Chain Of Responsibility

June 24, 2024

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.

Golang Implementation

Example: Patient goes to hospital

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

package main

type Department interface {
    execute(*Patient)
    setNext(Department)
}

reception.go

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

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

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

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

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)
}
// Reception registering patient
// Doctor checking patient
// Medical giving medicine to patient
// Cashier getting money from patient patient

Rust Implementation

Example: Patient goes to hospital

The example demonstrates processing a patient through a chain of departments. The chain of responsibility is constructed as follows:

Patient -> Reception -> Doctor -> Medical -> Cashier

The chain is constructed using Box pointers, which means dynamic dispatch in runtime. Why? It seems quite difficult to narrow down implementation to a strict compile-time typing using generics: in order to construct a type of a full chain Rust needs full knowledge of the “next of the next” link in the chain. Thus, it would look like this:

let mut reception = Reception::<Doctor::<Medical::<Cashier>>>::new(doctor); // 😱

Instead, Box allows chaining in any combination:

let mut reception = Reception::new(doctor); // 👍
let mut reception = Reception::new(cashier); // 🕵️‍♀️

patient.rs

#[derive(Default)]
pub struct Patient {
    pub name: String,
    pub registration_done: bool,
    pub doctor_check_up_done: bool,
    pub medicine_done: bool,
    pub payment_done: bool,
}

department.rs

mod cashier;
mod doctor;
mod medical;
mod reception;

pub use cashier::Cashier;
pub use doctor::Doctor;
pub use medical::Medical;
pub use reception::Reception;

use crate::patient::Patient;

/// A single role of objects that make up a chain.
/// A typical trait implementation must have `handle` and `next` methods,
/// while `execute` is implemented by default and contains a proper chaining
/// logic.
pub trait Department {
    fn execute(&mut self, patient: &mut Patient) {
        self.handle(patient);

        if let Some(next) = &mut self.next() {
            next.execute(patient);
        }
    }

    fn handle(&mut self, patient: &mut Patient);
    fn next(&mut self) -> &mut Option<Box<dyn Department>>;
}

/// Helps to wrap an object into a boxed type.
pub fn into_next(department: impl Department + Sized + 'static) -> Option<Box<dyn Department>> {
    Some(Box::new(department))
}

department/cashier.rs

use super::{Department, Patient};

#[derive(Default)]
pub struct Cashier {
    next: Option<Box<dyn Department>>,
}

impl Department for Cashier {
    fn handle(&mut self, patient: &mut Patient) {
        if patient.payment_done {
            println!("Payment done");
        } else {
            println!("Cashier getting money from a patient {}", patient.name);
            patient.payment_done = true;
        }
    }

    fn next(&mut self) -> &mut Option<Box<dyn Department>> {
        &mut self.next
    }
}

department/doctor.rs

use super::{into_next, Department, Patient};

pub struct Doctor {
    next: Option<Box<dyn Department>>,
}

impl Doctor {
    pub fn new(next: impl Department + 'static) -> Self {
        Self {
            next: into_next(next),
        }
    }
}

impl Department for Doctor {
    fn handle(&mut self, patient: &mut Patient) {
        if patient.doctor_check_up_done {
            println!("A doctor checkup is already done");
        } else {
            println!("Doctor checking a patient {}", patient.name);
            patient.doctor_check_up_done = true;
        }
    }

    fn next(&mut self) -> &mut Option<Box<dyn Department>> {
        &mut self.next
    }
}

department/medical.rs

use super::{into_next, Department, Patient};

pub struct Medical {
    next: Option<Box<dyn Department>>,
}

impl Medical {
    pub fn new(next: impl Department + 'static) -> Self {
        Self {
            next: into_next(next),
        }
    }
}

impl Department for Medical {
    fn handle(&mut self, patient: &mut Patient) {
        if patient.medicine_done {
            println!("Medicine is already given to a patient");
        } else {
            println!("Medical giving medicine to a patient {}", patient.name);
            patient.medicine_done = true;
        }
    }

    fn next(&mut self) -> &mut Option<Box<dyn Department>> {
        &mut self.next
    }
}

department/reception.rs

use super::{into_next, Department, Patient};

#[derive(Default)]
pub struct Reception {
    next: Option<Box<dyn Department>>,
}

impl Reception {
    pub fn new(next: impl Department + 'static) -> Self {
        Self {
            next: into_next(next),
        }
    }
}

impl Department for Reception {
    fn handle(&mut self, patient: &mut Patient) {
        if patient.registration_done {
            println!("Patient registration is already done");
        } else {
            println!("Reception registering a patient {}", patient.name);
            patient.registration_done = true;
        }
    }

    fn next(&mut self) -> &mut Option<Box<dyn Department>> {
        &mut self.next
    }
}

main.rs

mod department;
mod patient;

use department::{Cashier, Department, Doctor, Medical, Reception};
use patient::Patient;

fn main() {
    let cashier = Cashier::default();
    let medical = Medical::new(cashier);
    let doctor = Doctor::new(medical);
    let mut reception = Reception::new(doctor);

    let mut patient = Patient {
        name: "John".into(),
        ..Patient::default()
    };

    // Reception handles a patient passing him to the next link in the chain.
    // Reception -> Doctor -> Medical -> Cashier.
    reception.execute(&mut patient);

    println!("\nThe patient has been already handled:\n");

    reception.execute(&mut patient);
}
// Reception registering a patient John
// Doctor checking a patient John
// Medical giving medicine to a patient John
// Cashier getting money from a patient John
//
// The patient has been already handled:
//
// Patient registration is already done
// A doctor checkup is already done
// Medicine is already given to a patient
// Payment done