klag

Know why your Kafka consumer lag is growing — in 5 seconds from the terminal

Compared to existing tools

| | Burrow | Kafka UI | klag | |--|--------|----------|---------------| | Lag measurement | ✅ | ✅ | ✅ | | Root cause detection | ❌ | ❌ | ✅ | | CLI (npx) | ❌ | ❌ | ✅ | | Requires separate server | ✅ | ✅ | ❌ |

Run without installation

npx @closeup1202/klag --broker localhost:9092 --group my-service

Output example

⚡ klag 0.5.0

🔍 Consumer Group: my-service
   Broker:         localhost:9092
   Collected At:   2026-03-28 17:27:27 (Asia/Seoul)

   Group Status : 🚨 CRITICAL   Total Lag : 1,234   Drain : ∞

┌────────┬───────────┬──────────────────┬────────────────┬───────┬─────────┬──────┬──────────────┬──────────────┐
│ Topic  │ Partition │ Committed Offset │ Log-End Offset │  Lag  │ Status  │ Drain│ Produce Rate │ Consume Rate │
├────────┼───────────┼──────────────────┼────────────────┼───────┼─────────┼──────┼──────────────┼──────────────┤
│ orders │         0 │            8,796 │         10,000 │ 1,204 │ 🔴 HIGH │    ∞ │  40.0 msg/s  │   0.0 msg/s  │
│ orders │         1 │            9,988 │         10,000 │    12 │ 🟢 OK   │    — │   0.0 msg/s  │   0.0 msg/s  │
│ orders │         2 │            9,982 │         10,000 │    18 │ 🟢 OK   │    — │   0.0 msg/s  │   0.0 msg/s  │
└────────┴───────────┴──────────────────┴────────────────┴───────┴─────────┴──────┴──────────────┴──────────────┘

🔎 Root Cause Analysis
   [PRODUCER_BURST] orders
   → produce rate 40.0 msg/s vs consume rate 0.0 msg/s (∞x difference)
   → Suggestion: consider increasing consumer instances or partition count

   [HOT_PARTITION] orders
   → partition-0 holds 98% of lag (1,204 / 1,234) — skewed to 1 of 3 partitions
   → Suggestion: review partition key distribution strategy or consider increasing partition count

Installation

npm install -g @closeup1202/klag

Usage

# Basic usage
klag --broker localhost:9092 --group my-service

# Fast mode without rate sampling
klag --broker localhost:9092 --group my-service --no-rate

# Watch mode (auto-refresh every N seconds)
klag --broker localhost:9092 --group my-service --watch
klag --broker localhost:9092 --group my-service --watch --interval 3000

# JSON output (CI/pipeline integration)
klag --broker localhost:9092 --group my-service --json

# SSL (system CA trust)
klag --broker kafka.prod:9092 --group my-service --ssl

# SSL with custom certificates
klag --broker kafka.prod:9092 --group my-service \
  --ssl --ssl-ca /etc/kafka/ca.pem \
  --ssl-cert /etc/kafka/client.crt --ssl-key /etc/kafka/client.key

# SASL authentication (password via environment variable — recommended)
KLAG_SASL_PASSWORD=secret klag --broker kafka.prod:9092 --group my-service \
  --sasl-mechanism scram-sha-256 --sasl-username kafka-user

# SSL + SASL combined
KLAG_SASL_PASSWORD=secret klag --broker kafka.prod:9092 --group my-service \
  --ssl --sasl-mechanism scram-sha-256 --sasl-username kafka-user

Config file (.klagrc)

Create .klagrc in the current directory or ~/.klagrc to store default options. CLI arguments always take precedence over the config file.

{
  "broker": "kafka.prod.internal:9092",
  "group": "my-service",
  "interval": 3000,
  "ssl": {
    "enabled": true,
    "caPath": "/etc/kafka/ca.pem"
  },
  "sasl": {
    "mechanism": "scram-sha-256",
    "username": "kafka-user"
  }
}

With this file in place, you only need:

KLAG_SASL_PASSWORD=secret klag

Options

| Option | Description | Default | |--------|-------------|---------| | -b, --broker <host:port> | Kafka broker address | localhost:9092 | | -g, --group <groupId> | Consumer group ID | (required) | | -i, --interval <ms> | Rate sampling interval | 5000 | | -t, --timeout <ms> | Connection timeout | 5000 | | -w, --watch | Watch mode | false | | --no-rate | Skip rate sampling | false | | --json | JSON output | false | | --ssl | Enable SSL/TLS | false | | --ssl-ca <path> | CA certificate PEM file | - | | --ssl-cert <path> | Client certificate PEM file | - | | --ssl-key <path> | Client key PEM file | - | | --sasl-mechanism <type> | plain, scram-sha-256, scram-sha-512 | - | | --sasl-username <user> | SASL username | - | | --sasl-password <pass> | SASL password (prefer KLAG_SASL_PASSWORD env var) | - |

Detectable root causes

HOT_PARTITION

When 80% or more of total lag is concentrated on a single partition. Occurs when producer key distribution is uneven (key skew).

PRODUCER_BURST

When produce rate is at least 2x the consume rate (and the consumer is still running). Occurs when traffic spikes and the consumer cannot keep up.

SLOW_CONSUMER

When the produce rate is active but the consume rate has dropped to near zero. Occurs when the consumer process has stalled, crashed, or is blocked (e.g., long GC pause).

REBALANCING

When the consumer group is in PreparingRebalance or CompletingRebalance state. All consumption pauses during rebalancing, which can cause a temporary lag spike.

OFFSET_NOT_MOVING

When the committed offset has not moved between samples while lag remains non-zero and the consumer group is Stable. Indicates the consumer is alive but failing to commit — caused by auto-commit being disabled with manual commits skipped, a processing loop that is stuck, or a crash-loop between message processing and offset commit.

Requirements

• Node.js >= 18
• Kafka >= 2.0

Roadmap

• [x] v0.1.0 — lag collection, hot partition, producer burst, slow consumer, rebalancing detection, watch mode with lag trend
• [x] v0.2.0 — SSL/SASL authentication, .klagrc config file support
• [x] v0.3.0 — time-to-drain severity classification, Drain column per partition
• [x] v0.3.1 — interactive consumer group picker
• [x] v0.4.1 — OFFSET_NOT_MOVING detector (offset commit stall detection)
• [x] v0.5.0 — honest --no-rate mode: Status column removed when rate data is unavailable
• [ ] v0.6.0 — Slack alerts, Prometheus export

License

MIT © closeup1202