public interface LoadBalancingPolicy
Two methods need to be implemented:
distance(com.datastax.driver.core.Host)
: returns the "distance" of an host for that balancing
policy.
newQueryPlan(java.lang.String, com.datastax.driver.core.Statement)
: it is used for each query to find which host to
query first, and which hosts to use as failover.
The LoadBalancingPolicy
is informed of hosts up/down events. For efficiency purposes,
the policy is expected to exclude down hosts from query plans.
Modifier and Type | Method and Description |
---|---|
void |
close()
Gets invoked at cluster shutdown.
|
HostDistance |
distance(Host host)
Returns the distance assigned by this policy to the provided host.
|
void |
init(Cluster cluster,
Collection<Host> hosts)
Initialize this load balancing policy.
|
Iterator<Host> |
newQueryPlan(String loggedKeyspace,
Statement statement)
Returns the hosts to use for a new query.
|
void |
onAdd(Host host)
Called when a new node is added to the cluster.
|
void |
onDown(Host host)
Called when a node is determined to be down.
|
void |
onRemove(Host host)
Called when a node is removed from the cluster.
|
void |
onUp(Host host)
Called when a node is determined to be up.
|
void init(Cluster cluster, Collection<Host> hosts)
Note that the driver guarantees that it will call this method exactly once per policy object and will do so before any call to another of the methods of the policy.
cluster
- the Cluster
instance for which the policy is created.hosts
- the initial hosts to use.HostDistance distance(Host host)
The distance of an host influence how much connections are kept to the node (see HostDistance
). A policy should assign a LOCAL
distance to nodes that are susceptible
to be returned first by newQueryPlan
and it is useless for newQueryPlan
to
return hosts to which it assigns an IGNORED
distance.
The host distance is primarily used to prevent keeping too many connections to host in remote datacenters when the policy itself always picks host in the local datacenter first.
host
- the host of which to return the distance of.host
.Iterator<Host> newQueryPlan(String loggedKeyspace, Statement statement)
Each new query will call this method. The first host in the result will then be used to
perform the query. In the event of a connection problem (the queried host is down or appear to
be so), the next host will be used. If all hosts of the returned Iterator
are down, the
query will fail.
loggedKeyspace
- the currently logged keyspace (the one set through either Cluster.connect(String)
or by manually doing a USE
query) for the session on which
this plan need to be built. This can be null
if the corresponding session has no
keyspace logged in.statement
- the query for which to build a plan.void onAdd(Host host)
The newly added node should be considered up.
host
- the host that has been newly added.void onUp(Host host)
host
- the host that has been detected up.void onDown(Host host)
host
- the host that has been detected down.void onRemove(Host host)
host
- the removed host.void close()
This gives the policy the opportunity to perform some cleanup, for instance stop threads that it might have started.
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