ISA (Independent Systems Architecture) is a collection of best practices. This page explains the reasoning behind each of the principles it comprises.
Modularization has been recognized as an important tool for handling large and complex software systems for a very long time. Using the term “module” reuses these ideas. Modules imply e.g. high cohesion and low coupling as goals of the architecture, or the single responsibility principle. Also information hiding implies that the data and the database must not be accessed by any other module. So there is no need to state these ideas explicitly.
The architecture proposed here allows for a lot of freedom. However, all the modules still need to appear as one. So some decisions need to be done on the level of the Macro Architecture. Once defined, Macro and Micro Architecture clearly state what has to be decided on which level. The goal is to define as little as possible on the Macro Architecture level so that the freedom is not limited without a good reason. Not every detail should be defined in the Macro Architecture. Also the Macro Architecture should be focused on decisions that are pretty stable in the long run while the Micro Architecture decisions are not as stable. Macro Architecture influences all modules and is therefore harder to change. So it should not be changed too frequently.
Separating the modules into processes, containers, or virtual
machines allows e.g. each module to be implemented in a different
programming language on a different platform. Technical decisions
are therefore specific to just one module.
Also this kind of separation means that each module can crash without any of the other modules crashing. This would be different if all modules were just part of one process. This benefits resilience (see below).
Modules are separated into processes (see 2). Therefore local
method calls cannot be used to integrate them but a different means
of integration is needed.
A standard means that a true system is created. If each module is integrated differently, it is hard to even consider the result a system.
At first sight it might seem that one way of integration should be enough. But in fact integration will usually be done on the UI layer and on the logic layer. That already requires two different types of integration. Also there might be different cases in the same system where e.g. synchronous and asynchronous integration might be useful. Communication defines the low-level protocol that modules use to interact. Of course there is a relation between communication and integration. But e.g. REST allows for synchronous as well as asynchronous integration. Just as with integration, one option for communication might not be sufficient. For communication as well as integration the principles only take into account the communication among modules inside the system. Communication to other systems might be done with a different set of integration and communication technologies. Sometimes the communication and integration is already defined so it cannot be changed by the system. The public interface of the system is also different from an architectural point of view: It is harder to change because changes will influence other systems.
Users should not have to log in to each module but rather log in once to the whole system. So transferring authentication information including the principals and their roles is part of the standardization, too. Authorization is so closely linked to the domain logic that it should be done in each module. Therefore it is up to each module to allow access or specific actions. Also other metadata besides authentication information might also need to be standardized. For example, to trace calls between microservices, a unique id for a call and all dependent calls must be transfered.
Real independence is only possible if each module can be deployed by itself. The architecture is just a prerequisite. In other words: If the architecture allows independent modules but they still need to be deployed as a whole, the approach provides only part of the desired independence but the major effort will still have to be spent. In some cases, there is a shared integration test stage. Each module is eventually propagated to this stage and tested exclusively i.e. all other modules have to wait until that module passes the integration test stage. This is an example of a dependency bottleneck in the deployment pipelines: The other pipelines cannot continue until the module passes the stage. This has to be avoided e.g. by using consumer-driven contract tests and mocks to test the interfaces between modules, or by separated integration test stages.
Each module is a separate process, container, or virtual machine (2). Operations has to deal with each of them and is therefore facing a huge challenge. Standardization helps dealing with this complexity. However, there might still be services with special requirements that might use a different approach for operations. Also: In a “You build it - you run it” organization each team should be responsible for choosing the operations approach that fits them best. Most often that will still be a standardized approach because that causes less effort for the team. Note that the standard only covers the technology. Of course each module might have its own set of metrics, alerts, etc. ISA needs a high level of maturity concerning operations: Most operations procedures have to be automated. Lots of modules have to be handled. Lacking this level of maturity might make it impossible to implement ISA successfully.
In a way this is a result of the modularization: Modules might only be accessed though the interface. This should also be the case for any standards e.g. for operations. Also standards on the level of technology limit the free choice of technology which is a major benefit of this architecture. However, in some cases it might not be possible to limit the standard to the interface level. So the principle is just “should”.
Resilience at the level of the modules tremendously helps to obtain high availability for the overall distributed system. Also a scheduler might choose to restart modules or move them to a different server. Modules must be able to handle this.