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The magic of the cloud is that it can do anything. It is both robust and flexible, the best of both worlds. Yes, the cloud is highly flexible and it can do almost anything, but to get the most out of a cloud investment, it is important to define how the cloud will be used by creating and testing use cases. This is the chapter that describes the thought process behind how to design a cloud architecture that best suits the intended use. The diagram shows at a very abstract level the process for capturing requirements and building use cases. Once a set of use cases has been defined, it can then be used to design the cloud architecture. Use case planning can seem counter-intuitive. After all, it takes about five minutes to sign up for a server with Amazon. Amazon does not know in advance what any given user is planning on doing with it, right? Wrong. Amazon's product management department spends plenty of time figuring out exactly what would be attractive to their typical customer and honing the service to deliver it. For the enterprise, the planning process is no different, but instead of planning for an external paying customer, for example, the use could be for internal application developers or a web portal. The following is a list of the high level objectives that need to be incorporated into the thinking about creating a use case. Overall business objectives Develop clear definition of business goals and requirements Increase project support and engagement with business, customers and end users. Technology Coordinate the OpenStack architecture across the project and leverage OpenStack community efforts more effectively. Architect for automation as much as possible to speed development and deployment. Use the appropriate tools for the development effort. Create better and more test metrics and test harnesses to support continuous and integrated development, test processes and automation. Organization Better messaging of management support of team efforts Develop better cultural understanding of Open Source, cloud architectures, Agile methodologies, continuous development, test and integration, overall development concepts in general As an example of how this works, consider a business goal of using the cloud for the company's E-commerce website. This goal means planning for applications that will support thousands of sessions per second, variable workloads, and lots of complex and changing data. By identifying the key metrics, such as number of concurrent transactions per second, size of database, and so on, it is possible to then build a method for testing the assumptions. Develop functional user scenarios that can be used to develop test cases that can be used to measure overall project trajectory. If the organization is not ready to commit to an application or applications that can be used to develop user requirements, it needs to create requirements to build valid test harnesses and develop usable metrics. Once the metrics are established, as requirements change, it is easier to respond to the changes quickly without having to worry overly much about setting the exact requirements in advance. Think of this as creating ways to configure the system, rather than redesigning it every time there is a requirements change. Create requirements that address the pain points, but do not recreate the entire OpenStack tool suite. The requirement to build OpenStack, only better, is self-defeating. It is important to limit scope creep by concentrating on developing a platform that will address tool limitations for the requirements, but not recreating the entire suite of tools. Work with technical product owners to establish critical features that are needed for a successful cloud deployment.

Although the cloud is designed to make things easier, it is important to realize that "using cloud" is more than just firing up an instance and dropping an application on it. The "lift and shift" approach works in certain situations, but there is a fundamental difference between clouds and traditional bare-metal-based environments, or even traditional virtualized environments. In traditional environments, with traditional enterprise applications, the applications and the servers that run on them are "pets". They're lovingly crafted and cared for, the servers have names like Gandalf or Tardis, and if they get sick, someone nurses them back to health. All of this is designed so that the application does not experience an outage. In cloud environments, on the other hand, servers are more like cattle. There are thousands of them, they get names like NY-1138-Q, and if they get sick, they get put down and a sysadmin installs another one. Traditional applications that are unprepared for this kind of environment, naturally will suffer outages, lost data, or worse. There are other reasons to design applications with cloud in mind. Some are defensive, such as the fact that applications cannot be certain of exactly where or on what hardware they will be launched, they need to be flexible, or at least adaptable. Others are proactive. For example, one of the advantages of using the cloud is scalability, so applications need to be designed in such a way that they can take advantage of those and other opportunities.

There are several factors to take into consideration when looking at whether an application is a good fit for the cloud. Structure A large, monolithic, single-tiered legacy application typically isn't a good fit for the cloud. Efficiencies are gained when load can be spread over several instances, so that a failure in one part of the system can be mitigated without affecting other parts of the system, or so that scaling can take place where the app needs it. Dependencies Applications that depend on specific hardware—such as a particular chip set or an external device such as a fingerprint reader—might not be a good fit for the cloud, unless those dependencies are specifically addressed. Similarly, if an application depends on an operating system or set of libraries that cannot be used in the cloud, or cannot be virtualized, that is a problem. Connectivity Self-contained applications or those that depend on resources that are not reachable by the cloud in question, will not run. In some situations, work around these issues with custom network setup, but how well this works depends on the chosen cloud environment. Durability and resilience Despite the existence of SLAs, things break: servers go down, network connections are disrupted, or too many tenants on a server make a server unusable. An application must be sturdy enough to contend with these issues.

Here are some guidelines to keep in mind when designing an application for the cloud: Be a pessimist: Assume everything fails and design backwards. Love your chaos monkey. Put your eggs in multiple baskets: Leverage multiple providers, geographic regions and availability zones to accommodate for local availability issues. Design for portability. Think efficiency: Inefficient designs will not scale. Efficient designs become cheaper as they scale. Kill off unneeded components or capacity. Be paranoid: Design for defense in depth and zero tolerance by building in security at every level and between every component. Trust no one. But not too paranoid: Not every application needs the platinum solution. Architect for different SLA's, service tiers and security levels. Manage the data: Data is usually the most inflexible and complex area of a cloud and cloud integration architecture. Don't short change the effort in analyzing and addressing data needs. Hands off: Leverage automation to increase consistency and quality and reduce response times. Divide and conquer: Pursue partitioning and parallel layering wherever possible. Make components as small and portable as possible. Use load balancing between layers. Think elasticity: Increasing resources should result in a proportional increase in performance and scalability. Decreasing resources should have the opposite effect. Be dynamic: Enable dynamic configuration changes such as auto scaling, failure recovery and resource discovery to adapt to changing environments, faults and workload volumes. Stay close: Reduce latency by moving highly interactive components and data near each other. Keep it loose: Loose coupling, service interfaces, separation of concerns, abstraction and well defined API's deliver flexibility. Be cost aware: Autoscaling, data transmission, virtual software licenses, reserved instances, and so on can rapidly increase monthly usage charges. Monitor usage closely.