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mberlofa merged 2 commits into from
Aug 28, 2026
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docs(mongodb): document arbiter support #541

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docs(mongodb): document arbiter support
mberlofa Aug 28, 2026
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fix(mongodb): validate playground arbiter topology
mberlofa Aug 28, 2026
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7 changes: 7 additions & 0 deletions src/data/playground-configs.ts
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Expand Up @@ -4928,6 +4928,13 @@ export const chartConfigs: Record<string, ChartConfig> = {
default: '3',
description: 'Data-bearing members in replica set mode',
},
{
label: 'Arbiter',
key: 'arbiter.enabled',
type: 'toggle',
default: false,
description: 'Add one voting arbiter; requires 2, 4, or 6 data-bearing members',
},
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{
label: 'Storage Size',
key: 'persistence.size',
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28 changes: 19 additions & 9 deletions src/pages/docs/charts/mongodb.mdx
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Expand Up @@ -168,7 +168,10 @@ persistence:

<Callout type="info" title="When to use an arbiter">
An arbiter participates in elections but holds no data. Use it when you have an even number of data-bearing members to
ensure a majority for elections. With 3 data members, an arbiter is not needed.
ensure a majority for elections. MongoDB recommends three data-bearing members when capacity permits because an
arbiter does not add data redundancy and can affect majority write availability. The chart supports one arbiter with
2, 4, or 6 data-bearing members. See the [official arbiter
considerations](https://www.mongodb.com/docs/manual/core/replica-set-arbiter/).
</Callout>

</div>
Expand Down Expand Up @@ -241,12 +244,13 @@ config:

### Architecture

| Parameter | Type | Default | Description |
| -------------------- | ------- | ------------ | -------------------------------------------------------- |
| `architecture` | string | `standalone` | `standalone`, `replicaset`, or `sharded`. |
| `replicaSet.name` | string | `rs0` | Replica set name. |
| `replicaSet.members` | integer | `3` | Number of data-bearing replica set members. |
| `arbiter.enabled` | boolean | `false` | Add an arbiter pod (votes in elections, stores no data). |
| Parameter | Type | Default | Description |
| -------------------- | ------- | ------------ | ---------------------------------------------------------------- |
| `architecture` | string | `standalone` | `standalone`, `replicaset`, or `sharded`. |
| `replicaSet.name` | string | `rs0` | Replica set name. |
| `replicaSet.members` | integer | `3` | Number of data-bearing replica set members. |
| `arbiter.enabled` | boolean | `false` | Add one non-data-bearing voting member in replica set mode. |
| `arbiter.resources` | object | `{}` | CPU and memory requests/limits for the arbiter mongod container. |

### Sharded Cluster

Expand Down Expand Up @@ -340,8 +344,14 @@ MongoDB administrative commands instead of changing chart values.
2. Deploy the chart in `replicaset` mode.
3. Restore data with `mongorestore`.

Changing `replicaSet.members` on an existing replica set triggers a reconfiguration.
Scaling down removes members — ensure no data is exclusively on the removed members.
Enabling or disabling `arbiter.enabled` reconciles the arbiter through the post-upgrade hook. Increasing
`replicaSet.members` adds missing members. Reducing it does not remove data-bearing members from MongoDB; follow MongoDB's
member-removal procedure before scaling down the StatefulSet.

When arbiter reconciliation would change MongoDB's implicit default write concern, the hook preserves the currently
effective value (`w: 1` or `majority`) by promoting it to a global default. Existing operator-managed global defaults are
left unchanged. MongoDB 5.0 and newer do not allow a global default write concern to be unset after it is established. See
the official [`setDefaultRWConcern` documentation](https://www.mongodb.com/docs/manual/reference/command/setDefaultRWConcern/).

</Callout>

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