Do you use Docker?
Does your containerized app have the need to discover both its own IP and one or more mapped ports?
How can another container access my exposed ports and how can I do the same of my peers?
As it stands today, simple self discovery of your container’s accessible IP and one or more of its mapped ports is not exposed to your Docker container process as a native feature of the engine itself.
If you’ve attempted to containerize an app that attempts to discover its peers in order to form its own peer-level cluster etc, you’ve likely run into this challenge.
That said there are several tools out there with can help you with this issue. One of which is Registrator which is a special container that listens for events from a Docker host and acts as service discovery bridge that relays this info into other tooling such as Consul and etcd etc. In short, when your container is launched, the Registrator container collects all the info about the docker host it is running on and its exposed ports and registers this under a named service in one of the aforementioned backends.
This is all fine and great, however this still puts a lot of work on you, the container developer who needs to collect this info and then act upon it in order to form a higher level cluster between your containers.
I had this exact same problem for a Java based service that needed to form a Hazelcast cluster dynamically. Out of that use case I came up with a generic library that you can drop into your Java container application called docker-discovery-registrator-consul which is available at: https://github.com/bitsofinfo/docker-discovery-registrator-consul
The purpose of this library is for “self-discovery” from within your JVM based Docker application where you need to discover what your accessible docker-host bound IP and mapped port(s) are, as well as your peers within the same service. As noted above this is critical if your container has to do further peer discovery for other services it provides or clustering groups it must form.
You can read all the details of how it works and how to use it here: https://github.com/bitsofinfo/docker-discovery-registrator-consul
Hopefully it will be of use to you as well.
I’ve used Hazelcast for years and have generally relied upon the availability of multicast for Hazelcast cluster discovery and formation (within a single data-center). Recently was faced with two things, expand the footprint into a non-multicast enabled data-center and secondly pre-prep the service for containerization where nodes will come and go as scaling policies dictate it…. hardwired Hazelcast clustering via an XML configuration and/or reliance on multicast is a no-go.
With Hazelcast 3.6, they now support a pluggable implementation for a cluster discovery mechanism called the Discovery SPI. (Discovery Strategy) Perfect timing, given we are already playing with Etcd as part of our Docker container strategy, this was an opportunity to let our application’s native clustering mechanism (coded on top of Hazelcast) to leverage Etcd as well as discover/remove peers both within, and potentially across data-centers.
So I coded up hazelcast-etcd-discovery-spi available on GitHub.
This works with Hazelcast 3.6-EA+ and Etcd to provide (optional) automatic registration of your hazelcast nodes as Etcd services and automatic peer discovery of the Hazelcast cluster.
Note that the automatic registration of each hazelcast instance as a Etcd service is OPTIONAL if you want to manually maintain these key-paths in etcd. I added that in simply because I think it will be convenient for folks, especially when containerizing a Hazelcast enabled app (such as via Docker) where the less “dependencies” and manual things to do (i.e. register your hazelcast nodes manually).. the better. You can totally embedded this functionality with this discovery strategy SPI.
I hope others find this helpful, and please leave your feedback, pull-requests or issues on the project!
NOTE, if you are running your app in Docker you have a separate issue where you need to determine your own externally accessible IP/PORT that the docker host has mapped for you on 5701… well how can you determine that so that you can publish the correct IP/PORT info to Etcd? Check out: https://github.com/bitsofinfo/docker-discovery-registrator-consul
NOTE! Interested in consul? There is a separate project which is built around Consul for your discovery strategy located here: https://github.com/bitsofinfo/hazelcast-consul-discovery-spi