Architecting for scale is about more than just handling large numbers of users.

We have covered a lot of material in a lot of different topics in this book that, when taken together, is designed to help you scale your applications. We focused on five tenets:

· Tenet #1—Availability: Maintaining Availability in Modern Applications

· Tenet #2—Modern Application Architecture: Using Services

· Tenet #3—Organization: Scaling Your Organization for Modern Applications

· Tenet #4—Risk: Risk Management for Modern Applications

· Tenet #5—Cloud: Utilizing the Cloud

Tenet #1—Availability

Availability is the ability of your application to perform the tasks it is capable of doing. This differs from reliability, which is the ability of your application to not make mistakes. A system that adds 2 + 3 and returns 6 has poor reliability. A system that adds 2 + 3 and never returns a result has poor availability. Poor availability is caused by many things, including the following:

· Resource exhaustion

· Unplanned load-based changes

· Increased number of moving parts

· Outside dependencies

· Technical debt

Application availability is often the first casualty as an application tries to scale beyond its capabilities. We learned what availability was about, how to measure it, and how to apply tools for improving availability in highly scaled applications, even in light of continuously increasing scaling needs.

Tenet #2—Architecture

A service is a distinct enclosed system that provides business functionality in support of building one or more larger products. Services provide an application architecture pattern that facilitates building systems in a manner that promotes improved system and development team scalability.

When building highly scaled applications, services provide the ability to make improved scaling decisions, accommodate improved team focus and control, reduce complexity at the local level, and improve testing and deployment capabilities.

We provided tools and suggestions for how to build high availability into your application at the service level and reduce the effect of service failures on your application and its users.

Tenet #3—Organization

Scaling impacts your organization, not just your application. We looked at the Single Team Owned Service Architecture paradigm, or STOSA. This provides a model for scaling your development organization as your application scales, making it possible for a larger number of engineers to effectively work on a single application without sacrificing application scalability or availability. This involves defining what it means to be a service owner and organizing your application around these principles.

We talked about using tools for managing service dependencies to maintain application quality even during times of hypergrowth, including internal SLAs and service tiers.

Tenet #4—Risk

You cannot possibly manage the risk in your system if you cannot identify the risk in your system. This is the critical lesson from Tenet #4—Risk Management for Modern Applications. Understanding your risk is the first and most important step in operating a highly available, highly scalable application.

After you understand your risk, you must manage that risk. Although removing risk is always desirable, often the cost of doing so is unacceptably high, both from an actual cost standpoint and from the standpoint of the opportunity cost to your application. You certainly have more important, more customer-focused things to do that are better for your customers, your company, and your bottom line than to remove every ounce of risk you know from your application.

Managing risk involves evaluating two values with every risk: the risk’s likelihood and the risk’s severity. Generally, severity is the cost to you if a risk happens, whereas likelihood is the chance of the risk happening. A risk that can cause a very serious problem in your application but is improbable might not be one that you want to try removing. Similarly, a risk that is highly likely to happen but would have very little impact on your application is probably not a risk you will need to prioritize removing. But a risk that is somewhat likely to happen and can cause a reasonably serious problem might in fact be the most important risk for you to work on resolving.

We introduced a tool called the risk matrix, which can be quite effective in helping you manage the risks of your application and determine which risks need to be mitigated or removed.

We discussed techniques for mitigating risk, techniques for validating mitigation action plans, and techniques for building applications with reduced risk.

Tenet #5—Cloud

Finally, we looked at the cloud and how you can use it to build highly scaled applications.

We looked at how the cloud has changed the way we think about computing and the way we think about building applications. We discussed building geographic and network topographical diversity into your application using the cloud, and how to avoid pitfalls where you believe your application is geographically and network-topologically diverse when in fact it might have built-in dependencies that increase your risk of problems.

We addressed the use of managed infrastructure and how you can utilize it in highly scaled applications. We covered how cloud-based resources are allocated, and the role you need to play in ensuring that your applications have the cloud resources they need to keep operating.

We then discussed compute options available to you when using the cloud. We looked at AWS Lambda, and the revolutionary future in scalable development it enables.

Architecting for Scale

Architecting an application for scalability is more than building an application that handles lots of users at the same time. There are many things involved in making an application scalable:

· It must handle a large and growing number of customers; a large and growing quantity of data used by your customers; and a growing complexity in what your customers want to accomplish with your application.

· You need to add more developers to work on your application as your company’s needs expand, and you must do so without sacrificing development speed, efficiency, or application quality.

· Your application must be kept online and functioning, even in light of all of the aforementioned changes and improvements.

These aren’t easy problems to solve. The techniques discussed in this book are designed to help you solve these and many more of your application scalability concerns.