It's a fairly common scenario that services are released to production and, at some point in the future, it becomes evident that service versioning is needed. The question now becomes, "How do I add API versioning without breaking existing clients?"

Before API versioning was applied to your service, clients were already bound to some version of the service; they just don't know which version. A client in this situation doesn't have any flexibility to go backward. If the service changes, hopefully that carries forward without breaking any clients. When you're ready to introduce formal API versioning semantics into your service, then any previously unversioned services snap to a single, default API version.

The default API versioning semantics require that all clients explicitly request an API version for a service. This would break backward compatibility with existing clients, so we need a way to address this. The API versioning options provide a way to change the default behaviors that will enable supporting services that don't explicitly declare API versions.

The bare minimum requirement to enable backward compatibility is to assume the default API version when a client does not explicitly request an API version. This will allow a client to continue making requests to existing services without providing API version information. Your existing controller implementations that back these services do not require any attribution or configuration to enable this behavior. Clients wishing to upgrade to new versions of a service must begin explicitly specifying an API version.

config.AddApiVersioning( options => options.AssumeDefaultVersionWhenUnspecified = true );

services.AddApiVersioning( options => options.AssumeDefaultVersionWhenUnspecified = true );

The assumed API version is 1.0 by default. From a client's perspective, the default API version is inconsequential. As a service author, however, you may want to choose a different default API version so that it aligns with your overall API versioning scheme and instrumentation requirements.

configuration.AddApiVersioning(
    options =>
    {
        options.AssumeDefaultVersionWhenUnspecified = true;
        options.DefaultApiVersion = new ApiVersion( new DateTime( 2016, 7, 1 ) );
    } );

services.AddApiVersioning(
    options =>
    {
        options.AssumeDefaultVersionWhenUnspecified = true;
        options.DefaultApiVersion = new ApiVersion( new DateOnly( 2016, 7, 1 ) );
    } );

If these basic configuration settings are still insufficient for your needs, then you will need to use or create an API version selector and register it in the API versioning options.