What is the Azure Active Directory architecture?

• Article
• 08/18/2022
• 6 minutes to read

In this article

Azure Active Directory (Azure AD) enables you to securely manage access to Azure services and resources for your users. Included with Azure AD is a full suite of identity management capabilities. For information about Azure AD features, see What is Azure Active Directory?

With Azure AD, you can create and manage users and groups, and enable permissions to allow and deny access to enterprise resources. For information about identity management, see The fundamentals of Azure identity management.

Azure AD architecture

Azure AD’s geographically distributed architecture combines extensive monitoring, automated rerouting, failover, and recovery capabilities, which deliver company-wide availability and performance to customers.

The following architecture elements are covered in this article:

• Service architecture design
• Scalability
• Continuous availability
• Datacenters

Service architecture design

The most common way to build an accessible and usable, data-rich system is through independent building blocks or scale units. For the Azure AD data tier, scale units are called partitions.

The data tier has several front-end services that provide read-write capability. The diagram below shows how the components of a single-directory partition are delivered throughout geographically distributed datacenters.

The components of Azure AD architecture include a primary replica and secondary replicas.

Primary replica

The primary replica receives all writes for the partition it belongs to. Any write operation is immediately replicated to a secondary replica in a different datacenter before returning success to the caller, thus ensuring geo-redundant durability of writes.

Secondary replicas

All directory reads are serviced from secondary replicas, which are at datacenters that are physically located across different geographies. There are many secondary replicas, as data is replicated asynchronously. Directory reads, such as authentication requests, are serviced from datacenters that are close to customers. The secondary replicas are responsible for read scalability.

Scalability

Scalability is the ability of a service to expand to meet increasing performance demands. Write scalability is achieved by partitioning the data. Read scalability is achieved by replicating data from one partition to multiple secondary replicas distributed throughout the world.

Requests from directory applications are routed to the closest datacenter. Writes are transparently redirected to the primary replica to provide read-write consistency. Secondary replicas significantly extend the scale of partitions because the directories are typically serving reads most of the time.

Directory applications connect to the nearest datacenters. This connection improves performance, and therefore scaling out is possible. Since a directory partition can have many secondary replicas, secondary replicas can be placed closer to the directory clients. Only internal directory service components that are write-intensive target the active primary replica directly.

Continuous availability

Availability (or uptime) defines the ability of a system to perform uninterrupted. The key to Azure AD’s high-availability is that the services can quickly shift traffic across multiple geographically distributed datacenters. Each datacenter is independent, which enables de-correlated failure modes. Through this high availability design, Azure AD requires no downtime for maintenance activities.

Azure AD’s partition design is simplified compared to the enterprise AD design, using a single-master design that includes a carefully orchestrated and deterministic primary replica failover process.

Fault tolerance

A system is more available if it is tolerant to hardware, network, and software failures. For each partition on the directory, a highly available master replica exists: The primary replica. Only writes to the partition are performed at this replica. This replica is being continuously and closely monitored, and writes can be immediately shifted to another replica (which becomes the new primary) if a failure is detected. During failover, there could be a loss of write availability typically of 1-2 minutes. Read availability isn’t affected during this time.

Read operations (which outnumber writes by many orders of magnitude) only go to secondary replicas. Since secondary replicas are idempotent, loss of any one replica in a given partition is easily compensated by directing the reads to another replica, usually in the same datacenter.

Data durability

A write is durably committed to at least two datacenters prior to it being acknowledged. This happens by first committing the write on the primary, and then immediately replicating the write to at least one other datacenter. This write action ensures that a potential catastrophic loss of the datacenter hosting the primary doesn’t result in data loss.

Azure AD maintains a zero Recovery Time Objective (RTO) to not lose data on failovers. This includes:

• Token issuance and directory reads
• Allowing only about 5 minutes RTO for directory writes

Datacenters

Azure AD’s replicas are stored in datacenters located throughout the world. For more information, see Azure global infrastructure.

Azure AD operates across datacenters with the following characteristics:

• Authentication, Graph, and other AD services reside behind the Gateway service. The Gateway manages load balancing of these services. It will fail over automatically if any unhealthy servers are detected using transactional health probes. Based on these health probes, the Gateway dynamically routes traffic to healthy datacenters.
• For reads, the directory has secondary replicas and corresponding front-end services in an active-active configuration operating in multiple datacenters. If a datacenter fails, traffic is automatically routed to a different datacenter.
• For writes, the directory will fail over the primary replica across datacenters via planned (new primary is synchronized to old primary) or emergency failover procedures. Data durability is achieved by replicating any commit to at least two datacenters.

Data consistency

The directory model is one of eventual consistencies. One typical problem with distributed asynchronously replicating systems is that the data returned from a “particular” replica may not be up-to-date.

Azure AD provides read-write consistency for applications targeting a secondary replica by routing its writes to the primary replica, and synchronously pulling the writes back to the secondary replica.

Application writes using the Microsoft Graph API of Azure AD are abstracted from maintaining affinity to a directory replica for read-write consistency. The Microsoft Graph API service maintains a logical session, which has affinity to a secondary replica used for reads; affinity is captured in a “replica token” that the service caches using a distributed cache in the secondary replica datacenter. This token is then used for subsequent operations in the same logical session. To continue using the same logical session, subsequent requests must be routed to the same Azure AD datacenter. It isn’t possible to continue a logical session if the directory client requests are being routed to multiple Azure AD datacenters; if this happens then the client has multiple logical sessions that have independent read-write consistencies.

Note

Writes are immediately replicated to the secondary replica to which the logical session’s reads were issued.

Service-level backup

Azure AD implements daily backup of directory data and can use these backups to restore data if there is any service-wide issue.

The directory also implements soft deletes instead of hard deletes for selected object types. The tenant administrator can undo any accidental deletions of these objects within 30 days. For more information, see the API to restore deleted objects.

Metrics and monitors

Running a high availability service requires world-class metrics and monitoring capabilities. Azure AD continually analyzes and reports key service health metrics and success criteria for each of its services. There is also continuous development and tuning of metrics and monitoring and alerting for each scenario, within each Azure AD service and across all services.

If any Azure AD service isn’t working as expected, action is immediately taken to restore functionality as quickly as possible. The most important metric Azure AD tracks is how quickly live site issues can be detected and mitigated for customers. We invest heavily in monitoring and alerts to minimize time to detect (TTD Target: <5 minutes) and operational readiness to minimize time to mitigate (TTM Target: <30 minutes).

Secure operations

Using operational controls such as multi-factor authentication (MFA) for any operation, and auditing of all operations. In addition, using a just-in-time elevation system to grant necessary temporary access for any operational task-on-demand on an ongoing basis. For more information, see The Trusted Cloud.

Next steps

Azure Active Directory developer’s guide