Introduction

1. Introduction#

The enhanced communication abstraction layer (eCAL) is a middleware that enables scalable, high performance interprocess communication on a single computer node or between different nodes in a computer network. eCAL is designed for typical cloud computing scenarios where different processes exchange there I/O’s using a publish/subscribe pattern.

If you have ever worked with other publish subscribe middlewares like MQTT or ROS, you should be familiar with the terminology. But just in case you are new to this topic, let us clarify the basic terminology:

  • eCAL Process / Node:

    The term eCAL Process (or eCAL node) describes one small entity of your system that offers or consumes data. Typically, this is an executable (or a script) running on a PC.

  • eCAL Topic:

    eCAL Processes do not sent data directly to the receiver. Instead, they offer the data as a topic, which is identified by its name. The process is now a publisher of that topic. Another process that is interested in the data can now subscribe to the topic and will automatically receive all messages from it.

    Publishers and subscribers do not have to know anything about each other; the dataflow is entirely managed by eCAL.

Let us look at the following example:

eCAL concept

The image shows two PCs. Proc1 and Proc2 are running on one PC, while Proc3 runs on another PC that is connected via Ethernet.

Proc1 publishes Topic A. That topic is subscribed by Proc2 and Proc3, so both will receive all messages that are sent to that topic. Proc2 also publishes Topic B, which is subscribed by Proc3, while Proc3 publishes Topic C which is subscribed by Proc 2.

What are the advantages of such a public-subscribe system?

  • Components of your system can be implemented independent from each other; they only have to agree on the data format

  • You can add and remove publishers and subscribers, use different versions or replace the publisher with a replay

  • Publishers and subscribers can run on different machines that may even run different operating systems

Note

eCAL works entirely decentral. It does not rely on a central broker, like the MQTT Broker or the roscore from ROS1

Now, if you think you understood the basic concept, advance to the next section!