Map one AWS ELB load balancer port to balance to multiple instance ports

amazon-elbamazon-web-services

Lets say I have multiple processes running on multiple EC2 instances each on different ports. Can I map a single AWS ELB to balance to any of these processes across any instance?

For example, I'd ideally like a setup like this:

HTTPS 443 -> Instance 1 Process 1 Port 8081
       or -> Instance 1 Process 2 Port 8082
       or -> Instance 2 Process 1 Port 8081
       or -> Instance 2 Process 2 Port 8082

Is there any way to configure ELB like this? I'm trying to avoid running a load balancer on each instance to balance across that instance's processes.

Best Answer

Important Update

The service offering has evolved, so the following information (the original answer) is only applicable to ELB/1.0, which is now called an ELB Classic Load Balancer:

No, this isn't possible with ELB.

You can map multiple listeners on the front-side of an ELB to hit a single port on the back-end instances, but you can't map a single listener on the font-side of an ELB to hit multiple ports on the back-end instances.

ELB/2.0, called Application Load Balancer was released in late 2016 and does support multiple instance ports, using "target groups."

It also allows selective request routing by path pattern matching, supports web sockets in HTTP mode, and HTTP/2 on the front side.

When launching a balancer, you can still launch a Classic (it's still available, and the single instance port limit per listener still applies) or you can launch an ALB to get the new functionality.

Related Solutions

Elastic Load Balancer for multiple webapps

A single ELB routes traffic to exactly one set of instances, and distributes the incoming traffic to all the instances "behind" it. It does not selectively route traffic based on any layer 7 analysis of the traffic, such as the Host: header.

You need one ELB for each set of instances. As you describe it, that's one ELB for each webapp.

If your primary purpose for running ELB is offloading the SSL using a wildcard certificate (I have one system designed like this, with dozens of apps living at many-different-domains.my-wildcard-cert-domain.com), then the instances "behind" the ELB could be running a reverse proxy such as HAProxy (or several other alternatives, like Varnish) that can make layer-7 routing decisions and then forward the traffic to the appropriate subset of machines behind them, which also allows more sophisticated load balancing and has the advantage of providing you with stats and traffic counters, aggregate and separate.

       /-- HAProxy \  /----- instances hosting app #1  
ELB ---|            >> ----- instances hosting app #2 
       \-- HAProxy /  \----- instances hosting app #n

The intermediate ^^^^ instances can evaluate the Host: headers (among other things) and even capture the value of the session cookie in their logs for analysis.

This setup also allows me to run multiple apps on overlapping subsets of instances, where appropriate, and do a lot of other things that ELB by itself doesn't directly support. It also returns a custom "503" page in the case where an application gets overloaded or otherwise becomes unavailable, which ELB does not do on its own. I've depicted 2 proxy servers here, for no particular reason other than your mention of the number 2 in the question. My setup actually has 3, one for each availability zone in the region where this is deployed.

Amazon ELB – Setting Up ELB Health Checks with Multiple Applications on EC2 Instances

Here are two ways to solve this;

First option is to add another health check on the host that validates the health and returns HTTP 200s to the ELB if the logic says that you want to keep the host online. The logic there is, of course, up to you. The disadvantage here would be that if App 2 deployed successfully on some hosts all hosts would still be 'healthy' and receiving traffic.

Another option is to use an additional ELB for each application. You can point several ELBs to the same backend EC2 instances and the cost is pretty minor to do so. That way you can health check per application and drop hosts with issues at a per-application level rather than an all-or-nothing approach.

Edit: Please note this is an older answer and is specific to ELB not ALB. ALB supports separate targets on one host natively.

Best Answer

Related Solutions

Elastic Load Balancer for multiple webapps

Amazon ELB – Setting Up ELB Health Checks with Multiple Applications on EC2 Instances

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