Detect Conflict When Uploading Document To Google Drive

My Android app updates a Google Drive document. The file can be modified also elsewhere (e.g. through Drive web interface), so there may be a conflict on file upload. However, this

Solution 1:

Set the If-Match HTTP header:

finalUpdateupdate= mService.files().update(mDriveFile.getId(), mDriveFile, byteContent);
final HttpHeaders headers = update.getRequestHeaders();
headers.setIfMatch(mDriveFile.getEtag());
update.setRequestHeaders(headers);
mDriveFile = update.execute();

In case the document has changed meanwhile, the update will get rejected with a response something like:

com.google.api.client.googleapis.json.GoogleJsonResponseException: 412 Precondition Failed
{
  "code": 412,
  "errors": [
    {
      "domain": "global",
      "location": "If-Match",
      "locationType": "header",
      "message": "Precondition Failed",
      "reason": "conditionNotMet"
    }
  ],
  "message": "Precondition Failed"
}

Note that ETag might change even if the content does not.

Solution 2:

"Is this the correct way to use ETag?"

Yes

Also, for non-Docs files, you should also check md5Checksum for changes to the content.