Building a ‘memory layer’ for an online service often forces a tough choice: prioritize the consumer app or the public API? This guide breaks down three real-world strategies, from the quick-and-dirty to the API-first purist approach, helping you decide which technical debt is worth taking on.

Consumer App, Public API, or Both? The Memory Layer Dilemma

I remember a project back in 2018, codenamed “Magpie.” The goal was simple: build a smart shopping assistant that remembered everything you’d ever looked at, liked, or put in a cart, even across different sessions. The UX mockups were gorgeous. The product manager was selling a vision of a seamless, almost psychic shopping experience. We dove in headfirst, building a slick React front-end and a tight, monolithic Go backend to power it. We shipped in six months. High-fives all around. Then, the first big B2B partner came knocking. “We love it,” they said, “we just need API access to that ‘memory’ service for our own app.” We froze. The “memory layer” wasn’t a service; it was a series of tangled database calls and caching logic welded directly into our main application’s backend. We had built a beautiful house with no doors for anyone else to enter. It took us another four months of painful refactoring to decouple the logic into a real API, all while trying not to break the live consumer app. It was a classic case of building for the *what* without thinking about the *how* for the future.

The “Why”: It’s a Product Strategy Problem in Disguise

This isn’t just a technical fork in the road. When you’re asked to build a feature that has both an internal use (your own app) and a potential external one (a public API), you’re really being asked a fundamental business question: What is our core product here? Is it the consumer-facing application, or is it the underlying data platform and its capabilities?

• If the app is the product, the API is just a feature, an add-on.
• If the platform is the product, your app is just the first, most important customer of your own API.

Getting this wrong upfront means you either build yourself into a corner like we did with “Magpie,” or you over-engineer a massive platform for a product that never finds market fit. Your job as an engineer is to force this conversation early and then choose the architecture that matches the answer.

The Fixes: Three Paths, Three Different Kinds of Pain

There’s no single right answer, only the right answer for your team, your timeline, and your budget. Here are the three main strategies I’ve seen in the wild.

1. The Quick Fix: “The App-First Facade”

This is the “ship it now” approach. You build the consumer application and its dedicated backend. The “memory layer” is tightly coupled to this backend. For the public API, you create a thin layer, a facade, that essentially translates external requests into the language your internal backend understands. It’s pragmatic, fast, and gets a product into users’ hands.

How it works: Your consumer app talks directly to its backend service. The public API Gateway routes traffic to a separate, simple translation service (or even a module within your main backend) that calls the same internal functions the app does.

# Simplified view of the request flow

# Consumer App Request
# GET /internal/v1/user/123/memory
# -> monolith-backend-prod-01 (Directly accesses user_memory_table)
# <- { "viewed_items": ["abc", "xyz"] }

# Public API Request
# GET /public/v1/memory?user_id=123 (API Key validated at Gateway)
# -> api-facade-service-prod-01
# -> Translates request and calls the monolith's internal function
# -> monolith-backend-prod-01
# <- { "data": { "viewed_items": ["abc", "xyz"] } }

Warning: This approach is pure technical debt. You're kicking the can down the road. The moment your public API needs features or data models that your consumer app doesn't, you'll start writing ugly, conditional logic in the core backend. It gets messy, fast.

2. The Permanent Fix: "The API-First Mandate"

This is the purist's choice and the one that pays dividends in the long run. You treat the API as the one and only product. Your "memory layer" is a standalone microservice with a clearly defined contract (like an OpenAPI spec). Everyone, including your own consumer app team, is a client of this API. There is no "back door."

How it works: You design the API contract first. All logic, all data storage (e.g., `user-memory-postgres` and `session-cache-redis-01`), and all business rules live inside this new, isolated service. Your consumer app and your external partners both authenticate against the same API Gateway and hit the same endpoints.

An initial API definition might look something like this:

openapi: 3.0.0
info:
  title: TechResolve Memory Layer API
  version: 1.0.0
paths:
  /users/{userId}/memory:
    get:
      summary: Retrieve a user's memory state
      parameters:
        - name: userId
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: object
                properties:
                  lastViewed:
                    type: array
                    items:
                      type: string
                  savedForLater:
                    type: array
                    items:
                      type: string

This forces discipline. Your frontend team can't just ask the backend team to "add a quick field to the response." It has to be a proper versioned change to the API contract. It's slower to start but infinitely more scalable and maintainable.

3. The 'Nuclear' Option: "The BFF Monorepo"

Sometimes you need the speed of the App-First approach but the discipline of the API-First model. The Backend-for-Frontend (BFF) pattern, often managed within a monorepo, is the complex but powerful middle ground. You build a core, headless "memory" service, but then you build separate, lightweight backends (BFFs) for each "head" or client.

How it works:

• Core Memory Service: A standalone service that does one thing well: manage memory state. It has no knowledge of who is calling it.
• Consumer App BFF: A backend service that calls the Core Memory Service and other services, then aggregates and shapes the data specifically for the consumer app.
• Public API BFF: Another backend service that handles public authentication, rate limiting, and data shaping for external partners before calling the same Core Memory Service.

Pro Tip: This pattern shines when you have multiple, very different clients (e.g., a web app, a mobile app, and a public API) that all need to consume the same core data but in different ways. It prevents your core service from getting bloated with client-specific logic.

My Take: A Comparison

So, which one do you choose? It depends on where the real risk is for your business.

Ultimately, don't let perfect be the enemy of good. My mistake on "Project Magpie" wasn't choosing the app-first model; it was failing to acknowledge the debt we were taking on and having a plan to pay it back. Whatever path you choose, document the trade-offs and make sure everyone, from the junior dev to the CTO, understands the decision you made and why. Now go build something.

Darian Vance

Lead Cloud Architect & DevOps Strategist

With over 12 years in system architecture and automation, Darian specializes in simplifying complex cloud infrastructures. An advocate for open-source solutions, he founded TechResolve to provide engineers with actionable, battle-tested troubleshooting guides and robust software alternatives.

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