Overview of load-balancing options in Azure

The term load balancing refers to the distribution of workloads across multiple computing resources. Load balancing aims to optimize resource use, maximize throughput, minimize response time, and avoid overloading any single resource. It can also improve availability by sharing a workload across redundant computing resources.

Overview

Azure load balancing services can be categorized along two dimensions: global versus regional, and HTTP(S) versus non-HTTP(S).

Global versus regional

  • Global load-balancing services distribute traffic across regional backends, clouds, or hybrid on-premises services. These services route end-user traffic to the closest available backend. They also react to changes in service reliability or performance, in order to maximize availability and performance. You can think of them as systems that load balance between application stamps, endpoints, or scale-units hosted across different regions/geographies.

  • Regional load-balancing services distribute traffic within virtual networks across virtual machines (VMs) or zonal and zone-redundant service endpoints within a region. You can think of them as systems that load balance between VMs, containers, or clusters within a region in a virtual network.

HTTP(S) versus non-HTTP(S)

  • HTTP(S) load-balancing services are Layer 7 load balancers that only accept HTTP(S) traffic. They are intended for web applications or other HTTP(S) endpoints. They include features such as SSL offload, web application firewall, path-based load balancing, and session affinity.

  • Non-HTTP/S load-balancing services can handle non-HTTP(S) traffic and are recommended for non-web workloads.

The following table summarizes the Azure load balancing services by these categories:

Service Global/regional Recommended traffic
Azure Front Door Global HTTP(S)
Traffic Manager Global non-HTTP(S)
Application Gateway Regional HTTP(S)
Azure Load Balancer Regional non-HTTP(S)

Azure load balancing services

Here are the main load-balancing services currently available in Azure:

Front Door is an application delivery network that provides global load balancing and site acceleration service for web applications. It offers Layer 7 capabilities for your application like SSL offload, path-based routing, fast failover, caching, etc. to improve performance and high-availability of your applications.

Traffic Manager is a DNS-based traffic load balancer that enables you to distribute traffic optimally to services across global Azure regions, while providing high availability and responsiveness. Because Traffic Manager is a DNS-based load-balancing service, it load balances only at the domain level. For that reason, it can't fail over as quickly as Front Door, because of common challenges around DNS caching and systems not honoring DNS TTLs.

Application Gateway provides application delivery controller (ADC) as a service, offering various Layer 7 load-balancing capabilities. Use it to optimize web farm productivity by offloading CPU-intensive SSL termination to the gateway.

Azure Load Balancer is an integral part of the Azure SDN stack, providing high-performance, low-latency Layer 4 load-balancing services (inbound and outbound) for all UDP and TCP protocols.

Decision tree for load balancing in Azure

When selecting the load-balancing options, here are some factors to consider:

  • Traffic type. Is it a web (HTTP/HTTPS) application? Is it public facing or a private application?
  • Global versus. regional. Do you need to load balance VMs or containers within a virtual network, or load balance scale unit/deployments across regions, or both?
  • Availability. What is the service SLA?
  • Cost. See Azure pricing. In addition to the cost of the service itself, consider the operations cost for managing a solution built on that service.
  • Features and limits. What are the overall limitations of each service? See Service limits.

The following flowchart will help you to choose a load-balancing solution for your application. The flowchart guides you through a set of key decision criteria to reach a recommendation.

Treat this flowchart as a starting point. Every application has unique requirements, so use the recommendation as a starting point. Then perform a more detailed evaluation.

If your application consists of multiple workloads, evaluate each workload separately. A complete solution may incorporate two or more load-balancing solutions.

Decision tree for load balancing in Azure

Definitions

  • Internet facing. Applications that are publicly accessible from the internet. As a best practice, application owners apply restrictive access policies or protect the application by setting up offerings like web application firewall and DDoS protection.

  • Global. End users or clients are located beyond a small geographical area. For example, users across multiple continents, across countries within a continent, or even across multiple metropolitan areas within a larger country.

  • PaaS. Platform as a service (PaaS) services provide a managed hosting environment, where you can deploy your application without needing to manage VMs or networking resources. In this case, PaaS refers to services that provide integrated load balancing within a region. See Choosing a compute service – Scalability.

  • IaaS. Infrastructure as a service (IaaS) is a computing option where you provision the VMs that you need, along with associated network and storage components. IaaS applications require internal load balancing within a virtual network, using Azure Load Balancer.

  • Application-layer processing refers to special routing within a virtual network. For example, path-based routing within the virtual network across VMs or virtual machine scale sets. For more information, see When should we deploy an Application Gateway behind Front Door?.