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KlagKlag

About Klag

Klag is an open-source Kafka consumer lag exporter. It watches the consumer groups on a Kafka cluster and turns their progress into metrics your existing observability stack can alert on: Prometheus, Datadog, or any OTLP endpoint.

It is a single small service. You run it next to your cluster, point it at your brokers with read-only credentials, and it does nothing else — it never produces, consumes, or commits offsets, and it has no UI, no database, and no control plane.

Why it exists

Section titled “Why it exists”

The tool most teams reached for, kafka-lag-exporter, stopped being maintained. Its Scala/Akka stack also carried a licensing change and a heavy runtime for what is, in the end, a periodic AdminClient poll. Klag was written to be the boring replacement: same job, current dependencies, equivalent metric names so dashboards and alerts survive the swap.

Along the way it grew the signals that a raw lag number does not give you:

  • Lag velocity — whether the gap is growing or shrinking, so you can tell a backlog that is recovering from one that is not.
  • Time-based lag — how far behind a group is in seconds, which is what an SLA is usually written against.
  • Commit staleness — a group holding lag while its committed offset stops moving. That is a stuck consumer, and a flat lag value hides it.
  • Hot partitions — one partition carrying statistically more than its peers, usually a partition-key problem rather than a consumer problem.
  • Retention risk — how much of the retention window the lag has eaten, so you get warned before Kafka deletes data the consumer has not read.
  • Under-replicated partitions — fault-tolerance loss, from metadata Klag already fetches.

Technical shape

Section titled “Technical shape”

Java 21 on Vert.x with Micrometer for metrics. Collection is deliberately batched: request volume against the brokers stays independent of how many topics exist, so adding topics does not add load. It ships as a container image, a Helm chart, a fat JAR, and a GraalVM native image that starts in about 70-100 ms and holds about 44 MB of RSS.

Maintainers, license, support

Section titled “Maintainers, license, support”

Klag is maintained by themoah and released under the Apache License 2.0. It is free, with no hosted tier, no account, and no paid features — see pricing.

Support is community support, through GitHub issues. There is no commercial support contract and no SLA; if you need one, the license lets you run and modify Klag however you like.

For AI agents

Section titled “For AI agents”

The documentation is published in machine-readable form at /llms.txt and /llms-full.txt, every page has a markdown twin (append .md to any URL), and klag.dev hosts a read-only documentation MCP server. See Developers and Agent setup.