Choosing Delivery Semantics¶

Selecting the appropriate delivery semantic involves balancing reliability requirements against performance and complexity costs. This document provides a decision framework for choosing between at-most-once, at-least-once, and exactly-once semantics.

Decision Framework¶

Primary Decision Tree¶

Decision Matrix¶

Requirement	At-Most-Once	At-Least-Once	Exactly-Once
No data loss (acks=all, ISR ok, unclean leader election disabled)	❌	✅	✅
No duplicates	✅	❌	✅
High throughput	✅	✅	⚠️
Low latency	✅	✅	⚠️
Simple implementation	✅	✅	❌
Kafka-only	✅	✅	✅
External systems	✅	✅	⚠️

Legend: ✅ Supported | ⚠️ With constraints | ❌ Not supported

Kafka exactly-once applies to Kafka clients and Kafka Streams transactions. End-to-end exactly-once with external systems still requires idempotent or transactional integration.

Use Case Analysis¶

At-Most-Once Use Cases¶

Use Case	Why At-Most-Once
IoT telemetry	High-volume; individual readings expendable
Application metrics	Aggregate accuracy tolerates occasional loss
Real-time gaming	Next update arrives quickly
Log streaming	Missing lines rarely block debugging

At-Least-Once Use Cases¶

Use Case	Why At-Least-Once	Duplicate Handling
Order processing	Orders must not be lost	Order ID deduplication
User events	User actions must be captured	Event ID + timestamp
Financial transactions	Money movement must be recorded	Transaction ID
Email notifications	Users must receive communications	Email dedup by user+type

Exactly-Once Use Cases¶

Use Case	Why Exactly-Once	Alternative
Kafka Streams aggregates	SUM/COUNT must be exact	None (use EOS)
Balance calculations	Duplicate credits/debits cause errors	Strong idempotency
Usage metering	Billing must be accurate	Dedup with strong guarantees
Vote counting	Each vote counted once	Dedup table with unique constraint

Semantic Selection by Data Type¶

Classification Guide¶

Data Type	Typical Semantic	Rationale
Metrics/telemetry	At-most-once	Volume, expendability
Logs	At-most-once	Volume, non-critical
User activity	At-least-once	Cannot lose, naturally deduped
Business transactions	At-least-once	Critical, idempotent design
Financial records	Exactly-once or strong at-least-once	Accuracy critical
Aggregated state	Exactly-once	Correctness required

Data Criticality Assessment¶

Cost-Benefit Analysis¶

Performance Cost¶

Semantic	Latency Overhead	Throughput Impact	Resource Usage
At-most-once	Baseline	Baseline	Low
At-least-once	+2-10ms (workload-dependent)	85-95% (workload-dependent)	Medium
Exactly-once	+10-50ms (workload-dependent)	50-80% (workload-dependent)	High

Actual performance impact depends on batching, compression, replication, and disk/network latency.

Implementation Cost¶

Aspect	At-Most-Once	At-Least-Once	Exactly-Once
Code complexity	Simple	Moderate	Complex
Testing effort	Low	Medium	High
Debugging difficulty	Easy	Moderate	Challenging
Operational overhead	Low	Medium	High

Total Cost of Ownership¶

Semantic	Development	Operations	Infrastructure
At-most-once	Low	Low	Low
At-least-once	Medium	Medium	Medium
Exactly-once	High	High	High

Migration Strategies¶

Upgrading Semantics¶

At-Most-Once → At-Least-Once¶

Producer changes:

# Before
acks=0
retries=0

# After
acks=all
retries=2147483647
enable.idempotence=true

Consumer changes:

// Before: commit first
consumer.poll(timeout);
consumer.commitSync();
for (record : records) process(record);

// After: process first
consumer.poll(timeout);
for (record : records) process(record);
consumer.commitSync();

Additional requirements: - Implement idempotent consumer logic - Add message ID tracking or natural idempotence - Idempotence requires acks=all and max.in.flight.requests.per.connection<=5

At-Least-Once → Exactly-Once¶

Producer changes:

# Before
acks=all
enable.idempotence=true

# After
acks=all
enable.idempotence=true
transactional.id=my-app-instance-1

Consumer changes:

# Before
enable.auto.commit=false

# After
enable.auto.commit=false
isolation.level=read_committed

Code changes:

// Before
for (record : records) {
    process(record);
    producer.send(output);
}
consumer.commitSync();

// After
producer.initTransactions();

producer.beginTransaction();
for (record : records) {
    process(record);
    producer.send(output);
}
producer.sendOffsetsToTransaction(offsets, consumer.groupMetadata());
producer.commitTransaction();

Hybrid Approaches¶

Topic-Level Semantics¶

Different topics may require different semantics within the same application.

// Multiple producers with different configurations
Producer<String, String> metricsProducer = createProducer(acks=0);
Producer<String, String> ordersProducer = createProducer(acks=all);
Producer<String, String> paymentsProducer = createTransactionalProducer();

// Route by data type
switch (eventType) {
    case METRIC:
        metricsProducer.send(record);  // Fire and forget
        break;
    case ORDER:
        ordersProducer.send(record, callback);  // With retry
        break;
    case PAYMENT:
        paymentsProducer.beginTransaction();
        paymentsProducer.send(record);
        paymentsProducer.commitTransaction();
        break;
}

Tiered Processing¶

Anti-Patterns¶

Over-Engineering¶

Under-Engineering¶

Inconsistent Semantics¶

Decision Checklist¶

Before Choosing¶

[ ] What is the business impact of losing a message?
[ ] What is the business impact of processing duplicates?
[ ] What is the acceptable latency?
[ ] What is the required throughput?
[ ] Does the consumer have natural idempotence?
[ ] Are external systems involved?
[ ] What is the team's operational capability?

Semantic Selection¶

If...	Then use...
Loss acceptable, throughput critical	At-most-once
Loss unacceptable, can handle duplicates	At-least-once
Loss and duplicates unacceptable, Kafka-only	Exactly-once
Loss and duplicates unacceptable, external systems	At-least-once + idempotent sink

Post-Selection Validation¶

[ ] Performance tested under expected load
[ ] Failure scenarios tested
[ ] Monitoring in place for semantic violations
[ ] Runbooks for common issues
[ ] Team trained on operational procedures

Summary Table¶

Semantic	Best For	Avoid For
At-most-once	Telemetry, logs, real-time updates	Transactions, orders, critical events
At-least-once	Most business events, general purpose	When duplicates cause financial impact
Exactly-once	Aggregations, billing, financial calculations	Logs, metrics, non-critical data

At-Most-Once - Fire and forget patterns
At-Least-Once - Retry with idempotent consumers
Exactly-Once - Transactional processing
Delivery Semantics Overview - Semantic definitions