This is a sub-issue of HBase-14070. This JIRA is a small step towards completely adding Hybrid Logical Clocks(HLC) to HBase. The main idea of HLC is described in HBase-14070 along with the motivation of adding it to HBase.
This patch in this issue takes the code from the patch in the parent.
The parent patch is pretty big to review at once. So, plan is to get code reviewed in smaller patches and
in the process take suggestions and change things if necessary.
What is this patch/issue about ?
This issue attempts to add a timestamp class to hbase-common and timestamp type to HTable.
This is a part of the attempt to get HLC into HBase. This patch does not interfere with the current working of HBase.
Why Timestamp Class ?
Timestamp class can be as an abstraction to represent time in Hbase in 64 bits.
It is just used for manipulating with the 64 bits of the timestamp and is not concerned about the actual time.
There are three types of timestamps. System time, Custom and HLC. Each one of it has methods to manipulate the 64 bits of timestamp.
HTable changes: Added a timestamp type property to HTable. This will help HBase exist in conjunction with old type of timestamp and also the HLC which will be introduced. The default is set to custom timestamp(current way of usage of timestamp). default unset timestamp is also custom timestamp as it should be so. The default timestamp will be changed to HLC when HLC feature is introduced completely in HBase.
Update: Based on the suggestions, made timestamp enum. Here is the description of the new changes.
Check the HBASE-16210.master.8.1.patch
1. Changed the Timestamp Implementation to Enum.
2. Changed the Timestamp semantics. Instead of HLC, System monotonic and custom, we now have Hybrid and Physical. System monotonic clock and Custom clocks can map their timestamps to Physical. HLC clock can map its timestamp to Hybrid.
3. The HTableDescriptor will contain clock type(not implemented yet) instead of timestamp type. As clocks convey the semantics of monotonic increasing and non decreasing etc. TimestampType doesn't have those semantics enforced, it just knows what to do with given 64 bits. Therefore, I removed the timestamp type field in the HTableDescriptor.
Open for suggestions.