ZooKeeper

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Integrations

ZooKeeper

Monitor ZooKeeper request latency, connections, sessions, server state, file descriptors, data size, and network traffic.

Guides

Guides overview 2 pages

Apache ZooKeeper Operations Guides

Practical guides for running, troubleshooting, and monitoring Apache ZooKeeper in production.

ClickHouse Operations Guides

Practical guides for running, troubleshooting, and monitoring ClickHouse in production.

ClickHouse Operations Guides 7 pages

Open the ClickHouse Operations Guides hub

ClickHouse cannot connect to ZooKeeper/Keeper: diagnosing the coordination layer

Diagnose and recover when ClickHouse loses its ZooKeeper or ClickHouse Keeper coordination layer, including connection checks, 4lw probes, and replica readonly states.

ClickHouse distributed DDL stuck: ON CLUSTER queries that never finish

Diagnose and fix ON CLUSTER DDL queries that hang or never finish in ClickHouse. Identify stuck hosts, reconcile schema drift, and restore coordination with ZooKeeper or Keeper.

ClickHouse Keeper saturation spiral: too many tables, DDL storms, and cluster freeze

Diagnose and break the ZooKeeper/Keeper saturation spiral that turns replicated ClickHouse clusters read-only.

ClickHouse monitoring checklist: the signals every production cluster needs

A production monitoring checklist for ClickHouse organized by maturity level, from survival signals to expert diagnostics.

ClickHouse query error rate high: reading exception codes 241, 252, and 999

Triage a ClickHouse query error spike by exception code. Diagnose memory limits, too many parts, ZooKeeper issues, and schema drift using system.query_log and system.events.

ClickHouse Table is in readonly mode: is_readonly on replicated tables and how to fix it

Diagnose and recover replicated ClickHouse tables stuck in readonly mode caused by lost Keeper sessions, coordination failures, or quorum loss.

ClickHouse ZooKeeper session has expired: causes, recovery, and tuning

Diagnose and recover when ClickHouse replicas lose ZooKeeper sessions, become read-only, and stop accepting writes.

Kafka Operations Guides 3 pages

Open the Kafka Operations Guides hub

Kafka ActiveControllerCount not equal to 1: no controller or split brain

Diagnose and recover when the cluster-wide sum of Kafka ActiveControllerCount deviates from 1, indicating no active controller or split brain.

Kafka controller event queue backing up: overwhelmed controller and stalled metadata

Diagnose and recover when the Kafka controller's event queue backs up, stalling leader elections and metadata propagation.

Kafka monitoring checklist: the signals every production cluster needs

A production monitoring checklist for Apache Kafka organized by maturity level: survival, operational, mature, and expert signals every SRE should watch.

Apache ZooKeeper Operations Guides 20 pages

Open the Apache ZooKeeper Operations Guides hub

ZooKeeper "Cannot open channel to N at election address": the blocked election port

Diagnose the asymmetric connectivity trap where a blocked 3888 leader election port stays invisible until the next leader election kills the ensemble.

ZooKeeper "Client session timed out, have not heard from server": the heartbeat miss

Why the ZooKeeper client logs a session timeout heartbeat miss, what it means for the session, and how to isolate server GC, client GC, or network as the root cause.

ZooKeeper "Detected pause in JVM or host machine (eg GC)": the pause-monitor warning

Operator reference for ZooKeeper's JvmPauseMonitor warning: how to distinguish Stop-the-World GC from host-level freezes and which signals to correlate during recovery.

ZooKeeper "fsync-ing the write ahead log took too long": the disk warning behind most write stalls

What the ZooKeeper fsync warning really means, why it is the most critical disk signal in any ensemble, and how to triage it before it costs you quorum.

ZooKeeper "Packet len is out of range": jute.maxbuffer and oversized znodes

Diagnose and fix ZooKeeper's jute.maxbuffer rejection, from oversized znode blobs to huge getChildren responses blocking follower sync.

ZooKeeper "Too many connections from /IP - max is 60": maxClientCnxns rejecting clients

Diagnose ZooKeeper maxClientCnxns rejections: why the per-IP limit silently denies clients behind NAT and how to catch it with zk_connection_rejected.

ZooKeeper "Unable to load database on disk": corrupt snapshot on startup

Why a ZooKeeper node refuses to start with Unable to load database on disk, how to triage the corrupt file, and how to recover without losing the ensemble.

ZooKeeper "X is not executed because it is not in the whitelist": four-letter-word commands blocked

Why ZooKeeper 3.5.3+ blocks four-letter-word commands by default and how to restore monitoring visibility without re-introducing the DoS surface.

ZooKeeper autopurge not configured: snapshots and logs filling the disk over months

Diagnose and fix ZooKeeper disk exhaustion caused by disabled autopurge, with safe checks for snapshot and transaction log accumulation.

ZooKeeper avg_latency hides write stalls: why the headline number lies

Why zk_avg_latency hides write stalls on read-heavy ensembles, and how to monitor zk_updatelatency and zk_readlatency separately to catch them.

ZooKeeper data tree digest mismatch: detecting corruption before it spreads

What zk_digest_mismatches_count means, the four causes to investigate first, and how to triage single-node corruption from a ZAB replication bug.

ZooKeeper dataLogDir sharing a disk with snapshots: the #1 fsync-latency footgun

Why ZooKeeper fsync latency spikes when the transaction log shares a disk with snapshots, how to confirm it, and how to move the log to a dedicated device safely.

ZooKeeper KeeperErrorCode = ConnectionLoss: the transient disconnect every client hits

What ConnectionLoss actually means, how to tell it from SessionExpired, and how to diagnose a storm of disconnects across clients.

ZooKeeper KeeperErrorCode = NoAuth: ACL denials on protected znodes

Diagnose and resolve ZooKeeper KeeperErrorCode = NoAuth errors from missing addAuthInfo calls, digest and SASL credential mismatches, and the 3.9.2 exists() ACL change.

ZooKeeper KeeperErrorCode = NoNode: operating on a path that doesn't exist

Triage ZooKeeper NoNode errors: separate application bugs from session-expiration cascades with read-only checks and mntr signals.

ZooKeeper KeeperErrorCode = Session expired: ephemeral nodes gone, clients evicted

Diagnose ZooKeeper session expiration, its cascading effects on ephemeral nodes and watches, and the GC, network, and failover causes behind mass client eviction.

ZooKeeper monitoring checklist: the signals every production ensemble needs

A four-level maturity checklist for monitoring production ZooKeeper ensembles, from liveness and quorum to fsync latency, GC pauses, and data integrity signals.

ZooKeeper OutOfMemoryError: Java heap space - the OOM that kills the whole ensemble at once

Diagnose and stop the ZooKeeper Java heap space OOM that takes down an entire ensemble at once: data tree growth, GC death spiral, and heap sizing.

ZooKeeper quorum loss: no leader elected and every write is failing

Recover from ZooKeeper quorum loss: confirm no leader is elected, identify the cause, and restore enough voting members to form quorum without losing data.

ZooKeeper transaction log disk full: the crash with no graceful degradation

Diagnose and recover from a full ZooKeeper dataLogDir partition, including the crash-loop and silent data loss variants, plus the disk headroom rules that prevent recurrence.

Academy

Kafka ZooKeeper Monitoring

Learn everything about monitoring & troubleshooting Kafka ZooKeeper, what metrics are important to monitor and why, and how to monitor Kafka ZooKeeper with Netdata.

ZooKeeper Monitoring

Learn everything about monitoring & troubleshooting ZooKeeper, what metrics are important to monitor and why, and how to monitor ZooKeeper with Netdata.

Resources

Best ZooKeeper Monitoring Tools: 10 Ranked for 2026

We ranked the 10 best ZooKeeper monitoring tools for 2026 on metric depth, resolution, alerting, and cost. See where Netdata, Prometheus, and Datadog land.

Blog

Release 1.18: What’s new with the database engine?

Introducing New Innovations and Improvements for Comprehensive Monitoring