Understanding the principles of messaging systems

Continuing our focus on messaging systems, you may have seen applications where one application uses data that gets processed by other external applications or applications consuming data from one or more data sources. In such scenarios, messaging systems can be used as an integration channel for information exchange between different applications. If you haven't built such an application yet, then don't worry about it. We will build it in upcoming chapters.

In any application integration system design, there are a few important principles that should be kept in mind, such as loose coupling, common interface definitions, latency, and reliability. Let's look into some of these one by one:

• Loose coupling between applications ensures minimal dependencies on each other. This ensures that any changes in one application do not affect other applications. Tightly coupled applications are coded as per predefined specifications of other applications. Any change in specification would break or change the functionality of other dependent applications.
• Common interface definitions ensure a common agreed-upon data format for exchange between applications. This not only helps in establishing message exchange standards among applications but also ensures that some of the best practices of information exchange can be enforced easily. For example, you can choose to use the Avro data format to exchange messages. This can be defined as your common interface standard for information exchange. Avro is a good choice for message exchanges as it serializes data in a compact binary format and supports schema evolution.
• Latency is the time taken by messages to traverse between the sender and receiver. Most applications want to achieve low latency as a critical requirement. Even in an asynchronous mode of communication, high latency is not desirable as significant delay in receiving messages could cause significant loss to any organization.
• Reliability ensures that temporary unavailability of applications does not affect dependent applications that need to exchange information. In general, the when source application sends a message to the remote application, sometimes the remote application may be running slow or it may not be running due to some failure. Reliable, asynchronous message communication ensures that the source application continues its work and feels confident that the remote application will resume its task later.