docs(mongodb): document arbiter support

e2e/playground.spec.ts

@@ -42,6 +42,33 @@ test.describe('Playground', () => {
     await expect(page.locator('select[data-field-key="notediscovery.defaultTheme"]')).toHaveValue('light');
   });
 
+  test('mongodb arbiter output stays within supported topology combinations', async ({ page }) => {
+    await page.goto('/playground');
+    await page.locator('.playground-chart-btn[data-slug="mongodb"]').click();
+
+    const architecture = page.locator('select[data-field-key="architecture"]');
+    const members = page.locator('input[data-field-key="replicaSet.members"]');
+    const arbiter = page.locator('button[data-field-key="arbiter.enabled"]');
+    const code = page.locator('#playground-code');
+
+    await arbiter.click();
+    await expect(architecture).toHaveValue('replicaset');
+    await expect(members).toHaveValue('2');
+    await expect(code).toContainText('architecture=replicaset');
+    await expect(code).toContainText('replicaSet.members=2');
+    await expect(code).toContainText('arbiter.enabled=true');
+
+    await members.fill('3');
+    await expect(code).not.toContainText('arbiter.enabled');
+
+    await arbiter.click();
+    await architecture.selectOption('standalone');
+    await expect(code).not.toContainText('arbiter.enabled');
+
+    await page.goto('/playground?chart=mongodb&architecture=replicaset&replicaSet.members=3&arbiter.enabled=true');
+    await expect(page.locator('#playground-code')).not.toContainText('arbiter.enabled');
+  });
+
   test('ntfy defaults stay aligned with the chart', async ({ page }) => {
     await page.goto('/playground');
     await page.locator('.playground-chart-btn[data-slug="ntfy"]').click();

src/data/playground-configs.ts

@@ -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',
+        },
         {
           label: 'Storage Size',
           key: 'persistence.size',

src/pages/docs/charts/mongodb.mdx

@@ -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>
@@ -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
 
@@ -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>
 

src/scripts/playground.ts

@@ -81,6 +81,7 @@ const coupledRuleKeysBySlug: Record<string, string[][]> = {
 };
 
 const giteaPostgresqlPassword = 'change-me-gitea-postgresql';
+const mongodbArbiterMemberCounts = new Set(['2', '4', '6']);
 
 function getGroups(slug: string): GroupConfig[] {
   return configs[slug] ?? configs['_default'] ?? [];
@@ -232,7 +233,7 @@ function autoEnableField(key: string) {
 function setControlValue(key: string, value: string) {
   currentValues[key] = value;
   const control = controlsEl?.querySelector<HTMLInputElement | HTMLSelectElement | HTMLButtonElement>(
-    `[data-field-key="${key}"]`,
+    `input[data-field-key="${key}"], select[data-field-key="${key}"], button[data-field-key="${key}"]`,
   );
 
   if (!control) return;
@@ -259,6 +260,22 @@ function clearGiteaExternalDatabaseDetectionFields() {
 }
 
 function applyFieldSideEffects(key: string, value: string) {
+  if (selectedSlug === 'mongodb') {
+    if (key === 'arbiter.enabled' && value === 'true') {
+      setControlValue('architecture', 'replicaset');
+      if (!mongodbArbiterMemberCounts.has(currentValues['replicaSet.members'])) {
+        setControlValue('replicaSet.members', '2');
+      }
+    } else if (
+      currentValues['arbiter.enabled'] === 'true' &&
+      ((key === 'architecture' && value !== 'replicaset') ||
+        (key === 'replicaSet.members' && !mongodbArbiterMemberCounts.has(value)))
+    ) {
+      updateToggleField('arbiter.enabled', false);
+    }
+    return;
+  }
+
   if (selectedSlug !== 'gitea') return;
 
   if (key === 'database.mode' && value === 'postgresql') {
@@ -607,6 +624,7 @@ function buildFieldControl(field: FieldConfig): HTMLElement {
     input.addEventListener('input', () => {
       setFieldValue(field.key, input.value);
       if (field.enables) autoEnableField(field.enables);
+      applyFieldSideEffects(field.key, input.value);
       updateOutput();
     });
     controlDiv.appendChild(input);
@@ -997,6 +1015,14 @@ function setTreeValue(tree: Record<string, any>, key: string, value: boolean | n
 
 function updateOutput() {
   if (!codeEl || !selectedSlug) return;
+  if (
+    selectedSlug === 'mongodb' &&
+    currentValues['arbiter.enabled'] === 'true' &&
+    (currentValues['architecture'] !== 'replicaset' ||
+      !mongodbArbiterMemberCounts.has(currentValues['replicaSet.members']))
+  ) {
+    updateToggleField('arbiter.enabled', false);
+  }
   if (copyBtn) copyBtn.disabled = false;
   if (shareBtn) shareBtn.disabled = false;