code: @Override
      public Object store(String projectId, String eventCollection,
                          String event) throws IOException {
          // Prepare the collection cache directory.
          File collectionCacheDir = prepareCollectionDir(projectId, eventCollection);
  
          // Create the cache file.
          Calendar timestamp = Calendar.getInstance();
          File cacheFile = getFileForEvent(collectionCacheDir, timestamp);
  
          // Write the event to the cache file.
          Writer writer = null;
          try {
              OutputStream out = new FileOutputStream(cacheFile);
              writer = new OutputStreamWriter(out, ENCODING);
              writer.write(event);
          } finally {
              KeenUtils.closeQuietly(writer);
          }
  
          // Return the file as the handle to use for retrieving/removing the event.
          return cacheFile;
      }
docs: {@inheritDoc}