What Is an Application Instance?

This chapter defines an Application Instance and explains how it differs from the Application itself.

Definition

An Application Instance is a uniquely identifiable, configured, and deployed occurrence of an Application within a specific IT Operating Environment Instance that, when active, is instantiated as one or more in-computer-memory processes that may interact with one another and with technical components outside the Application’s own boundary, including other Application Instances.

Understanding Application Instances

An Application is a single logical entity. An Application Instance is one concrete, deployed occurrence of it. A typical Application might have one Instance in each of five environments: development, Systems Integration Testing (SIT), User Acceptance Testing (UAT), Pre-Production Staging, and Production. That is one Application with five Application Instances.

Several points clarify what counts as an Application Instance:

• Instances are counted per deployment. Each separately deployed and configured occurrence of the Application is its own Instance. An enterprise that runs two separate Production deployments of the same Application, for example one per region, has two Production Instances.

• Scaled replicas are not separate Instances. When a single deployment runs as many identical processes to handle load, those processes together make up one Instance, not many.

• Inactive Instances are still Instances. An Instance that is deployed but stopped, such as a cold Disaster Recovery standby, a paused Instance, or one that scales to zero when idle, remains an Application Instance.

• Instances differ. Instances of the same Application frequently differ in configuration, infrastructure, data, Integrations, security settings, and the Release they are running. Those differences are exactly what Application Documentation must capture, as later chapters explain.

Best Practice: Identify and Document Every Application Instance

For every Application, identify each of its Application Instances, the IT Operating Environment Instance each one runs in, and the Release each one is running, and document them in the Application Documentation. Later chapters describe how to name Instances consistently and how to document the ways in which Instances differ.

Benefits: Knowing exactly which Instances exist, where they run, and which Release each one is running lets operations staff find the right Instance during an incident, lets security teams confirm that a fix has reached every Instance, and prevents forgotten Instances from lingering unpatched and unmonitored.

Connections to Related IF4IT Practices and Inventories

The IT Operating Environments Best Practices defines IT Operating Environments and Environment Instances, the settings in which Application Instances are deployed. The Release Management Best Practices describes how Releases move through those environments.