Chain of Responsibility in Go
Chain of Responsibility is behavioral design pattern that allows passing request along the chain of potential handlers until one of them handles request.
The pattern allows multiple objects to handle the request without coupling sender class to the concrete classes of the receivers. The chain can be composed dynamically at runtime with any handler that follows a standard handler interface.
Conceptual Example
Let’s look at the Chain of Responsibility pattern with the case of a hospital app. A hospital could have multiple departments such as:
- Reception
- Doctor
- Medical examination room
- Cashier
Whenever any patient arrives, they first get to Reception, then to Doctor, then to Medicine Room, and then to Cashier (and so on). The patient is being sent through a chain of departments, where each department sends the patient further down the chain once their function is completed.
The pattern is applicable when there are multiple candidates to process the same request. It is also useful when you don’t want the client to choose the receiver as there are multiple objects can handle the request. Another useful case is when you want to decouple the client from receivers—the client will only need to know the first element in the chain.
As in the example of the hospital, a patient first goes to the reception. Then, based upon a patient’s current status, reception sends up to the next handler in the chain.
department.go: Handler interface
package main
type Department interface {
execute(*Patient)
setNext(Department)
}
reception.go: Concrete handler
package main
import "fmt"
type Reception struct {
next Department
}
func (r *Reception) execute(p *Patient) {
if p.registrationDone {
fmt.Println("Patient registration already done")
r.next.execute(p)
return
}
fmt.Println("Reception registering patient")
p.registrationDone = true
r.next.execute(p)
}
func (r *Reception) setNext(next Department) {
r.next = next
}
doctor.go: Concrete handler
package main
import "fmt"
type Doctor struct {
next Department
}
func (d *Doctor) execute(p *Patient) {
if p.doctorCheckUpDone {
fmt.Println("Doctor checkup already done")
d.next.execute(p)
return
}
fmt.Println("Doctor checking patient")
p.doctorCheckUpDone = true
d.next.execute(p)
}
func (d *Doctor) setNext(next Department) {
d.next = next
}
medical.go: Concrete handler
package main
import "fmt"
type Medical struct {
next Department
}
func (m *Medical) execute(p *Patient) {
if p.medicineDone {
fmt.Println("Medicine already given to patient")
m.next.execute(p)
return
}
fmt.Println("Medical giving medicine to patient")
p.medicineDone = true
m.next.execute(p)
}
func (m *Medical) setNext(next Department) {
m.next = next
}
cashier.go: Concrete handler
package main
import "fmt"
type Cashier struct {
next Department
}
func (c *Cashier) execute(p *Patient) {
if p.paymentDone {
fmt.Println("Payment Done")
}
fmt.Println("Cashier getting money from patient patient")
}
func (c *Cashier) setNext(next Department) {
c.next = next
}
patient.go
package main
type Patient struct {
name string
registrationDone bool
doctorCheckUpDone bool
medicineDone bool
paymentDone bool
}
main.go: Client code
package main
func main() {
cashier := &Cashier{}
//Set next for medical department
medical := &Medical{}
medical.setNext(cashier)
//Set next for doctor department
doctor := &Doctor{}
doctor.setNext(medical)
//Set next for reception department
reception := &Reception{}
reception.setNext(doctor)
patient := &Patient{name: "abc"}
//Patient visiting
reception.execute(patient)
}
output.txt: Execution result
Reception registering patient
Doctor checking patient
Medical giving medicine to patient
Cashier getting money from patient patient
