Simple way for message passing in distributed system

Question

I am implementing a small distributed system (in Python) with nodes behind firewalls. What is the easiest way to pass messages between the nodes under the following restrictions:

1. I don't want to open any ports or punch holes in the firewall
2. Also, I don't want to export/forward any internal ports outside my network
3. Time delay less than, say 5 minutes, is acceptable, but closer to real time would be nice, if possible.
4. 1+2 → I need to use a third party, accessible by all my nodes. From this follows, that I probably also want to use encryption

Solutions considered:

• Email - by setting up separate or a shared free email accounts (e.g. Gmail) which each client connects to using IMAP/SMTP
• Google docs - using a shared online spreadsheet (e.g. Google docs) and some python library for accessing/changing cells using a polling mechanism
• XMPP using connections to a third party server
• IRC
• Renting a cheap 5$ VPS and setting up a Zero-MQ publish-subscribe node (or any other protocol) forwarded over SSH and having all nodes connect to it

Are there any other public (free) accessible message queues available (or platforms that can be misused as a message queue)?

I am aware of the solution of setting up my own message broker (RabbitMQ, Mosquito) etc and make it accessible to my nodes somehow (ssh-forwardning to a third host etc). But my questions is primarily about any solution that doesn't require me to do that, i.e. any solutions that utilizes already available/accessible third party infrastructure. (i.e. are there any public message brokers I can use?)

Answer 1

How about Mosquitto: message broker that implements the MQ Telemetry Transport protocol versions 3.1 and 3.1.1. MQTT provides a lightweight method of carrying out messaging using a publish/subscribe model. This makes it suitable for "machine to machine" messaging. It supports encryption. Time to setup: approximatively 15 mins you should be up and running. Since it is a message broker, you can write your own code to ensure you can communicate with 3rd party solutions. Also, it achieves soft real-time, but depending on your setup you can achieve hard real-time. After you look into Mosquitto have a look at Paho, which is a port of Mosquito to Eclipse Foundation.

Paho also provides a Python Client, which offers support for both MQTT v3.1 and v3.1.1 on Python 2.7 or 3.x. It also provides some helper functions to make publishing one off messages to an MQTT server very straightforward. Plenty of documentation and examples to get you up and running.