Why do I need a port.mapping property when clustering WSO2 API Manager/WSO2 ESB?

In a dynamically clustered set up where you front a carbon instance using a WSO2 ELB, it is the responsibility of a Carbon server to send its information to ELB. You can visualize this as a "member object somehow getting passed to ELB from the Carbon server instance". In the Carbon server's clustering section, under properties, you can define any Member property. This way, you can let ELB know about the information other than the basic ones. You might think, what is the basic information? Typically those are host names, http port, https port, etc.

WSO2 ESB, WSO2 API Manager etc. are somewhat special with regard to ports as they usually have two http ports (compare with one http port of WSO2 AS). Hence, here we have to somehow send this additional information to ELB. The easiest way to do this is by setting a member property. Here, we use port.mapping property. Also, in order to front these special servers, we need two http ports in ELB too, which are exposed to the outside. There's a deployment decision to be made here, i.e. which http port of ELB should map to which http port of the server (i.e. servlet http port or Nhttp http port). With that in mind, let me explain how you should use this port.mapping property.
Let's consider only the http scenario. Say, in your API-M instance, you have used 8280 as the Nhttp transport port (axis2.xml) and 9763 as the Servlet transport port (catalina-server.xml). Also, ELB has 2 http ports one is 8280 and the other is 8290. Imagine there's a Member object, and in this case Member's http port would be 8280 (usually the port defined in axis2.xml gets here). But since ELB has 2 ports, there's no way to correctly map ports, by only specifying Member's http port. There arises the importance of port.mapping property. You have to think this property from the perspective of ELB.

Let's assume we define the above property, now this means, if a request comes to ELB, in its 8290 port (see... we're thinking from ELB's perspective), forward that request to the 9763 port of the Member. Having only this property is enough, we do not need following property,

Let me explain why. The logic was written in a way that port.mapping properties get higher precedence over the default ports. This means, that when a request comes to ELB, ELB will first check whether the port it received the request from is specified as a port.mapping property. If it is, it will grab the target port from that property. If not, it will send the request to the default http port. Hence, if a request received to the 8280 port of ELB, it will be automatically get redirected to 8280 port of the Member (since it's the http port of Member).

Similarly, we should define a mapping for https servlet port.

Source: https://nirmalfdo.blogspot.com/2013/09/why-i-need-portmapping-property-when.html