HColumnDescriptor and HTableDescriptor Support

This section describes the supported fields in the HColumnDescriptor and the HTableDescriptor classes.

MapR-DB supports all of the methods that are in these classes. However, it supports only a subset of their fields.

HColumnDescriptor Class

Field Description
BLOCKSIZE Size of blocks in files stored to the filesytem (hfiles).
BLOOMFILTER Whether or not to use bloomfilters.
COMPRESSION Compression type.
IN_MEMORY Whether to serve from memory or not.
MIN_VERSIONS Minimum number of versions to keep.
NAME Name of the column family.
TTL Time to live of cell contents.
VERSIONS Number of versions to keep.

HTableDescriptor Class

Field Description
AUTOSPLIT

Specifies whether to split the table into regions automatically as the table grows. The average size of each region is determined by theregionsizemb parameter.

The default value is true.

BULKLOAD Boolean. Specifies whether to perform a full bulk load of the table. The default is false. For more information, see Bulk Loading and MapR Tables.
DELETE_TTL

Used for multi-master replication.

Normally, delete operations are purged after the affected table cells are updated. Whereas the result of an update is saved in a table until another change overwrites or deletes it, the result of a delete is not saved. In multi-master replication, this difference can lead to tables being unsynchronized.

Example

Suppose that you have set up multi-master replication between table customers in the cluster sanfrancisco and table customers in the cluster newyork. Client applications then make these two changes:

  1. On /mapr/sanfrancisco/customers, put row A at 10:00:00 AM.
  2. On /mapr/newyork/customers, delete row A at 10:00:01 AM.

On /mapr/sanfrancisco/customers, the order of operations is:

  1. Put row A with a timestamp of 10:00:00 AM
  2. Delete row A with a timestamp of 10:00:01 AM (This operation is repllicated from /mapr/newyork/customers.)

On /mapr/newyork/customers, the order of operations is:

  1. Delete row A with a timestamp of 10:00:01 AM
  2. Put row A with a timestamp of 10:00:00 AM (This operation is replicated from /mapr/sanfrancisco/customers.)

Now, though the put happened on /mapr/sanfrancisco/customers at 10:00:00 AM, the put reaches /mapr/newyork/customers several seconds after that. Suppose that the actual time that the put arrives at /mapr/newyork/customers is 10:00:03 AM.

To ensure that both tables stay synchronized, /mapr/newyork/customers should preserve the delete until after the put is replicated. Then, the delete can be applied after the put. Therefore, the time-to-live for the delete should be at least long enough for the put to arrive at /mapr/newyork/customers. In this case, the time-to-live should be at least 3 seconds.

In general, the time-to-live for deletes should be greater than the amount of time that it takes replicated operations to reach replicas. By default, the value is 24 hours.

For example, suppose (to extend the scenario above) that you pause replication during weekdays and resume it on weekends. The put takes place on Monday morning /mapr/sanfrancisco/customers at 10:00:00 AM and the delete takes place at /mapr/newyork/customers at 10:00:01 AM. Replication does not resume until 12:00:00 AM Saturday morning. Given the volume of operations to be replicated and the potential for network problems, it is possible that these operations will not be replicated until Sunday. In this scenario, a value of 7 days for DELETE_TTL (7 multiplied by 24 hours) should provide sufficient margin.

NAME Name of the table.