> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vortexiq.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Active Replicas, ClickHouse

> Active Replicas for ClickHouse instances. Tracked live in Vortex IQ Nerve Centre. How to read it, why it matters, and how to act on it.

**Card class:** [Non-Hero](/nerve-centre/overview#card-classes-explained)  •  **Category:** [Replication](/nerve-centre/connectors#connectors-by-type)

## At a glance

> **Active Replicas** is the count of replicas currently online and participating in replication for the selected period. On a ReplicatedMergeTree setup, every replica should be active and talking to ClickHouse Keeper (or ZooKeeper). A drop below the expected count means a node has gone read-only or lost its Keeper session, which puts redundancy and write availability at risk.

## What it tracks

The card reads the active-replica picture from `system.replicas`, where each row represents one replicated table on this node and the `is_session_expired`, `is_readonly`, and `active_replicas` columns describe replication health. A healthy cluster shows `active_replicas` equal to `total_replicas` for every table; any shortfall means a peer replica is offline, read-only, or has an expired Keeper session and is no longer keeping its copy current. Because this is a real-time (`RT`) state read, the value reflects the cluster as it stands at the moment of refresh, not an average over time. It carries no alert threshold of its own, but it is the context number you check first when [Replication Lag (absolute\_delay)](/nerve-centre/kpi-cards/clickhouse/replication-lag-absolute-delay) or [Replication Queue Size (Top Tables)](/nerve-centre/kpi-cards/clickhouse/replication-queue-size-top-tables) starts climbing: a missing replica is the usual root cause.

## Reconciling against the source

Confirm the value directly in `clickhouse-client` with `SELECT database, table, active_replicas, total_replicas, is_readonly, is_session_expired FROM system.replicas`. On ClickHouse Cloud, the same query runs in the SQL console and the managed service surfaces node status in its monitoring view.

***

### Tracked live in Vortex IQ Nerve Centre

*Active Replicas* is one of hundreds of KPI pulses Vortex IQ tracks across ClickHouse and 70+ other ecommerce connectors. Nerve Centre runs the detection layer; Vortex Mind investigates the cause when something moves; Ask Viq lets you interrogate any number in plain English.

[Start for free](https://app.vortexiq.ai/login) or [book a demo](https://www.vortexiq.ai/contact-us) to see this metric running on your own data.