✍️ 필사 모드: Event Streaming Deep Dive Guide 2025: Kafka Internals, Flink, Exactly-Once, Schema Evolution

Topic: orders (3 partitions)

Partition 0: [msg0] [msg3] [msg6] [msg9]  ... → offset increases
Partition 1: [msg1] [msg4] [msg7] [msg10] ... → offset increases
Partition 2: [msg2] [msg5] [msg8] [msg11] ... → offset increases

- Ordering guaranteed only within the same partition
- Partition count = max parallel consumers in a Consumer Group
- Partition key routes messages with the same key to the same partition

/kafka-logs/orders-0/
├── 00000000000000000000.log      # Segment file (actual messages)
├── 00000000000000000000.index    # Offset index
├── 00000000000000000000.timeindex # Timestamp index
├── 00000000000005242880.log      # Next segment
├── 00000000000005242880.index
├── 00000000000005242880.timeindex
└── leader-epoch-checkpoint

Segment File Structure
========================

.log file (message storage)
┌──────────────────────────────────────┐
│ Batch Header                         │
│  - Base Offset: 0                    │
│  - Batch Length: 256 bytes           │
│  - Magic: 2 (Kafka 0.11+)           │
│  - CRC: checksum                     │
│  - Attributes: compression, txn      │
│  - Producer ID: 12345                │
│  - Producer Epoch: 0                 │
│  - Base Sequence: 0                  │
├──────────────────────────────────────┤
│ Record 0                             │
│  - Offset Delta: 0                   │
│  - Timestamp Delta: 0               │
│  - Key: "order-123"                  │
│  - Value: (serialized payload)       │
│  - Headers: [("source", "web")]      │
├──────────────────────────────────────┤
│ Record 1                             │
│  - Offset Delta: 1                   │
│  - ...                               │
└──────────────────────────────────────┘

.index file (sparse index)
┌────────────────────────┐
│ Relative Offset → File │
│ Position               │
├────────────────────────┤
│ 0    → 0               │
│ 4096 → 32768           │
│ 8192 → 65536           │
└────────────────────────┘

.timeindex file
┌────────────────────────────────┐
│ Timestamp → Relative Offset   │
├────────────────────────────────┤
│ 1700000000000 → 0             │
│ 1700000060000 → 4096          │
└────────────────────────────────┘

Before compaction:
offset 0: key=A, value=v1
offset 1: key=B, value=v1
offset 2: key=A, value=v2  ← latest for A
offset 3: key=C, value=v1
offset 4: key=B, value=v2  ← latest for B
offset 5: key=A, value=v3  ← latest for A

After compaction:
offset 3: key=C, value=v1  (only value for C)
offset 4: key=B, value=v2  (latest for B)
offset 5: key=A, value=v3  (latest for A)

# Log Compaction configuration
log.cleanup.policy=compact
log.cleaner.min.cleanable.ratio=0.5
log.cleaner.min.compaction.lag.ms=0
min.cleanable.dirty.ratio=0.5

Partition 0 (Replication Factor = 3)
======================================

Broker 1 (Leader):      [0] [1] [2] [3] [4] [5]  ← LEO: 6
Broker 2 (Follower):    [0] [1] [2] [3] [4]       ← LEO: 5
Broker 3 (Follower):    [0] [1] [2] [3] [4] [5]  ← LEO: 6

ISR = {Broker 1, Broker 2, Broker 3}
HW (High Watermark) = 5 (minimum LEO within ISR)

- Consumers can only read up to HW
- If Broker 2 lags too much, it is removed from ISR
- Removed from ISR when exceeding replica.lag.time.max.ms

# ISR-related configuration
min.insync.replicas=2           # Minimum in-sync replicas
replica.lag.time.max.ms=30000   # ISR removal threshold
unclean.leader.election.enable=false  # Prevent non-ISR replica leader election

ZooKeeper Mode (Legacy)
==========================
[ZooKeeper Ensemble]
  ↕ metadata
[Controller (one of the Brokers)]
  ↕ leader election
[Broker 1] [Broker 2] [Broker 3]

Issues:
- Requires separate ZooKeeper operations
- Metadata synchronization delays
- Performance degradation as partition count increases


KRaft Mode (Kafka 3.x+)
==========================
[Controller Quorum]
  Broker 1 (Controller + Broker)
  Broker 2 (Controller + Broker)
  Broker 3 (Controller + Broker)

Advantages:
- Eliminates ZooKeeper dependency
- Improved metadata management performance
- Supports millions of partitions
- Reduced operational complexity

# KRaft configuration
process.roles=broker,controller
node.id=1
controller.quorum.voters=1@broker1:9093,2@broker2:9093,3@broker3:9093
controller.listener.names=CONTROLLER

// Default partitioner behavior
public class DefaultPartitioner implements Partitioner {
    // With key: murmur2 hash
    // Without key: Sticky Partitioner (changes partition per batch)

    public int partition(String topic, Object key, byte[] keyBytes,
                         Object value, byte[] valueBytes, Cluster cluster) {
        List<PartitionInfo> partitions = cluster.partitionsForTopic(topic);
        int numPartitions = partitions.size();

        if (keyBytes == null) {
            // Sticky Partitioner: same partition until batch is full
            return stickyPartitionCache.partition(topic, cluster);
        }

        // Key-based partitioning: same key = same partition
        return Utils.toPositive(Utils.murmur2(keyBytes)) % numPartitions;
    }
}

// Custom partitioner example: region-based
public class RegionPartitioner implements Partitioner {
    private Map<String, Integer> regionMapping;

    public int partition(String topic, Object key, byte[] keyBytes,
                         Object value, byte[] valueBytes, Cluster cluster) {
        String region = extractRegion(value);
        int numPartitions = cluster.partitionsForTopic(topic).size();

        return regionMapping.getOrDefault(region, 0) % numPartitions;
    }
}

# Producer batch settings
batch.size=16384                  # Batch size (bytes)
linger.ms=5                       # Batch wait time
buffer.memory=33554432            # Total buffer memory

# Compression settings
compression.type=lz4              # none, gzip, snappy, lz4, zstd

Producer Batch Processing Flow
==========================

send() ──▶ [Serializer] ──▶ [Partitioner] ──▶ [RecordAccumulator]
                                                     │
                                         ┌───────────┴───────────┐
                                         │  Partition 0 batch    │
                                         │  [msg1][msg2][msg3]   │
                                         ├───────────────────────┤
                                         │  Partition 1 batch    │
                                         │  [msg4][msg5]         │
                                         └───────────┬───────────┘
                                                     │
                                              batch.size reached
                                              or linger.ms elapsed
                                                     │
                                                     ▼
                                              [Sender Thread]
                                                     │
                                              Compress + network send
                                                     │
                                                     ▼
                                              [Kafka Broker]

# Idempotent producer settings
enable.idempotence=true    # Default since Kafka 3.0+
acks=all                   # Required for idempotency
retries=2147483647         # Infinite retries
max.in.flight.requests.per.connection=5  # Max 5 (idempotency range)

Idempotent Producer Operation
==========================

Producer                          Broker
   │                                │
   │── ProducerID: 1000            │
   │   Epoch: 0                    │
   │   Sequence: 0                 │
   │   Message: "order-1"   ──────▶│  ✓ Stored
   │                                │
   │── ProducerID: 1000            │
   │   Epoch: 0                    │
   │   Sequence: 1                 │
   │   Message: "order-2"   ──────▶│  ✓ Stored
   │                                │
   │── (Resend due to network timeout) │
   │   ProducerID: 1000            │
   │   Epoch: 0                    │
   │   Sequence: 1                 │
   │   Message: "order-2"   ──────▶│  ✗ Duplicate! Ignored
   │                                │

// Transactional producer example
Properties props = new Properties();
props.put("bootstrap.servers", "broker1:9092,broker2:9092");
props.put("transactional.id", "order-processor-1");
props.put("enable.idempotence", "true");

KafkaProducer<String, String> producer = new KafkaProducer<>(props);
producer.initTransactions();

try {
    producer.beginTransaction();

    // Atomically send to multiple topics/partitions
    producer.send(new ProducerRecord<>("orders", "key1", "order-created"));
    producer.send(new ProducerRecord<>("inventory", "key1", "stock-reserved"));
    producer.send(new ProducerRecord<>("notifications", "key1", "email-queued"));

    // Consumer offsets can be included in the transaction
    producer.sendOffsetsToTransaction(offsets, consumerGroupId);

    producer.commitTransaction();
} catch (Exception e) {
    producer.abortTransaction();
    throw e;
}

Topic: orders (6 partitions)

Consumer Group: order-processor
===================================

Case 1: 3 Consumers
  Consumer A: [P0] [P1]
  Consumer B: [P2] [P3]
  Consumer C: [P4] [P5]

Case 2: 6 Consumers (ideal)
  Consumer A: [P0]
  Consumer B: [P1]
  Consumer C: [P2]
  Consumer D: [P3]
  Consumer E: [P4]
  Consumer F: [P5]

Case 3: 8 Consumers (2 idle)
  Consumer A: [P0]
  Consumer B: [P1]
  Consumer C: [P2]
  Consumer D: [P3]
  Consumer E: [P4]
  Consumer F: [P5]
  Consumer G: (idle)
  Consumer H: (idle)

// Partition assignment strategy comparison
Properties props = new Properties();

// 1. Range Assignor (default)
props.put("partition.assignment.strategy",
    "org.apache.kafka.clients.consumer.RangeAssignor");
// Divides partitions into ranges per topic
// Topic A: P0,P1,P2 -> C1: [P0,P1], C2: [P2]
// Topic B: P0,P1,P2 -> C1: [P0,P1], C2: [P2]
// Issue: skew toward C1

// 2. RoundRobin Assignor
props.put("partition.assignment.strategy",
    "org.apache.kafka.clients.consumer.RoundRobinAssignor");
// Distributes all topic partitions in round-robin
// C1: [A-P0, B-P1], C2: [A-P1, B-P0], C3: [A-P2, B-P2]

// 3. Sticky Assignor
props.put("partition.assignment.strategy",
    "org.apache.kafka.clients.consumer.StickyAssignor");
// Maximizes existing assignment retention + even distribution
// Moves minimum partitions during rebalance

// 4. CooperativeSticky Assignor (recommended)
props.put("partition.assignment.strategy",
    "org.apache.kafka.clients.consumer.CooperativeStickyAssignor");
// Sticky + incremental rebalancing
// Reassigns partitions without full stop

Eager Rebalancing (legacy)
================================
1. Consumer C3 added
2. All consumers release partitions (Stop-the-World)
   C1: [] C2: [] C3: []
3. Calculate new assignment
4. Reassign partitions
   C1: [P0,P1] C2: [P2,P3] C3: [P4,P5]
→ Full processing stop occurs!


Incremental Cooperative Rebalancing (recommended)
================================
1. Consumer C3 added
2. 1st rebalance: only release partitions that need to move
   C1: [P0,P1,P2] → C1: [P0,P1]  (P2 released)
   C2: [P3,P4,P5] → C2: [P3,P4]  (P5 released)
3. 2nd rebalance: assign released partitions to C3
   C3: [] → C3: [P2,P5]
→ Remaining partitions continue processing without interruption!

// Auto commit (default)
props.put("enable.auto.commit", "true");
props.put("auto.commit.interval.ms", "5000");
// Risk: if committed before processing, messages can be lost

// Manual commit (recommended)
props.put("enable.auto.commit", "false");

while (true) {
    ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));

    for (ConsumerRecord<String, String> record : records) {
        // Process message
        processRecord(record);
    }

    // Synchronous commit (after processing complete)
    consumer.commitSync();

    // Or async commit (performance improvement)
    consumer.commitAsync((offsets, exception) -> {
        if (exception != null) {
            log.error("Commit failed", exception);
        }
    });
}

Offset Storage: __consumer_offsets topic
============================================

Key: (group_id, topic, partition)
Value: (offset, metadata, timestamp)

Example:
Key: (order-processor, orders, 0)
Value: (offset=42, metadata="", timestamp=1700000000)

At-Most-Once
================================
Producer ──send()──▶ Broker
  (acks=0, no confirmation)
- Messages can be lost
- No duplicates
- Fastest

At-Least-Once
================================
Producer ──send()──▶ Broker ──ack──▶ Producer
  (acks=all, with retries)
- No message loss
- Duplicates possible
- Most default configurations

Exactly-Once
================================
Producer ──txn──▶ Broker ──commit──▶ Consumer(read_committed)
  (idempotent producer + transactions + read_committed)
- No message loss
- No duplicates
- Strongest but with overhead

// Exactly-Once: Consume-Transform-Produce pattern
Properties producerProps = new Properties();
producerProps.put("transactional.id", "eos-processor-1");
producerProps.put("enable.idempotence", "true");

Properties consumerProps = new Properties();
consumerProps.put("group.id", "eos-group");
consumerProps.put("isolation.level", "read_committed");  // Key!
consumerProps.put("enable.auto.commit", "false");

KafkaProducer<String, String> producer = new KafkaProducer<>(producerProps);
KafkaConsumer<String, String> consumer = new KafkaConsumer<>(consumerProps);

producer.initTransactions();
consumer.subscribe(Collections.singleton("input-topic"));

while (true) {
    ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));

    if (!records.isEmpty()) {
        producer.beginTransaction();

        try {
            for (ConsumerRecord<String, String> record : records) {
                // Transform
                String result = transform(record.value());

                // Produce to output topic
                producer.send(new ProducerRecord<>(
                    "output-topic", record.key(), result));
            }

            // Include consumer offsets in transaction
            Map<TopicPartition, OffsetAndMetadata> offsets = new HashMap<>();
            for (TopicPartition partition : records.partitions()) {
                List<ConsumerRecord<String, String>> partRecords =
                    records.records(partition);
                long lastOffset = partRecords.get(
                    partRecords.size() - 1).offset();
                offsets.put(partition,
                    new OffsetAndMetadata(lastOffset + 1));
            }

            producer.sendOffsetsToTransaction(offsets,
                consumer.groupMetadata());
            producer.commitTransaction();

        } catch (Exception e) {
            producer.abortTransaction();
        }
    }
}

Exactly-Once Transaction Flow
================================

1. beginTransaction()
2. send(output-topic, msg1)     → Temporarily stored on Broker (uncommitted)
3. send(output-topic, msg2)     → Temporarily stored
4. sendOffsetsToTransaction()   → Consumer offsets included in transaction
5. commitTransaction()          → All messages + offsets atomically committed

read_committed consumers only read committed messages
→ If failure occurs mid-way, abortTransaction() rolls back everything

// Version 1: Initial schema
{
  "type": "record",
  "name": "Order",
  "namespace": "com.company.events",
  "fields": [
    {"name": "order_id", "type": "string"},
    {"name": "customer_id", "type": "string"},
    {"name": "amount", "type": "double"},
    {"name": "currency", "type": "string", "default": "USD"},
    {"name": "created_at", "type": "long"}
  ]
}

// Version 2: Backward Compatible evolution
// New fields with default values → existing consumers can read new data
{
  "type": "record",
  "name": "Order",
  "namespace": "com.company.events",
  "fields": [
    {"name": "order_id", "type": "string"},
    {"name": "customer_id", "type": "string"},
    {"name": "amount", "type": "double"},
    {"name": "currency", "type": "string", "default": "USD"},
    {"name": "created_at", "type": "long"},
    {"name": "status", "type": "string", "default": "CREATED"},
    {"name": "region", "type": ["null", "string"], "default": null}
  ]
}

BACKWARD (default, recommended)
================================
- New schema can read data written with old schema
- New fields: default value required
- Field removal: allowed
- Upgrade consumers first → then producers

FORWARD
================================
- Old schema can read data written with new schema
- New fields: default value required
- Field removal: only fields with default values
- Upgrade producers first → then consumers

FULL
================================
- Both BACKWARD and FORWARD satisfied
- Safest but most restrictive

NONE
================================
- No compatibility checks
- Dangerous! Do not use in production

# Schema Registry API usage
import requests

SCHEMA_REGISTRY_URL = "http://schema-registry:8081"

# Register schema
schema = {
    "type": "record",
    "name": "Order",
    "fields": [
        {"name": "order_id", "type": "string"},
        {"name": "amount", "type": "double"}
    ]
}

response = requests.post(
    f"{SCHEMA_REGISTRY_URL}/subjects/orders-value/versions",
    json={"schema": json.dumps(schema), "schemaType": "AVRO"},
)

# Check compatibility
response = requests.post(
    f"{SCHEMA_REGISTRY_URL}/compatibility/subjects/orders-value/versions/latest",
    json={"schema": json.dumps(new_schema), "schemaType": "AVRO"},
)
compatibility = response.json()
print(f"Compatible: {compatibility['is_compatible']}")

# Set compatibility mode
requests.put(
    f"{SCHEMA_REGISTRY_URL}/config/orders-value",
    json={"compatibility": "BACKWARD"},
)

// Version 1
syntax = "proto3";
package com.company.events;

message Order {
  string order_id = 1;
  string customer_id = 2;
  double amount = 3;
  string currency = 4;
  int64 created_at = 5;
}

// Version 2 (maintaining compatibility)
message Order {
  string order_id = 1;
  string customer_id = 2;
  double amount = 3;
  string currency = 4;
  int64 created_at = 5;
  // New fields: never reuse old field numbers!
  string status = 6;          // newly added
  string region = 7;          // newly added
  // reserved 2;              // reserve deleted field numbers
}

┌─────────────────────────────────────────────┐
│              Flink Cluster                   │
├─────────────────────────────────────────────┤
│                                             │
│  [JobManager]                               │
│    - Job scheduling                         │
│    - Checkpoint coordination                │
│    - Failure recovery                       │
│                                             │
│  [TaskManager 1]     [TaskManager 2]        │
│    - Task Slot 1       - Task Slot 1        │
│    - Task Slot 2       - Task Slot 2        │
│    - Task Slot 3       - Task Slot 3        │
│                                             │
├─────────────────────────────────────────────┤
│  [State Backend]                            │
│    - RocksDB (large state)                  │
│    - HashMap (small state)                  │
│                                             │
│  [Checkpoint Storage]                       │
│    - S3 / HDFS / GCS                        │
└─────────────────────────────────────────────┘

// Flink DataStream API example
StreamExecutionEnvironment env =
    StreamExecutionEnvironment.getExecutionEnvironment();

// Kafka Source
KafkaSource<Order> source = KafkaSource.<Order>builder()
    .setBootstrapServers("broker1:9092")
    .setTopics("orders")
    .setGroupId("flink-order-processor")
    .setStartingOffsets(OffsetsInitializer.committedOffsets(
        OffsetResetStrategy.EARLIEST))
    .setValueOnlyDeserializer(new OrderDeserializer())
    .build();

DataStream<Order> orders = env.fromSource(
    source, WatermarkStrategy
        .<Order>forBoundedOutOfOrderness(Duration.ofSeconds(10))
        .withTimestampAssigner((order, ts) -> order.getCreatedAt()),
    "Kafka Source"
);

// Processing pipeline
DataStream<OrderStats> stats = orders
    .filter(order -> order.getStatus().equals("COMPLETED"))
    .keyBy(Order::getRegion)
    .window(TumblingEventTimeWindows.of(Time.minutes(5)))
    .aggregate(new OrderStatsAggregator());

// Kafka Sink
KafkaSink<OrderStats> sink = KafkaSink.<OrderStats>builder()
    .setBootstrapServers("broker1:9092")
    .setRecordSerializer(KafkaRecordSerializationSchema.builder()
        .setTopic("order-stats")
        .setValueSerializationSchema(new OrderStatsSerializer())
        .build())
    .setDeliveryGuarantee(DeliveryGuarantee.EXACTLY_ONCE)
    .setTransactionalIdPrefix("flink-order-stats")
    .build();

stats.sinkTo(sink);
env.execute("Order Statistics Pipeline");

Tumbling Window
================================
Time:   ──|──1──2──3──|──4──5──6──|──7──8──9──|──
Windows:  [  Window 1  ][  Window 2  ][  Window 3  ]
- Fixed size, non-overlapping
- Example: aggregate every 5 minutes


Sliding Window
================================
Time:   ──|──1──2──3──4──5──6──7──8──|──
Windows:  [    Window 1    ]
            [    Window 2    ]
                [    Window 3    ]
- Fixed size, slides by interval
- Example: 10-minute window, 5-minute slide


Session Window
================================
Time:   ──1─2──3─────────5─6──7─────────9──
Windows:  [Session 1]     [Session 2]     [S3]
                  gap          gap
- Activity-based, variable size
- Window closes after gap of inactivity

// Window examples
// Tumbling Window
orders.keyBy(Order::getRegion)
    .window(TumblingEventTimeWindows.of(Time.minutes(5)))
    .sum("amount");

// Sliding Window
orders.keyBy(Order::getRegion)
    .window(SlidingEventTimeWindows.of(Time.minutes(10), Time.minutes(5)))
    .sum("amount");

// Session Window
orders.keyBy(Order::getCustomerId)
    .window(EventTimeSessionWindows.withGap(Time.minutes(30)))
    .aggregate(new SessionAggregator());

Event Time vs Processing Time
================================

Event generation time(Event Time):   t=10  t=12  t=11  t=15  t=13
Processing time(Processing Time):    T=20  T=21  T=22  T=23  T=24

Events do not arrive in order!
→ Watermarks indicate "all events up to this point should have arrived"


Watermark Operation
================================

Events:     [t=10] [t=12] [t=11] [t=15] [t=13]
Watermarks: W(10)  W(12)  W(12)  W(15)  W(15)
            (with maxOutOfOrderness = 5 seconds)
            Actual watermarks: W(5)  W(7)  W(7)  W(10) W(10)

Window [0, 10): fires when watermark exceeds 10

// Watermark strategy
WatermarkStrategy<Order> strategy = WatermarkStrategy
    // Allow up to 10 seconds of late arrivals
    .<Order>forBoundedOutOfOrderness(Duration.ofSeconds(10))
    .withTimestampAssigner((order, timestamp) -> order.getCreatedAt())
    // Advance watermark if no events for 5 minutes
    .withIdleness(Duration.ofMinutes(5));

// Late data handling
OutputTag<Order> lateOutputTag = new OutputTag<>("late-orders") {};

SingleOutputStreamOperator<OrderStats> stats = orders
    .keyBy(Order::getRegion)
    .window(TumblingEventTimeWindows.of(Time.minutes(5)))
    .allowedLateness(Time.minutes(1))  // Wait 1 more minute
    .sideOutputLateData(lateOutputTag)  // After that, to side output
    .aggregate(new OrderStatsAggregator());

// Handle late data separately
DataStream<Order> lateOrders = stats.getSideOutput(lateOutputTag);
lateOrders.addSink(new LateOrderAlertSink());

// Keyed State example
public class FraudDetector extends KeyedProcessFunction<String, Transaction, Alert> {

    // ValueState: single value per key
    private ValueState<Boolean> flagState;

    // ListState: list per key
    private ListState<Transaction> recentTransactions;

    // MapState: map per key
    private MapState<String, Integer> merchantCounts;

    @Override
    public void open(Configuration parameters) {
        ValueStateDescriptor<Boolean> flagDescriptor =
            new ValueStateDescriptor<>("flag", Boolean.class);
        flagState = getRuntimeContext().getState(flagDescriptor);

        ListStateDescriptor<Transaction> listDescriptor =
            new ListStateDescriptor<>("recent-txns", Transaction.class);
        recentTransactions = getRuntimeContext().getListState(listDescriptor);

        MapStateDescriptor<String, Integer> mapDescriptor =
            new MapStateDescriptor<>("merchant-counts", String.class, Integer.class);
        merchantCounts = getRuntimeContext().getMapState(mapDescriptor);
    }

    @Override
    public void processElement(Transaction txn, Context ctx, Collector<Alert> out) {
        Boolean flag = flagState.value();

        if (flag != null && txn.getAmount() > 10000) {
            out.collect(new Alert(txn.getCustomerId(), "HIGH_AMOUNT_AFTER_FLAG"));
        }

        if (txn.getAmount() < 1.0) {
            flagState.update(true);
            // Timer to clear flag after 1 minute
            ctx.timerService().registerEventTimeTimer(
                txn.getTimestamp() + 60000);
        }
    }

    @Override
    public void onTimer(long timestamp, OnTimerContext ctx, Collector<Alert> out) {
        flagState.clear();
    }
}

Checkpoint Operation
================================

[Source] ──▶ [Map] ──▶ [KeyBy] ──▶ [Window] ──▶ [Sink]

1. JobManager: inject checkpoint barrier
   Source ──▶ |barrier| ──▶ Map ──▶ KeyBy ──▶ Window ──▶ Sink

2. State snapshot taken as barrier passes each operator
   Source(snapshot) ──▶ Map(snapshot) ──▶ ...

3. All operator snapshots complete → checkpoint success

4. On failure: restore from last successful checkpoint

// Checkpoint configuration
env.enableCheckpointing(60000);  // 60-second interval
env.getCheckpointConfig().setCheckpointingMode(CheckpointingMode.EXACTLY_ONCE);
env.getCheckpointConfig().setMinPauseBetweenCheckpoints(30000);
env.getCheckpointConfig().setCheckpointTimeout(120000);
env.getCheckpointConfig().setMaxConcurrentCheckpoints(1);

// State Backend configuration
env.setStateBackend(new EmbeddedRocksDBStateBackend());
env.getCheckpointConfig().setCheckpointStorage("s3://checkpoints/flink/");

use_kafka_streams_when:
  - "Simple Kafka-to-Kafka processing"
  - "Separate cluster operations are burdensome"
  - "Embedding inside microservices"
  - "Medium throughput is sufficient"
  - "Team is familiar with Kafka ecosystem"

use_flink_when:
  - "Complex event processing (CEP)"
  - "Connecting diverse sources/sinks"
  - "Large-scale state management needed"
  - "Advanced window functions needed"
  - "SQL-based stream processing"
  - "Ultra-high throughput required"

// Debezium MySQL Source Connector
{
  "name": "mysql-source-orders",
  "config": {
    "connector.class": "io.debezium.connector.mysql.MySqlConnector",
    "database.hostname": "mysql-primary",
    "database.port": "3306",
    "database.user": "debezium",
    "database.password": "dbz-password",
    "database.server.id": "1",
    "topic.prefix": "cdc",
    "database.include.list": "order_service",
    "table.include.list": "order_service.orders,order_service.order_items",
    "schema.history.internal.kafka.bootstrap.servers": "broker:9092",
    "schema.history.internal.kafka.topic": "schema-changes.orders",
    "transforms": "route",
    "transforms.route.type": "org.apache.kafka.connect.transforms.RegexRouter",
    "transforms.route.regex": "cdc\\.order_service\\.(.*)",
    "transforms.route.replacement": "order.$1.cdc"
  }
}

// Elasticsearch Sink Connector
{
  "name": "es-sink-orders",
  "config": {
    "connector.class": "io.confluent.connect.elasticsearch.ElasticsearchSinkConnector",
    "connection.url": "http://elasticsearch:9200",
    "topics": "order.orders.cdc",
    "type.name": "_doc",
    "key.ignore": "false",
    "schema.ignore": "true",
    "behavior.on.null.values": "delete",
    "write.method": "upsert",
    "transforms": "extractKey",
    "transforms.extractKey.type": "org.apache.kafka.connect.transforms.ExtractField$Key",
    "transforms.extractKey.field": "order_id"
  }
}

Debezium CDC Flow
================================

[MySQL] ──binlog──▶ [Debezium Connector] ──▶ [Kafka Topic]
                                                    │
                                    ┌───────────────┤
                                    ▼               ▼
                              [Flink Job]    [ES Sink Connector]
                                    │               │
                                    ▼               ▼
                              [Kafka Topic]  [Elasticsearch]

CDC Event Structure:
- before: record before change (UPDATE/DELETE)
- after: record after change (INSERT/UPDATE)
- source: source DB metadata
- op: operation type (c=create, u=update, d=delete, r=read)

# Producer performance optimization
batch.size=65536              # 64KB (default 16KB)
linger.ms=10                  # Wait 10ms (batch efficiency)
buffer.memory=67108864        # 64MB
compression.type=lz4          # Compression (saves network/disk)
acks=all                      # Reliability
max.in.flight.requests.per.connection=5  # Pipelining
send.buffer.bytes=131072      # Socket buffer

# Consumer performance optimization
fetch.min.bytes=1048576       # 1MB (batch efficiency)
fetch.max.wait.ms=500         # Wait up to 500ms
max.poll.records=500          # Records per poll
max.partition.fetch.bytes=1048576  # 1MB per partition
session.timeout.ms=30000      # Session timeout
heartbeat.interval.ms=10000   # Heartbeat interval
max.poll.interval.ms=300000   # Max poll interval 5min

# Broker performance optimization
num.partitions=12                    # Default partition count
default.replication.factor=3         # Replication factor
num.io.threads=8                     # I/O threads
num.network.threads=3                # Network threads
num.replica.fetchers=4               # Replica fetchers
socket.send.buffer.bytes=102400      # Socket send buffer
socket.receive.buffer.bytes=102400   # Socket receive buffer
log.segment.bytes=1073741824         # 1GB segments
log.retention.hours=168              # 7-day retention
log.retention.bytes=-1               # No size limit

Partition Count Formula
================================

Required throughput: 100MB/s
Single partition throughput: ~10MB/s (Producer)
                             ~5MB/s (Consumer)

Producer perspective: 100MB/s / 10MB/s = 10 partitions
Consumer perspective: 100MB/s / 5MB/s = 20 partitions

→ Minimum 20 partitions needed

Additional considerations:
- Match consumer instance count (partitions >= consumers)
- Partition count can increase but cannot decrease
- Too many partitions: metadata overhead, rebalancing delay
- Recommendation: 6-50 per topic

# Key Kafka metrics
broker_metrics:
  - name: "UnderReplicatedPartitions"
    description: "Partitions fallen out of ISR"
    alert_threshold: "> 0"
    severity: critical

  - name: "ActiveControllerCount"
    description: "Number of active controllers"
    alert_threshold: "!= 1"
    severity: critical

  - name: "OfflinePartitionsCount"
    description: "Number of offline partitions"
    alert_threshold: "> 0"
    severity: critical

  - name: "RequestsPerSec"
    description: "Requests per second"
    alert_threshold: "> 10000"
    severity: warning

producer_metrics:
  - name: "record-send-rate"
    description: "Records sent per second"

  - name: "record-error-rate"
    description: "Record errors per second"
    alert_threshold: "> 0"

  - name: "request-latency-avg"
    description: "Average request latency"
    alert_threshold: "> 100ms"

consumer_metrics:
  - name: "records-lag-max"
    description: "Maximum consumer lag"
    alert_threshold: "> 10000"
    severity: warning

  - name: "records-consumed-rate"
    description: "Records consumed per second"

  - name: "commit-latency-avg"
    description: "Average commit latency"

Consumer Lag Calculation
================================

Partition 0:
  Log End Offset (LEO): 1000  (latest message)
  Consumer Offset:       850  (last committed)
  Lag: 1000 - 850 = 150

Partition 1:
  LEO: 2000
  Consumer Offset: 1950
  Lag: 50

Total Lag = 150 + 50 = 200 messages

# Burrow (LinkedIn's Consumer Lag Monitoring)
burrow_config:
  general:
    pidfile: "/var/run/burrow.pid"
    stdout-logfile: "/var/log/burrow.log"

  cluster:
    kafka-prod:
      class-name: kafka
      servers: ["broker1:9092", "broker2:9092", "broker3:9092"]
      topic-refresh: 120
      offset-refresh: 30

  consumer:
    kafka-prod:
      class-name: kafka
      cluster: kafka-prod
      servers: ["broker1:9092", "broker2:9092"]
      group-denylist: "^(console-consumer|_)"
      start-latest: true

  notifier:
    slack:
      class-name: http
      url-open: "https://hooks.slack.com/services/xxx/yyy/zzz"
      template-open: |
        Consumer group lag alert:
        Group: {{.GroupName}}
        Status: {{.Status}}
      send-close: true

Kafka Monitoring Dashboard Panels
================================

1. Cluster Overview
   - Broker count / status
   - Total partition count
   - Under-replicated partitions
   - Active Controller

2. Throughput
   - Messages In/Out per sec
   - Bytes In/Out per sec
   - Requests per sec

3. Consumer Groups
   - Consumer Lag (per group)
   - Consume Rate
   - Rebalance count

4. Latency
   - Produce Request Latency (p50, p99)
   - Fetch Request Latency
   - End-to-End Latency

5. Resources
   - Disk Usage per Broker
   - Network I/O
   - CPU / Memory