Package com.datastax.driver.core.policies

Class ErrorAwarePolicy

  • All Implemented Interfaces:
    ChainableLoadBalancingPolicy, LoadBalancingPolicy
    @Beta
public class ErrorAwarePolicy
extends Object
implements ChainableLoadBalancingPolicy
    Chainable load balancing policy that filters out hosts based on their error rates.

    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.

    • Method Detail

      • init

        public void init​(Cluster cluster,
                 Collection<Host> hosts)
        Description copied from interface: LoadBalancingPolicy
        Initialize this load balancing policy.

        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.

        Specified by:
        init in interface LoadBalancingPolicy
        Parameters:
        cluster - the Cluster instance for which the policy is created.
        hosts - the initial hosts to use.

      • distance

        public HostDistance distance​(Host host)
        Description copied from interface: LoadBalancingPolicy
        Returns the distance assigned by this policy to the provided 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.

        Specified by:
        distance in interface LoadBalancingPolicy
        Parameters:
        host - the host of which to return the distance of.
        Returns:
        the HostDistance to host.

      • newQueryPlan

        public Iterator<Host> newQueryPlan​(String loggedKeyspace,
                                   Statement statement)
        Description copied from interface: LoadBalancingPolicy
        Returns the hosts to use for a new query.

        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.

        Specified by:
        newQueryPlan in interface LoadBalancingPolicy
        Parameters:
        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.
        Returns:
        an iterator of Host. The query is tried against the hosts returned by this iterator in order, until the query has been sent successfully to one of the host.

      • onAdd

        public void onAdd​(Host host)
        Description copied from interface: LoadBalancingPolicy
        Called when a new node is added to the cluster.

        The newly added node should be considered up.

        Specified by:
        onAdd in interface LoadBalancingPolicy
        Parameters:
        host - the host that has been newly added.

      • onUp

        public void onUp​(Host host)
        Description copied from interface: LoadBalancingPolicy
        Called when a node is determined to be up.
        Specified by:
        onUp in interface LoadBalancingPolicy
        Parameters:
        host - the host that has been detected up.

      • onDown

        public void onDown​(Host host)
        Description copied from interface: LoadBalancingPolicy
        Called when a node is determined to be down.
        Specified by:
        onDown in interface LoadBalancingPolicy
        Parameters:
        host - the host that has been detected down.

      • builder

        public static ErrorAwarePolicy.Builder builder​(LoadBalancingPolicy childPolicy)
        Creates a new error aware policy builder given the child policy that the resulting policy should wrap.
        Parameters:
        childPolicy - the load balancing policy to wrap with error awareness.
        Returns:
        the created builder.

      • close

        public void close()
        Description copied from interface: LoadBalancingPolicy
        Gets invoked at cluster shutdown.

        This gives the policy the opportunity to perform some cleanup, for instance stop threads that it might have started.

        Specified by:
        close in interface LoadBalancingPolicy