Allow different EPMD ports

[jonenst]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I can't freely choose the ports of listening servers. https://www.rabbitmq.com/docs/networking#epmd-port says "All hosts in a cluster must use the same [EPMD] port.". But for various reasons, you may want to use different ports for the different servers:

• on one computer a port is available but on another computer it is not. This can be frustrating if you decide to use a computer later on and its port is not available, you either have to move the port for everyone or find another computer.
• when running on a single computer, you may want to run multiple servers to simulate having multiple servers
• when running on a single computer using docker, you may want to force all traffic to go through the host with mapped ports.

Describe the solution you'd like

specify the epmd port of each target server:

rabbitmqctl join_cluster rabbit@my-rabbit-01.localdomain:4370

Describe alternatives you've considered

when using docker on a single computer, use a docker network for open access between nodes without going through the host

Rejected solutions:

• when running on a single computer, allow the official community rabbitmq docker image from dockerhub to use a single external shared epmd server and provide clear instructions on how to set it up.
• write my own epmd reverse-proxy that can correctly dispatch connections to upstream servers running on different ports based on the name
• run virtual machines
• buy more servers
• use another message broker

Additional context

No response

[lukebakken]

Hello, thanks for using RabbitMQ.

This is the first time I've seen this request come up, so it's very unlikely we will spend time on it unless a customer that pays for a commercial license requests it.

More likely, we would remove the epmd requirement from RabbitMQ altogether at some point.

@michaelklishin anything to add?

[mkuratczyk]

I was planning on suggesting the same: we basically don't need epmd at this point (except, perhaps, for local development). However, I'm not sure whether this solves the problem - we currently use epmd to "discover" the port which RabbitMQ uses, which is basically always 25672. Therefore, we don't need epmd (we know the answer it'll provide) but perhaps it just moves the problem to a different place - all nodes use 25672.

[lukebakken]

Yep. We could allow per-node disterl ports, but then we'd have to have a way to configure the other nodes with that information (peer discovery) and really that's not worth working on unless someone wants to pay for it.

[jonenst]

Thank you for your answer. I totally understand that you don't want to add this feature. I just wanted to share what felt like a valid use case (see my other reply for more details). Thanks again

[michaelklishin]

@jonenst you can override the port used by RabbitMQ nodes to receive traffic from peers and CLI tools. You can also override epmd port for any Erlang node, including RabbitMQ.

Both are documented:

1. Inter-node Communication Port Range
2. ERL_EPMD_PORT.

Now that we require a recent version of Erlang, and that supports "epmd-less" clustering, we can simply move to use that in 4.1 or 4.2.

[jonenst]

hi @michaelklishin thanks again for your answer. I had seen the docs you linked to. When you are running local clusters outside of docker on a single computer, are you using a single epmd server for multiple rabbitmq nodes ? In this case you don't have the problem of having to bind multiple times on the the epmd port.

If you use containers, you can't forward them differently because the epmd port must be the same for all empd servers in the cluster (which is what the docs say here https://www.rabbitmq.com/docs/networking#epmd-port or is that wrong ?) and you have one epmd server per container. Could you give an example of using docker on a single computer without using docker networks and having traffic going through differently mapped port on the host ?

If you don't have time that's ok too, I don't want to impose.

Thanks.

[michaelklishin]

@jonenst yes, all nodes use the same epmd and register different ports with it (the inter-node communication port).

[jonenst]

and so when running multiple docker containers on a single computer without creating a docker network (only using mapped ports on the host), what do you do ? A single shared epmd daemon but how do you get all nodes to use it from their respective containers ? Multiple epmd daemons but how do you deal with the fact that they must all bind on the same port on the host ?

[jonenst]

also https://www.rabbitmq.com/docs/networking#what-is-epmd-and-how-is-it-used says "epmd runs alongside every RabbitMQ node" and sounds like each node must always have its own epmd (not the case as you described when running multiple rabbitmq nodes on the same computer with a shared epmd) (even though with careful reading one can understand that the epmd may be shared, but the logic of when rabbitmq decides to spawn an epmd daemon is opaque). Maybe rephrase this documentation ? or maybe it's clear enough already. or even maybe this is too niche of use case and not worth talking about in the docs, not sure.

[michaelklishin]

@jonenst in 99.9% of production clusters you do not colocate N nodes on a single host, so each node does have its own local epmd. If N nodes run on the same host, they will all use the same local epmd perfectly fine.

I don't see any need to rephrase the documentation.