ->
                val docRef = db.collection("items").document()
                batch.set(docRef, item)
            }

            // Also can include deletes and updates
            batch.delete(db.collection("temp").document("staging"))
            batch.update(db.collection("config").document("settings"), "lastBatch", System.currentTimeMillis())

            batch.commit().await() // Atomic commit
        }
    }

    // Batch update multiple fields across documents
    suspend fun updateScores(scores: Map<String, Long>) {
        val batch = db.batch()

        scores.entries.chunked(499).forEach { chunk ->
            val b = db.batch()
            chunk.forEach { (userId, score) ->
                val docRef = db.collection("users").document(userId)
                b.update(docRef, "score", FieldValue.increment(score))
            }
            b.commit().await()
        }
    }

    // Run both batch operations with WriteBatch
    suspend fun complexBatchWrite() {
        // A WriteBatch can contain set, update, and delete operations
        val batch = db.batch()

        // Set a new document
        val newDoc = db.collection("orders").document()
        batch.set(newDoc, Order("pending", userId))

        // Update existing
        val userDoc = db.collection("users").document(userId)
        batch.update(userDoc, "orderCount", FieldValue.increment(1))

        // Delete old temp data
        val tempDoc = db.collection("temp").document("order_$userId")
        batch.delete(tempDoc)

        batch.commit().await()
    }
}

Output: Atomic batch writes with no partially-committed data.

Prevention

  • Max 500 operations per batch. Chunk larger batches.
  • Batch operations are atomic — all succeed or all fail.
  • Use batch() for related writes that must be consistent.
  • Batches count as a single write operation for billing (each document still counts individually).

Common Mistakes with firestore batch

  1. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
  2. Using return to exit a function early instead of wrapping a pure value in the monad
  3. Mixing let bindings with <- bindings in do notation, producing type errors

These mistakes appear frequently in real-world Android code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

### What is the difference between a batch and a Transaction?

A batch commits without reading current data (fire-and-forget). A Transaction reads data before writing (optimistic concurrency). Use batches for independent writes, transactions for read-then-write.

### How many operations can a batch have?

Maximum 500 operations (set, update, delete) per batch. Exceeding this throws an error. Chunk your data into batches of 499 to be safe.

### Can I commit the same batch twice?

No. A WriteBatch can only be committed once. Create a new WriteBatch for each commit.

← Previous Firebase Remote Config — Complete Guide Next → Firestore Composite Indexes — Complete Guide

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