Currently flushing of timeline writes is not really handled. For example, HBaseTimelineWriterImpl relies on HBase's BufferedMutator to batch and write puts asynchronously. However, BufferedMutator may not flush them to HBase unless the internal buffer fills up.
We do need a flush functionality first to ensure that data are written in a reasonably timely manner, and to be able to ensure some critical writes are done synchronously (e.g. key lifecycle events).