openstack-manuals/doc/arch-design/source/introduction-methodology.rst

Methodology

The best way to design your cloud architecture is through creating and testing use cases. Planning for applications that support thousands of sessions per second, variable workloads, and complex, changing data, requires you to identify the key meters. Identifying these key meters, such as number of concurrent transactions per second, and size of database, makes it possible to build a method for testing your assumptions.

Use a functional user scenario to develop test cases, and to measure overall project trajectory.

Note

If you do not want to use an application to develop user requirements automatically, you need to create requirements to build test harnesses and develop usable meters.

Establishing these meters allows you to respond to changes quickly without having to set exact requirements in advance. This creates ways to configure the system, rather than redesigning it every time there is a requirements change.

Important

It is important to limit scope creep. Ensure you address tool limitations, but do not recreate the entire suite of tools. Work with technical product owners to establish critical features that are needed for a successful cloud deployment.

Application cloud readiness

The cloud does more than host virtual machines and their applications. This 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 are 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, 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 may suffer outages, loss of data, or complete failure.

There are other reasons to design applications with the cloud in mind. Some are defensive, such as the fact that because 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. Applications need to be designed in such a way that they can take advantage of these and other opportunities.

Determining whether an application is cloud-ready

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 is not 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, you can 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.

Designing for the cloud

Here are some guidelines to keep in mind when designing an application for the cloud:

• Be a pessimist: Assume everything fails and design backwards.
• 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. Do not 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 similar costs can rapidly increase monthly usage charges. Monitor usage closely.