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{-| Implementation of lock allocation.
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-}
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{-
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Copyright (C) 2014 Google Inc.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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02110-1301, USA.
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-}
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module Ganeti.Locking.Allocation
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( LockAllocation
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, emptyAllocation
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, OwnerState(..)
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, listLocks
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) where
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import qualified Data.Map as M
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import Data.Maybe (fromMaybe)
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import qualified Data.Set as S
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{-
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This module is parametric in the type of locks and lock owners.
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While we only state minimal requirements for the types, we will
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consistently use the type variable 'a' for the type of locks and
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the variable 'b' for the type of the lock owners throughout this
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module.
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-}
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-- | The state of a lock that is taken.
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data AllocationState a = Exclusive a | Shared (S.Set a) deriving (Eq, Show)
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-- | Data type describing the way a lock can be owned.
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data OwnerState = OwnShared | OwnExclusive deriving (Ord, Eq, Show)
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{-| Representation of a Lock allocation
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To keep queries for locks efficient, we keep two
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associations, with the invariant that they fit
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together: the association from locks to their
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allocation state, and the association from an
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owner to the set of locks owned. As we do not
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export the constructor, the problem of keeping
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this invariant reduces to only exporting functions
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that keep the invariant.
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-}
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data LockAllocation a b =
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LockAllocation { laLocks :: M.Map a (AllocationState b)
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, laOwned :: M.Map b (M.Map a OwnerState)
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}
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deriving (Eq, Show)
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-- | A state with all locks being free.
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emptyAllocation :: (Ord a, Ord b) => LockAllocation a b
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emptyAllocation =
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LockAllocation { laLocks = M.empty
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, laOwned = M.empty
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}
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-- | Obtain the set of locks held by a given owner.
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listLocks :: Ord b => b -> LockAllocation a b -> M.Map a OwnerState
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listLocks owner = fromMaybe M.empty . M.lookup owner . laOwned
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