Building Domain-Driven Microservices in the Pharmacy Industry: A Python Example

Designing and Implementing a Microservices Architecture with DDD Principles for Efficient Pharmacy Management

In recent years, the popularity of microservices architecture has grown considerably. Microservices offer several benefits over traditional monolithic architectures, such as scalability, resilience, and agility. However, designing and implementing a microservices architecture can be challenging, especially when it comes to defining the boundaries between services and ensuring that each service’s responsibilities are well-defined and understood. This is where Domain-Driven Design (DDD) comes in. DDD provides a set of principles and practices that can help developers design and implement microservices that are aligned with the business domain.

In this blog, we will explore how to implement domain-driven microservices using Python in the pharmacy domain.

Domain-Driven Design (DDD)

DDD is a set of principles and practices that help developers design software that is aligned with the business domain. The core idea behind DDD is to focus on the domain model, which is a representation of the business domain concepts and rules. The domain model should be designed collaboratively with domain experts, and it should be the central point around which the software architecture revolves.

DDD provides a set of patterns and practices that help developers implement the domain model in code. Some of these patterns include:

• Bounded contexts: A bounded context is a specific area of the business domain with well-defined boundaries. In a microservices architecture, each service should correspond to a bounded context.
• Aggregates: An aggregate is a cluster of domain objects that are treated as a single unit for the purpose of data consistency and transactional boundaries.
• Entities: An entity is an object with a unique identity that can change over time.
• Value objects: A value object is an object that has no identity and is defined by its attributes.
• Domain events: A domain event is an object that represents something that has happened in the domain and can be used to trigger side-effects in other parts of the system.

Pharmacy Domain Example

Let’s consider a simple example in the pharmacy domain. Suppose we are building a system that allows patients to order medication online and have it delivered to their homes. The system should have the following features:

• Patients can browse a catalog of available medications and add them to their cart.
• Patients can specify their shipping address and payment information.
• The system should check the patient’s eligibility for the prescribed medication based on their medical history and any other medications they are taking.
• Once the order is confirmed, the system should send a notification to the pharmacy to prepare the medication and another notification to the delivery service to schedule the delivery.

To implement this system using microservices, we need to identify the bounded contexts and define the domain model for each context. Here are the bounded contexts we can identify:

• Catalog: This context is responsible for managing the catalog of available medications.
• Ordering: This context is responsible for managing the order process, including eligibility checking, notifications, and payment processing.
• Pharmacy: This context is responsible for managing the pharmacy’s inventory and preparing the medication for delivery.
• Delivery: This context is responsible for managing the delivery process.

Each of these bounded contexts can be implemented as a separate microservice. Let’s focus on the Ordering context and define the domain model for this context.

Ordering Bounded Context

The Ordering bounded context is responsible for managing the order process. Here is the domain model for this context:

• Order: An order is an entity that represents a patient’s order. It has the following attributes:
  - Order ID: A unique identifier for the order.
  - Patient ID: A unique identifier for the patient who placed the order.
  - Medication ID: A unique identifier for the medication being ordered.
  - Quantity: The quantity of the medication being ordered.
  - Status: The status of the order, which can be one of the following: New, EligibilityChecking, EligibilityApproved, EligibilityRejected, PaymentProcessing, Completed, or Cancelled.
  - Shipping Address: The shipping address for the order.
  - Payment Information: The payment information for the order.
• EligibilityCheck: An eligibility check is a value object that represents the patient’s eligibility to receive the medication. It has the following attributes:
  - Patient ID: The ID of the patient being checked.
  - Medication ID: The ID of the medication being checked.
  - Eligibility: A boolean indicating whether the patient is eligible to receive the medication.
  - Reason: A string explaining the reason for the eligibility status (e.g., drug interaction, allergy, etc.).
• OrderPlaced: An order placed event is a domain event that is triggered when a new order is placed. It contains the order ID and patient ID.
• OrderCancelled: An order cancelled event is a domain event that is triggered when an order is cancelled. It contains the order ID and patient ID.
• EligibilityChecked: An eligibility checked event is a domain event that is triggered when an eligibility check is performed. It contains the patient ID, medication ID, eligibility status, and reason.
• PaymentProcessed: A payment processed event is a domain event that is triggered when a payment is processed. It contains the order ID and payment information.

To implement the Ordering microservice, we can define the following Python classes:

class Order:
    def __init__(self, order_id, patient_id, medication_id, quantity, status, shipping_address, payment_information):
        self.order_id = order_id
        self.patient_id = patient_id
        self.medication_id = medication_id
        self.quantity = quantity
        self.status = status
        self.shipping_address = shipping_address
        self.payment_information = payment_information

class EligibilityCheck:
    def __init__(self, patient_id, medication_id, eligibility, reason):
        self.patient_id = patient_id
        self.medication_id = medication_id
        self.eligibility = eligibility
        self.reason = reason

class OrderPlaced:
    def __init__(self, order_id, patient_id):
        self.order_id = order_id
        self.patient_id = patient_id

class OrderCancelled:
    def __init__(self, order_id, patient_id):
        self.order_id = order_id
        self.patient_id = patient_id

class EligibilityChecked:
    def __init__(self, patient_id, medication_id, eligibility, reason):
        self.patient_id = patient_id
        self.medication_id = medication_id
        self.eligibility = eligibility
        self.reason = reason

class PaymentProcessed:
    def __init__(self, order_id, payment_information):
        self.order_id = order_id
        self.payment_information = payment_information

We can use these classes to define the API for the Ordering microservice, which would include endpoints for placing an order, cancelling an order, checking eligibility, and processing payments.

In summary, by using Domain-Driven Design principles and practices, we can design and implement microservices that are well-aligned with the business domain. In the pharmacy domain example, we identified four bounded contexts — Catalog, Ordering, Pharmacy, and Delivery — and defined the domain model for the Ordering bounded context. We also provided an example of how to implement the Ordering microservice using Python and the defined domain model. By following these practices, we can build microservices that are more maintainable, scalable, and resilient. Additionally, by working closely with domain experts, we can ensure that the software we build meets the needs of the business and is aligned with the business goals.