REST API between services in Microservices - Deep Dive

Overview - REST API between services

What is it?

A REST API between services is a way for different software parts, called microservices, to talk to each other over the internet using simple rules. It uses standard web methods like GET, POST, PUT, and DELETE to request or send data. Each service has its own address and can respond with data in a format like JSON. This makes it easy for services to work together without being tightly connected.

Why it matters

Without REST APIs, microservices would struggle to communicate clearly and reliably, leading to tightly linked systems that are hard to change or fix. REST APIs solve this by creating a simple, common language for services to exchange information, making systems more flexible, scalable, and easier to maintain. This means faster updates, better reliability, and smoother user experiences.

Where it fits

Before learning REST APIs between services, you should understand basic web concepts like HTTP methods and URLs, and the idea of microservices architecture. After this, you can explore advanced topics like API gateways, service meshes, and asynchronous communication patterns to build more resilient and efficient systems.

Mental Model

Core Idea

REST APIs let independent services talk over the web using simple, standard requests and responses to share data and commands.

Think of it like...

It's like ordering food at a restaurant: you (client service) tell the waiter (REST API) what you want using a menu (standard methods), and the kitchen (another service) prepares and sends back your order (response).

┌───────────────┐       HTTP Request       ┌───────────────┐
│ Client Service│ ───────────────────────▶ │ Server Service│
└───────────────┘                          └───────────────┘
       ▲                                         │
       │             HTTP Response               │
       └─────────────────────────────────────────┘

Build-Up - 7 Steps

1

Foundation

Understanding HTTP Basics

Concept: Learn the core HTTP methods and how they are used to request and send data.

HTTP is the language of the web. The main methods are GET (to get data), POST (to create data), PUT (to update data), and DELETE (to remove data). Each method tells the server what action the client wants. URLs specify the resource to act on. Responses include status codes like 200 (success) or 404 (not found) and data in formats like JSON.

Result

You can identify what each HTTP method does and how clients and servers communicate using requests and responses.

Understanding HTTP methods and status codes is essential because REST APIs rely on these to clearly express actions and results between services.

2

Foundation

What is a REST API?

Concept: REST is a set of rules for building APIs that use HTTP methods and URLs to access resources.

REST stands for Representational State Transfer. It means each resource (like a user or order) has a unique URL. Clients use HTTP methods to perform actions on these resources. REST APIs are stateless, meaning each request is independent and contains all needed information. Responses usually use JSON to send data back.

Result

You understand REST as a simple, standard way for services to exchange data over HTTP.

Knowing REST principles helps you design APIs that are easy to use, scalable, and maintainable.

3

Intermediate

Designing REST APIs for Microservices

🤔Before reading on: do you think each microservice should expose many endpoints or just a few focused ones? Commit to your answer.

Concept: Microservices expose REST APIs that focus on their specific domain, keeping endpoints clear and limited.

Each microservice owns a specific part of the system, like users or payments. Its REST API should only expose endpoints related to that domain. For example, a User service might have /users and /users/{id}. This keeps services independent and easier to change. APIs should use clear, consistent naming and HTTP methods to match actions.

Result

You can design REST APIs that keep microservices loosely coupled and focused on their responsibilities.

Understanding domain-focused APIs prevents tight coupling and makes the system easier to evolve and debug.

4

Intermediate

Handling Data Formats and Status Codes

🤔Before reading on: do you think all services must use the same data format or can they differ? Commit to your answer.

Concept: Services usually agree on common data formats like JSON and use standard HTTP status codes to communicate results.

JSON is the most common format for REST APIs because it's easy to read and write. Services agree on the structure of JSON data to understand each other. HTTP status codes like 200 (OK), 201 (Created), 400 (Bad Request), and 500 (Server Error) tell clients what happened. Proper use of these codes helps clients handle responses correctly.

Result

You know how to make services communicate clearly using shared data formats and status codes.

Consistent data formats and status codes reduce errors and improve communication between services.

5

Intermediate

Securing REST APIs Between Services

🤔Before reading on: do you think REST APIs between services should be open or protected? Commit to your answer.

Concept: REST APIs between services must be secured to prevent unauthorized access and data leaks.

Services use authentication (proving who they are) and authorization (what they can do). Common methods include API keys, OAuth tokens, or mutual TLS. Communication often happens over HTTPS to encrypt data. Securing APIs protects sensitive data and prevents malicious actions.

Result

You understand how to protect REST APIs to keep service communication safe.

Knowing security basics prevents data breaches and service misuse in distributed systems.

6

Advanced

Managing API Versioning and Compatibility

🤔Before reading on: do you think changing an API always breaks clients? Commit to your answer.

Concept: API versioning allows services to evolve without breaking existing clients by supporting multiple versions.

When a service changes its API, older clients might break if they expect the old format. Versioning strategies include URL versioning (/v1/users), header versioning, or query parameters. Services can run multiple versions side by side during transitions. This ensures smooth upgrades and backward compatibility.

Result

You can design REST APIs that evolve safely without disrupting other services.

Understanding versioning avoids costly downtime and client failures during API changes.

7

Expert

Optimizing REST APIs for Performance and Scalability

🤔Before reading on: do you think REST APIs always respond instantly or can they handle delays gracefully? Commit to your answer.

Concept: Advanced REST API design includes caching, pagination, rate limiting, and asynchronous patterns to improve performance and scalability.

Caching stores responses to reduce repeated work. Pagination breaks large data into smaller chunks. Rate limiting prevents overload by limiting requests per time. Sometimes, services use asynchronous calls or webhooks to handle long tasks without blocking. These techniques keep systems fast and reliable under heavy load.

Result

You know how to build REST APIs that perform well and scale as demand grows.

Mastering these optimizations ensures your microservices remain responsive and stable in real-world conditions.

Under the Hood

REST APIs work by sending HTTP requests from one service to another over the network. Each request includes a method, URL, headers, and optionally a body with data. The receiving service processes the request, performs actions like reading or updating data, and sends back an HTTP response with a status code and data. This interaction is stateless, meaning each request is independent and contains all needed information. Network protocols like TCP/IP handle data transport, while HTTP defines the message format and rules.

Why designed this way?

REST was designed to use the existing web infrastructure and protocols to simplify communication between distributed systems. By using standard HTTP methods and stateless interactions, REST APIs avoid complex connection management and allow services to scale independently. Alternatives like SOAP were more complex and heavyweight. REST's simplicity and use of web standards made it popular for microservices and web APIs.

┌───────────────┐       HTTP Request       ┌───────────────┐
│ Client Service│ ───────────────────────▶ │ Server Service│
│ (Sends Method,│                          │ (Processes    │
│ URL, Headers, │                          │ Request, Sends│
│ Body)         │                          │ Response)     │
└───────────────┘       HTTP Response      └───────────────┘
       ▲                                         │
       │                                         │
       │           Stateless Interaction        │
       └─────────────────────────────────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Do REST APIs require maintaining session state between requests? Commit to yes or no.

Common Belief: REST APIs need to keep session state to remember clients between calls.

Reality: REST APIs are stateless; each request must contain all information needed to process it without relying on past requests.

Why it matters: Assuming statefulness leads to designs that don't scale well and cause errors when requests are handled by different servers.

Quick: Do you think REST APIs always use JSON for data? Commit to yes or no.

Common Belief: REST APIs must use JSON as the data format.

Reality: REST APIs can use any format like XML, plain text, or others, but JSON is popular for its simplicity and readability.

Why it matters: Believing JSON is mandatory limits flexibility and integration with systems that use other formats.

Quick: Do you think REST APIs between microservices should be open to the public? Commit to yes or no.

Common Belief: All REST APIs between services can be open without security since they are internal.

Reality: Internal REST APIs must be secured to prevent unauthorized access and protect sensitive data.

Why it matters: Ignoring security risks leads to data leaks, service disruptions, and vulnerabilities to attacks.

Quick: Do you think changing an API endpoint always breaks all clients immediately? Commit to yes or no.

Common Belief: Any change to an API endpoint breaks all clients using it.

Reality: With proper versioning and backward compatibility, APIs can evolve without breaking existing clients.

Why it matters: Misunderstanding this leads to fear of change and stagnation in system improvements.

Expert Zone

1

Some REST APIs use hypermedia (HATEOAS) to guide clients dynamically, but many practical APIs skip this for simplicity.

2

Choosing between synchronous REST calls and asynchronous messaging depends on latency tolerance and system complexity.

3

API gateways often handle cross-cutting concerns like authentication, rate limiting, and logging, simplifying individual services.

When NOT to use

REST APIs are not ideal for high-frequency, low-latency communication or streaming data. Alternatives like gRPC or message queues (Kafka, RabbitMQ) are better for these cases.

Production Patterns

In production, REST APIs between microservices often use API gateways for routing and security, implement circuit breakers to handle failures gracefully, and use centralized logging and monitoring to track API health and performance.

Connections

Message Queues

Alternative communication pattern

Understanding REST APIs helps contrast synchronous request-response with asynchronous messaging, clarifying when to use each for service communication.

API Gateway

Builds on REST APIs to manage traffic

Knowing REST APIs is essential to appreciate how API gateways route, secure, and monitor service calls in complex systems.

Human Conversation

Similar pattern of request and response

Seeing REST API calls as conversations helps grasp the importance of clear requests, responses, and context in communication.

Common Pitfalls

#1 Tightly coupling services by sharing internal data models directly in REST APIs.

Wrong approach:

Service A sends its internal database object structure directly in API responses without abstraction.

Correct approach:

Service A defines clear API contracts with only necessary fields and hides internal details.

Root cause:

Confusing internal data representation with external API design leads to fragile, hard-to-change interfaces.

#2 Ignoring error handling and always returning 200 OK status.

Wrong approach:

Always respond with HTTP 200 even when an error occurs, putting error info only in the body.

Correct approach:

Use proper HTTP status codes like 400 for bad requests or 500 for server errors to signal problems clearly.

Root cause:

Not using HTTP status codes properly causes clients to misinterpret responses and complicates debugging.

#3 Exposing too many endpoints and mixing unrelated concerns in one service.

Wrong approach:

A single microservice exposes dozens of unrelated REST endpoints for different domains.

Correct approach:

Design services with focused responsibilities and limited, clear REST endpoints per domain.

Root cause:

Lack of domain-driven design leads to bloated services that are hard to maintain and scale.

Key Takeaways

REST APIs use standard web methods and URLs to let microservices communicate clearly and independently.

Statelessness and consistent use of HTTP methods and status codes make REST APIs scalable and reliable.

Designing focused, domain-specific APIs prevents tight coupling and eases system evolution.

Security, versioning, and performance optimizations are essential for production-ready REST APIs.

Understanding REST APIs helps choose the right communication patterns and tools for building flexible distributed systems.

Practice

1. What is the main purpose of using a REST API between microservices?

easy

A. To allow services to communicate over HTTP using standard methods

B. To store data permanently in a database

C. To run services on the same machine only

D. To replace all backend logic with frontend code

Solution

1. Step 1: Understand REST API role in microservices

   REST APIs enable communication between independent services using HTTP methods like GET, POST, PUT, DELETE.

2. Step 2: Identify the correct purpose

   Storing data or running services on the same machine are not the main goals of REST APIs; they focus on communication.

3. Final Answer:

   To allow services to communicate over HTTP using standard methods -> Option A

4. Quick Check:

   REST API = communication over HTTP [OK]

Hint: REST APIs connect services via HTTP methods [OK]

Common Mistakes:

• Confusing REST API with database storage
• Thinking REST APIs run services locally only
• Assuming REST replaces backend logic

2. Which HTTP method is typically used to update an existing resource in a REST API between microservices?

easy

A. GET

B. PUT

C. POST

D. DELETE

Solution

1. Step 1: Recall HTTP methods and their purposes

   GET retrieves data, POST creates new data, PUT updates existing data, DELETE removes data.

2. Step 2: Match method to update action

   Updating a resource is done with PUT, which replaces or modifies the resource at the given URL.

3. Final Answer:

   PUT -> Option B

4. Quick Check:

   Update = PUT [OK]

Hint: PUT method updates resources in REST APIs [OK]

Common Mistakes:

• Using GET to update data
• Confusing POST with update instead of create
• Using DELETE to update resources

3. Consider this REST API call between two services:

GET /users/123

What is the expected result of this request?

medium

A. Retrieve details of the user with ID 123

B. Create a new user with ID 123

C. Delete the user with ID 123

D. Update the user with ID 123

Solution

1. Step 1: Analyze the HTTP method and URL

   The method is GET, which is used to retrieve data. The URL targets user with ID 123.

2. Step 2: Determine the action based on method

   GET requests fetch data without changing it, so it retrieves user details.

3. Final Answer:

   Retrieve details of the user with ID 123 -> Option A

4. Quick Check:

   GET /users/123 = retrieve user data [OK]

Hint: GET fetches data, not modifies it [OK]

Common Mistakes:

• Thinking GET creates or deletes data
• Confusing URL path with action
• Assuming GET updates resources

4. A microservice sends a POST request to another service's REST API but receives a 405 Method Not Allowed error. What is the most likely cause?

medium

A. The request body is missing required fields

B. The URL endpoint does not exist

C. The HTTP method POST is not supported by the endpoint

D. The server is down

Solution

1. Step 1: Understand 405 Method Not Allowed meaning

   This error means the server recognizes the URL but does not allow the HTTP method used.

2. Step 2: Match error to cause

   If POST is not supported on that endpoint, the server rejects it with 405. Other issues cause different errors.

3. Final Answer:

   The HTTP method POST is not supported by the endpoint -> Option C

4. Quick Check:

   405 error = method not allowed [OK]

Hint: 405 means method not allowed on endpoint [OK]

Common Mistakes:

• Confusing 405 with 404 (not found)
• Assuming missing fields cause 405
• Thinking server down causes 405

5. You design two microservices: Service A calls Service B's REST API to get user data. To improve scalability and reduce latency, which design choice is best?

hard

A. Service A calls Service B synchronously for every user request

B. Service A and B share a database to avoid API calls

C. Service B pushes user data to Service A via REST POST requests

D. Service A caches user data locally and refreshes periodically

Solution

1. Step 1: Evaluate synchronous calls impact

   Calling Service B synchronously for every request adds latency and load, reducing scalability.

2. Step 2: Consider caching benefits

   Caching user data locally in Service A reduces calls to Service B, improving response time and scalability.

3. Step 3: Assess other options

   Service B pushing data is complex and not typical REST usage; sharing a database breaks microservice independence.

4. Final Answer:

   Service A caches user data locally and refreshes periodically -> Option D

5. Quick Check:

   Caching improves scalability and latency [OK]

Hint: Cache data locally to reduce cross-service calls [OK]

Common Mistakes:

• Using synchronous calls for every request
• Sharing databases between microservices
• Expecting REST APIs to push data automatically