At-Most-Once Delivery

At-most-once delivery guarantees that messages are delivered zero or one time, never more. Messages may be lost but are never duplicated. This semantic provides the highest performance but lowest reliability.

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Semantic Definition

Property	Guarantee
Delivery count	0 or 1
Message loss	Possible
Duplicates	Never
Ordering	Preserved within partition

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Producer Implementation

Fire-and-Forget Pattern

The producer sends messages without waiting for acknowledgment.

Configuration

# At-most-once producer configuration
acks=0                              # No broker acknowledgment
retries=0                           # No retry on failure
buffer.memory=67108864              # 64MB buffer
linger.ms=5                         # Batch for 5ms
batch.size=32768                    # 32KB batch size
max.in.flight.requests.per.connection=1000  # High parallelism

Configuration Explanation

Configuration	Value	Rationale
acks=0	0	Producer does not wait for broker acknowledgment
retries=0	0	No retry; failed sends are lost
linger.ms	5	Small batch window for throughput
batch.size	32KB	Reasonable batch size
max.in.flight.requests.per.connection	1000	Maximum parallelism

Java Producer Example

Properties props = new Properties();
props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "kafka:9092");
props.put(ProducerConfig.ACKS_CONFIG, "0");
props.put(ProducerConfig.RETRIES_CONFIG, 0);
props.put(ProducerConfig.LINGER_MS_CONFIG, 5);
props.put(ProducerConfig.BATCH_SIZE_CONFIG, 32768);
props.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
props.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer.class);

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

// Fire and forget - no callback, no Future.get()
for (Metric metric : metrics) {
    producer.send(new ProducerRecord<>("metrics", metric.key(), metric.toJson()));
}

// Buffer flush (optional, at shutdown)
producer.flush();
producer.close();

Performance Characteristics

Metric	At-Most-Once	At-Least-Once	Exactly-Once
Latency (p50)	~1ms	~5ms	~20ms
Throughput	Highest	High	Moderate
CPU overhead	Lowest	Low	Higher
Network round trips	0	1+	Multiple

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Consumer Implementation

Commit-Before-Process Pattern

At-most-once consumers commit offsets before processing, ensuring no redelivery but risking message loss on failure.

Consumer Configuration

# At-most-once consumer configuration
enable.auto.commit=false            # Manual commit control
auto.offset.reset=latest            # Skip old messages on new consumer
max.poll.records=500                # Batch size

Java Consumer Example

Properties props = new Properties();
props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "kafka:9092");
props.put(ConsumerConfig.GROUP_ID_CONFIG, "metrics-processor");
props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, false);
props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "latest");
props.put(ConsumerConfig.MAX_POLL_RECORDS_CONFIG, 500);
props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);

KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
consumer.subscribe(Collections.singletonList("metrics"));

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

    if (!records.isEmpty()) {
        // Commit BEFORE processing (at-most-once)
        consumer.commitSync();

        // Process after commit - failure here loses messages
        for (ConsumerRecord<String, String> record : records) {
            processMetric(record.value());
        }
    }
}

Auto-Commit Alternative

Using auto-commit for at-most-once semantics (not recommended due to timing complexity):

enable.auto.commit=true
auto.commit.interval.ms=100         # Very frequent commits

Auto-Commit Timing

Auto-commit timing does not guarantee at-most-once semantics. Manual commit-before-process provides explicit control.

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Failure Scenarios

Producer Failures

Failure	Outcome	Mitigation
Network timeout	Message lost	Accept loss
Broker unavailable	Message lost	Accept loss
Serialization error	Message dropped	Log error
Buffer full	Block or exception	Increase buffer
Producer crash	Buffer lost	Accept loss

Consumer Failures

Failure Point	Records Affected	Outcome
After commit, before process	All polled records	Lost
During processing	Remaining unprocessed	Lost
After processing	None	No impact

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Use Cases

High-Volume Telemetry

Why at-most-once works:

• High data volume means individual points are expendable
• Aggregates (averages, percentiles) remain accurate
• Next reading arrives within seconds
• Throughput matters more than individual reliability

Real-Time Gaming

Data Type	Loss Impact	Semantic
Player position updates	Old data stale anyway	At-most-once
Kill/death events	Must be recorded	At-least-once
Chat messages	Must not be lost	At-least-once
Heartbeats	Expendable	At-most-once

Log Streaming

# Log forwarder configuration (at-most-once acceptable)
acks=0
retries=0
compression.type=lz4               # High compression
linger.ms=100                      # Batch aggressively
batch.size=1048576                 # 1MB batches

Rationale:

• Log volume often exceeds retention capacity
• Missing a few log lines rarely impacts debugging
• Throughput and cost are primary concerns
• Duplicates would complicate log analysis

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Monitoring

Producer Metrics

Metric	Alert Condition	Action
record-send-total	Dropping significantly	Check connectivity
record-error-rate	> 1%	Investigate errors
buffer-available-bytes	< 10% of total	Increase buffer or reduce rate
batch-size-avg	Very small	Increase linger.ms

Consumer Metrics

Metric	Alert Condition	Action
records-consumed-total	Lower than expected	Check producer health
commit-latency-avg	High	Check broker health
records-lag	Growing	Scale consumers

Estimating Loss Rate

At-most-once systems should monitor approximate loss:

// Producer-side counter
AtomicLong sendAttempts = new AtomicLong(0);
AtomicLong sendErrors = new AtomicLong(0);

producer.send(record, (metadata, exception) -> {
    sendAttempts.incrementAndGet();
    if (exception != null) {
        sendErrors.incrementAndGet();
    }
});

// Estimated loss rate
double lossRate = (double) sendErrors.get() / sendAttempts.get();

Loss Estimation

With acks=0, the callback exception only captures local errors. Network and broker-side losses are not reported and must be estimated through consumer-side analysis.

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Anti-Patterns

Using At-Most-Once for Critical Data

// WRONG: Financial transactions must not be lost
producer.send(new ProducerRecord<>("payments", payment.toJson()));
// No ack, no retry - payment may be lost!

Instead: Use at-least-once with idempotent processing:

props.put(ProducerConfig.ACKS_CONFIG, "all");
props.put(ProducerConfig.RETRIES_CONFIG, Integer.MAX_VALUE);
props.put(ProducerConfig.ENABLE_IDEMPOTENCE_CONFIG, true);

Mixing Semantics Within Topic

Instead: Separate topics by reliability requirement:

metrics-realtime    → at-most-once (high volume, low value)
metrics-critical    → at-least-once (auditing, billing)

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Comparison with Other Semantics

Aspect	At-Most-Once	At-Least-Once	Exactly-Once
Message loss	Possible	Never	Never
Duplicates	Never	Possible	Never
Latency	Lowest	Low	Higher
Throughput	Highest	High	Moderate
Complexity	Simple	Moderate	Complex
Use case	Telemetry, logs	Most workloads	Financial, billing

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Migration to Stronger Semantics

When requirements change, migrating from at-most-once to stronger semantics:

Step 1: Enable Acknowledgments

# Change from acks=0 to acks=1 or acks=all
acks=all

Step 2: Enable Retries

retries=2147483647
retry.backoff.ms=100

Step 3: Consider Idempotence

enable.idempotence=true

Step 4: Update Consumer

// Change from commit-before-process to process-before-commit
for (ConsumerRecord<String, String> record : records) {
    process(record);  // Process first
}
consumer.commitSync();  // Then commit

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Related Documentation

• Delivery Semantics Overview - Semantic comparison
• At-Least-Once - Retry-based delivery
• Exactly-Once - Transactional processing
• Producer Configuration - Producer settings