Apollo Configuration Change Notification Process and Long‑Polling Mechanism

When an Apollo client starts, it launches an asynchronous thread that continuously sends an HTTP long‑polling request to the Config Service’s /notifications/v2 endpoint; the request times out after 90 seconds, longer than the server’s 60‑second long‑poll timeout.

The Config Service receives the request and, instead of replying immediately, suspends it using Spring’s DeferredResult , storing the DeferredResult in a local cache keyed by client information.

If no relevant configuration change occurs within 60 seconds, the service returns HTTP 304; otherwise, when a change is published, the Admin Service writes a notification message to the database, which the Config Service’s background scanner picks up.

The scanner retrieves the matching DeferredResult from the cache and invokes setResult , causing the long‑poll request to return the changed namespace; the client then immediately requests the latest configuration for that namespace.

private final Multimap
deferredResults =
      Multimaps.synchronizedSetMultimap(
          TreeMultimap.create(String.CASE_INSENSITIVE_ORDER, Ordering.natural()));