Often an organization wants to provide reliability for its services by deploying one or more backup services in case their primary service goes down. The 'Priority' traffic-routing method allows Azure customers to easily implement this failover pattern.

The Traffic Manager profile contains a prioritized list of service endpoints. By default, Traffic Manager sends all traffic to the primary (highest-priority) endpoint. If the primary endpoint is not available, Traffic Manager routes the traffic to the second endpoint. If both the primary and secondary endpoints are not available, the traffic goes to the third, and so on. Availability of the endpoint is based on the configured status (enabled or disabled) and the ongoing endpoint monitoring.

Configuring endpoints

With Azure Resource Manager, you configure the endpoint priority explicitly using the 'priority' property for each endpoint. This property is a value between 1 and 1000. Lower values represent a higher priority. Endpoints cannot share priority values. Setting the property is optional. When omitted, a default priority based on the endpoint order is used.

The 'Weighted' traffic-routing method allows you to distribute traffic evenly or to use a pre-defined weighting.

In the Weighted traffic-routing method, you assign a weight to each endpoint in the Traffic Manager profile configuration. The weight is an integer from 1 to 1000. This parameter is optional. If omitted, Traffic Managers uses a default weight of '1'. The higher weight, the higher the priority.

For each DNS query received, Traffic Manager randomly chooses an available endpoint. The probability of choosing an endpoint is based on the weights assigned to all available endpoints. Using the same weight across all endpoints results in an even traffic distribution. Using higher or lower weights on specific endpoints causes those endpoints to be returned more or less frequently in the DNS responses.

The weighted method enables some useful scenarios:

• Gradual application upgrade: Allocate a percentage of traffic to route to a new endpoint, and gradually increase the traffic over time to 100%.
• Application migration to Azure: Create a profile with both Azure and external endpoints. Adjust the weight of the endpoints to prefer the new endpoints.
• Cloud-bursting for additional capacity: Quickly expand an on-premises deployment into the cloud by putting it behind a Traffic Manager profile. When you need extra capacity in the cloud, you can add or enable more endpoints and specify what portion of traffic goes to each endpoint.

The Resource Manager Azure portal supports the configuration of weighted traffic routing. You can configure weights using the Resource Manager versions of Azure PowerShell, CLI, and the REST APIs.

It is important to understand that DNS responses are cached by clients and by the recursive DNS servers that the clients use to resolve DNS names. This caching can have an impact on weighted traffic distributions. When the number of clients and recursive DNS servers is large, traffic distribution works as expected. However, when the number of clients or recursive DNS servers is small, caching can significantly skew the traffic distribution.

Common use cases include:

• Development and testing environments
• Application-to-application communications
• Applications aimed at a narrow user-base that share a common recursive DNS infrastructure (for example, employees of company connecting through a proxy)

These DNS caching effects are common to all DNS-based traffic routing systems, not just Azure Traffic Manager. In some cases, explicitly clearing the DNS cache may provide a workaround. In other cases, an alternative traffic-routing method may be more appropriate.

Deploying endpoints in two or more locations across the globe can improve the responsiveness of many applications by routing traffic to the location that is 'closest' to you. The 'Performance' traffic-routing method provides this capability.

The 'closest' endpoint is not necessarily closest as measured by geographic distance. Instead, the 'Performance' traffic-routing method determines the closest endpoint by measuring network latency. Traffic Manager maintains an Internet Latency Table to track the round-trip time between IP address ranges and each Azure datacenter.

Traffic Manager looks up the source IP address of the incoming DNS request in the Internet Latency Table. Traffic Manager chooses an available endpoint in the Azure datacenter that has the lowest latency for that IP address range, then returns that endpoint in the DNS response.

As explained in How Traffic Manager Works, Traffic Manager does not receive DNS queries directly from clients. Rather, DNS queries come from the recursive DNS service that the clients are configured to use. Therefore, the IP address used to determine the 'closest' endpoint is not the client's IP address, but it is the IP address of the recursive DNS service. In practice, this IP address is a good proxy for the client.

Traffic Manager regularly updates the Internet Latency Table to account for changes in the global Internet and new Azure regions. However, application performance varies based on real-time variations in load across the Internet. Performance traffic-routing does not monitor load on a given service endpoint. However, if an endpoint becomes unavailable, Traffic Manager does not include it in DNS query responses.

Points to note:

• If your profile contains multiple endpoints in the same Azure region, then Traffic Manager distributes traffic evenly across the available endpoints in that region. If you prefer a different traffic distribution within a region, you can use nested Traffic Manager profiles.
• If all enabled endpoints in the closest Azure region are degraded, Traffic Manager moves traffic to the endpoints in the next closest Azure region. If you want to define a preferred failover sequence, use nested Traffic Manager profiles.
• When using the Performance traffic routing method with external endpoints or nested endpoints, you need to specify the location of those endpoints. Choose the Azure region closest to your deployment. Those locations are the values supported by the Internet Latency Table.
• The algorithm that chooses the endpoint is deterministic. Repeated DNS queries from the same client are directed to the same endpoint. Typically, clients use different recursive DNS servers when traveling. The client may be routed to a different endpoint. Routing can also be affected by updates to the Internet Latency Table. Therefore, the Performance traffic-routing method does not guarantee that a client is always routed to the same endpoint.
• When the Internet Latency Table changes, you may notice that some clients are directed to a different endpoint. This routing change is more accurate based on current latency data. These updates are essential to maintain the accuracy of Performance traffic-routing as the Internet continually evolves.

Traffic Manager profiles can be configured to use the Geographic routing method so that users are directed to specific endpoints (Azure, External or Nested) based on which geographic location their DNS query originates from. This empowers Traffic Manager customers to enable scenarios where knowing a user’s geographic region and routing them based on that is important. Examples include complying with data sovereignty mandates, localization of content & user experience and measuring traffic from different regions. When a profile is configured for geographic routing, each endpoint associated with that profile needs to have a set of geographic regions assigned to it. A geographic region can be at following levels of granularity

• World– any region
• Regional Grouping – for example, Africa, Middle East, Australia/Pacific etc.
• Country/Region – for example, Ireland, Peru, Hong Kong SAR etc.
• State/Province – for example, USA-California, Australia-Queensland, Canada-Alberta etc. (note: this granularity level is supported only for states / provinces in Australia, Canada, UK, and USA).

When a region or a set of regions is assigned to an endpoint, any requests from those regions gets routed only to that endpoint. Traffic Manager uses the source IP address of the DNS query to determine the region from which a user is querying from – usually this is the IP address of the local DNS resolver doing the query on behalf of the user.

Traffic Manager reads the source IP address of the DNS query and decides which geographic region it is originating from. It then looks to see if there is an endpoint that has this geographic region mapped to it. This lookup starts at the lowest granularity level (State/Province where it is supported, else at the Country/Region level) and goes all the way up to the highest level, which is World. The first match found using this traversal is designated as the endpoint to return in the query response. When matching with a Nested type endpoint, an endpoint within that child profile is returned, based on its routing method. The following points are applicable to this behavior:

• A geographic region can be mapped only to one endpoint in a Traffic Manager profile when the routing type is Geographic Routing. This ensures that routing of users is deterministic, and customers can enable scenarios that require unambiguous geographic boundaries.
• If a user’s region comes under two different endpoints’ geographic mapping, Traffic Manager selects the endpoint with the lowest granularity and does not consider routing requests from that region to the other endpoint. For example, consider a Geographic Routing type profile with two endpoints - Endpoint1 and Endpoint2. Endpoint1 is configured to receive traffic from Ireland and Endpoint2 is configured to receive traffic from Europe. If a request originates from Ireland, it is always routed to Endpoint1.

• Since a region can be mapped only to one endpoint, Traffic Manager returns it regardless of whether the endpoint is healthy or not.

  Important

  It is strongly recommended that customers using the geographic routing method associate it with the Nested type endpoints that has child profiles containing at least two endpoints within each.

• If an endpoint match is found and that endpoint is in the Stopped state, Traffic Manager returns a NODATA response. In this case, no further lookups is made higher up in the geographic region hierarchy. This behavior is also applicable for nested endpoint types when the child profile is in the Stopped or Disabled state.
• If an endpoint displays a Disabled status, it won’t be included in the region matching process. This behavior is also applicable for nested endpoint types when the endpoint is in the Disabled state.
• If a query is coming from a geographic region that has no mapping in that profile, Traffic Manager returns a NODATA response. Therefore, it is strongly recommended that customers use geographic routing with one endpoint, ideally of type Nested with at least two endpoints within the child profile, with the region World assigned to it. This also ensures that any IP addresses that do not map to a region are handled.

As explained in How Traffic Manager Works, Traffic Manager does not receive DNS queries directly from clients. Rather, DNS queries come from the recursive DNS service that the clients are configured to use. Therefore, the IP address used to determine the region is not the client's IP address, but it is the IP address of the recursive DNS service. In practice, this IP address is a good proxy for the client.