Independent Systems Architecture describes the concept quite well: Build many small systems instead of a large monolith. Make those systems as independent as possible, and therefore aim for decoupling.
ISA is one possible approach to building microservices. However, there is little agreement over what microservices actually are and how you should build a microservices system. ISA defines some best practices that have been working in many projects.
Self-contained Systems define a specific type of systems because each SCS and therefore the system has to have a UI. Also SCS should be integrated asynchronously or on the UI level. ISA is broader because a system that just provides an API and no UI can be a ISA, but not an SCS. Also ISA does not recommend a specific type of integration.
ISA covers topics like operations and micro / macro architecture that are not part of SCS.
So SCS and ISA are two different concepts that might be combined.
No. Like any other architecture, ISA is a trade-off that fits some scenarios quite well while it might not be a great fit for others. The fundamental idea of ISA is to separate a system into very loosely coupled modules that can be deployed and developed individually. This works great for applications that can be deployed frequently and fine grained, i.e. they are hosted in a private data center or the public cloud. It might not work too well in environment that focuses on less frequent deployments and monolithic deployment units, e.g. mobile applications distributed via an app store or games. Note that this is not clear cut. Also it is possible to use at least some of the ISA ideas for these scenarios.
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