What is mediation engine?

The mediation engine reference in this context is originating from the topic of Enterprise Application Integration and is closely related to what GoF Mediator Pattern does - That is encapsulate communication between entities. In the case of EAI, a mediator/mediation engine sits between multiple disparate systems and acts as a broker between them, instead of letting the systems communicate directly.

Mediation approach in EAI offers capabilities like

1. Reduced coupling between systems. For instance, you do not have to learn and implement a legacy mainframe protocol in modern systems, just because you want to get some data from main frames. A mediation engine like Apache Camel could talk REST over HTTPS at one end and some archaic mainframe protocol on the other end.
2. Ease of migration: Once the mainframes area replaced with something else, you could just change the mediation layer to do something about it, instead of modifying multiple impacted systems that used to talk to mainframes.
3. Access to single resource/service via multiple channels: Let's say you have an old system that currently does SOAP over HTTP but you would like to offer REST with JSON payload to some of your new customers. Instead of building completely new systems up-front for this purpose, you could throw Apache Camel as a mediator and it would accept JSON payloads at one end and SOAP on the other end. Whoever wants to talk JSON can go through Camel and whoever wants to do SOAP may continue in a direct connection to the legacy system. Someday if some hypothetical FooBar protocol becomes popular, and if Apache Camel provides you a FooBar component, your users who demands FooBar support could be routed through Camel, to the system that still speaks SOAP.

All these stuff is discussed in detail in the Enterprise Integration Patterns site and book. Apache Camel implements truck loads of patterns as described in the EIP Book. I hope this answer helped you to understand the mediation role Apache Camel could play in Enterprise IT ecosystems.

From Camel in action

The core feature of Camel is its routing and mediation engine. A routing engine selectively moves a message around, based on the route’s configuration. In Camel’s case, routes are configured with a combination of enterprise integration patterns and a domain-specific language.

In this link, https://camel.apache.org/manual/latest/faq/what-is-camel.html indicated projects that can be components of the Camel route to and from where messages can be sent and consumed (https://camel.apache.org/components/latest/activemq-component.html, https://camel.apache.org/components/latest/cxf-component.html).

Apache camel is kind of a ESB middleware. Mediation with respect to Camel would mean the following

Data format transformation : If Application A speaks JSON and Application B understands CSV format. You can use Apache Camel to transfer JSON to CSV.

Protocol transformation : If Application A knows only to call webservices but Application B prefers reading data from a message queue. You can use Apache Camel to receive this data by exposing a webservice and then push it to a queue for application B to consume.

Content transformation - Filtering or Enriching data : During this transformation process, you can also transform the data by filtering or enriching data fields based on what application B needs. In this way no change is required in A as it sends what it has and no change is required in application B as it gets what it needs.

Connectors : Many ESB now have in-built connectors to connect directly with ERP or SAS based applications. For example, a kafka connector. https://camel.apache.org/blog/Camel-Kafka-connector-intro/