MongoDB vs PostgreSQL — What DevOps Engineers Need to Know in 2026

-- PostgreSQL: user in a table
SELECT u.name, o.total 
FROM users u 
JOIN orders o ON u.id = o.user_id
WHERE u.id = 123;

// MongoDB: user as a document
db.users.findOne({ _id: ObjectId("...") })
// Returns: { name: "Shubham", orders: [{total: 150}, {total: 230}] }
// No JOIN needed — data embedded in document

patronictl -c /etc/patroni.yml list
patronictl -c /etc/patroni.yml switchover

# Check replica set status
mongosh --eval "rs.status()"
 
# Initiate replica set
mongosh --eval 'rs.initiate({
  _id: "rs0",
  members: [
    {_id: 0, host: "mongo1:27017"},
    {_id: 1, host: "mongo2:27017"},
    {_id: 2, host: "mongo3:27017"}
  ]
})'

apiVersion: postgresql.cnpg.io/v1
kind: Cluster
metadata:
  name: pg-cluster
spec:
  instances: 3
  storage:
    size: 50Gi
  backup:
    barmanObjectStore:
      destinationPath: s3://my-bucket/pg-backups
      s3Credentials:
        accessKeyId:
          name: s3-creds
          key: ACCESS_KEY_ID
        secretAccessKey:
          name: s3-creds
          key: SECRET_ACCESS_KEY

apiVersion: mongodbcommunity.mongodb.com/v1
kind: MongoDBCommunity
metadata:
  name: mongodb-cluster
spec:
  members: 3
  type: ReplicaSet
  version: "7.0.0"
  security:
    authentication:
      modes: ["SCRAM"]
  users:
  - name: appuser
    db: admin
    passwordSecretRef:
      name: mongodb-secret
    roles:
    - name: readWrite
      db: myapp
  statefulSet:
    spec:
      volumeClaimTemplates:
      - metadata:
          name: data-volume
        spec:
          accessModes: [ReadWriteOnce]
          resources:
            requests:
              storage: 50Gi

apiVersion: atlas.mongodb.com/v1
kind: AtlasDeployment
metadata:
  name: my-cluster
spec:
  projectRef:
    name: my-atlas-project
  deploymentSpec:
    name: my-cluster
    clusterType: REPLICASET
    providerSettings:
      instanceSizeName: M10
      providerName: AWS
      regionName: AP_SOUTH_1

# pg_dump — logical backup
pg_dump -h localhost -U postgres -d myapp -Fc -f myapp.dump
 
# Restore
pg_restore -h localhost -U postgres -d myapp myapp.dump
 
# pgBackRest — production backup with PITR
pgbackrest --stanza=myapp backup --type=full
pgbackrest --stanza=myapp restore --target="2026-04-28 10:00:00"

# mongodump — logical backup
mongodump --uri="mongodb://user:pass@localhost:27017/myapp" \
  --out /backup/$(date +%Y%m%d)
 
# Restore
mongorestore --uri="mongodb://user:pass@localhost:27017" \
  /backup/20260428/myapp
 
# For production: use MongoDB Ops Manager or Atlas Backup
# Atlas backup: point-in-time recovery up to 1 second granularity

# Start backup
pbm backup
 
# List backups
pbm list
 
# Restore to point in time
pbm restore --time="2026-04-28T10:00:00"

# postgres-exporter for Prometheus
docker run -d \
  -e DATA_SOURCE_NAME="postgresql://postgres:pass@localhost:5432/postgres?sslmode=disable" \
  -p 9187:9187 \
  prometheuscommunity/postgres-exporter

# Connection pool saturation
pg_stat_activity_count{state="idle"} / pg_settings_max_connections > 0.8
 
# Replication lag
pg_replication_lag_seconds > 30
 
# Long running queries (> 5 minutes)
pg_stat_activity_max_tx_duration{state="active"} > 300
 
# Cache hit rate (should be > 99%)
pg_stat_database_blks_hit / (pg_stat_database_blks_hit + pg_stat_database_blks_read) * 100 < 99

# mongodb-exporter for Prometheus
docker run -d \
  -e MONGODB_URI="mongodb://exporter:pass@localhost:27017" \
  -p 9216:9216 \
  percona/mongodb_exporter:latest

# Replication lag (secondary behind primary)
mongodb_rs_member_optime_date{state="SECONDARY"} 
- on() mongodb_rs_member_optime_date{state="PRIMARY"} > 30
 
# Connection pool usage
mongodb_connections{state="current"} / mongodb_connections{state="available"} > 0.8
 
# Page faults (memory pressure)
rate(mongodb_extra_info_page_faults_total[5m]) > 100