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Private access in Azure Cosmos DB for PostgreSQL

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APPLIES TO: Azure Cosmos DB for PostgreSQL (powered by the Citus database extension to PostgreSQL)

Azure Cosmos DB for PostgreSQL supports three networking options:

  • No access
    • This is the default for a newly created cluster if public or private access is not enabled. No computers, whether inside or outside of Azure, can connect to the database nodes.
  • Public access
    • A public IP address is assigned to the coordinator node.
    • Access to the coordinator node is protected by firewall.
    • Optionally, access to all worker nodes can be enabled. In this case, public IP addresses are assigned to the worker nodes and are secured by the same firewall.
  • Private access
    • Only private IP addresses are assigned to the cluster’s nodes.
    • Each node requires a private endpoint to allow hosts in the selected virtual network to access the nodes.
    • Security features of Azure virtual networks such as network security groups can be used for access control.

When you create a cluster, you may enable public or private access, or opt for the default of no access. Once the cluster is created, you can choose to switch between public or private access, or activate them both at once.

This page describes the private access option. For public access, see here.

Definitions

Virtual network. An Azure Virtual Network (VNet) is the fundamental building block for private networking in Azure. Virtual networks enable many types of Azure resources, such as database servers and Azure Virtual Machines (VM), to securely communicate with each other. Virtual networks support on-premises connections, allow hosts in multiple virtual networks to interact with each other through peering, and provide added benefits of scale, security options, and isolation. Each private endpoint for a cluster requires an associated virtual network.

Subnet. Subnets segment a virtual network into one or more subnetworks. Each subnetwork gets a portion of the address space, improving address allocation efficiency. You can secure resources within subnets using Network Security Groups. For more information, see Network security groups.

When you select a subnet for a cluster’s private endpoint, make sure enough private IP addresses are available in that subnet for your current and future needs.

Private endpoint. A private endpoint is a network interface that uses a private IP address from a virtual network. This network interface connects privately and securely to a service powered by Azure Private Link. Private endpoints bring the services into your virtual network.

Enabling private access for Azure Cosmos DB for PostgreSQL creates a private endpoint for the cluster’s coordinator node. The endpoint allows hosts in the selected virtual network to access the coordinator. You can optionally create private endpoints for worker nodes too.

Private DNS zone. An Azure private DNS zone resolves hostnames within a linked virtual network, and within any peered virtual network. Domain records for nodes are created in a private DNS zone selected for their cluster. Be sure to use fully qualified domain names (FQDN) for nodes' PostgreSQL connection strings.

You can use private endpoints for your clusters to allow hosts on a virtual network (VNet) to securely access data over a Private Link.

The cluster's private endpoint uses an IP address from the virtual network's address space. Traffic between hosts on the virtual network and nodes goes over a private link on the Microsoft backbone network, eliminating exposure to the public Internet.

Applications in the virtual network can connect to the nodes over the private endpoint seamlessly, using the same connection strings and authorization mechanisms that they would use otherwise.

You can select private access during cluster creation, and you can switch from public access to private access at any point.

Using a private DNS zone

A new private DNS zone is automatically provisioned for each private endpoint, unless you select one of the private DNS zones previously created by Azure Cosmos DB for PostgreSQL. For more information, see the private DNS zones overview.

The Azure Cosmos DB for PostgreSQL service creates DNS records such as c-mygroup01.12345678901234.privatelink.postgres.cosmos.azure.com in the selected private DNS zone for each node with a private endpoint. When you connect to a node from an Azure VM via private endpoint, Azure DNS resolves the node’s FQDN into a private IP address.

Private DNS zone settings and virtual network peering are independent of each other. If you want to connect to a node in the cluster from a client that's provisioned in another virtual network (from the same region or a different region), you have to link the private DNS zone with the virtual network. For more information, see Link the virtual network.

Note

The service also always creates public CNAME records such as c-mygroup01.12345678901234.postgres.cosmos.azure.com for every node. However, selected computers on the public internet can connect to the public hostname only if the database administrator enables public access to the cluster.

If you're using a custom DNS server, you must use a DNS forwarder to resolve the FQDN of nodes. The forwarder IP address should be 168.63.129.16. The custom DNS server should be inside the virtual network or reachable via the virtual network's DNS server setting. To learn more, see Name resolution that uses your own DNS server.

Recommendations

When you enable private access for your cluster, consider:

  • Subnet size: When selecting subnet size for a cluster, consider current needs such as IP addresses for coordinator or all nodes in that cluster, and future needs such as growth of that cluster. Make sure you have enough private IP addresses for the current and future needs. Keep in mind, Azure reserves five IP addresses in each subnet. See more details in this FAQ.
  • Private DNS zone: DNS records with private IP addresses are going to be maintained by Azure Cosmos DB for PostgreSQL service. Make sure you don’t delete private DNS zone used for clusters.

Limits and limitations

See Azure Cosmos DB for PostgreSQL limits and limitations page.

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