Designing for Change: API Versioning and Evolution Strategies in Phoenix

It starts simple.

One mobile app.

One clean set of JSON endpoints.

One happy Phoenix API.

Everything talks to everything.

The world makes sense.

Until it doesn’t.

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The Moment Your API Succeeds, Change Becomes Inevitable

The second your API gets real users — partners, clients, apps — you inherit a new job:

Preserving stability while making progress.

Because in APIs, the future means:

• New fields
• New formats
• New consumers
• New constraints

And the question is:

How do you evolve without breaking what’s already working?

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Phoenix Gives You Power — And Responsibility

Phoenix won’t enforce a versioning strategy.

That’s your job.

But it gives you the tools:

• Explicit routing
• Flexible views
• Strong separation between business logic and HTTP interface

To design for evolution from day one.

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The Simplest Versioning: Route Prefixes

Start with /api/v1 in your router:

scope "/api/v1", MyAppWeb.V1 do
  pipe_through :api
  resources "/users", UserController
end

Clients know what version they’re using.

You know what they expect.

No surprises.

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Views = JSON Stability

Phoenix doesn't return raw structs.

You shape your JSON deliberately.

That gives you control to:

• Add fields
• Rename values
• Deprecate old keys
• Change formats gradually

def render("user.json", %{user: user}) do
  %{
    id: user.id,
    name: user.full_name,
    username: user.username,
    legacy_name: user.old_field # Mark as deprecated
  }
end

Stage changes. Don’t delete things.

Add new formats alongside old ones. Deprecate, don’t detonate.

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When It’s Time for /api/v2

Some changes are too big:

• New auth model
• New response formats
• Rewritten data flows

That’s when you introduce /api/v2.

But don’t clone everything.

Reuse context modules.

# v1 controller
defmodule MyAppWeb.V1.UserController do
  def show(conn, %{"id" => id}) do
    user = Accounts.get_user!(id)
    render(conn, "show.json", user: user)
  end
end

# v2 controller
defmodule MyAppWeb.V2.UserController do
  def show(conn, %{"id" => id}) do
    user = Accounts.get_user!(id)
    render(conn, "show_v2.json", user: user)
  end
end

Business logic stays the same. Views evolve.

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What Needs a Version?

🚫 You don’t always need a new version.

✅ Safe changes:

• Adding optional fields
• Optimizing queries
• Internal naming tweaks

⚠️ Risky changes (should version):

• Removing fields
• Changing field meaning
• Altering relationship structures

If it breaks a client silently — version it.

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Deprecation Is a Conversation

Don’t just pull the plug.

Communicate.

• Add "deprecated": true in responses
• Include Warning headers
• Document timelines
• Announce deprecations visibly

Give clients time. Show empathy. Build trust.

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Documentation = Versioned Contracts

You’re not just building endpoints.

You’re building expectations.

So your documentation must match:

• Versioned paths: /api/v1/products, /api/v2/products
• Versioned docs: docs/v1.html, docs/v2.html

Tools like PhoenixSwagger can help. Or roll your own using annotations in views and controllers.

Keep it honest. Keep it in sync.

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Security Applies Across Versions

Just because /api/v1 is old doesn’t mean it’s soft.

Apply consistent standards:

• Token-based auth
• Rate limiting
• Logging and audit trails

Version-aware logging is especially powerful:

Logger.metadata(api_version: "v1", user_id: user.id)
Logger.info("GET /api/v1/orders")

Track who’s still on v1. Who’s ready for v2. Who’s abusing limits.

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One Day, You’ll Sunset a Version

And that’s okay — if you do it right:

1. Announce end-of-life
2. Communicate timelines
3. Monitor usage
4. Reach out to clients
5. Remove endpoints cleanly

Versioning isn’t a graveyard. It’s a cycle.

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Phoenix Is Built for APIs That Last

The magic of Phoenix isn’t just fast JSON rendering.

It’s architectural maturity.

You can build:

• /api/v1
• /api/v2
• /api/v3

… all using the same:

• Context modules
• Request pipelines
• Plug architecture

Different surfaces, same core.

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Build for Change, Not Just for Speed

You won’t get it perfect.

But you can get it right.

• Plan for evolution
• Version intentionally
• Document clearly
• Respect your clients

Because your API isn’t just a contract — it’s a promise.

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Ready to Take Phoenix Further?

If you're serious about using Phoenix LiveView like a pro, download the guide:

Phoenix LiveView: The Pro's Guide to Scalable Interfaces and UI Patterns

✅ 20-page PDF

✅ Real-world examples

✅ Battle-tested architecture

✅ Reusable UI patterns

Make your next interface the last one you need to rebuild.