How to Implement API Aggregation in API Gateways

Introduction

Modern applications are powered by microservices, which increase scalability and flexibility but also introduce complexity in client-server communication. When a frontend needs to retrieve data from multiple microservices, it must often make several API calls, each to a different backend. This results in increased latency, higher network overhead, and complex frontend logic.

API aggregation is a pattern where the API gateway composes responses from multiple backend services into a single response. This approach reduces roundtrips, simplifies frontend code, and centralizes logic at the gateway.

This guide will walk you through everything you need to know about implementing API aggregation in API gateways—from conceptual foundations to production-ready examples.

What Is API Aggregation?

API aggregation refers to combining responses from multiple backend APIs into a single response. It is a common architectural pattern, particularly in systems built with microservices.

Example Scenario

A mobile app homepage needs the following data:

• Latest products (/product/latest)
• User profile (/user/me)
• Cart summary (/cart/summary)

Without aggregation:

• The frontend makes 3 HTTP calls
• Has to handle retries and partial failures
• Needs logic to synchronize results

With aggregation:

• A single call to /api/homepage returns all the data
• Logic is centralized at the gateway or aggregation layer

Why API Aggregation is Critical in Microservices Architectures

In monolithic systems, backend data often comes from a single source. In microservices, data is spread across services:

Challenge	How Aggregation Helps
Multiple roundtrips	Combines into one
Network latency	Reduced
Frontend complexity	Simplified
Inconsistent response formats	Standardized
Partial failures	Centralized fallback logic

When to Use API Aggregation

API aggregation should be used when:

• The frontend frequently calls multiple services in one user interaction
• The latency cost of multiple calls is too high
• You want to offload aggregation logic from clients
• A BFF (Backend-for-Frontend) is needed for a mobile or web app

But avoid overusing aggregation for:

• Services that change independently (tight coupling)
• Situations where caching or CDN can solve latency
• Systems that require fine-grained access control at the microservice level

Common Aggregation Patterns

1. Fan-out Aggregation

• Gateway sends requests to multiple services in parallel and combines the results.

2. Chained Aggregation

• The result from one service is used as input to another.

3. Conditional Aggregation

• Based on client request or token, different sets of services are called.

How API Gateways Implement Aggregation

Gateways that support aggregation typically:

• Accept a single client request
• Fan out to multiple backends (often async)
• Wait for all responses (with timeout/fallback)
• Compose and return the aggregated response

Technologies Supporting Aggregation:

• Apache APISIX: via plugins
• Kong: with custom plugins or Kong Gateway Enterprise
• NGINX: with Lua or NGINX Plus JavaScript
• Envoy: with filters or via a BFF service

Architecture of API Aggregation in a Gateway

Aggregation without Gateway

sequenceDiagram
    participant Client
    participant Service A
    participant Service B
    participant Service C
    Client->>Service A: GET /user
    Client->>Service B: GET /orders
    Client->>Service C: GET /recommendations

Aggregation with Gateway

sequenceDiagram
    participant Client
    participant API Gateway
    participant Service A
    participant Service B
    participant Service C
    Client->>API Gateway: GET /homepage
    API Gateway->>Service A: GET /user
    API Gateway->>Service B: GET /orders
    API Gateway->>Service C: GET /recommendations
    API Gateway-->>Client: Aggregated Response

GraphQL vs REST Aggregation

While GraphQL is often pitched as a solution to API composition, it has trade-offs.

Feature	REST Aggregation	GraphQL
Gateway-based	✅	❌ (usually server-based)
Schema flexibility	Low	High
Caching simplicity	High	Medium
Tooling	Mature (REST)	Growing
Debugging	Easier	Requires introspection tools

REST with gateway-based aggregation provides fine control and gateway-level visibility, which aligns well with cloud-native observability practices.

API Aggregation Implementation Approaches

1. Gateway-level aggregation (recommended)

• Aggregation logic is placed inside the API gateway.
• Easier to manage, observable, and centralized.

2. Middleware/BFF approach

• Separate service acts as an aggregator.
• More flexible logic, but introduces a new component.

3. Client-side aggregation (anti-pattern in microservices)

• Client does orchestration; poor UX, high latency.

Conclusion

API aggregation is a key enabler for modern application performance and user experience. By offloading response orchestration to API gateways or dedicated services, teams can reduce frontend complexity and improve system efficiency.

Whether you use gateway-native plugins or backend orchestrators, the goal is to provide fast, coherent, and reliable APIs to your clients. Evaluate your use case, choose the right pattern, and apply best practices to ensure robust and maintainable aggregation logic.

For scalable, pluggable, and cloud-native API gateways like Apache APISIX, implementing API aggregation is both flexible and powerful.

FAQs

1. What's the difference between API aggregation and API composition?

They are often used interchangeably, but composition may imply a broader set of orchestration, transformation, and logic layers.

2. Is it okay to aggregate APIs at the frontend?

For simple apps, yes. For complex, latency-sensitive, or security-aware apps, prefer server-side aggregation.

3. Which API gateway supports aggregation out of the box?

Apache APISIX, Kong (via plugins), Tyk, and Gravitee support aggregation through plugins or scripting.

Next Steps

Stay tuned for our upcoming column on the API gateway Guide, where you'll find the latest updates and insights!

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