@Override
            public void run() {
              try {
                writer.close();
              } catch (Exception e) {
                throw new SystemProducerException("Close failed for topic " + stream, e);
              }
            }
// find the writer in the writerMap else create one
@param source for which to find/create the writer
@param messageEnvelope to fetch the schema from if writer needs to be created
@return an AzureBlobWriter object
samza-azure/src/main/java/org/apache/samza/system/azureblob/producer/AzureBlobSystemProducer.java