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DEVANSHU PATIL
DEVANSHU PATIL

Posted on AI-assisted

Why Your Microservice Architecture Should Probably Be a Modular Monolith First

#softwareengineering #microservices #architecture #backend

Why Your Microservice Architecture Should Probably Be a Modular Monolith First

Over the last decade, "Microservices" became the default architectural buzzword. Startups with two engineers and twenty daily active users were creating thirty microservices running on Kubernetes clusters.

A few months later, reality hits:

  • A simple user checkout requires tracing RPC calls through four different services.
  • Database consistency requires complex Saga orchestrators.
  • Local development requires Docker Desktop consuming 16GB of RAM.
  • CI/CD deployment pipelines take 45 minutes to run.

Before you jump into the distributed systems swamp, there is a vastly superior starting architecture that 90% of development teams should adopt: the Modular Monolith.

The Hidden Tax of Microservices

Microservices do not solve architectural problems; they turn architectural problems into distributed systems problems.

Single Monolithic Call:
Function A() -------- (In-Memory Pointer Call: 5 nanoseconds) --------> Function B()

Microservice Network Call:
Service A --(Serialize JSON)--> [TCP / TLS] --> [Router] --> [Service B] --(Deserialize)--> DB
(Total Latency: 25 milliseconds = 5,000,000x slower!)
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When you split an application prematurely:

  1. Network Fallibility: Network packets drop. Services timeout. You now need Circuit Breakers, Retries, Idempotency Keys, and Dead Letter Queues for basic operations.
  2. Data Consistency: You lose single-database ACID transactions. Joining data across two tables now requires in-memory application joins or asynchronous event sync.
  3. Operational Drag: Instead of monitoring one binary, your team spends half its week maintaining Kubernetes ingresses, Helm charts, Service Meshes, and Docker registries.

What Is a Modular Monolith?

A Modular Monolith is a single deployable application binary whose codebase is rigorously structured into isolated domain modules with strict public interfaces.

+--------------------------------------------------------+
| Application Binary (Single Process, Single DB)        |
|                                                        |
|  +--------------------+        +--------------------+  |
|  |  Identity Module   | <====> |   Billing Module   |  |
|  |  (Internal DB Sch) |   API  |  (Internal DB Sch) |  |
|  +--------------------+        +--------------------+  |
|            ^                              ^            |
|            |                              |            |
|            v                              v            |
|  +--------------------------------------------------+  |
|  |             Catalog & Orders Module              |  |
|  +--------------------------------------------------+  |
+--------------------------------------------------------+
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The Golden Rules of Modular Monoliths:

  1. Modules Communicate via Public Interfaces Only: Module A never directly queries Module B's internal database tables or internal classes.
  2. Logical Database Isolation: Each module owns its designated tables or database schema. Foreign keys across module boundaries are forbidden!
  3. Single Process Deployment: Everything compiles into one binary and deploys to one server in seconds.

Summary

Start with a Modular Monolith. Keep your deployments simple, your transactions local, your latency in nanoseconds, and your developer productivity high. Only break things apart when scale demands it.

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