API Composition Pattern in Microservices



1592 views Designing μ-services



Say, we have 3 microservices - Order, Payments, and Logistics - and to get the order details we need data from all of them, merge it, and then respond to the client. A common pattern to achieve this is API Composition.

API Composition

It is a high-level pattern to query microservices. It puts a composer right in the middle abstracting out the microservices.

With the composer sitting in between, the request from the client first hits the composer, and the composer then talks to the relevant services to get the response. It then merges the responses before sending them to the client.

Implementing API Composition

Instead of building it from scratch, we can use tools that specialize in achieving this - ex: API Gateways like KrakenD, Kong, and AWS API Gateway.

Improving user’s experience using composer

An API Composer not only helps in making the backend simpler, but it also helps in gaining a good UX.

If we do not have an API composer, the client (browser/app) would have to make multiple API calls to microservices to get the information and render the interface. The multiple calls would require multiple round trips of the data increasing the latency and will also eat up the user’s data.

By having an API composer sitting in between the client would only need to make one API call and the fan-out happening at composer will be within the infra. This would reduce the latency for clients and improve the UX.

Branch Composition

For a complex usecase, it is quite possible that a downstream service may use another composer to reach out to another set of services to get things done. A dependency like this would create a multi-level API composition also called Branch composition.

This would create a hierarchical dependency between services solved through multiple API composers and it is a common pattern observed in complex e-commerce platforms.

Advantages of using API Composition

  • Simple to implement
  • Client has a single point to interact
  • Hides the implementation complexities
  • Security and Limiting applied only to the composer
  • Can cover the “bad” design decisions with a shiny new interface
  • Hides legacy system allowing us to gradually move out of it

Disadvantages of using API Composition

  • If the dataset we fetch from microservices is large, it would make the composer in-efficient
  • Overall availability is challenged as the number of services increase
  • Having a transactional data consistency is difficult
  • Composer needs to be managed and maintained
  • Composer may become a bottleneck at scale

Arpit Bhayani

Arpit's Newsletter

CS newsletter for the curious engineers

❤️ by 17000+ readers

If you like what you read subscribe you can always subscribe to my newsletter and get the post delivered straight to your inbox. I write essays on various engineering topics and share it through my weekly newsletter.




Other essays that you might like


BFF - Backend for Frontend - Pattern in Microservices

2225 views 114 likes 2022-07-04

As your application evolves, supporting multiple types of clients like Desktop, Mobile apps, etc becomes tricky. The bac...

Best practices that make microservices integration easy

850 views 50 likes 2022-06-27

Running microservices in isolation does not make any sense. To get something done, multiple microservices need to talk t...

Things to remember while building Microservices

856 views 35 likes 2022-06-20

An engineer working on Microservices should not only just focus on engineering; there are so many other aspects to look ...

Why should we have a standard way of building Microservices?

631 views 27 likes 2022-06-17

We all love creating microservices, but what if every team creates its own microservice uniquely and uses its own conven...


Be a better engineer

A set of courses designed to make you a better engineer and excel at your career; no-fluff, pure engineering.


System Design Masterclass

A masterclass that helps you become great at designing scalable, fault-tolerant, and highly available systems.

Enrolled by 700+ learners

Details →

Designing Microservices

A free course to help you understand Microservices and their high-level patterns in depth.

Enrolled by 17+ learners

Details →

GitHub Outage Dissections

A free course to help you learn core engineering from outages that happened at GitHub.

Enrolled by 67+ learners

Details →

Hash Table Internals

A free course to help you learn core engineering from outages that happened at GitHub.

Enrolled by 25+ learners

Details →

BitTorrent Internals

A free course to help you understand the algorithms and strategies that power P2P networks and BitTorrent.

Enrolled by 42+ learners

Details →

Topics I talk about

Being a passionate engineer, I love to talk about a wide range of topics, but these are my personal favourites.




Arpit's Newsletter read by 17000+ engineers

🔥 Thrice a week, in your inbox, an essay about system design, distributed systems, microservices, programming languages internals, or a deep dive on some super-clever algorithm, or just a few tips on building highly scalable distributed systems.



  • v12.4.4
  • © Arpit Bhayani, 2022

Powered by this tech stack.