How to Register Go Package Implementations Without Creating Cyclic Dependencies?

Registering Packages in Go without Cyclic Dependency

When designing a Go application, it's crucial to avoid cyclic dependencies between packages, where one package depends on another, which in turn depends on the first. This can arise when defining interfaces and their implementations across multiple packages.

Question:

How can we register client implementations without creating a cyclic dependency between our central package (with the client interface) and the packages containing the implementations (e.g., UDPClient, TCPClient)?

Answer:

The standard library offers two main approaches to address this issue:

1. Without a Central Registry

Define the client interface in a central package. For implementation, explicitly specify the desired implementation when instantiating clients. For example:

import (
    "hash/md5"
    "hash/sha256"
)

func main() {
    h1 := md5.New()
    h2 := sha256.New()
}

2. With a Central Registry

Create a registry mechanism where implementations register themselves. This approach is similar to the spontaneous registration discussed in the question. It's used in the image package, where various image format decoders (e.g., GIF, JPEG, PNG) register themselves.

3. Proposed Custom Registry

Introduce a separate factory package that provides the NewClient() methods for creating clients. This package can refer to the interface and implementation packages without causing a circular dependency. For instance:

package pi

// Client represents the client interface.
type Client interface { ... }

package pa

// UDPClient implements the Client interface.
type UDPClient struct { ... }

package pb

// TCPClient implements the Client interface.
type TCPClient struct { ... }

package pf

import (
    "pi"
    "pa"
    "pb"
)

// NewClient creates a client based on the implementation.
func NewClient(impl string) pi.Client {
    switch impl {
    case "udp":
        return &pa.UDPClient{}
    case "tcp":
        return &pb.TCPClient{}
    default:
        panic("Unsupported implementation")
    }
}

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