Database.CQL.IO.Cluster.Policies

Documentation

data Policy

A policy defines a load-balancing strategy and generally handles host visibility.

Constructors

Policy

Fields

• setup :: [Host] -> [Host] -> IO ()

  Initialise the policy with two sets of hosts. The first parameter are hosts known to be available, the second are other nodes. Note that a policy may be re-initialised at any point through this function.

• onEvent :: HostEvent -> IO ()

  Event handler. Policies will be informed about cluster changes through this function.

• select :: IO (Maybe Host)

  Host selection. The driver will ask for a host to use in a query through this function. A policy which has no available nodes may return Nothing.

• current :: IO [Host]

  Return all currently alive hosts.

• acceptable :: Host -> IO Bool

  During startup and node discovery, the driver will ask the policy if a dicovered host should be ignored.

• hostCount :: IO Word

  During query processing, the driver will ask the policy for a rough estimate of alive hosts. The number is used to repeatedly invoke select (with the underlying assumption that the policy returns mostly different hosts).

• display :: IO String

  Like having an effectful Show instance for this policy.

random :: IO Policy

Return hosts in random order.

roundRobin :: IO Policy

Iterate over hosts one by one.