Get specific versions of resources

Every resource has a version field that changes every time the resource changes—the etag field. ETags are a standard part of HTTP and are supported in the Google Calendar API for two cases:

• On resource modifications, to ensure no other write to this resource occurred in the interim (conditional modification).
• On resource retrieval, to retrieve data only if the resource has changed (conditional retrieval).

Conditional modification

If you want to update or delete a resource only if it hasn't changed since you last retrieved it, you can specify an If-Match header that contains the value of the ETag from the previous retrieval. This helps prevent lost modifications on resources. Clients can re-retrieve the resource and reapply the changes.

If the entry (and its ETag) hasn't changed since the last retrieval, the modification succeeds and the server returns the new version of the resource with the new ETag. Otherwise, the server returns a 412 Precondition Failed response code.

Important: The Calendar API doesn't support conditional modifications for insert operations. Instead, if you provide a custom resource ID, the operation succeeds only if no existing entry has that ID.

The following sample demonstrates how to perform conditional modifications with the Java client library:

  private static void run() throws IOException {
    // Create a test event.
    Event event = Utils.createTestEvent(client, "Test Event");
    System.out.println(String.format("Event created: %s", event.getHtmlLink()));

    // Pause while the user modifies the event in the Calendar UI.
    System.out.println("Modify the event's description and hit enter to continue.");
    System.in.read();

    // Modify the local copy of the event.
    event.setSummary("Updated Test Event");

    // Update the event, making sure that we don't overwrite other changes.
    int numAttempts = 0;
    boolean isUpdated = false;
    do {
      Calendar.Events.Update request = client.events().update("primary", event.getId(), event);
      request.setRequestHeaders(new HttpHeaders().setIfMatch(event.getEtag()));
      try {
        event = request.execute();
        isUpdated = true;
      } catch (GoogleJsonResponseException e) {
        if (e.getStatusCode() == 412) {
          // A 412 status code, "Precondition failed", indicates that the etag values didn't
          // match, and the event was updated on the server since we last retrieved it. Use
          // {@link Calendar.Events.Get} to retrieve the latest version.
          Event latestEvent = client.events().get("primary", event.getId()).execute();

          // You may want to have more complex logic here to resolve conflicts. In this sample we're
          // simply overwriting the summary.
          latestEvent.setSummary(event.getSummary());
          event = latestEvent;
        } else {
          throw e;
        }
      }
      numAttempts++;
    } while (!isUpdated && numAttempts <= MAX_UPDATE_ATTEMPTS);

    if (isUpdated) {
      System.out.println("Event updated.");
    } else {
      System.out.println(String.format("Failed to update event after %d attempts.", numAttempts));
    }
  }
ConditionalModificationSample.java

Conditional retrieval

If you want to retrieve a resource only if it has changed since you last retrieved it, you can specify an If-None-Match header that contains the value of the ETag from the previous retrieval. If the entry (and thus its ETag) has changed since the last retrieval, the server returns the new version of the resource with the new ETag. Otherwise, the server returns a 304 Not Modified response code.

Note: There are special cases in which ETags won't change, such as when one of the read-only fields of a `calendarList` entry changes (such as calendar properties or ACLs).

The following sample demonstrates how to perform conditional retrieval with the Java client library:

  private static void run() throws IOException {
    // Create a test event.
    Event event = Utils.createTestEvent(client, "Test Event");
    System.out.println(String.format("Event created: %s", event.getHtmlLink()));

    // Pause while the user modifies the event in the Calendar UI.
    System.out.println("Modify the event's description and hit enter to continue.");
    System.in.read();

    // Fetch the event again if it's been modified.
    Calendar.Events.Get getRequest = client.events().get("primary", event.getId());
    getRequest.setRequestHeaders(new HttpHeaders().setIfNoneMatch(event.getEtag()));
    try {
      event = getRequest.execute();
      System.out.println("The event was modified, retrieved latest version.");
    } catch (GoogleJsonResponseException e) {
      if (e.getStatusCode() == 304) {
        // A 304 status code, "Not modified", indicates that the etags match, and the event has
        // not been modified since we last retrieved it.
        System.out.println("The event was not modified, using local version.");
      } else {
        throw e;
      }
    }
  }
ConditionalRetrievalSample.java