registrar

Registrar

A Go library implementing a variant of the registration pattern

Description

The registration, driver, strategy or singleton pattern is the idea that implementations of the same functionality are built up into a registry for use and/or selection at runtime. This pattern is used in a few places in the standard library with the net/http, database/sql, and flag packages. In my experience, most notably with the registration of database/sql drivers.

This library is a variant of this pattern. Its use case is to build up a registry of implementations which can be filtered or ordered and returned as a generic slice of interface{}. These interfaces can then be passed along to functions that can type assert or switch case on them to run concrete interface methods. The registration of a driver in this package is explicit (Registrar.Register()). All magic is purposefully avoided. See the examples directory for code snippets.

Background

This library is produced as a general library but was built from its original use-case in bmclib. The purpose of bmclib is to interact with Baseboard Management Controllers (BMC). There are many different ways to interact with a single BMC, so a registry of the different implementations is built up and then when a request to perform a single action (power state for example) is tried with all the different implementations until one works.

Usage

// create a registry
reg := registrar.NewRegistry()

// registry drivers
one := &driverOne{name: "driverOne", protocol: "tcp", metadata: "this is driver one", features: registrar.Features{registrar.Feature("always double checking")}}
two := &driverTwo{name: "driverTwo", protocol: "udp", metadata: "this is driver two", features: registrar.Features{registrar.Feature("set and forget")}}
reg.Register(one.name, one.protocol, one.features, one.metadata, one)
reg.Register(two.name, two.protocol, two.features, two.metadata, two)

// do some filtering
ctx := context.Background()
reg.Drivers = reg.Using("tcp")
reg.Drivers = reg.FilterForCompatible(ctx)

// get the interfaces for use
interfaces := reg.GetDriverInterfaces()

References

• https://dave.cheney.net/2017/06/11/go-without-package-scoped-variables
• https://eli.thegreenplace.net/2019/design-patterns-in-gos-databasesql-package/
• https://peter.bourgon.org/blog/2017/06/09/theory-of-modern-go.html