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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Apache Kafka Masterclass — The Distributed Log</title>
<meta property="og:title" content="Apache Kafka Masterclass — The Distributed Log">
<meta property="og:description" content="Comprehensive guide to Apache Kafka — distributed event streaming, topics, partitions, and real-time data pipelines.">
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.stat:last-child { border-right: none; }
.stat:hover { background: var(--void3); }
.stat-num { font-family: var(--display); font-size: 2.1rem; font-weight: 700; line-height: 1; margin-bottom: 0.2rem; }
.stat-label { font-family: var(--mono); font-size: 0.6rem; color: var(--text3); letter-spacing: 0.1em; text-transform: uppercase; }
/* ── PARTITION DIAGRAM ── */
.partition-vis {
background: var(--void);
border: 1px solid var(--border);
padding: 1.25rem;
font-family: var(--mono);
font-size: 0.72rem;
overflow-x: auto;
}
.part-row { display: flex; align-items: center; gap: 6px; margin-bottom: 8px; }
.part-label { color: var(--text3); width: 100px; flex-shrink: 0; font-size: 0.68rem; letter-spacing: 0.06em; }
.part-seg {
height: 32px;
display: flex; align-items: center; justify-content: center;
font-size: 0.62rem;
font-weight: 500;
border: 1px solid;
flex-shrink: 0;
min-width: 44px;
position: relative;
transition: all 0.15s;
}
.part-seg:hover { transform: translateY(-2px); }
.ps-a { background: var(--sig-dim); border-color: var(--signal2); color: var(--signal); }
.ps-b { background: var(--amb-dim); border-color: var(--amber2); color: var(--amber); }
.ps-c { background: var(--plas-dim); border-color: var(--plasma); color: var(--plasma); }
.ps-r { background: rgba(170,85,255,0.1); border-color: var(--violet); color: var(--violet); font-size: 0.55rem; }
.ps-ho { background: rgba(255,68,85,0.08); border-color: var(--coral); color: var(--coral); }
.part-arrow { color: var(--text3); font-size: 0.9rem; }
.part-end { padding: 6px 10px; border: 1px solid var(--border); color: var(--text3); font-size: 0.65rem; background: var(--void3); white-space: nowrap; }
.part-offset { position: absolute; top: 1px; right: 2px; font-size: 0.5rem; opacity: 0.6; color: var(--text3); }
/* ── FLOW DIAGRAM ── */
.flow { display: flex; flex-direction: column; gap: 2px; }
.flow-row { display: flex; align-items: center; gap: 6px; }
.flow-box {
padding: 7px 14px;
border: 1px solid;
font-family: var(--mono);
font-size: 0.72rem;
display: flex; align-items: center; gap: 6px;
transition: all 0.15s; cursor: default;
white-space: nowrap;
}
.flow-box:hover { transform: translateY(-1px); }
.fb-sig { background: var(--sig-dim); border-color: var(--signal2); color: var(--signal); }
.fb-amber { background: var(--amb-dim); border-color: var(--amber2); color: var(--amber); }
.fb-plasma { background: var(--plas-dim); border-color: var(--plasma); color: var(--plasma); }
.fb-violet { background: var(--vio-dim); border-color: var(--violet); color: var(--violet); }
.fb-coral { background: var(--cor-dim); border-color: var(--coral); color: var(--coral); }
.fb-dim { background: var(--void3); border-color: var(--border); color: var(--text2); }
.flow-arr { color: var(--text3); font-size: 0.9rem; flex-shrink: 0; }
.flow-note { font-family: var(--mono); font-size: 0.62rem; color: var(--text3); padding-left: 4px; }
/* ── PERF METER ── */
.perf-row { display: grid; grid-template-columns: 180px 1fr 100px; gap: 1rem; align-items: center; padding: 0.55rem 0; border-bottom: 1px solid var(--border2); }
.perf-label { font-family: var(--mono); font-size: 0.73rem; color: var(--text2); }
.perf-track { height: 6px; background: var(--void3); border: 1px solid var(--border); overflow: hidden; }
.perf-fill { height: 100%; transition: width 1.2s cubic-bezier(0.4,0,0.2,1); }
.perf-val { font-family: var(--mono); font-size: 0.7rem; color: var(--text3); text-align: right; }
/* ── SEVERITY ROW ── */
.sev-row { display: grid; grid-template-columns: 120px 1fr auto; gap: 1rem; align-items: center; padding: 0.6rem 0.85rem; border-bottom: 1px solid var(--border2); border-left: 2px solid; }
.sev-row:hover { background: var(--void3); }
.sev-name { font-family: var(--mono); font-size: 0.75rem; }
.sev-desc { font-size: 0.83rem; color: var(--text2); }
.sev-row.s-sig { border-left-color: var(--signal); }
.sev-row.s-amber { border-left-color: var(--amber); }
.sev-row.s-coral { border-left-color: var(--coral); }
.sev-row.s-violet { border-left-color: var(--violet); }
.sev-row.s-plasma { border-left-color: var(--plasma); }
/* ── SEPARATOR ── */
.sep { border: none; border-top: 1px solid var(--border); margin: 2rem 0; }
.sep-glow { border-top-color: rgba(0,255,136,0.2); }
/* ── TEXT ── */
p { margin-bottom: 0.9rem; color: var(--text2); font-weight: 300; }
p:last-child { margin-bottom: 0; }
h3 { font-family: var(--display); font-size: 1.5rem; font-weight: 600; letter-spacing: 0.05em; color: var(--text); margin: 1.75rem 0 0.6rem; }
h3:first-child { margin-top: 0; }
h4 { font-family: var(--mono); font-size: 0.72rem; color: var(--text3); letter-spacing: 0.12em; text-transform: uppercase; margin: 1.2rem 0 0.5rem; }
ul, ol { padding-left: 1.5rem; color: var(--text2); font-weight: 300; }
ul li, ol li { margin-bottom: 0.35rem; }
strong { color: var(--text); font-weight: 600; }
::-webkit-scrollbar { width: 5px; height: 5px; }
::-webkit-scrollbar-track { background: var(--void2); }
::-webkit-scrollbar-thumb { background: var(--void4); }
</style>
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<button class="nav-btn active" onclick="show('signal',this)">SIGNAL</button>
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<!-- ══════════════════════════════════════════════
SECTION 1 — SIGNAL: ARCHITECTURE
══════════════════════════════════════════════ -->
<section id="signal" class="active">
<div class="section-header">
<div class="section-eyebrow">Architecture</div>
<div class="section-title">THE <em>DISTRIBUTED LOG</em></div>
<div class="section-sub">Kafka is not a message queue. It is an immutable, append-only, distributed commit log. Every event ever written is retained, replayable, and reprocessable by any number of consumers independently. This one design decision makes it fundamentally different from RabbitMQ, SQS, and every traditional broker.</div>
</div>
<div class="stat-strip">
<div class="stat"><div class="stat-num" style="color:var(--signal)">10M+</div><div class="stat-label">Msgs/sec/broker</div></div>
<div class="stat"><div class="stat-num" style="color:var(--amber)">∞</div><div class="stat-label">Consumer Replay</div></div>
<div class="stat"><div class="stat-num" style="color:var(--plasma)">O(1)</div><div class="stat-label">Read Complexity</div></div>
<div class="stat"><div class="stat-num" style="color:var(--violet)">RPF</div><div class="stat-label">Replicated Partitioned Log</div></div>
<div class="stat"><div class="stat-num" style="color:var(--signal)">KRaft</div><div class="stat-label">No ZooKeeper (3.x+)</div></div>
<div class="stat"><div class="stat-num" style="color:var(--coral)">EOS</div><div class="stat-label">Exactly-Once</div></div>
</div>
<div class="g2">
<div>
<h3>Anatomy of a Topic</h3>
<div class="partition-vis">
<div style="font-family:var(--mono);font-size:0.65rem;color:var(--signal);margin-bottom:1rem;letter-spacing:0.1em">TOPIC: orders — 3 partitions, RF=2, retention=7d</div>
<div class="part-row">
<div class="part-label">Partition 0</div>
<div class="part-seg ps-a" style="width:44px">0<div class="part-offset">off</div></div>
<div class="part-seg ps-a" style="width:44px">1</div>
<div class="part-seg ps-a" style="width:44px">2</div>
<div class="part-seg ps-a" style="width:44px">3</div>
<div class="part-seg ps-a" style="width:44px">4</div>
<div class="part-seg ps-a" style="width:44px">5 ←</div>
<div class="part-arrow">→</div>
<div class="part-end">LEO:6 | Broker 1 (Leader)</div>
</div>
<div class="part-row">
<div class="part-label" style="color:var(--text3)"> ↳ replica</div>
<div class="part-seg ps-r" style="width:44px">0</div>
<div class="part-seg ps-r" style="width:44px">1</div>
<div class="part-seg ps-r" style="width:44px">2</div>
<div class="part-seg ps-r" style="width:44px">3</div>
<div class="part-seg ps-r" style="width:44px">4</div>
<div class="part-seg ps-r" style="width:44px">5</div>
<div class="part-arrow"> </div>
<div class="part-end">ISR | Broker 2</div>
</div>
<div class="part-row" style="margin-top:6px">
<div class="part-label">Partition 1</div>
<div class="part-seg ps-b" style="width:44px">0</div>
<div class="part-seg ps-b" style="width:44px">1</div>
<div class="part-seg ps-b" style="width:44px">2</div>
<div class="part-seg ps-b" style="width:44px">3 ←</div>
<div class="part-arrow">→</div>
<div class="part-end">LEO:4 | Broker 2 (Leader)</div>
</div>
<div class="part-row">
<div class="part-label" style="color:var(--text3)"> ↳ replica</div>
<div class="part-seg ps-r" style="width:44px">0</div>
<div class="part-seg ps-r" style="width:44px">1</div>
<div class="part-seg ps-r" style="width:44px">2</div>
<div class="part-seg ps-r" style="width:44px">3</div>
<div class="part-arrow"> </div>
<div class="part-end">ISR | Broker 3</div>
</div>
<div class="part-row" style="margin-top:6px">
<div class="part-label">Partition 2</div>
<div class="part-seg ps-c" style="width:44px">0</div>
<div class="part-seg ps-c" style="width:44px">1</div>
<div class="part-seg ps-c" style="width:44px">2</div>
<div class="part-seg ps-c" style="width:44px">3</div>
<div class="part-seg ps-c" style="width:44px">4</div>
<div class="part-seg ps-c" style="width:44px">5</div>
<div class="part-seg ps-c" style="width:44px">6 ←</div>
<div class="part-arrow">→</div>
<div class="part-end">LEO:7 | Broker 3 (Leader)</div>
</div>
<div style="margin-top:1rem;font-size:0.62rem;color:var(--text3);display:flex;gap:1.5rem">
<span><span style="color:var(--signal)">■</span> Leader</span>
<span><span style="color:var(--violet)">■</span> ISR Replica</span>
<span><span style="color:var(--amber)">←</span> HW (High Watermark)</span>
<span>LEO = Log End Offset</span>
</div>
</div>
<div class="callout c-amber">
<strong>High Watermark — The Visibility Contract</strong>
Consumers can only read up to the <strong>High Watermark</strong> (HW) — the last offset replicated to ALL in-sync replicas (ISR). A message written to the leader but not yet replicated is invisible to consumers. This is not a bug — it's the consistency guarantee that prevents reading data that might be lost on leader failover.
</div>
</div>
<div>
<h3>The Full Cluster Topology</h3>
<div class="card" style="padding:1rem;margin-bottom:1rem">
<div class="flow">
<div class="flow-row">
<div class="flow-box fb-amber">⚡ Producer App</div>
<div class="flow-arr">──────────────────→</div>
<div class="flow-box fb-sig">BROKER 1 (Controller)</div>
</div>
<div class="flow-row" style="margin-left:180px">
<div class="flow-arr">↕ KRaft Replication</div>
</div>
<div class="flow-row" style="justify-content:center;gap:8px">
<div class="flow-box fb-dim">BROKER 2</div>
<div class="flow-box fb-dim">BROKER 3</div>
<div class="flow-box fb-dim">BROKER N</div>
</div>
<div class="flow-row" style="justify-content:center">
<div class="flow-arr">↓ Poll</div>
</div>
<div class="flow-row" style="justify-content:center;gap:8px">
<div class="flow-box fb-plasma">Consumer A</div>
<div class="flow-box fb-plasma">Consumer B</div>
<div class="flow-box fb-plasma">Consumer C</div>
</div>
<div class="flow-note" style="text-align:center;margin-top:6px">↑ Same group = partitions split between them</div>
</div>
</div>
<h3>Message Anatomy</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">ProducerRecord fields</span><span class="code-lang">java/python</span></div>
<pre><span class="cm"># Every Kafka message has exactly this structure:</span>
ProducerRecord(
topic <span class="op">=</span> <span class="st">'orders'</span>, <span class="cm"># destination topic</span>
partition <span class="op">=</span> <span class="kw">None</span>, <span class="cm"># None = hash(key) % num_partitions</span>
key <span class="op">=</span> b<span class="st">'customer-uuid'</span>, <span class="cm"># bytes — routing + ordering unit</span>
value <span class="op">=</span> b<span class="st">'{"id":...}'</span>, <span class="cm"># bytes — your payload</span>
headers <span class="op">=</span> [ <span class="cm"># metadata KV pairs</span>
(<span class="st">'X-Source'</span>, b<span class="st">'checkout-svc'</span>),
(<span class="st">'X-Trace-ID'</span>, b<span class="st">'abc-123-def'</span>),
(<span class="st">'Content-Type'</span>,b<span class="st">'application/json'</span>),
],
timestamp <span class="op">=</span> <span class="kw">None</span> <span class="cm"># None = broker-assigned (CreateTime)</span>
)
<span class="cm"># KEY INSIGHT: Key determines partition assignment.
# Same key → same partition → guaranteed ordering for that key.
# No key → round-robin across all partitions.
# Null key after sticky partitioning (batching) completes.</span></pre>
</div>
<h3>KRaft — Kafka Without ZooKeeper</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">KRaft mode (Kafka 3.3+ production-ready)</span><span class="code-lang">properties</span></div>
<pre><span class="cm"># server.properties — combined broker+controller node</span>
process.roles=broker,controller <span class="cm"># this node does both</span>
node.id=1
controller.quorum.voters=1@host1:9093,2@host2:9093,3@host3:9093
listeners=PLAINTEXT://:9092,CONTROLLER://:9093
inter.broker.listener.name=PLAINTEXT
controller.listener.names=CONTROLLER
<span class="cm"># Generate cluster UUID (once, before first start):</span>
<span class="cm"># kafka-storage.sh format --config server.properties \</span>
<span class="cm"># --cluster-id $(kafka-storage.sh random-uuid)</span>
<span class="cm"># What KRaft eliminates:
# ✓ ZooKeeper cluster (3-5 extra VMs)
# ✓ ZK latency on metadata changes
# ✓ Split-brain between ZK and Kafka state
# ✓ 2-phase shutdown/startup coordination
# New limit: 1M+ partitions (ZK limit was ~200K)</span></pre>
</div>
</div>
</div>
</section>
<!-- ══════════════════════════════════════════════
SECTION 2 — PRODUCERS
══════════════════════════════════════════════ -->
<section id="producers">
<div class="section-header">
<div class="section-eyebrow">Write Path</div>
<div class="section-title"><em>PRODUCERS</em> — WRITE GUARANTEE MATRIX</div>
<div class="section-sub">The producer is deceptively complex. Every config choice is a tradeoff between throughput, latency, durability, and ordering. Most production outages trace back to misunderstood producer settings.</div>
</div>
<div class="g2">
<div>
<h3>The Three Delivery Guarantees</h3>
<div class="sev-row s-coral">
<div class="sev-name" style="color:var(--coral)">acks=0</div>
<div class="sev-desc"><strong>Fire and forget.</strong> Producer sends, doesn't wait for acknowledgement. Fastest throughput, zero durability. Data loss is guaranteed on any broker failure. Use only for: metrics, telemetry, logs where loss is acceptable.</div>
<div><span class="badge b-coral">AT-MOST-ONCE</span></div>
</div>
<div class="sev-row s-amber">
<div class="sev-name" style="color:var(--amber)">acks=1</div>
<div class="sev-desc"><strong>Leader acknowledges.</strong> Message written to leader log, ack sent. If leader dies before replication, message is lost. Default in many libraries — a dangerous default for financial data.</div>
<div><span class="badge b-amber">RISKY</span></div>
</div>
<div class="sev-row s-sig">
<div class="sev-name" style="color:var(--signal)">acks=all</div>
<div class="sev-desc"><strong>All ISR replicas acknowledge.</strong> Zero data loss assuming <code>min.insync.replicas ≥ 2</code>. Combined with idempotent producer: exactly-once within a partition. Use for everything that matters.</div>
<div><span class="badge b-sig">DURABLE</span></div>
</div>
<div class="callout c-amber">
<strong>min.insync.replicas — The Hidden Config That Matters</strong>
<code>acks=all</code> is meaningless without <code>min.insync.replicas=2</code> on the topic. With RF=3 and min.insync.replicas=1 (the default), one broker = full ISR and <code>acks=all</code> behaves like <code>acks=1</code>. Set this at the broker AND topic level.
</div>
<h3>Batching & Throughput Tuning</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">High-throughput producer config</span><span class="code-lang">python</span></div>
<pre><span class="py">from</span> confluent_kafka <span class="py">import</span> Producer
p <span class="op">=</span> Producer({
<span class="cm"># Durability</span>
<span class="st">'acks'</span>: <span class="st">'all'</span>,
<span class="st">'enable.idempotence'</span>: <span class="kw">True</span>, <span class="cm"># requires acks=all + retries>0</span>
<span class="cm"># Throughput vs Latency tradeoff</span>
<span class="st">'linger.ms'</span>: <span class="nu">20</span>, <span class="cm"># wait 20ms to batch more msgs</span>
<span class="st">'batch.size'</span>: <span class="nu">1048576</span>, <span class="cm"># 1MB batch (default 16KB)</span>
<span class="st">'buffer.memory'</span>: <span class="nu">67108864</span>,<span class="cm"># 64MB producer buffer</span>
<span class="cm"># Compression (huge throughput win)</span>
<span class="st">'compression.type'</span>: <span class="st">'zstd'</span>, <span class="cm"># best ratio; lz4 for low latency</span>
<span class="cm"># Retry behavior</span>
<span class="st">'retries'</span>: <span class="nu">2147483647</span>, <span class="cm"># max retries (idempotent = safe)</span>
<span class="st">'retry.backoff.ms'</span>: <span class="nu">100</span>,
<span class="st">'delivery.timeout.ms'</span>: <span class="nu">120000</span>, <span class="cm"># 2min max for any one message</span>
<span class="st">'request.timeout.ms'</span>: <span class="nu">30000</span>,
<span class="cm"># Ordering guarantee within partition</span>
<span class="st">'max.in.flight.requests.per.connection'</span>: <span class="nu">5</span>,
<span class="cm"># With idempotence: safe up to 5 in-flight.
# Without idempotence: must be 1 to prevent reordering on retry.</span>
<span class="st">'bootstrap.servers'</span>: <span class="st">'broker1:9092,broker2:9092,broker3:9092'</span>,
})
<span class="cm"># Async produce with delivery callback</span>
<span class="py">def</span> <span class="fn">on_delivery</span>(err, msg):
<span class="py">if</span> err: <span class="fn">log_error</span>(err)
<span class="py">else</span>: <span class="fn">metrics.increment</span>(<span class="st">'produced'</span>)
p.produce(
topic <span class="op">=</span> <span class="st">'orders'</span>,
key <span class="op">=</span> order.customer_id.encode(),
value <span class="op">=</span> order.to_json().encode(),
headers <span class="op">=</span> {<span class="st">'source'</span>: b<span class="st">'checkout'</span>},
callback <span class="op">=</span> on_delivery
)
p.poll(<span class="nu">0</span>) <span class="cm"># trigger callbacks without blocking</span>
<span class="cm"># Flush before shutdown — critical!</span>
p.flush(timeout<span class="op">=</span><span class="nu">30</span>) <span class="cm"># wait up to 30s for all outstanding messages</span></pre>
</div>
</div>
<div>
<h3>Partitioning Strategy</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Key selection — the most important decision</span><span class="code-lang">python</span></div>
<pre><span class="cm">"""
KEY SELECTION RULES:
1. Same key → same partition → ordering guaranteed
2. No key → round-robin (or sticky batch)
3. High-cardinality keys = even distribution
4. Low-cardinality keys = hot partition risk
"""</span>
<span class="cm"># ✅ GOOD: customer_id as key
# Orders per customer are ordered
# Millions of customers → even distribution</span>
p.produce(topic<span class="op">=</span><span class="st">'orders'</span>, key<span class="op">=</span>order.customer_id.encode(), value<span class="op">=</span>v)
<span class="cm"># ✅ GOOD: composite key for finer ordering</span>
key <span class="op">=</span> f<span class="st">"{customer_id}:{order_id}"</span>.encode()
<span class="cm"># ❌ BAD: country_code as key
# Only ~200 unique values → 200 partitions max
# "US" gets 40% of traffic → hot partition</span>
<span class="cm"># ❌ BAD: timestamp as key
# New partition every millisecond if hash collides
# No ordering guarantee you actually need</span>
<span class="cm"># ❌ BAD: null key for ordered events
# Round-robin destroys per-entity ordering</span>
<span class="cm">─────────────────────────────────────────────────</span>
<span class="cm"># Custom partitioner (Python)</span>
<span class="cm"># Useful for: geographic routing, tenant isolation</span>
<span class="py">def</span> <span class="fn">geography_partitioner</span>(key_bytes, all_partitions, available):
region_map <span class="op">=</span> {b<span class="st">'us'</span>: <span class="nu">0</span>, b<span class="st">'eu'</span>: <span class="nu">1</span>, b<span class="st">'ap'</span>: <span class="nu">2</span>}
region <span class="op">=</span> key_bytes[:2]
<span class="py">if</span> region <span class="py">in</span> region_map:
idx <span class="op">=</span> region_map[region] <span class="op">%</span> len(all_partitions)
<span class="py">return</span> all_partitions[idx]
<span class="cm"># default: hash</span>
<span class="py">return</span> all_partitions[hash(key_bytes) <span class="op">%</span> len(all_partitions)]</pre>
</div>
<h3>Transactional Producer</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Atomic multi-topic writes</span><span class="code-lang">python</span></div>
<pre><span class="cm"># Transactional API: write to multiple topics atomically.
# Either ALL messages commit or NONE do.
# Essential for: read-process-write pipelines (stream processing).</span>
p <span class="op">=</span> Producer({
<span class="st">'bootstrap.servers'</span>: <span class="st">'broker:9092'</span>,
<span class="st">'transactional.id'</span>: <span class="st">'checkout-producer-1'</span>, <span class="cm"># unique per instance</span>
<span class="st">'enable.idempotence'</span>: <span class="kw">True</span>,
})
p.init_transactions()
<span class="py">try</span>:
p.begin_transaction()
<span class="cm"># Write to multiple topics in one atomic batch:</span>
p.produce(<span class="st">'orders'</span>, key<span class="op">=</span>k, value<span class="op">=</span>order_msg)
p.produce(<span class="st">'inventory'</span>, key<span class="op">=</span>k, value<span class="op">=</span>reserve_msg)
p.produce(<span class="st">'notifications'</span>, key<span class="op">=</span>k, value<span class="op">=</span>email_msg)
<span class="cm"># Atomically commit consumer offsets WITH the transaction:</span>
p.send_offsets_to_transaction(
consumer.position(partitions),
consumer.consumer_group_metadata()
)
p.commit_transaction()
<span class="py">except</span> KafkaException <span class="py">as</span> e:
p.abort_transaction() <span class="cm"># all three writes rolled back</span>
<span class="py">raise</span></pre>
</div>
</div>
</div>
</section>
<!-- ══════════════════════════════════════════════
SECTION 3 — CONSUMERS
══════════════════════════════════════════════ -->
<section id="consumers">
<div class="section-header">
<div class="section-eyebrow">Read Path</div>
<div class="section-title"><em>CONSUMERS</em> — THE POLL LOOP</div>
<div class="section-sub">The consumer poll loop is the most misunderstood piece of the Kafka API. Get it wrong and you get rebalance storms, offset commit races, and ghost consumers that hold partitions without processing them.</div>
</div>
<div class="g2">
<div>
<h3>The Correct Poll Loop</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Production consumer — full pattern</span><span class="code-lang">python</span></div>
<pre><span class="py">from</span> confluent_kafka <span class="py">import</span> Consumer, KafkaError
<span class="py">import</span> signal, sys
c <span class="op">=</span> Consumer({
<span class="st">'bootstrap.servers'</span>: <span class="st">'broker1:9092'</span>,
<span class="st">'group.id'</span>: <span class="st">'order-processor-v3'</span>,
<span class="st">'auto.offset.reset'</span>: <span class="st">'earliest'</span>, <span class="cm"># 'latest' for new groups</span>
<span class="cm"># CRITICAL: disable auto-commit</span>
<span class="st">'enable.auto.commit'</span>: <span class="kw">False</span>,
<span class="cm"># Auto-commit commits on a timer, NOT after processing.
# You can commit before processing → message loss on crash.
# Always commit manually after confirmed processing.</span>
<span class="cm"># Session & heartbeat</span>
<span class="st">'session.timeout.ms'</span>: <span class="nu">45000</span>, <span class="cm"># 45s — time before assumed dead</span>
<span class="st">'heartbeat.interval.ms'</span>: <span class="nu">3000</span>, <span class="cm"># 1/3 of session.timeout</span>
<span class="st">'max.poll.interval.ms'</span>: <span class="nu">300000</span>, <span class="cm"># 5min — max time between polls</span>
<span class="cm"># Fetch tuning</span>
<span class="st">'fetch.min.bytes'</span>: <span class="nu">1024</span>, <span class="cm"># wait for 1KB before returning</span>
<span class="st">'fetch.max.wait.ms'</span>: <span class="nu">500</span>, <span class="cm"># max wait for fetch.min.bytes</span>
<span class="st">'max.partition.fetch.bytes'</span>:<span class="nu">1048576</span>, <span class="cm"># 1MB per partition per fetch</span>
<span class="st">'max.poll.records'</span>: <span class="nu">500</span>, <span class="cm"># max messages per poll()</span>
})
running <span class="op">=</span> <span class="kw">True</span>
signal.signal(signal.SIGTERM, <span class="py">lambda</span> *_: globals().update(running<span class="op">=False</span>))
c.subscribe([<span class="st">'orders'</span>], on_assign<span class="op">=</span>on_assign, on_revoke<span class="op">=</span>on_revoke)
<span class="py">try</span>:
<span class="py">while</span> running:
msgs <span class="op">=</span> c.consume(num_messages<span class="op">=</span><span class="nu">500</span>, timeout<span class="op">=</span><span class="nu">1.0</span>)
<span class="py">if not</span> msgs: <span class="py">continue</span>
<span class="py">for</span> msg <span class="py">in</span> msgs:
<span class="py">if</span> msg.error():
<span class="py">if</span> msg.error().code() <span class="op">==</span> KafkaError._PARTITION_EOF:
<span class="py">continue</span> <span class="cm"># end of partition, not an error</span>
<span class="py">raise</span> KafkaException(msg.error())
process(msg) <span class="cm"># ← your business logic</span>
<span class="cm"># Commit AFTER processing the batch — at-least-once</span>
c.commit(asynchronous<span class="op">=False</span>) <span class="cm"># sync commit = no gaps</span>
<span class="py">finally</span>:
c.close() <span class="cm"># triggers graceful rebalance, commits offsets</span></pre>
</div>
</div>
<div>
<h3>Consumer Groups & Rebalancing</h3>
<div class="callout c-coral">
<strong>REBALANCE STORM — The #1 Production Problem</strong>
A rebalance pauses ALL consumers in a group while partitions are redistributed. Causes: slow processing (exceeds <code>max.poll.interval.ms</code>), GC pauses, DB queries in the hot path, or too many consumers for too few partitions. Symptoms: lag spikes, duplicate processing, offset commit failures.
</div>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Rebalance callbacks + static membership</span><span class="code-lang">python</span></div>
<pre><span class="py">from</span> confluent_kafka <span class="py">import</span> Consumer
<span class="cm"># Static group membership — eliminates rebalance on restart</span>
c <span class="op">=</span> Consumer({
<span class="st">'group.id'</span>: <span class="st">'order-processor'</span>,
<span class="st">'group.instance.id'</span>: <span class="st">'processor-us-east-1a'</span>, <span class="cm"># STATIC</span>
<span class="st">'session.timeout.ms'</span>: <span class="nu">300000</span>, <span class="cm"># 5min — safe rolling restarts</span>
<span class="cm"># Static member holds its partitions for up to session.timeout
# after disconnect. Rolling deploys trigger zero rebalances.</span>
})
<span class="py">def</span> <span class="fn">on_assign</span>(consumer, partitions):
<span class="cm"># Called when partitions are assigned — good place to:</span>
<span class="cm"># - Seek to a custom offset</span>
<span class="cm"># - Reset a per-partition state machine</span>
<span class="cm"># - Load per-partition warm cache from DB</span>
<span class="py">for</span> p <span class="py">in</span> partitions:
log.info(f<span class="st">"Assigned partition {p.partition}"</span>)
consumer.assign(partitions)
<span class="py">def</span> <span class="fn">on_revoke</span>(consumer, partitions):
<span class="cm"># Called BEFORE partitions are taken away.
# MUST commit all pending offsets here — your last chance.</span>
consumer.commit(asynchronous<span class="op">=False</span>)
<span class="py">for</span> p <span class="py">in</span> partitions:
flush_partition_state(p.partition)
<span class="cm">─────────────────────────────────────────────────
# Partition assignment strategies (broker-side)</span>
<span class="cm"># partition.assignment.strategy=cooperative-sticky (Kafka 2.4+)</span>
<span class="cm"># cooperative-sticky: incremental rebalance — only moves
# partitions that need to move. Other partitions keep consuming.
# vs range/roundrobin: STOP-THE-WORLD — all partitions revoked.</span>
<span class="cm"># Always set cooperative-sticky in new deployments:</span>
<span class="st">'partition.assignment.strategy'</span>: <span class="st">'cooperative-sticky'</span></pre>
</div>
<h3>Consumer Lag — Reading the Signal</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Lag monitoring + seek patterns</span><span class="code-lang">python</span></div>
<pre><span class="py">from</span> confluent_kafka <span class="py">import</span> Consumer, TopicPartition
c <span class="op">=</span> Consumer({<span class="st">'group.id'</span>: <span class="st">'monitor'</span>, <span class="st">'bootstrap.servers'</span>: <span class="st">'...'</span>})
<span class="cm"># Get consumer group lag for a topic:</span>
md <span class="op">=</span> c.list_topics(<span class="st">'orders'</span>)
partitions <span class="op">=</span> [
TopicPartition(<span class="st">'orders'</span>, p) <span class="py">for</span> p <span class="py">in</span> md.topics[<span class="st">'orders'</span>].partitions
]
committed <span class="op">=</span> c.committed(partitions)
high_water <span class="op">=</span> c.get_watermark_offsets
<span class="py">for</span> tp <span class="py">in</span> committed:
_, hw <span class="op">=</span> c.get_watermark_offsets(tp)
lag <span class="op">=</span> hw <span class="op">-</span> (tp.offset <span class="py">if</span> tp.offset <span class="op">>=</span> <span class="nu">0</span> <span class="py">else</span> <span class="nu">0</span>)
<span class="py">print</span>(f<span class="st">"Partition {tp.partition}: lag={lag}"</span>)
<span class="cm"># Seek to specific offset (replay from point in time):</span>
c.seek(TopicPartition(<span class="st">'orders'</span>, partition<span class="op">=</span><span class="nu">0</span>, offset<span class="op">=</span><span class="nu">12345</span>))
<span class="cm"># Seek to timestamp (replay from a specific time):</span>
ts_ms <span class="op">=</span> <span class="nu">1704067200000</span> <span class="cm"># 2024-01-01 00:00:00 UTC in ms</span>
offsets_for_ts <span class="op">=</span> c.offsets_for_times([TopicPartition(<span class="st">'orders'</span>, <span class="nu">0</span>, ts_ms)])
c.seek(offsets_for_ts[<span class="nu">0</span>])</pre>
</div>
</div>
</div>
</section>
<!-- ══════════════════════════════════════════════
SECTION 4 — KAFKA STREAMS
══════════════════════════════════════════════ -->
<section id="streams">
<div class="section-header">
<div class="section-eyebrow">Stream Processing</div>
<div class="section-title">KAFKA <em>STREAMS</em></div>
<div class="section-sub">Kafka Streams is a library (not a cluster) that runs inside your Java/JVM application. No separate processing cluster. It reads from Kafka, transforms, and writes back to Kafka — with stateful windowed aggregations backed by RocksDB.</div>
</div>
<div class="g2">
<div>
<h3>Topology Building Blocks</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Kafka Streams DSL — Java</span><span class="code-lang">java</span></div>
<pre><span class="ty">StreamsBuilder</span> builder <span class="op">=</span> <span class="kw">new</span> <span class="ty">StreamsBuilder</span>();
<span class="cm">// KStream: unbounded stream of events</span>
<span class="ty">KStream</span><<span class="ty">String</span>, <span class="ty">Order</span>> orders <span class="op">=</span>
builder.stream(<span class="st">"orders"</span>, <span class="ty">Consumed</span>.with(
<span class="ty">Serdes</span>.String(), orderSerde
));
<span class="cm">// Filter + branch</span>
<span class="ty">KStream</span><<span class="ty">String</span>, <span class="ty">Order</span>>[] branches <span class="op">=</span> orders
.filter((k, v) <span class="op">-></span> v.getAmount() <span class="op">></span> <span class="nu">0</span>)
.branch(
(k, v) <span class="op">-></span> v.getStatus().equals(<span class="st">"COMPLETED"</span>),
(k, v) <span class="op">-></span> v.getStatus().equals(<span class="st">"REFUNDED"</span>),
(k, v) <span class="op">-></span> <span class="kw">true</span> <span class="cm">// catch-all</span>
);
<span class="ty">KStream</span> completed <span class="op">=</span> branches[<span class="nu">0</span>];
<span class="ty">KStream</span> refunded <span class="op">=</span> branches[<span class="nu">1</span>];