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Caching

Some API methods have the option to cache responses on the client side by headers Etag, Cache-Control and Expires.

If Etag is returned when requesting a resource in API, for example, the request:

GET /areas/1 HTTP/1.1
Host: api.hh.ru
User-Agent: MyApp/1.0 (my-app-feedback@example.com)

returned the answer:

HTTP/1.1 200 OK
Etag: W/"ai-356a192-57E047847BCE15-RU10e33"
Content-Type: application/json; charset=UTF-8
Content-Length: 61
Expires: Tue, 03 Oct 2017 07:01:04 GMT
Cache-Control: max-age=1200

{
    "areas": [],
    "id": "1",
    "name": "Москва",
    "parent_id": "113"
}

The client can remember the meaning of Etag with the response data and next time when this resource is needed, request information about changes in the resource by sending Etag value in the header If-None-Match:

GET /areas/1 HTTP/1.1
Host: api.hh.ru
If-None-Match: W/"ai-356a192-57E047847BCE15-RU10e33"
User-Agent: MyApp/1.0 (my-app-feedback@example.com)

If the resource has not changed, it will return a response with a code 304 Not Modified and without a body:

HTTP/1.1 304 Not Modified
Etag: W/"ai-356a192-57E047847BCE15-RU10e33"
Cache-Control: max-age=1200
Expires: Tue, 03 Oct 2017 07:05:43 GMT

If the resource changes, it will return new contents of the resource:

HTTP/1.1 200 OK
Etag: W/"ai-34e7018-356a192b7913b0-RU73fda"
Content-Type: application/json; charset=UTF-8
Expires: Tue, 03 Oct 2017 07:10:31 GMT
Cache-Control: max-age=1200

{
    "areas": [],
    "id": "1",
    "name": "Москва",
    "parent_id": "777"
}

The HEAD request can be used when checking by Etag; it will work similarly, but even if the resource has changed, it will return only headers without a response body.

In addition, when requesting the resource, the response contains headers Cache-Control and Expires for the client to cache the response and use it before the indicated expiration date.

More details in RFC-7232.

Caching is supported by the majority of directories. You can determine support by the presence of Etag in the response.