API Versioning

Core Principles

• Version APIs from the start — retrofitting is painful
• URL segment versioning (/api/v1/orders) is the most common and recommended
• Report available and deprecated versions via response headers
• Maintain backward compatibility within a version
• Use sunset policies to communicate deprecation timelines

Patterns

URL Segment Versioning (Recommended)

// 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;
});

Controller Versioning

[ApiController]
[Route("api/v{version:apiVersion}/orders")]
[ApiVersion("1.0")]
[ApiVersion("2.0")]
public sealed class OrdersController(ISender sender) : ControllerBase
{
    [HttpGet]
    [MapToApiVersion("1.0")]
    public async Task<ActionResult<List<OrderV1Response>>> GetOrdersV1(
        CancellationToken ct)
    {
        var result = await sender.Send(new ListOrdersV1Query(), ct);
        return Ok(result);
    }

    [HttpGet]
    [MapToApiVersion("2.0")]
    public async Task<ActionResult<PagedList<OrderV2Response>>> GetOrdersV2(
        [FromQuery] OrderFilter filter, CancellationToken ct)
    {
        var result = await sender.Send(new ListOrdersV2Query(filter), ct);
        return Ok(result);
    }
}

// Or separate controllers per version
[ApiController]
[Route("api/v{version:apiVersion}/orders")]
[ApiVersion("1.0", Deprecated = true)]
public sealed class OrdersV1Controller : ControllerBase { }

[ApiController]
[Route("api/v{version:apiVersion}/orders")]
[ApiVersion("2.0")]
public sealed class OrdersV2Controller : ControllerBase { }

Minimal API Versioning

var versionSet = app.NewApiVersionSet()
    .HasApiVersion(new ApiVersion(1, 0))
    .HasApiVersion(new ApiVersion(2, 0))
    .ReportApiVersions()
    .Build();

var v1 = app.MapGroup("/api/v{version:apiVersion}")
    .WithApiVersionSet(versionSet);

v1.MapGet("/orders", GetOrdersV1)
    .MapToApiVersion(new ApiVersion(1, 0));

v1.MapGet("/orders", GetOrdersV2)
    .MapToApiVersion(new ApiVersion(2, 0));

Header Versioning (Alternative)

builder.Services.AddApiVersioning(options =>
{
    options.ApiVersionReader = new HeaderApiVersionReader("X-Api-Version");
});

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

Query String Versioning (Alternative)

builder.Services.AddApiVersioning(options =>
{
    options.ApiVersionReader = new QueryStringApiVersionReader("api-version");
});

// Client requests: /api/orders?api-version=2.0

Combined Version Reader

builder.Services.AddApiVersioning(options =>
{
    options.ApiVersionReader = ApiVersionReader.Combine(
        new UrlSegmentApiVersionReader(),
        new HeaderApiVersionReader("X-Api-Version"),
        new QueryStringApiVersionReader("api-version"));
});

Sunset Policy

builder.Services.AddApiVersioning(options =>
{
    options.Policies.Sunset(1.0)
        .Effective(new DateTimeOffset(2025, 6, 1, 0, 0, 0, TimeSpan.Zero))
        .Link("https://docs.{Company}.com/api/migration-guide")
            .Title("v1 to v2 Migration Guide")
            .Type("text/html");
});

// Response header: Sunset: Sat, 01 Jun 2025 00:00:00 GMT
// Response header: Link: <https://docs.{Company}.com/api/migration-guide>; ...

OpenAPI Document Per Version

builder.Services.AddOpenApi("v1");
builder.Services.AddOpenApi("v2");

// Each document at /openapi/v1.json and /openapi/v2.json
app.MapOpenApi();

Anti-Patterns

• Not versioning from the start (forces breaking changes on clients)
• Using version in the response body instead of URL/header
• Major behavior changes within the same version
• Forgetting to deprecate old versions
• Different versioning strategies across the same API

Detect Existing Patterns

1. Search for Asp.Versioning package reference in .csproj
2. Look for AddApiVersioning in Program.cs
3. Check for [ApiVersion] attributes on controllers
4. Look for v{version:apiVersion} in route templates
5. Check for version query parameters in existing URLs

Adding to Existing Project

1. Install Asp.Versioning.Http (minimal API) or Asp.Versioning.Mvc.ApiExplorer (controllers)
2. Configure AddApiVersioning with default version and reader
3. Add [ApiVersion] to existing controllers (start with "1.0")
4. Update routes to include version segment
5. Create v2 endpoints for new behavior; keep v1 for backward compat
6. Add sunset policies for deprecated versions

Decision Guide

Strategy	Pros	Cons	Use When
URL segment	Visible, cacheable, simple	Version in URL	Default choice
Header	Clean URLs	Hidden, harder to test	Internal APIs
Query string	Easy to add	Pollutes query params	Legacy compat

References

• https://github.com/dotnet/aspnet-api-versioning
• https://learn.microsoft.com/en-us/aspnet/core/web-api/advanced/custom-formatters