Before a client sends a request to your load balancer, it resolves the load balancer’s domain name using a Domain Name System (DNS) server. The DNS entry is controlled by Amazon, because your load balancers are in the amazonaws.com domain. The Amazon DNS servers return one or more IP addresses to the client. These are the IP addresses of the load balancer nodes for your load balancer. With Network Load Balancers, Elastic Load Balancing creates a network interface for each Availability Zone that you enable, and uses it to get a static IP address. You can optionally associate one Elastic IP address with each network interface when you create the Network Load Balancer.
As traffic to your application changes over time, Elastic Load Balancing scales your load balancer and updates the DNS entry. The DNS entry also specifies the time-to-live (TTL) of 60 seconds. This helps ensure that the IP addresses can be remapped quickly in response to changing traffic.
The client determines which IP address to use to send requests to the load balancer. The load balancer node that receives the request selects a healthy registered target and sends the request to the target using its private IP address.
Routing Alogorithm
With Application Load Balancers, the load balancer node that receives the request uses the following process:
- Evaluates the listener rules in priority order to determine which rule to apply.
- Selects a target from the target group for the rule action, using the routing algorithm configured for the target group. The default routing algorithm is round robin. Routing is performed independently for each target group, even when a target is registered with multiple target groups.
With Network Load Balancers, the load balancer node that receives the connection uses the following process:
- Selects a target from the target group for the default rule using a flow hash algorithm. It bases the algorithm on:
- The protocol
- The source IP address and source port
- The destination IP address and destination port
- The TCP sequence number
- Routes each individual TCP connection to a single target for the life of the connection. The TCP connections from a client have different source ports and sequence numbers, and can be routed to different targets.
With Classic Load Balancers, the load balancer node that receives the request selects a registered instance as follows:
- Uses the round robin routing algorithm for TCP listeners
- Uses the least outstanding requests routing algorithm for HTTP and HTTPS listeners
HTTP Connections
Classic Load Balancers use pre-open connections, but Application Load Balancers do not. Both Classic Load Balancers and Application Load Balancers use connection multiplexing. This means that requests from multiple clients on multiple front-end connections can be routed to a given target through a single backend connection. Connection multiplexing improves latency and reduces the load on your applications. To prevent connection multiplexing, disable HTTP keep-alive headers by setting the Connection: close header in your HTTP responses.
Load Balancer Scheme
When you create a load balancer, you must choose whether to make it an internal load balancer or an internet-facing load balancer.
The nodes of an internet-facing load balancer have public IP addresses. The DNS name of an internet-facing load balancer is publicly resolvable to the public IP addresses of the nodes. Therefore, internet-facing load balancers can route requests from clients over the internet.
The nodes of an internal load balancer have only private IP addresses. The DNS name of an internal load balancer is publicly resolvable to the private IP addresses of the nodes. Therefore, internal load balancers can only route requests from clients with access to the VPC for the load balancer.
Both internet-facing and internal load balancers route requests to your targets using private IP addresses. Therefore, your targets do not need public IP addresses to receive requests from an internal or an internet-facing load balancer.
If your application has multiple tiers, you can design an architecture that uses both internal and internet-facing load balancers. For example, this is true if your application uses web servers that must be connected to the internet, and application servers that are only connected to the web servers. Create an internet-facing load balancer and register the web servers with it. Create an internal load balancer and register the application servers with it. The web servers receive requests from the internet-facing load balancer and send requests for the application servers to the internal load balancer. The application servers receive requests from the internal load balancer.