api-versioning

---
name: api-versioning
description: >
license: MIT
---

# API Versioning

## Core Principles

1. **Version from day one** — Adding versioning later is painful. Start with a version in the URL even if you only have v1.
2. **URL segment versioning is the default** — `/api/v1/orders` is the most discoverable and cache-friendly strategy.
3. **Never break existing versions** — Add a new version for breaking changes. Deprecate the old version with a timeline.
4. **Version the API, not individual endpoints** — All endpoints in a version group share the same version number.

## Patterns

### Setup with Asp.Versioning

```csharp
// Program.cs
builder.Services.AddApiVersioning(options =>
{
    options.DefaultApiVersion = new ApiVersion(1, 0);
    options.AssumeDefaultVersionWhenUnspecified = true;
    options.ReportApiVersions = true;
    options.ApiVersionReader = new UrlSegmentApiVersionReader();
})
.AddApiExplorer(options =>
{
    options.GroupNameFormat = "'v'VVV";
    options.SubstituteApiVersionInUrl = true;
});
```

### URL Segment Versioning (Recommended)

```csharp
var v1 = app.NewApiVersionSet()
    .HasApiVersion(new ApiVersion(1, 0))
    .Build();

var v2 = app.NewApiVersionSet()
    .HasApiVersion(new ApiVersion(2, 0))
    .Build();

app.MapGroup("/api/v{version:apiVersion}/orders")
    .WithApiVersionSet(v1)
    .WithTags("Orders")
    .MapOrderEndpointsV1();

app.MapGroup("/api/v{version:apiVersion}/orders")
    .WithApiVersionSet(v2)
    .WithTags("Orders")
    .MapOrderEndpointsV2();
```

### Header Versioning (Alternative)

```csharp
options.ApiVersionReader = new HeaderApiVersionReader("X-Api-Version");

// Client sends: X-Api-Version: 2.0
```

### Deprecating a Version

```csharp
var v1 = app.NewApiVersionSet()
    .HasDeprecatedApiVersion(new ApiVersion(1, 0))
    .HasApiVersion(new ApiVersion(2, 0))
    .Build();

// Response headers will include: api-deprecated-versions: 1.0
```

### Version-Specific Endpoint Groups

```csharp
public static class OrderEndpointsV1
{
    public static RouteGroupBuilder MapOrderEndpointsV1(this RouteGroupBuilder group)
    {
        group.MapGet("/{id:guid}", GetOrderV1);
        group.MapPost("/", CreateOrderV1);
        return group;
    }

    private static async Task<Results<Ok<OrderResponseV1>, NotFound>> GetOrderV1(
        Guid id, ISender sender, CancellationToken ct)
    {
        // V1 response shape
        var result = await sender.Send(new GetOrder.Query(id), ct);
        return result.IsSuccess
            ? TypedResults.Ok(result.Value.ToV1())
            : TypedResults.NotFound();
    }
}

public static class OrderEndpointsV2
{
    public static RouteGroupBuilder MapOrderEndpointsV2(this RouteGroupBuilder group)
    {
        group.MapGet("/{id:guid}", GetOrderV2);
        group.MapPost("/", CreateOrderV2);
        return group;
    }

    private static async Task<Results<Ok<OrderResponseV2>, NotFound>> GetOrderV2(
        Guid id, ISender sender, CancellationToken ct)
    {
        // V2 response shape — includes new fields
        var result = await sender.Send(new GetOrder.Query(id), ct);
        return result.IsSuccess
            ? TypedResults.Ok(result.Value.ToV2())
            : TypedResults.NotFound();
    }
}
```

## Anti-patterns

### Don't Version Individual Endpoints

```csharp
// BAD — inconsistent versioning within a group
app.MapGet("/api/v1/orders", ListOrdersV1);
app.MapGet("/api/v2/orders/{id}", GetOrderV2); // V2 only for this endpoint?

// GOOD — version the entire group
app.MapGroup("/api/v1/orders").MapOrderEndpointsV1();
app.MapGroup("/api/v2/orders").MapOrderEndpointsV2();
```

### Don't Use Query String Versioning as Default

```csharp
// BAD for REST APIs — version hidden in query string, not cache-friendly
GET /api/orders?api-version=2.0

// GOOD — version in URL, discoverable and cacheable
GET /api/v2/orders
```

## Decision Guide

| Scenario | Recommendation |
|----------|---------------|
| New public API | URL segment versioning from day one |
| Internal API between services | Header versioning (cleaner URLs) |
| Breaking response shape change | New version |
| Adding new optional fields | Same version (backwards compatible) |
| Deprecating a version | Mark deprecated, set sunset date, document migration path |