Singleton pattern

As explained in Infrastructure as Code, Patterns and Practices: With examples in Python and Terraform

A singleton pattern is a design pattern in which a resource or set of resources has one authoritative instance within a system.

A singleton pattern declares a set of infrastructure resources as singular instances in a system and deploys them with one command. A static file or directory holds environment-specific parameters and expresses relationships among resources in one place. section 3.1

Use the pattern deliberately when:

• A resource is unique, changes rarely, and has no need for multiple independently managed instances. section 3.6 section 3.7
• A small configuration benefits from a single reference for provisioning order, required parameters, debugging, and troubleshooting. section 3.1
• Expect the configuration to become a coordination and scaling liability when many resources share state, changes overlap, or one update affects a broad blast radius. section 9.0 section 10.0

Database infrastructure in GCP

One configuration defines a GCP project, a database network using 10.0.3.0/16, a server template that installs MySQL packages, and a server group containing three database servers. The configuration captures their relationships and provisions them together. section 3.1

Figure 3.1 A singleton pattern expresses the configuration of an initial set of resources—such as a project, network, and database—in one file. This pattern captures the relationships among resources in one place.

Shows how a singleton configuration places a project, network, and database resources in one file and captures their relationships. section 3.1

How Infrastructure as Code, Patterns and Practices: With examples in Python and Terraform covers it

How deep it goes

Defines the pattern, demonstrates a GCP configuration, and defers detailed modular decomposition to factory, builder, and composite patterns.

Examples

Uses a GCP project, database network, MySQL server template, three-server group, and a testing workflow for an application server.

What it assumes

Assumes familiarity with infrastructure resources and configuration while teaching resource relationships, parameters, and provisioning order through one configuration.

What you can do after

Operate/configure a small set of unique infrastructure resources from one static configuration and one deployment command.

How it connects

Connects singleton configurations with infrastructure as code, factory pattern, builder pattern, composite pattern, immutability, and end-to-end testing.

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Also explained in

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  Software Design for Python ProgrammersThe Singleton Design Pattern is a creational design pattern that restricts a class to at most one object instance and provides access to that instance.
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  Object-Oriented Software Design in C++The Singleton design pattern is a creational design pattern that restricts a class to one instance and provides a controlled access point to it.
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Related concepts

• design patternkind of · A design pattern is a reusable, general solution structure for a recurring software design problem in a particular context.
• static fielduses · A named variable whose storage is associated with a class or has static duration, rather than being allocated separately for each object instance or function call.
• lazy initializationuses · Lazy initialization is a technique that postpones creating, retrieving, computing, or loading a value or resource until it is needed.
• AddSingletonconfigured by · AddSingleton is an ASP.NET Core dependency-injection registration method that reuses one service instance for the application's lifetime.
• global variablecontrasts with · A global variable is a variable whose binding is accessible from broad program or module scope according to the language’s name-resolution rules.
• object declarationproduced by · An object declaration is a Kotlin declaration that defines a singleton object and creates its sole instance as part of the declaration.

This explainer was generated by Manning's knowledge graph from the text of Infrastructure as Code, Patterns and Practices: With examples in Python and Terraform. The section links point to the parts of the book it draws on.