Kafka Event-Driven Design Review

Quick Reference

If you need to…	Go to
Understand what this skill covers	§1 Scope
Check mandatory prerequisites	§2 Mandatory Gates
Choose review depth	§3 Depth Selection
Handle incomplete context	§4 Degradation Modes
Evaluate design item by item	§5 Design Checklist
Choose partition and key strategy	§6 Partition Design
Avoid common Kafka mistakes	§7 Anti-Examples
Score the review result	§8 Scorecard
Format review output	§9 Output Contract
Deep-dive event schema patterns	references/event-schema-patterns.md
Understand consumer failure modes	references/consumer-failure-modes.md

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§1 Scope

In scope — Kafka event-driven architecture for production backend services:

• Topic design (naming, partition count, replication factor, retention)
• Partition key strategy (ordering guarantees, hot partition avoidance)
• Event schema design (Avro/Protobuf/JSON Schema, schema evolution, compatibility)
• Producer configuration (acks, retries, idempotence, transactional producers)
• Consumer group design (assignment strategy, rebalance handling, commit strategy)
• Idempotent consumption (deduplication, idempotency keys, exactly-once semantics)
• Dead letter queue (DLQ) and retry patterns
• Backpressure and consumer lag management
• Schema Registry integration and compatibility modes

Out of scope — delegate to dedicated skills:

• Kafka cluster operations, broker config, ZooKeeper/KRaft migration → Kafka ops
• Application code changes unrelated to Kafka → go-code-reviewer
• General API design → api-design

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§2 Mandatory Gates

Execute gates sequentially. Each gate has a STOP condition.

Gate 1: Context Collection

Item	Why it matters	If unknown
Kafka version (2.x / 3.x)	Exactly-once, cooperative rebalance features vary	Assume 2.8 (conservative)
Schema format (Avro / Protobuf / JSON / none)	Determines evolution strategy and registry needs	Must clarify
Ordering requirement	Per-entity? Per-tenant? Global? None?	Must clarify — drives partition key
Delivery guarantee needed	At-most-once / At-least-once / Exactly-once	Assume at-least-once
Throughput estimate (events/sec)	Determines partition count and consumer scaling	Ask; assume moderate
Consumer count / group topology	Single consumer group? Multiple? Fan-out?	Must clarify
Retention policy	Time-based / size-based / compacted	Assume 7 days time-based
Schema Registry	Confluent / AWS Glue / Apicurio / none	Ask; critical for evolution

STOP: Cannot determine what events are being produced/consumed (no domain context). Clarify before proceeding.

PROCEED: At least event type, ordering requirement, and delivery guarantee are known.

Gate 2: Scope Classification

Mode	Trigger	Output
review	User provides existing Kafka config/code	Safety analysis with findings
design	User describes event-driven requirements	Complete topic + schema + consumer design
troubleshoot	User reports issues (lag, duplication, ordering)	Root cause + fix plan

STOP: Not Kafka-related (e.g., REST API design). Redirect to appropriate skill.

PROCEED: Kafka event-driven intent confirmed.

Gate 3: Risk Classification

Risk	Definition	Required action
SAFE	Single topic, simple consumer, at-least-once	Standard review
WARN	Multi-topic transactions, schema evolution, exactly-once	Off-peak deployment + monitoring
UNSAFE	Partition key change on live topic, consumer group migration, schema breaking change	Staged rollout + rollback plan mandatory

STOP: Any UNSAFE item without mitigation plan.

PROCEED: Every component has risk level and mitigation.

Gate 4: Output Completeness

Before delivering output, verify all §9 Output Contract sections present. §9.9 Uncovered Risks must never be empty.

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§3 Depth Selection

Depth	When to use	Gates	References to load
Lite	Single producer/consumer pair, simple schema	1–4	None
Standard	Multi-consumer topology, schema evolution, DLQ design	1–4	event-schema-patterns.md
Deep	Cross-service event mesh, exactly-once, CQRS/ES pattern	1–4	Both reference files

Force Standard or higher when any signal appears: schema evolution requirement, exactly-once semantics, multi-consumer-group fan-out, partition key redesign, consumer group migration, compacted topics for event sourcing.

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§4 Degradation Modes

When context is incomplete, degrade gracefully — never fabricate assumptions about ordering requirements.

Available context	Mode	What you can do	What you cannot do
Full (version, schema, ordering, delivery, throughput)	Full	Complete design with quantified guarantees	—
Event type + ordering known, infra unknown	Degraded	Schema + consumer design; flag infra unknowns	Partition count, replication recommendations
Only code snippets, no architecture context	Minimal	Static review of producer/consumer patterns	Full topology design
No code (greenfield design)	Planning	Propose event architecture from requirements	Review existing implementation

Hard rule: Never claim "exactly-once" without verifying transactional producer + consumer read-committed isolation + idempotent processing. In Degraded/Minimal mode, flag "delivery guarantee unverified" in §9.9.

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§5 Design Checklist

Execute every item. Mark PASS / WARN / FAIL with evidence.

5.1 Topic Design

1. Topic naming convention — follow {domain}.{entity}.{event-type} or {team}.{service}.{event} pattern. Names should be greppable, meaningful, and avoid generic names like "events" or "messages".

2. Partition count sized for throughput — each partition is the unit of parallelism. Rule of thumb: partitions ≥ max expected consumer instances. Over-partitioning wastes resources; under-partitioning caps throughput. Partition count cannot be decreased (only increased, which breaks key-based ordering).

3. Replication factor ≥ 3 for production topics — min.insync.replicas = 2 with acks = all ensures no data loss on single broker failure. RF=1 is unacceptable for any non-ephemeral data.

4. Retention policy matches use case — time-based (default 7 days) for event streaming; compacted for entity-state topics (latest value per key retained indefinitely); infinite retention for event sourcing.

5.2 Producer Design

5. acks=all + retries + enable.idempotence=true — this is the minimum safe producer config. acks=1 risks data loss on leader failure. acks=0 is fire-and-forget. Idempotent producer (Kafka 0.11+) prevents duplicate writes from retries.

6. Partition key chosen for ordering + distribution — key determines which partition an event goes to. Events with the same key are ordered. Common keys: entity ID (order_id, user_id). Avoid: timestamp (hot partition), null (round-robin = no ordering), high-cardinality unbounded (too many partitions).

7. Event schema includes metadata — every event should carry: event_id (UUID), event_type, timestamp, source_service, correlation_id, schema_version. Without these, debugging and deduplication become impossible.

8. Transactional producer for multi-topic atomicity — if producing to multiple topics must be atomic (e.g., event + outbox), use Kafka transactions (initTransactions, beginTransaction, commitTransaction). Without transactions, partial writes create inconsistency.

5.3 Consumer Design

9. Idempotent consumption — at-least-once delivery means consumers WILL receive duplicates (after rebalance, retry, or producer retry). Every consumer must handle duplicates: deduplication by event_id, idempotent database operations (INSERT ON CONFLICT), or exactly-once via transactional consumer.

10. Commit strategy explicit — enable.auto.commit=true (default) commits offsets periodically, risking reprocessing on crash. For at-least-once: manual commit after processing. For exactly-once: commit offsets within the transaction that processes the event.

11. Dead letter queue (DLQ) for poison messages — messages that repeatedly fail processing must be routed to a DLQ topic instead of blocking the partition. Without DLQ, a single bad message blocks all subsequent messages in that partition indefinitely.

12. Consumer lag monitoring — consumer_lag (latest offset - committed offset) per partition is the primary health metric. Alert on sustained lag > threshold. Use kafka-consumer-groups.sh or metrics exporter. Lag = events waiting to be processed.

5.4 Schema Evolution & Operations

13. Schema compatibility mode set — Schema Registry supports: BACKWARD (new reader, old data), FORWARD (old reader, new data), FULL (both). Choose based on deployment strategy. BACKWARD_TRANSITIVE is safest for most cases. Breaking changes require a new topic.

14. Backpressure handling defined — what happens when consumers can't keep up? Options: scale consumers (up to partition count), increase batch size, apply rate limiting on producer, drop low-priority events. "Consumer crashes under load" is not a strategy.

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§6 Partition Design (Standard + Deep)

Quick decision guide — for schema patterns load references/event-schema-patterns.md.

Ordering need	Partition key	Example
Per-entity ordering	Entity ID	order_id → all events for order 123 in same partition
Per-tenant ordering	Tenant ID	tenant_id → tenant isolation per partition
Per-user ordering	User ID	user_id → user action sequence preserved
No ordering needed	null (round-robin)	Metrics, logs, analytics events
Global ordering	Single partition	Only if throughput is very low (<100 events/sec)

Hot partition detection and mitigation

• Symptom: one partition has 10x the events of others; consumer for that partition lags
• Cause: skewed partition key (e.g., one tenant produces 90% of events)
• Fix: composite key (tenant_id + entity_id), or custom partitioner that spreads hot keys across N partitions while maintaining per-entity ordering

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§7 Anti-Examples

AE-1: Producer with acks=1 for critical business events

// WRONG: acks=1 — leader acknowledges before replication; data lost on leader failure
producer, _ := sarama.NewSyncProducer(brokers, config)
// config.Producer.RequiredAcks = sarama.WaitForLocal  // acks=1
// RIGHT: acks=all + idempotent
config.Producer.RequiredAcks = sarama.WaitForAll
config.Producer.Idempotent = true
config.Net.MaxOpenRequests = 1

AE-2: Consumer without idempotency handling

// WRONG: processes event and assumes it won't be delivered again
func handleOrderCreated(event OrderCreated) {
    db.Insert(event.Order)  // duplicate delivery → duplicate insert → constraint violation or double-charge
}
// RIGHT: idempotent processing
func handleOrderCreated(event OrderCreated) {
    db.Exec("INSERT INTO orders ... ON CONFLICT (id) DO NOTHING", event.Order)
}

AE-3: No dead letter queue — poison message blocks partition

// WRONG: bad message causes infinite retry loop, blocking all events behind it
func consume(msg *sarama.ConsumerMessage) {
    if err := process(msg); err != nil {
        log.Error(err)
        // message is not committed → redelivered forever
    }
}
// RIGHT: route to DLQ after N retries
if retryCount >= maxRetries {
    producer.Send(dlqTopic, msg)
    consumer.CommitMessage(msg)  // advance past the poison message
}

AE-4: Null partition key — loses ordering guarantee

// WRONG: null key → round-robin across partitions → order events scattered
producer.SendMessage(&sarama.ProducerMessage{
    Topic: "order.events",
    Value: sarama.ByteEncoder(data),
    // Key is nil → no ordering guarantee
})
// RIGHT: use entity ID as partition key
producer.SendMessage(&sarama.ProducerMessage{
    Topic: "order.events",
    Key:   sarama.StringEncoder(order.ID),
    Value: sarama.ByteEncoder(data),
})

AE-5: Schema change without compatibility check

// WRONG: removed required field — breaks all existing consumers
// v1: {"order_id": "123", "amount": 99.50, "currency": "USD"}
// v2: {"order_id": "123", "amount": 99.50}  // removed currency!
// RIGHT: add new optional fields, never remove required ones (BACKWARD compatible)

AE-6: Kafka issue reported as application logic bug

-- WRONG: "Bug: some orders processed twice causing double charge"
-- This is a consumer idempotency issue, not application logic.
-- RIGHT: "Consumer lacks idempotency: duplicate Kafka delivery causes double processing"

Extended anti-examples (AE-7 through AE-13) in references/consumer-anti-examples.md.

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§8 Kafka Design Scorecard

Critical — any FAIL means overall FAIL

• Producer uses acks=all + enable.idempotence=true for non-ephemeral events
• Consumer handles duplicate delivery (idempotent processing or exactly-once)
• Dead letter queue exists for poison messages (no infinite retry loops)

Standard — 4 of 5 must pass

• Partition key matches ordering requirement (not null for ordered events)
• Schema includes event metadata (event_id, event_type, timestamp, source)
• Schema compatibility mode configured in Schema Registry
• Consumer lag monitoring with alert threshold defined
• Commit strategy explicitly chosen (auto-commit disabled for at-least-once)

Hygiene — 3 of 4 must pass

• Topic naming follows convention ({domain}.{entity}.{event-type})
• Replication factor ≥ 3 with min.insync.replicas = 2
• Backpressure strategy defined (scale / rate-limit / drop)
• Retention policy matches use case (time-based / compacted / infinite)

Verdict: X/12; Critical: Y/3; Standard: Z/5; Hygiene: W/4. PASS requires: Critical 3/3 AND Standard ≥4/5 AND Hygiene ≥3/4.

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§9 Output Contract

Every design review MUST produce these sections. Write "N/A — [reason]" if inapplicable.

### 9.1 Context Gate
| Item | Value | Source |

### 9.2 Depth & Mode
[Lite/Standard/Deep] × [review/design/troubleshoot] — [rationale]

### 9.3 Risk Assessment
| Component | Risk | Notes |

### 9.4 Architecture Design (Standard/Deep; "N/A — Lite" for Lite)
- Topic topology + partition strategy
- Producer + consumer configuration
- Schema design + evolution strategy

### 9.5 Implementation (topic config, producer/consumer code patterns)

### 9.6 Validation Plan
- End-to-end event flow test
- Duplicate delivery test
- Schema evolution test (produce v2, consume with v1 reader)

### 9.7 Failure Handling
- DLQ routing + monitoring
- Consumer rebalance behavior
- Broker failure recovery

### 9.8 Monitoring & Alerts
- Consumer lag per partition
- DLQ message rate
- Producer error rate, batch size, latency

### 9.9 Uncovered Risks (MANDATORY — never empty)
| Area | Reason | Impact | Follow-up |

Volume rules:

• FAIL: always fully detailed
• WARN: up to 10; overflow to §9.9
• PASS: summary only
• §9.9 minimum: document all assumptions (especially delivery guarantee if unverified)

Scorecard summary (append after §9.9):

Scorecard: X/12 — Critical Y/3, Standard Z/5, Hygiene W/4 — PASS/FAIL
Data basis: [full context | degraded | minimal | planning]

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§10 Reference Loading Guide

Condition	Load
Standard or Deep depth	references/event-schema-patterns.md
Deep depth, or consumer failure/lag signals	references/consumer-failure-modes.md
Extended anti-example matching	references/consumer-anti-examples.md