public String getTaskAssignment() {
    return getMessageValue(CONTAINER_KEY);
  }
SteContainerToHostMapping is used to set the container to host mapping information.
@param source              the source of the message
@param taskName                 the taskName which is used to persist the message
@param containerId            the hostname of the container
samza-core/src/main/java/org/apache/samza/coordinator/stream/messages/SetTaskContainerMapping.java