tree 40fe0e9ef3ce57ee239ee1a5fd725368939d4c12
parent 40843bb56793e74eada6dd84c3eccff644f7c71e
author Huaxiang Sun <huaxiangsun@apache.com> 1604337436 -0800
committer stack <stack@apache.org> 1605859488 -0800

HBASE-25126 Add load balance logic in hbase-client to distribute read load over meta replica regions

It adds load balance support for meta lookup in AsyncTableRegionLocator.
The existing meta replica mode is renamed as "HedgedRead", client sends scan request to the primary meta replica region first,
if response is not back within a configured amount of time, it will send scan requests to all meta replica regions and
take the first response. On top of the existing mode, a new mode "LoadBalance" is introduced. In this mode, client first
choose a meta replica region to send scan request. If the response is stale, it may send the request to another meta replica region or
the primary region. In this mode, meta scan requests are load balanced across all replica regions with the primary mode as
the ultimate source of truth.

Two new config knobs are added:

1. hbase.locator.meta.replicas.mode
   Valid options are "None", "HedgedRead" and "LoadBalance", they are case insensitive. The default mode is "None".

2. hbase.locator.meta.replicas.mode.loadbalance.selector
   The load balance alogrithm to select a meta replica to send the requests.
   Only org.apache.hadoop.hbase.client.CatalogReplicaLoadBalanceReplicaSimpleSelector.class
   is supported for now, which is the default as well. The algorithm works as follows:
      a. Clients select a randome meta replica region to send the requests if there is no entry for the location in the stale
         location cache.
      b. If the location from one meta replica region is stale, a stale entry will be created in the statle location cache
         for the region.
      c. Clients select the primary meta region if the location is in the stale location cache.
      d. The stale location cache entries time out in 3 seconds.

If there is no "hbase.locator.meta.replicas.mode" configured, it will check the config knob "hbase.meta.replicas.use".
If "hbase.meta.replicas.use" is configured, the mode will be set to "HedgedRead".