Hands-On Design Patterns and Best Practices with Julia

The singleton pattern

The singleton pattern is used to create a single instance of an object and reuse it from anywhere. A singleton object is typically constructed when the application starts, or it can be created lazily on the first use of the object. An interesting requirement for the singleton pattern arises for multithreaded applications because the instantiation of the singleton object must happen only once. It can be a challenge if the object creation function is called lazily from many threads.

Suppose that we want to create a singleton called AppKey that is used for encryption in the application:

# AppKey contains an app id and encryption keystruct AppKey    appid::String    value::UInt128end

Initially, we may be tempted to use a global ...