Replication of Past Samples

:::tip

What You'll Learn

How Netdata automatically fills data gaps when Children reconnect to Parents, including replication limitations, configuration options, and monitoring progress.

:::

When your Netdata Child reconnects to a Parent after being offline, replication automatically kicks in. Your Parent gets the metric samples it missed while the Child was disconnected, ensuring you don't see gaps in your charts.

:::note

This same process works between Parents too. When Parents sync with each other, one acts as the sender and the other as the receiver.

:::

How Replication Works

When multiple Netdata Parents are available, the replication happens in sequence, like in the following diagram:

mermaid

flowchart TD
    C("Child")
    P1("Parent 1")
    P2("Parent 2")
    
    C --> P1
    P1 --> P2
    P1 --> C
    P2 --> P1

    %% Style definitions
    classDef alert fill:#ffeb3b,stroke:#000000,stroke-width:3px,color:#000000,font-size:18px
    classDef neutral fill:#f9f9f9,stroke:#000000,stroke-width:3px,color:#000000,font-size:18px
    classDef complete fill:#4caf50,stroke:#000000,stroke-width:3px,color:#000000,font-size:18px
    classDef database fill:#2196F3,stroke:#000000,stroke-width:3px,color:#000000,font-size:18px

    %% Apply styles
    class C alert
    class P1,P2 complete

Replication Process

As shown in the diagram:

Connections establish immediately after a Netdata child connects to any of the Netdata Parents.
Each connection pair completes replication (Child→Parent1, Parent1→Parent2) on the receiving side and then initiates replication on the sending side.
Replication fills gaps up to now, and the sending side immediately enters streaming mode, without leaving any gaps on the samples of the receiving side.
Each connection negotiates retention to back-fill as much data as necessary.

Understanding Limitations

:::important

Key Replication Constraints

The current implementation is optimized to replicate small durations and have minimal impact during reconnecting. Understanding these limitations helps you plan your monitoring setup effectively.

:::

<details> <summary>What Can and Can’t Be Replicated</summary>

Append-only replication. Replication can only append samples to metrics. Only missing samples at the end of each time-series are replicated.
Tier0 samples only. Only tier0 samples are replicated. Samples of higher tiers in Netdata are derived from tier0 samples, and therefore there is no mechanism for ingesting them directly. This means that the maximum retention that can be replicated across Netdata is limited by the samples available in tier0 of the sending Netdata.
Active metrics only. Only samples of metrics that are currently being collected are replicated. Archived metrics (or even archived nodes) will be replicated when and if they are collected again.

:::note

Netdata archives metrics 1 hour after they stop being collected, so Netdata Parents may miss data only if Netdata Children are disconnected for more than an hour from their Parents.

:::

</details>

Configuration Options

Configure these options in netdata.conf on the respective systems.

<details> <summary>Receiving Side Configuration (Netdata Parent)</summary>

Setting	Description	Default
`[db].replication period`	Sets the maximum time window for replication. Remember, you're also limited by how much tier0 data your Child systems have kept.	1 day

</details> <details> <summary>Sending Side Configuration (Netdata Children or clustered Parents)</summary>

Setting	Description	Default
`[db].replication threads`	Controls how many parallel threads handle replication. Each thread can handle about two million samples per second, so more threads can speed up replication between Parents with lots of data.	1 thread
`[db].cleanup obsolete charts after`	Controls how long metrics remain available for replication after collection stops. If you expect Parent maintenance to last longer than 1 hour, increase this setting. Just be aware that in dynamic environments with lots of short-lived metrics, this can increase RAM usage since metrics stay "active" longer.	1 hour
(3600 seconds)

</details>

Monitoring Replication Progress

:::note

Where to Check Replication Status

You can monitor how replication is progressing through both your dashboard and API endpoints to make sure your data synchronization is working correctly.

:::

Dashboard Monitoring

Check your replication progress right in your dashboard using the Netdata Function Netdata-streaming, under the Live tab.

API Monitoring

You can also get the same information via the API endpoint http://agent-ip:19999/api/v2/node_instances on both your Parents and Children.