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Co-Existent use case
This is the most complicated of the three, where some parts of the same application are on the public cloud, while the other parts might be on the private cloud. There are mainly the following motivations that will have you create this kind of an architecture:
- Cloud bursting
- Use of cognitive services from a public cloud
- Licensing restrictions
- Non availability of a service on the Public cloud
You may be wondering, if these are the application's needs, why not move the whole application to the public cloud? The reasons (one or a combination) would be the same as discussed in Chapter 2, Hybrid Cloud – Why Does It Matter?, and are noted here simply to refresh your memory:
- Cost of running an application perennially in the public cloud might be higher
- Compliance/security reasons
If the applications need to use the co-exist model, then they need to be architected appropriately. We will look at the application architecture principles a little later in the chapter.