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Write Operations: Core Sync Functionality

connector-writer/destination/step-by-step/4-write-operations.md

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Write Operations: Core Sync Functionality

Prerequisites: Complete 3-write-infrastructure.md - Your DI setup must be complete and you must understand test contexts.

What You'll Build: After completing this guide, you'll have a working connector with:

  • InsertBuffer with efficient batch writes
  • Aggregate and AggregateFactory
  • Writer orchestration
  • Append mode (direct insert)
  • Overwrite mode (temp table + atomic swap)
  • Generation ID tracking
  • --write operation working for basic syncs

Write Phase 1: Writer & Append Mode (Business Logic)

Goal: Implement actual data writing (Writer, Aggregate, InsertBuffer)

Checkpoint: Can write one record end-to-end

šŸ“‹ Dependency Context: Now that infrastructure exists (Phases 6-7), add business logic:

  • InsertBuffer (accumulates and flushes records to database)
  • Aggregate (processes transformed records)
  • AggregateFactory (creates Aggregate instances)
  • Writer (orchestrates setup and creates StreamLoaders)

Key insight: Infrastructure DI (Phase 7) is separate from business logic DI (Phase 8). Phase 7 validates "can we start?" Phase 8 validates "can we write data?"

Write Step 1: Create InsertBuffer

File: write/load/{DB}InsertBuffer.kt

kotlin
package io.airbyte.integrations.destination.{db}.write.load

import io.airbyte.cdk.load.data.AirbyteValue
import io.airbyte.cdk.load.schema.model.TableName
import io.airbyte.integrations.destination.{db}.client.{DB}AirbyteClient
import io.github.oshai.kotlinlogging.KotlinLogging

private val log = KotlinLogging.logger {}

/**
 * Accumulates records and flushes to database in batches.
 *
 * NOT a @Singleton - created per-stream by AggregateFactory
 */
class {DB}InsertBuffer(
    private val tableName: TableName,
    private val client: {DB}AirbyteClient,
    private val flushLimit: Int = 1000,
) {
    private val buffer = mutableListOf<Map<String, AirbyteValue>>()
    private var recordCount = 0

    fun accumulate(recordFields: Map<String, AirbyteValue>) {
        buffer.add(recordFields)
        recordCount++

        if (recordCount >= flushLimit) {
            kotlinx.coroutines.runBlocking { flush() }
        }
    }

    suspend fun flush() {
        if (buffer.isEmpty()) return

        try {
            log.info { "Flushing $recordCount records to ${tableName}..." }

            // Simple multi-row INSERT for now
            // (Optimize in Phase 15: CSV staging, COPY, bulk APIs)
            buffer.forEach { record ->
                insertRecord(tableName, record)
            }

            log.info { "Finished flushing $recordCount records" }
        } finally {
            buffer.clear()
            recordCount = 0
        }
    }

    private suspend fun insertRecord(
        tableName: TableName,
        record: Map<String, AirbyteValue>
    ) {
        val columns = record.keys.joinToString(", ") { "\"$it\"" }
        val placeholders = record.keys.joinToString(", ") { "?" }
        val sql = """
            INSERT INTO "${tableName.namespace}"."${tableName.name}" ($columns)
            VALUES ($placeholders)
        """

        client.executeInsert(sql, record.values.toList())
    }
}

Key points:

  • NOT @Singleton - one buffer per stream
  • Simple implementation: single-row inserts
  • Phase 15 (Optimization) replaces with bulk loading

Why not @Singleton?

  • Each stream needs its own buffer
  • Buffers hold stream-specific state (table name, accumulated records)
  • AggregateFactory creates one buffer per stream

Write Step 2: Add executeInsert() to Client

File: Update client/{DB}AirbyteClient.kt

kotlin
// Add this method to {DB}AirbyteClient
fun executeInsert(sql: String, values: List<AirbyteValue>) {
    dataSource.connection.use { connection ->
        connection.prepareStatement(sql).use { statement ->
            values.forEachIndexed { index, value ->
                setParameter(statement, index + 1, value)
            }
            statement.executeUpdate()
        }
    }
}

private fun setParameter(statement: PreparedStatement, index: Int, value: AirbyteValue) {
    when (value) {
        is StringValue -> statement.setString(index, value.value)
        is IntegerValue -> statement.setLong(index, value.value)
        is NumberValue -> statement.setBigDecimal(index, value.value)
        is BooleanValue -> statement.setBoolean(index, value.value)
        is TimestampValue -> statement.setTimestamp(index, Timestamp.from(value.value))
        is DateValue -> statement.setDate(index, Date.valueOf(value.value))
        is TimeValue -> statement.setTime(index, Time.valueOf(value.value.toLocalTime()))
        is ObjectValue -> statement.setString(index, value.toJson())  // JSON as string
        is ArrayValue -> statement.setString(index, value.toJson())   // JSON as string
        is NullValue -> statement.setNull(index, Types.VARCHAR)
        else -> statement.setString(index, value.toString())
    }
}

Note: For non-JDBC databases, use native client APIs (e.g., MongoDB insertOne, ClickHouse native client)

Write Step 3: Create Aggregate

File: dataflow/{DB}Aggregate.kt

kotlin
package io.airbyte.integrations.destination.{db}.dataflow

import io.airbyte.cdk.load.dataflow.aggregate.Aggregate
import io.airbyte.cdk.load.dataflow.transform.RecordDTO
import io.airbyte.integrations.destination.{db}.write.load.{DB}InsertBuffer

/**
 * Processes transformed records for a single stream.
 *
 * Dataflow pipeline: Raw record ā†’ Transform ā†’ RecordDTO ā†’ Aggregate.accept() ā†’ InsertBuffer
 *
 * NOT a @Singleton - created per-stream by AggregateFactory
 */
class {DB}Aggregate(
    private val buffer: {DB}InsertBuffer,
) : Aggregate {

    override fun accept(record: RecordDTO) {
        buffer.accumulate(record.fields)
    }

    override suspend fun flush() {
        buffer.flush()
    }
}

What this does:

  • Receives transformed records from CDK dataflow pipeline
  • Delegates to InsertBuffer for accumulation
  • Implements flush() for end-of-stream flushing

Dataflow pipeline:

Platform ā†’ JSONL records
  ā†“
AirbyteMessageDeserializer (CDK)
  ā†“
RecordTransformer (CDK, uses ColumnNameMapper from Phase 7)
  ā†“
RecordDTO (transformed record with mapped column names)
  ā†“
Aggregate.accept()  ā† YOUR CODE STARTS HERE
  ā†“
InsertBuffer.accumulate()
  ā†“
Database

Write Step 4: Create AggregateFactory

File: dataflow/{DB}AggregateFactory.kt

kotlin
package io.airbyte.integrations.destination.{db}.dataflow

import io.airbyte.cdk.load.dataflow.aggregate.Aggregate
import io.airbyte.cdk.load.dataflow.aggregate.AggregateFactory
import io.airbyte.cdk.load.state.StoreKey
import io.airbyte.cdk.load.table.directload.DirectLoadTableExecutionConfig
import io.airbyte.cdk.load.write.StreamStateStore
import io.airbyte.integrations.destination.{db}.client.{DB}AirbyteClient
import io.airbyte.integrations.destination.{db}.write.load.{DB}InsertBuffer
import io.micronaut.context.annotation.Factory
import jakarta.inject.Singleton

@Factory
class {DB}AggregateFactory(
    private val client: {DB}AirbyteClient,
    private val streamStateStore: StreamStateStore<DirectLoadTableExecutionConfig>,
) : AggregateFactory {

    @Singleton
    override fun create(key: StoreKey): Aggregate {
        // StreamStateStore contains execution config for each stream
        // Config includes table name, column mapping, etc.
        val tableName = streamStateStore.get(key)!!.tableName

        val buffer = {DB}InsertBuffer(
            tableName = tableName,
            client = client,
        )

        return {DB}Aggregate(buffer)
    }
}

What this does:

  • @Factory class provides factory method for creating Aggregates
  • create() called once per stream at start of sync
  • StreamStateStore provides table name for the stream
  • Creates InsertBuffer ā†’ Aggregate chain

Why factory pattern?

  • Aggregate needs per-stream state (table name)
  • Can't use constructor injection (dynamic stream list)
  • Factory receives StoreKey, looks up stream config, creates Aggregate

Write Step 5: Create Writer

File: write/{DB}Writer.kt

kotlin
package io.airbyte.integrations.destination.{db}.write

import io.airbyte.cdk.SystemErrorException
import io.airbyte.cdk.load.command.DestinationCatalog
import io.airbyte.cdk.load.command.DestinationStream
import io.airbyte.cdk.load.table.ColumnNameMapping
import io.airbyte.cdk.load.table.DatabaseInitialStatusGatherer
import io.airbyte.cdk.load.table.directload.DirectLoadInitialStatus
import io.airbyte.cdk.load.table.directload.DirectLoadTableAppendStreamLoader
import io.airbyte.cdk.load.table.directload.DirectLoadTableAppendTruncateStreamLoader
import io.airbyte.cdk.load.table.directload.DirectLoadTableExecutionConfig
import io.airbyte.cdk.load.write.DestinationWriter
import io.airbyte.cdk.load.write.StreamLoader
import io.airbyte.cdk.load.write.StreamStateStore
import io.airbyte.integrations.destination.{db}.client.{DB}AirbyteClient
import jakarta.inject.Singleton

@Singleton
class {DB}Writer(
    private val catalog: DestinationCatalog,
    private val stateGatherer: DatabaseInitialStatusGatherer<DirectLoadInitialStatus>,
    private val streamStateStore: StreamStateStore<DirectLoadTableExecutionConfig>,
    private val client: {DB}AirbyteClient,
) : DestinationWriter {

    private lateinit var initialStatuses: Map<DestinationStream, DirectLoadInitialStatus>

    override suspend fun setup() {
        // Create all namespaces
        catalog.streams
            .map { it.tableSchema.tableNames.finalTableName!!.namespace }
            .toSet()
            .forEach { client.createNamespace(it) }

        // Gather initial state (which tables exist, generation IDs, etc.)
        initialStatuses = stateGatherer.gatherInitialStatus()
    }

    override fun createStreamLoader(stream: DestinationStream): StreamLoader {
        val initialStatus = initialStatuses[stream]!!

        // Access schema directly from stream (modern CDK pattern)
        val realTableName = stream.tableSchema.tableNames.finalTableName!!
        val tempTableName = stream.tableSchema.tableNames.tempTableName!!
        val columnNameMapping = ColumnNameMapping(
            stream.tableSchema.columnSchema.inputToFinalColumnNames
        )

        // Choose StreamLoader based on sync mode
        return when (stream.minimumGenerationId) {
            0L ->
                // Append mode: just insert records
                DirectLoadTableAppendStreamLoader(
                    stream,
                    initialStatus,
                    realTableName = realTableName,
                    tempTableName = tempTableName,
                    columnNameMapping,
                    client,  // TableOperationsClient
                    client,  // TableSchemaEvolutionClient
                    streamStateStore,
                )
            stream.generationId ->
                // Overwrite/truncate mode: replace table contents
                DirectLoadTableAppendTruncateStreamLoader(
                    stream,
                    initialStatus,
                    realTableName = realTableName,
                    tempTableName = tempTableName,
                    columnNameMapping,
                    client,
                    client,
                    streamStateStore,
                )
            else ->
                throw SystemErrorException(
                    "Cannot execute a hybrid refresh - current generation ${stream.generationId}; minimum generation ${stream.minimumGenerationId}"
                )
        }
    }
}

What this does:

  • setup(): Creates namespaces, gathers initial table state
  • createStreamLoader(): Creates StreamLoader for each stream based on sync mode

Modern CDK pattern (stream.tableSchema):

  • Schema info is embedded in stream.tableSchema (set by CDK)
  • Access via stream.tableSchema.tableNames.finalTableName!!
  • Column mappings via stream.tableSchema.columnSchema.inputToFinalColumnNames
  • No need for defensive null checks (CDK guarantees schema exists)

StreamLoader selection:

  • minimumGenerationId == 0: Append mode (DirectLoadTableAppendStreamLoader)
  • minimumGenerationId == generationId: Overwrite mode (DirectLoadTableAppendTruncateStreamLoader)
  • Other combinations: Error (hybrid refresh not supported)

StreamLoader responsibilities:

  • start(): Create/prepare table
  • accept(): Add record to buffer
  • complete(): Flush and finalize

CDK provides implementations:

  • DirectLoadTableAppendStreamLoader
  • DirectLoadTableAppendTruncateStreamLoader
  • DirectLoadTableDedupStreamLoader
  • DirectLoadTableDedupTruncateStreamLoader

Write Step 6: Create ConnectorWiringSuite Test

File: src/test-integration/kotlin/.../component/{DB}WiringTest.kt

kotlin
package io.airbyte.integrations.destination.{db}.component

import io.airbyte.cdk.load.component.ConnectorWiringSuite
import io.airbyte.cdk.load.component.TableOperationsClient
import io.airbyte.cdk.load.dataflow.aggregate.AggregateFactory
import io.airbyte.cdk.load.write.DestinationWriter
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import org.junit.jupiter.api.Test

@MicronautTest(environments = ["component"])
class {DB}WiringTest(
    override val writer: DestinationWriter,
    override val client: TableOperationsClient,
    override val aggregateFactory: AggregateFactory,
) : ConnectorWiringSuite {

    // Optional: Override test namespace if different from "test"
    // override val testNamespace = "my_database"

    @Test
    override fun `all beans are injectable`() {
        super.`all beans are injectable`()
    }

    @Test
    override fun `writer setup completes`() {
        super.`writer setup completes`()
    }

    @Test
    override fun `can create append stream loader`() {
        super.`can create append stream loader`()
    }

    @Test
    override fun `can write one record`() {
        super.`can write one record`()
    }
}

What ConnectorWiringSuite does:

Test 1: all beans are injectable

  • Validates all DI beans exist
  • Catches missing @Singleton annotations
  • Catches circular dependencies

Test 2: writer setup completes

  • Calls Writer.setup()
  • Validates namespace creation works
  • Catches database connection errors

Test 3: can create append stream loader

  • Calls Writer.createStreamLoader()
  • Validates StreamLoader instantiation
  • Catches missing StreamLoader dependencies

Test 4: can write one record ā† MOST IMPORTANT

  • Creates test stream
  • Calls StreamLoader.start() ā†’ creates table
  • Calls Aggregate.accept() ā†’ buffers record
  • Calls Aggregate.flush() ā†’ writes to database
  • Validates record appears in database
  • END-TO-END validation of full write path!

Test context:

  • Uses MockDestinationCatalog (fast, no real catalog parsing)
  • Uses Testcontainers database
  • Component test (not integration test)

Why MockDestinationCatalog?

  • Fast iteration (no catalog JSON parsing)
  • Creates dynamic test streams
  • Focuses on write logic, not catalog parsing

Write Step 7: Validate ConnectorWiringSuite

Validate:

bash
$ ./gradlew :destination-{db}:testComponentAllBeansAreInjectable \
             :destination-{db}:testComponentWriterSetupCompletes \
             :destination-{db}:testComponentCanCreateAppendStreamLoader \
             :destination-{db}:testComponentCanWriteOneRecord  # 4 tests should pass
$ ./gradlew :destination-{db}:componentTest  # 9 tests should pass
$ ./gradlew :destination-{db}:integrationTest  # 3 tests should pass

If can write one record FAILS:

DI errors: ā†’ Check Phase 7 infrastructure (WriteOperationV2, DatabaseInitialStatusGatherer, ColumnNameMapper) ā†’ Check Phase 6 name generators all have @Singleton

Table creation errors: ā†’ Check TableOperationsClient.createTable() implementation (Phase 4) ā†’ Check SqlGenerator.createTable() SQL syntax

Insert errors: ā†’ Check InsertBuffer.insertRecord() implementation ā†’ Check client.executeInsert() and setParameter() logic ā†’ Check column name mapping

Record not found in database: ā†’ Check buffer.flush() is called ā†’ Check SQL INSERT statement is correct ā†’ Query database directly to debug

āœ… Checkpoint: First working sync + all previous phases still work

Write Phase 2: Generation ID Support

Goal: Track sync generations for refresh handling

Checkpoint: Can retrieve generation IDs

šŸ“‹ What's a Generation ID?

  • Unique identifier for each sync run
  • Used to distinguish "old data" from "new data" during refreshes
  • Stored in _airbyte_generation_id column

When used:

  • Full refresh: minimumGenerationId = generationId (replace all data)
  • Incremental: minimumGenerationId = 0 (keep all data)

Write Step 1: Enable Generation ID Test

File: Update src/test-integration/kotlin/.../component/{DB}TableOperationsTest.kt

kotlin
@Test
override fun `get generation id`() {
    super.`get generation id`()
}

What this tests:

  • TableOperationsClient.getGenerationId() returns correct value
  • Returns 0L for tables without generation ID
  • Returns actual generation ID from _airbyte_generation_id column

Write Step 2: Validate

Validate:

bash
$ ./gradlew :destination-{db}:testComponentGetGenerationId  # 1 test should pass
$ ./gradlew :destination-{db}:componentTest  # 10 tests should pass

āœ… Checkpoint: Generation ID tracking works + all previous phases still work

Write Phase 3: Overwrite Mode

Goal: Support full refresh (replace all data)

Checkpoint: Can replace table contents atomically

šŸ“‹ How Overwrite Works:

  1. Write new data to temp table
  2. Atomically swap temp table with final table
  3. Drop old table

Sync modes:

  • Append (Phase 8): INSERT into existing table
  • Overwrite (Phase 10): SWAP temp table with final table

Write Step 1: Implement overwriteTable() in SQL Generator

File: Update client/{DB}SqlGenerator.kt

kotlin
fun overwriteTable(source: TableName, target: TableName): List<String> {
    // Option 1: SWAP (Snowflake)
    return listOf(
        "ALTER TABLE ${fullyQualifiedName(target)} SWAP WITH ${fullyQualifiedName(source)}".andLog(),
        "DROP TABLE IF EXISTS ${fullyQualifiedName(source)}".andLog(),
    )

    // Option 2: EXCHANGE (ClickHouse)
    return listOf(
        "EXCHANGE TABLES ${fullyQualifiedName(target)} AND ${fullyQualifiedName(source)}".andLog(),
        "DROP TABLE IF EXISTS ${fullyQualifiedName(source)}".andLog(),
    )

    // Option 3: DROP + RENAME (fallback for most databases)
    return listOf(
        "DROP TABLE IF EXISTS ${fullyQualifiedName(target)}".andLog(),
        "ALTER TABLE ${fullyQualifiedName(source)} RENAME TO ${target.name.quote()}".andLog(),
    )

    // Option 4: BEGIN TRANSACTION + DROP + RENAME + COMMIT (for ACID guarantees)
    return listOf(
        "BEGIN TRANSACTION".andLog(),
        "DROP TABLE IF EXISTS ${fullyQualifiedName(target)}".andLog(),
        "ALTER TABLE ${fullyQualifiedName(source)} RENAME TO ${target.name.quote()}".andLog(),
        "COMMIT".andLog(),
    )
}

Database-specific notes:

  • Snowflake: SWAP is atomic and instant (metadata operation)
  • ClickHouse: EXCHANGE is atomic
  • Postgres/MySQL: DROP + RENAME requires transaction for atomicity
  • BigQuery: CREATE OR REPLACE TABLE (different pattern)

Write Step 2: Implement overwriteTable() in Client

File: Update client/{DB}AirbyteClient.kt

kotlin
override suspend fun overwriteTable(
    sourceTableName: TableName,
    targetTableName: TableName
) {
    val statements = sqlGenerator.overwriteTable(sourceTableName, targetTableName)
    statements.forEach { execute(it) }
}

Write Step 3: Update Writer for Truncate Mode

File: Update write/{DB}Writer.kt

kotlin
override fun createStreamLoader(stream: DestinationStream): StreamLoader {
    // Defensive: Handle streams not in catalog (for test compatibility)
    val initialStatus = if (::initialStatuses.isInitialized) {
        initialStatuses[stream] ?: DirectLoadInitialStatus(null, null)
    } else {
        DirectLoadInitialStatus(null, null)
    }

    val tableNameInfo = names[stream]
    val (realTableName, tempTableName, columnNameMapping) = if (tableNameInfo != null) {
        Triple(
            tableNameInfo.tableNames.finalTableName!!,
            tempTableNameGenerator.generate(tableNameInfo.tableNames.finalTableName!!),
            tableNameInfo.columnNameMapping
        )
    } else {
        val tableName = TableName(
            namespace = stream.mappedDescriptor.namespace ?: "test",
            name = stream.mappedDescriptor.name
        )
        Triple(tableName, tempTableNameGenerator.generate(tableName), ColumnNameMapping(emptyMap()))
    }

    // Choose StreamLoader based on sync mode
    return when (stream.minimumGenerationId) {
        0L -> DirectLoadTableAppendStreamLoader(
            stream, initialStatus, realTableName, tempTableName,
            columnNameMapping, client, client, streamStateStore
        )
        stream.generationId -> DirectLoadTableAppendTruncateStreamLoader(
            stream, initialStatus, realTableName, tempTableName,
            columnNameMapping, client, client, streamStateStore
        )
        else -> throw SystemErrorException("Hybrid refresh not supported")
    }
}

What changed:

  • Added when statement to choose StreamLoader based on minimumGenerationId
  • minimumGenerationId = 0: Append mode (keep old data)
  • minimumGenerationId = generationId: Truncate mode (replace old data)

StreamLoader behavior:

  • AppendStreamLoader: Writes directly to final table
  • AppendTruncateStreamLoader: Writes to temp table, then swaps

Write Step 4: Enable Tests

File: Update src/test-integration/kotlin/.../component/{DB}TableOperationsTest.kt

kotlin
@Test
override fun `overwrite tables`() {
    super.`overwrite tables`()
}

Write Step 5: Validate

Validate:

bash
$ ./gradlew :destination-{db}:testComponentOverwriteTables  # 1 test should pass
$ ./gradlew :destination-{db}:componentTest  # 11 tests should pass
$ ./gradlew :destination-{db}:integrationTest  # 3 tests should pass

āœ… Checkpoint: Full refresh mode works + all previous phases still work

Write Phase 4: Copy Operation

Goal: Support table copying (used internally by some modes)

Checkpoint: Can copy data between tables

šŸ“‹ When Copy is Used:

  • Dedupe mode: Copy deduplicated data from temp to final
  • Some overwrite implementations: Copy instead of swap
  • Schema evolution: Copy to new schema

Write Step 1: Implement copyTable() in SQL Generator

File: Update client/{DB}SqlGenerator.kt

kotlin
fun copyTable(
    columnMapping: ColumnNameMapping,
    source: TableName,
    target: TableName
): String {
    val columnList = columnMapping.values.joinToString(", ") { "\"$it\"" }

    return """
        INSERT INTO ${fullyQualifiedName(target)} ($columnList)
        SELECT $columnList
        FROM ${fullyQualifiedName(source)}
    """.trimIndent().andLog()
}

What this does:

  • Copies all rows from source to target
  • Only copies mapped columns (not all columns)
  • Preserves data types (SELECT ā†’ INSERT)

Alternative: Include Airbyte metadata columns explicitly:

kotlin
fun copyTable(
    columnMapping: ColumnNameMapping,
    source: TableName,
    target: TableName
): String {
    // Include Airbyte metadata + user columns
    val allColumns = listOf(
        "_airbyte_raw_id",
        "_airbyte_extracted_at",
        "_airbyte_meta",
        "_airbyte_generation_id"
    ) + columnMapping.values

    val columnList = allColumns.joinToString(", ") { "\"$it\"" }

    return """
        INSERT INTO ${fullyQualifiedName(target)} ($columnList)
        SELECT $columnList
        FROM ${fullyQualifiedName(source)}
    """.trimIndent().andLog()
}

Write Step 2: Implement copyTable() in Client

File: Update client/{DB}AirbyteClient.kt

kotlin
override suspend fun copyTable(
    columnNameMapping: ColumnNameMapping,
    sourceTableName: TableName,
    targetTableName: TableName
) {
    execute(sqlGenerator.copyTable(columnNameMapping, sourceTableName, targetTableName))
}

Write Step 3: Enable Test

File: Update src/test-integration/kotlin/.../component/{DB}TableOperationsTest.kt

kotlin
@Test
override fun `copy tables`() {
    super.`copy tables`()
}

Write Step 4: Validate

Validate:

bash
$ ./gradlew :destination-{db}:testComponentCopyTables  # 1 test should pass
$ ./gradlew :destination-{db}:componentTest  # 12 tests should pass
$ ./gradlew :destination-{db}:integrationTest  # 3 tests should pass

āœ… Checkpoint: Copy operation works + all previous phases still work

Next Steps

Next: Your connector now works for basic use cases! Continue to 5-advanced-features.md for production-ready features, or jump to 6-testing.md to run the full test suite.