@Beta public class ErrorAwarePolicy extends Object implements ChainableLoadBalancingPolicy
When creating a query plan, this policy gathers a list of candidate hosts from its child policy; for each candidate host, it then determines whether it should be included into or excluded from the final query plan, based on its current error rate (measured over the last minute, with a 5-second granularity).
Note that the policy should not blindly count all errors in its measurements: some type of
errors (e.g. CQL syntax errors) can originate from the client and occur on all hosts, therefore
they should not count towards the exclusion threshold or all hosts could become excluded. You can
provide your own ErrorAwarePolicy.ErrorFilter
to customize that logic.
The policy follows the builder pattern to be created, the ErrorAwarePolicy.Builder
class can be created
with builder(com.datastax.driver.core.policies.LoadBalancingPolicy)
method.
This policy is currently in BETA mode and its behavior might be changing throughout different driver versions.
Modifier and Type | Class and Description |
---|---|
static class |
ErrorAwarePolicy.Builder
Utility class to create a
ErrorAwarePolicy . |
static interface |
ErrorAwarePolicy.ErrorFilter
A filter for the errors considered by
ErrorAwarePolicy . |
Modifier and Type | Method and Description |
---|---|
static ErrorAwarePolicy.Builder |
builder(LoadBalancingPolicy childPolicy)
Creates a new error aware policy builder given the child policy that the resulting policy
should wrap.
|
void |
close()
Gets invoked at cluster shutdown.
|
HostDistance |
distance(Host host)
Returns the distance assigned by this policy to the provided host.
|
LoadBalancingPolicy |
getChildPolicy()
Returns the child policy.
|
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.
|
public LoadBalancingPolicy getChildPolicy()
ChainableLoadBalancingPolicy
getChildPolicy
in interface ChainableLoadBalancingPolicy
public void init(Cluster cluster, Collection<Host> hosts)
LoadBalancingPolicy
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.
init
in interface LoadBalancingPolicy
cluster
- the Cluster
instance for which the policy is created.hosts
- the initial hosts to use.public HostDistance distance(Host host)
LoadBalancingPolicy
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.
distance
in interface LoadBalancingPolicy
host
- the host of which to return the distance of.host
.public Iterator<Host> newQueryPlan(String loggedKeyspace, Statement statement)
LoadBalancingPolicy
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.
newQueryPlan
in interface LoadBalancingPolicy
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.public void onAdd(Host host)
LoadBalancingPolicy
The newly added node should be considered up.
onAdd
in interface LoadBalancingPolicy
host
- the host that has been newly added.public void onUp(Host host)
LoadBalancingPolicy
onUp
in interface LoadBalancingPolicy
host
- the host that has been detected up.public void onDown(Host host)
LoadBalancingPolicy
onDown
in interface LoadBalancingPolicy
host
- the host that has been detected down.public void onRemove(Host host)
LoadBalancingPolicy
onRemove
in interface LoadBalancingPolicy
host
- the removed host.public static ErrorAwarePolicy.Builder builder(LoadBalancingPolicy childPolicy)
childPolicy
- the load balancing policy to wrap with error awareness.public void close()
LoadBalancingPolicy
This gives the policy the opportunity to perform some cleanup, for instance stop threads that it might have started.
close
in interface LoadBalancingPolicy
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