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Every resource has a version field that changes every time the resource
changes — theetagfield. Etags are a standard part of HTTP and are
supported in the calendar API for two cases:
on resource modifications to ensure that there has been no other write to this resource in the meantime (conditional modification)
on resource retrieval to only retrieve resource data if the resource has changed (conditional retrieval)
Conditional modification
If you want to update or delete a resource only if it has not changed since
you last retrieved it, you can specify anIf-Matchheader that contains the
value of the etag from the previous retrieval. This is very useful to prevent
lost modifications on resources. The clients have the option of re-retrieving
the resource and re-applying the changes.
If the entry (and its etag) has not changed since the last retrieval, the
modification succeeds and the new version of the resource with the new etag is
returned. Otherwise, you will get a 412 (Precondition failed) response code.
The snippet of sample code below demonstrates how to perform conditional
modifications with theJava client library.
privatestaticvoidrun()throwsIOException{// Create a test event.Eventevent=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.intnumAttempts=0;booleanisUpdated=false;do{Calendar.Events.Updaterequest=client.events().update("primary",event.getId(),event);request.setRequestHeaders(newHttpHeaders().setIfMatch(event.getEtag()));try{event=request.execute();isUpdated=true;}catch(GoogleJsonResponseExceptione){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.EventlatestEvent=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{throwe;}}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));}}
If you want to retrieve a resource only if it has changed since you last
retrieved it, you can specify anIf-None-Matchheader which contains the
value of the etag from the previous retrieval. If the entry (and thus its etag)
has changed since the last retrieval, the new version of the resource with the
new etag will be returned. Otherwise you will get a 304 (Not Modified)
response code.
The snippet of sample code below demonstrates how to perform conditional
retrieval with theJava client library.
privatestaticvoidrun()throwsIOException{// Create a test event.Eventevent=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.GetgetRequest=client.events().get("primary",event.getId());getRequest.setRequestHeaders(newHttpHeaders().setIfNoneMatch(event.getEtag()));try{event=getRequest.execute();System.out.println("The event was modified, retrieved latest version.");}catch(GoogleJsonResponseExceptione){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{throwe;}}}
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Missing the information I need","missingTheInformationINeed","thumb-down"],["Too complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-28 UTC."],[],[],null,["# Get specific versions of resources\n\nEvery resource has a version field that changes every time the resource\nchanges --- the `etag` field. Etags are a standard part of HTTP and are\nsupported in the calendar API for two cases:\n\n- on resource modifications to ensure that there has been no other write to this resource in the meantime (conditional modification)\n- on resource retrieval to only retrieve resource data if the resource has changed (conditional retrieval)\n\nConditional modification\n------------------------\n\nIf you want to update or delete a resource only if it has not changed since\nyou last retrieved it, you can specify an `If-Match` header that contains the\nvalue of the etag from the previous retrieval. This is very useful to prevent\nlost modifications on resources. The clients have the option of re-retrieving\nthe resource and re-applying the changes.\n\nIf the entry (and its etag) has not changed since the last retrieval, the\nmodification succeeds and the new version of the resource with the new etag is\nreturned. Otherwise, you will get a 412 (Precondition failed) response code.\n| **Important:** There is no support for conditional modifications for insert operations. Instead, it is guaranteed that if you are allowed to provide a resource ID, then the operation will only succeed if no existing entry has that ID.\n\nThe snippet of sample code below demonstrates how to perform conditional\nmodifications with the\n[Java client library](/api-client-library/java/apis/calendar/v3). \n\n```java\n private static void run() throws IOException {\n // Create a test event.\n Event event = Utils.createTestEvent(client, \"Test Event\");\n System.out.println(String.format(\"Event created: %s\", event.getHtmlLink()));\n\n // Pause while the user modifies the event in the Calendar UI.\n System.out.println(\"Modify the event's description and hit enter to continue.\");\n System.in.read();\n\n // Modify the local copy of the event.\n event.setSummary(\"Updated Test Event\");\n\n // Update the event, making sure that we don't overwrite other changes.\n int numAttempts = 0;\n boolean isUpdated = false;\n do {\n Calendar.Events.Update request = client.events().update(\"primary\", event.getId(), event);\n request.setRequestHeaders(new HttpHeaders().setIfMatch(event.getEtag()));\n try {\n event = request.execute();\n isUpdated = true;\n } catch (GoogleJsonResponseException e) {\n if (e.getStatusCode() == 412) {\n // A 412 status code, \"Precondition failed\", indicates that the etag values didn't\n // match, and the event was updated on the server since we last retrieved it. Use\n // {@link Calendar.Events.Get} to retrieve the latest version.\n Event latestEvent = client.events().get(\"primary\", event.getId()).execute();\n\n // You may want to have more complex logic here to resolve conflicts. In this sample we're\n // simply overwriting the summary.\n latestEvent.setSummary(event.getSummary());\n event = latestEvent;\n } else {\n throw e;\n }\n }\n numAttempts++;\n } while (!isUpdated && numAttempts \u003c= MAX_UPDATE_ATTEMPTS);\n\n if (isUpdated) {\n System.out.println(\"Event updated.\");\n } else {\n System.out.println(String.format(\"Failed to update event after %d attempts.\", numAttempts));\n }\n }https://github.com/googleworkspace/java-samples/blob/26cb124371d51cb5cb8e6c4a3db6422bbef586fb/calendar/sync/src/main/java/com/google/api/services/samples/calendar/sync/ConditionalModificationSample.java\n```\n\nConditional retrieval\n---------------------\n\nIf you want to retrieve a resource only if it has changed since you last\nretrieved it, you can specify an `If-None-Match` header which contains the\nvalue of the etag from the previous retrieval. If the entry (and thus its etag)\nhas changed since the last retrieval, the new version of the resource with the\nnew etag will be returned. Otherwise you will get a 304 (Not Modified)\nresponse code.\n| **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 (e.g., calendar properties or ACLs).\n\nThe snippet of sample code below demonstrates how to perform conditional\nretrieval with the\n[Java client library](/api-client-library/java/apis/calendar/v3). \n\n```java\n private static void run() throws IOException {\n // Create a test event.\n Event event = Utils.createTestEvent(client, \"Test Event\");\n System.out.println(String.format(\"Event created: %s\", event.getHtmlLink()));\n\n // Pause while the user modifies the event in the Calendar UI.\n System.out.println(\"Modify the event's description and hit enter to continue.\");\n System.in.read();\n\n // Fetch the event again if it's been modified.\n Calendar.Events.Get getRequest = client.events().get(\"primary\", event.getId());\n getRequest.setRequestHeaders(new HttpHeaders().setIfNoneMatch(event.getEtag()));\n try {\n event = getRequest.execute();\n System.out.println(\"The event was modified, retrieved latest version.\");\n } catch (GoogleJsonResponseException e) {\n if (e.getStatusCode() == 304) {\n // A 304 status code, \"Not modified\", indicates that the etags match, and the event has\n // not been modified since we last retrieved it.\n System.out.println(\"The event was not modified, using local version.\");\n } else {\n throw e;\n }\n }\n }https://github.com/googleworkspace/java-samples/blob/26cb124371d51cb5cb8e6c4a3db6422bbef586fb/calendar/sync/src/main/java/com/google/api/services/samples/calendar/sync/ConditionalRetrievalSample.java\n```"]]
