Azure Load Balancer Types

The term load balancing refers to the distribution of workloads across multiple computing resources.

Azure provides various load-balancing services that you can use to distribute your workloads across multiple computing resources. These resources include Azure Application Gateway, Azure Front Door, Azure Load Balancer, and Azure Traffic Manager.

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

Azure Load Balancer operates at Layer 4, Application Gateway, Azure Front Door and Traffic Manager operates at Layer 7.

To understand difference between Layer 4 and Layer 7 check out link

Azure Load Balancer
Distributes Layer 4(TCP/UDP) connectivity within single region.
Internal (or Private) Load balancers are used to load balance traffic inside a virtual network.
External load balancer is for external users calling our resources from outside VNET.
External Load Balancer can also be used for Out-Bound connectivity as NAT Gateway.

Azure Standard Load Balancer supports cross-region load balancing enabling geo-redundant high availability.

Application Gateway
It’s Layer 7 load balancer, have access to application resources like headers, route etc.

Application Gateway can make routing decisions based on additional attributes of an HTTP request, for example URI path or host headers. For example, you can route traffic based on the incoming URL. So, if /images is in the incoming URL, you can route traffic to a specific set of servers (known as a pool) configured for images. If /video is in the URL, that traffic is routed to another pool that's optimized for videos.

Azure Traffic Manager
It works across regions using DNS.
For Instance, if we have users in East US and application is hosted in West US and Australia Central, when user tries to access the application then traffic manager ensures it route to nearest region in this case West US. If West Us is down or not accessible, then traffic is routed to Australia central.

The most important point to understand is that Traffic Manager works at the DNS level which is at the Application layer (Layer-7). Traffic Manager uses DNS to direct clients to specific service endpoints based on the rules of the traffic-routing method. Clients connect to the selected endpoint directly. Traffic Manager is not a proxy or a gateway. Traffic Manager does not see the traffic passing between the client and the service.

Azure Front Door
Front door is an Acceleration solution uses Microsoft global edge network to provide fast connectivity, across globe. It acts like a global application gateway.

Azure Front Door is a cloud-based service that delivers your applications faster and more reliably. It uses layer 7 load balancing to distribute traffic across multiple regions and endpoints.
It also offers dynamic site acceleration (DSA) to optimize web performance and near real-time failover to ensure high availability.
Azure Front Door is a fully managed service, so you don’t have to worry about scaling or maintenance.

Front door vs Application Gateway
Front Door is a global service that can distribute requests across regions, while Application Gateway is a regional service that can balance requests within a region. Azure Front Door works with scale units, clusters or stamp units, while Azure Application Gateway works with VMs, containers or other resources in the same scale unit.

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