Load Balancer and Availability Zones

Azure Load Balancer supports availability zones scenarios. You can use Standard Load Balancer to increase availability throughout your scenario by aligning resources with, and distribution across zones. Review this document to understand these concepts and fundamental scenario design guidance

A Load Balancer can either be zone redundant, zonal, or non-zonal. To configure the zone related properties (mentioned above) for your load balancer, select the appropriate type of frontend needed.

Zone redundant

In a region with Availability Zones, a Standard Load Balancer can be zone-redundant. This traffic is served by a single IP address.

A single frontend IP address will survive zone failure. The frontend IP may be used to reach all (non-impacted) backend pool members no matter the zone. One or more availability zones can fail and the data path survives as long as one zone in the region remains healthy.

The frontend's IP address is served simultaneously by multiple independent infrastructure deployments in multiple availability zones. Any retries or reestablishment will succeed in other zones not affected by the zone failure.

Figure depicts a zone-redundant standard load balancer directing traffic in three different zones to three different subnets in a zone redundant configuration.

Figure: Zone redundant load balancer

Zonal

You can choose to have a frontend guaranteed to a single zone, which is known as a zonal. This scenario means any inbound or outbound flow is served by a single zone in a region. Your frontend shares fate with the health of the zone. The data path is unaffected by failures in zones other than where it was guaranteed. You can use zonal frontends to expose an IP address per Availability Zone.

Additionally, the use of zonal frontends directly for load balanced endpoints within each zone is supported. You can use this configuration to expose per zone load-balanced endpoints to individually monitor each zone. For public endpoints, you can integrate them with a DNS load-balancing product like Traffic Manager and use a single DNS name.

Figure depicts three zonal standard load balancers each directing traffic in a zone to three different subnets in a zonal configuration.

Figure: Zonal load balancer

For a public load balancer frontend, you add a zones parameter to the public IP. This public IP is referenced by the frontend IP configuration used by the respective rule.

For an internal load balancer frontend, add a zones parameter to the internal load balancer frontend IP configuration. A zonal frontend guarantees an IP address in a subnet to a specific zone.

Non-Zonal

Load Balancers can also be created in a non-zonal configuration by use of a "no-zone" frontend (Public IP or Public IP Prefix). This option does not give a guarantee of redundancy. Note that all Public IP addresses that are upgraded will be of type "no-zone".

Design considerations

Now that you understand the zone related properties for Standard Load Balancer, the following design considerations might help as you design for high availability.

Tolerance to zone failure

  • A zone redundant Load Balancer can serve a zonal resource in any zone with one IP address. The IP can survive one or more zone failures as long as at least one zone remains healthy within the region.
  • A zonal frontend is a reduction of the service to a single zone and shares fate with the respective zone. If the zone your deployment is in goes down, your deployment will not survive this failure.

It is recommended you use zone-redundant Load Balancer for your production workloads.

Multiple frontends

Using multiple frontends allow you to load balance traffic on more than one port and/or IP address. When designing your architecture, it is important to account for the way zone redundancy and multiple frontends can interact. Note that if the goal is to always have every frontend be resilient to failure, then all IP addresses used assigned as frontends must be zone-redundant. If a set of frontends are intended to be associated with a single zone, then every IP address for that set must be associated with that specific zone. It is not required to have a load balancer for each zone; rather, each zonal frontend (or set of zonal frontends) could be associated with virtual machines in the backend pool that are part of that specific availability zone.

Transition between regional zonal models

In the case where a region is augmented to have availability zones would remain non-zonal. In order to ensure your architecture can take advantage of the new zones, it is recommended that new frontend IPs be created, and the appropriate rules and configurations be replicated to utilizes these new public IPs.

Control vs data plane implications

Zone-redundancy doesn't imply hitless data plane or control plane. Zone-redundant flows can use any zone and your flows will use all healthy zones in a region. In a zone failure, traffic flows using healthy zones aren't affected.

Traffic flows using a zone at the time of zone failure may be affected but applications can recover. Traffic continues in the healthy zones within the region upon retransmission when Azure has converged around the zone failure.

Review Azure cloud design patterns to improve the resiliency of your application to failure scenarios.

Limitations

  • Zones can't be changed, updated, or created for the resource after creation.
  • Resources can't be updated from zonal to zone-redundant or vice versa after creation.

Next steps