Add MongoDB instance scoping configuration page

config/_default/menus/main.en.yaml

@@ -4635,6 +4635,11 @@ menu:
       parent: dbm
       identifier: dbm_setup_architecture
       weight: 2
+    - name: MongoDB Configurations
+      url: database_monitoring/architecture/mongodb/
+      parent: dbm_setup_architecture
+      identifier: dbm_architecture_mongodb
+      weight: 2
     - name: Setting Up Postgres
       url: database_monitoring/setup_postgres/
       parent: dbm

content/en/database_monitoring/architecture/mongodb.md

@@ -0,0 +1,239 @@
+---
+title: MongoDB Configurations
+description: Configure MongoDB instance scoping for Database Monitoring across replica sets, standalone, and sharded cluster topologies.
+further_reading:
+- link: "/database_monitoring/architecture/"
+  tag: "Documentation"
+  text: "DBM Setup Architectures"
+- link: "/database_monitoring/setup_mongodb/"
+  tag: "Documentation"
+  text: "Setting up MongoDB"
+---
+
+## Overview
+
+Each check instance in the Datadog Agent configuration creates one Database Monitoring (DBM) instance. MongoDB's monitoring topology uses multiple check instances sharing a `cluster_name`, rather than a single entry with scope flags. MongoDB has no [database_identifier](/database_monitoring/guide/database_identifier/) template. Identity is driven by `cluster_name` combined with the hostname auto-detected from `serverStatus`.
+
+**Scenario**: Three deployment types:
+
+| Topology | Nodes |
+|---|---|
+| Standalone | `mongo-standalone.example.com:27017` |
+| Replica set | Primary: `mongo-rs1.example.com:27017`; Secondaries: `mongo-rs2.example.com:27017`, `mongo-rs3.example.com:27017` |
+| Sharded cluster | mongos: `mongos.example.com:27017`; Shard primary: `mongo-rs1.example.com:27017`; Config server: `configsvr.example.com:27017` |
+
+## Replica set
+
+<div class="alert alert-info">For replica sets, use one entry per replica member. This is the only way to get replication lag and per-node metrics. Do not use <code>replicaSet</code> in options. The Agent expects to always connect to the same host.</div>
+
+One check instance per replica set member. All members share the same `cluster_name` so the Agent builds a topology view.
+
+In this topology, this counts as three DBM instances.
+
+```yaml
+instances:
+  # Primary
+  - hosts:
+      - mongo-rs1.example.com:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    dbm: true
+    cluster_name: my-replica-set
+    database_autodiscovery:
+      enabled: true
+
+  # Secondary 1
+  - hosts:
+      - mongo-rs2.example.com:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    dbm: true
+    cluster_name: my-replica-set
+    database_autodiscovery:
+      enabled: true
+
+  # Secondary 2
+  - hosts:
+      - mongo-rs3.example.com:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    dbm: true
+    cluster_name: my-replica-set
+    database_autodiscovery:
+      enabled: true
+```
+
+## Standalone
+
+<div class="alert alert-info">For standalone deployments, one entry per host is sufficient. All databases are auto-discovered.</div>
+
+In this topology, this counts as one DBM instance.
+
+```yaml
+instances:
+  - hosts:
+      - mongo-standalone.example.com:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    dbm: true
+    cluster_name: my-standalone-cluster
+    database_autodiscovery:
+      enabled: true
+```
+
+## Sharded cluster
+
+<div class="alert alert-info">For sharded clusters, configure one entry per mongos router, one per shard member, and one per config server, all sharing the same <code>cluster_name</code>.</div>
+
+In this topology, this counts as three DBM instances.
+
+```yaml
+instances:
+  # mongos router
+  - hosts:
+      - mongos.example.com:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    dbm: true
+    cluster_name: my-sharded-cluster
+    options:
+      directConnection: "false"      # required when listing multiple mongos hosts
+    additional_metrics:
+      - jumbo_chunks
+      - sharded_data_distribution
+
+  # Shard primary
+  - hosts:
+      - mongo-rs1.example.com:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    dbm: true
+    cluster_name: my-sharded-cluster
+
+  # Config server replica set member
+  - hosts:
+      - configsvr.example.com:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    dbm: true
+    cluster_name: my-sharded-cluster
+```
+
+## Multiple mongod processes
+
+<div class="alert alert-info">Use <code>reported_database_hostname</code> to disambiguate multiple mongod processes running on the same host on different ports.</div>
+
+In this topology, this counts as two DBM instances.
+
+```yaml
+instances:
+  - hosts:
+      - mongo-standalone.example.com:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    dbm: true
+    cluster_name: my-standalone-cluster
+    reported_database_hostname: mongo-standalone.example.com:27017
+
+  - hosts:
+      - mongo-standalone.example.com:27018
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    dbm: true
+    cluster_name: my-standalone-cluster
+    reported_database_hostname: mongo-standalone.example.com:27018
+```
+
+## Recommendation
+
+MongoDB scoping is determined by deployment topology, not configuration preference:
+
+| Topology | Configuration |
+|---|---|
+| Standalone | One entry per host |
+| Replica set | One entry per replica member |
+| Sharded cluster | One entry per mongos + one per shard member + one per config server, all sharing `cluster_name` |
+
+The most important configuration decision is `cluster_name`. It must be unique per cluster and identical across all members of the same cluster.
+
+## Managed services
+
+### Amazon DocumentDB
+
+```yaml
+instances:
+  - hosts:
+      - mydocdb.cluster-cfxgae8cilcf.us-east-1.docdb.amazonaws.com:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    tls: true
+    tls_ca_file: /path/to/rds-combined-ca-bundle.pem
+    dbm: true
+    cluster_name: my-docdb-cluster
+    aws:
+      instance_endpoint: mydocdb.cfxgae8cilcf.us-east-1.docdb.amazonaws.com
+      cluster_identifier: mydocdb
+```
+
+**Note**: DocumentDB has MongoDB wire-protocol compatibility limitations. Operation samples (`$currentOp`) and query metrics (`$queryStats`) are not supported. Expect most DBM features to be non-functional or degraded.
+
+### MongoDB Atlas
+
+For dedicated tiers (M10+), use per-node connection strings:
+
+```yaml
+instances:
+  - hosts:
+      - atlas-shard-00-00.xxxxx.mongodb.net:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    tls: true
+    dbm: true
+    cluster_name: my-atlas-cluster
+
+  - hosts:
+      - atlas-shard-00-01.xxxxx.mongodb.net:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    tls: true
+    dbm: true
+    cluster_name: my-atlas-cluster
+
+  - hosts:
+      - atlas-shard-00-02.xxxxx.mongodb.net:27017
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    tls: true
+    dbm: true
+    cluster_name: my-atlas-cluster
+```
+
+For free/shared tiers (M0, M2, M5), use the SRV cluster hostname. Per-node replication metrics are not available at these tiers.
+
+```yaml
+instances:
+  - hosts:
+      - cluster0.xxxxx.mongodb.net
+    connection_scheme: mongodb+srv
+    username: datadog
+    password: "<PASSWORD>"
+    database: admin
+    tls: true
+    dbm: true
+    cluster_name: my-atlas-cluster
+```
+
+{{< partial name="whats-next/whats-next.html" >}}