If an API needs to support multiple versions without changing the URL path for existing clients, where would an expert tell the client to specify which API version it wants to use?

When an API needs to support multiple versions without altering the URL path segment for existing clients, an expert would typically advise the client to specify the desired API version using two primary mechanisms: the HTTP Accept header for media type negotiation, or a custom HTTP request header. A less preferred alternative is a query parameter.

First, the client can specify the API version within the HTTP Accept header. The Accept header is a standard HTTP request header sent by the client to indicate which media types, or data formats, it is capable of understanding and prefers in the server's response. For versioning, the version number is embedded directly into a custom media type. For example, a client could send an `Accept` header like `Accept: application/vnd.myapi.v2+json`. In this example, `application` denotes the top-level type, `vnd` signifies a vendor-specific type, `myapi` identifies the specific API, `v2` indicates version 2 of the API, and `json` specifies the desired data format. The Uniform Resource Locator (URL) path, such as `/users/123`, remains entirely unchanged, as the version information is encapsulated within the header. This approach leverages standard HTTP content negotiation, aligning well with RESTful principles.

Second, the client can use a custom HTTP request header. A custom request header is an additional HTTP header defined by the API provider, not part of standard HTTP headers, which clients include in their requests. For instance, the API could define a header named `X-API-Version` or `API-Version`. A client wishing to use version 2 of the API would then include `X-API-Version: 2` in its request headers. Similar to the Accept header method, the URL path, for example, `/products/456`, remains identical across all versions. The version specification is entirely contained within this custom header, making it a clear and straightforward method for both client and server implementation.

Finally, a third, though generally less preferred, method involves using a query parameter. A query parameter is a key-value pair appended to the end of a URL after a question mark (`?`), used to pass additional information to the server. For example, a client might request `GET /orders/789?version=2`. While this method does change the overall Uniform Resource Locator (URL), it does not alter the core URL *path segment(the part before the `?`). However, using query parameters for API versioning can introduce complexities with caching, as different query parameters for the same path segment are typically treated as distinct resources by caching proxies, potentially leading to reduced cache efficiency if the underlying data is similar across versions.