TabletBalancer (Apache Accumulo Core 2.1.1 API)

All Known Implementing Classes:

GroupBalancer, HostRegexTableLoadBalancer, RegexGroupBalancer, SimpleLoadBalancer, TableLoadBalancer
```
public interface TabletBalancer
```
This class is responsible for managing the distribution of tablets throughout an Accumulo cluster. In most cases, users will want a balancer implementation which ensures a uniform distribution of tablets, so that no individual tablet server is handling significantly more work than any other.
Implementations may wish to store configuration in Accumulo's system configuration using the Property.GENERAL_ARBITRARY_PROP_PREFIX. They may also benefit from using per-table configuration using Property.TABLE_ARBITRARY_PROP_PREFIX.

Since:

2.1.0

Nested Class Summary

Nested Classes
Modifier and Type	Interface	Description
`static interface`	`TabletBalancer.AssignmentParameters`	An interface for grouping parameters required for the balancer to assign unassigned tablets.
`static interface`	`TabletBalancer.BalanceParameters`	An interface for grouping parameters required for the balancer to balance tablets.

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`long`	`balance(TabletBalancer.BalanceParameters params)`	Ask the balancer if any migrations are necessary.
`void`	`getAssignments(TabletBalancer.AssignmentParameters params)`	Assign tablets to tablet servers.
`void`	`init(BalancerEnvironment balancerEnvironment)`	Initialize the TabletBalancer.

- Method Detail
  - init
```
void init(BalancerEnvironment balancerEnvironment)
```
    Initialize the TabletBalancer. This gives the balancer the opportunity to read the configuration.
  - getAssignments
```
void getAssignments(TabletBalancer.AssignmentParameters params)
```
    Assign tablets to tablet servers. This method is called whenever the manager finds tablets that are unassigned.
  - balance
```
long balance(TabletBalancer.BalanceParameters params)
```
    Ask the balancer if any migrations are necessary. If the balancer is going to self-abort due to some environmental constraint (e.g. it requires some minimum number of tservers, or a maximum number of outstanding migrations), it should issue a log message to alert operators. The message should be at WARN normally and at ERROR if the balancer knows that the problem can not self correct. It should not issue these messages more than once a minute. This method will not be called when there are unassigned tablets.
    
    Returns:
    
    the time, in milliseconds, to wait before re-balancing.