crdt-event-fold-1.2.0.0: src/Data/CRDT/EventFold.hs
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE NamedFieldPuns #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
{-# OPTIONS_GHC -Wmissing-deriving-strategies #-}
{- |
Description: Garbage collected event folding CRDT.
This module provides a CRDT data structure that collects and applies
operations (called "events") that mutate an underlying data structure
(like folding).
In addition to mutating the underlying data, each operation can also
produce an output that can be obtained by the client. The output can be
either totally consistent across all replicas (which is slower), or it
can be returned immediately and possibly reflect an inconsistent state.
The 'EventFold' name derives from a loose analogy to folding over a list of
events using plain old 'foldl'. The component parts of 'foldl' are:
- A binary operator, analogous to 'apply'.
- An accumulator value, analogous to 'infimumValue'.
- A list of values to fold over, loosely analogous to "the list of
all future calls to 'event'".
- A return value. There is no real analogy for the "return value".
Similarly to how you never actually obtain a return value if you
try to 'foldl' over an infinite list, 'EventFold's are meant to be
long-lived objects that accommodate an infinite number of calls
to 'event'. What you can do is inspect the current value of the
accumulator using 'infimumValue', or the "projected" value of the
accumulator using 'projectedValue' (where "projected" means "taking
into account all of the currently known calls to 'event' that have not
yet been folded into the accumulator, and which may yet turn out to to
have other events inserted into the middle or beginning of the list").
The 'EventFold' value itself can be thought of as an intermediate,
replicated, current state of the fold of an infinite list of events
that has not yet been fully generated. So you can, for instance,
check the current accumulator value.
In a little more detail, consider the type signature of 'foldl' (for lists).
> foldl
> :: (b -> a -> b) -- Analogous to 'apply', where 'a' is your 'Event'
> -- instance, and 'b' is 'State a'.
>
> -> b -- Loosely analogous to 'infimumValue' where
> -- progressive applications are accumulated. (I
> -- know that in the type signature of 'foldl'
> -- this is the "starting value", but imagine that
> -- for a recursive implementation of 'foldl',
> -- the child call's "starting value" is the parent
> -- call's accumulated value.)
>
> -> [a] -- Analogous to all outstanding or future calls to
> -- 'event'.
>
> -> b
-}
module Data.CRDT.EventFold (
-- * Basic API
-- ** Creating new CRDTs
new,
-- ** Adding new events
event,
-- ** Coordinating replica updates
{- |
Functions in this section are used to help merge foreign copies of
the CRDT, and transmit our own copy. (This library does not provide
any kind of transport support, except that all the relevant types
have 'Binary' instances. Actually arranging for these things to get
shipped across a wire is left to the user.)
In principal, the only function you need is 'fullMerge'. Everything
else in this section is an optimization. You can ship the full
'EventFold' value to a remote participant and it can incorporate
any changes using 'fullMerge', and vice versa. You can receive an
'EventFold' value from another participant and incorporate its
changes locally using 'fullMerge'.
However, if your underlying data structure is large, it may be more
efficient to just ship a sort of diff containing the information
that the local participant thinks the remote participant might be
missing. That is what 'events' and 'diffMerge' are for.
-}
fullMerge,
UpdateResult(..),
events,
diffMerge,
MergeError(..),
-- ** Participation
participate,
disassociate,
-- ** Defining your state and events
Event(..),
EventResult(..),
-- * Inspecting the 'EventFold'
isBlockedOnError,
projectedValue,
infimumValue,
infimumId,
infimumParticipants,
allParticipants,
projParticipants,
origin,
divergent,
-- * Underlying Types
EventFold,
EventId,
Diff,
) where
import Data.Bifunctor (first)
import Data.Binary (Binary(get, put))
import Data.Default.Class (Default(def))
import Data.DoubleWord (Word128(Word128), Word256(Word256))
import Data.Functor.Identity (Identity(Identity), runIdentity)
import Data.Map (Map, keys, toAscList, toDescList, unionWith)
import Data.Maybe (catMaybes)
import Data.Set ((\\), Set, member, union)
import Data.Word (Word64)
import GHC.Generics (Generic)
import qualified Data.Map as Map
import qualified Data.Map.Merge.Lazy as Map.Merge
import qualified Data.Set as Set
data EventFoldF o p e f = EventFoldF {
psOrigin :: o,
psInfimum :: Infimum (State e) p,
psEvents :: Map (EventId p) (f (Delta p e), Set p)
} deriving stock (Generic)
deriving stock instance
( Eq (f (Delta p e))
, Eq (Output e)
, Eq o
, Eq p
, Eq e
)
=>
Eq (EventFoldF o p e f)
instance
(
Binary (f (Delta p e)),
Binary o,
Binary p,
Binary e,
Binary (State e),
Binary (Output e)
)
=>
Binary (EventFoldF o p e f)
deriving stock instance
( Show (f (Delta p e))
, Show o
, Show p
, Show (State e)
)
=> Show (EventFoldF o p e f)
{- |
This type is a
<https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type CRDT>
into which participants can add 'Event's that are folded into a
base 'State'. You can also think of the "events" as operations that
mutate the base state, and the point of this CRDT is to coordinate
the application of the operations across all participants so that
they are applied consistently even if the operations themselves are
not commutative, idempotent, or monotonic.
Variables are:
- @o@ - Origin
- @p@ - Participant
- @e@ - Event
The "Origin" is a value that is more or less meant to identify the
"thing" being replicated, and in particular identify the historical
lineage of the 'EventFold'. The idea is that it is meaningless to
try and merge two 'EventFold's that do not share a common history
(identified by the origin value) and doing so is a programming error. It
is only used to try and check for this type of programming error and
throw an exception if it happens instead of producing undefined (and
difficult to detect) behavior.
-}
newtype EventFold o p e = EventFold { unEventFold :: EventFoldF o p e Identity}
deriving stock instance
(Show o, Show p, Show e, Show (Output e), Show (State e))
=>
Show (EventFold o p e)
deriving newtype instance
(Binary o, Binary p, Binary e, Binary (Output e), Binary (State e))
=>
Binary (EventFold o p e)
deriving newtype instance
(Eq o, Eq p, Eq e, Eq (Output e))
=>
Eq (EventFold o p e)
{- |
`Infimum` is the infimum, or greatest lower bound, of the possible
values of @s@.
-}
data Infimum s p = Infimum {
eventId :: EventId p,
participants :: Set p,
stateValue :: s
} deriving stock (Generic, Show)
instance (Binary s, Binary p) => Binary (Infimum s p)
instance (Eq p) => Eq (Infimum s p) where
Infimum s1 _ _ == Infimum s2 _ _ = s1 == s2
instance (Ord p) => Ord (Infimum s p) where
compare (Infimum s1 _ _) (Infimum s2 _ _) = compare s1 s2
{- |
`EventId` is a monotonically increasing, totally ordered identification
value which allows us to lend the attribute of monotonicity to event
application operations which would not naturally be monotonic.
-}
data EventId p
= BottomEid
| Eid Word256 p
deriving stock (Generic, Eq, Ord, Show)
instance (Binary p) => Binary (EventId p) where
put = put . toMaybe
where
toMaybe :: EventId p -> Maybe (Word64, Word64, Word64, Word64, p)
toMaybe BottomEid =
Nothing
toMaybe (Eid (Word256 (Word128 a b) (Word128 c d)) p) =
Just (a, b, c, d, p)
get = do
theThing <- get
return $ case theThing of
Nothing -> BottomEid
Just (a, b, c, d, p) -> Eid (Word256 (Word128 a b) (Word128 c d)) p
instance Default (EventId p) where
def = BottomEid
{- |
This is the exception type for illegal merges. These errors indicate
a serious programming bugs.
-}
data MergeError o p e
= DifferentOrigins o o
{- ^
The 'EventFold's do not have the same origin. It makes no sense
to merge 'EventFold's that have different origins because they
do not share a common history.
-}
| DiffTooNew (EventFold o p e) (Diff o p e)
{- ^
The `Diff`'s infimum is greater than any event known to 'EventFold'
into which it is being merged. This should be impossible and
indicates that either the local 'EventFold' has rolled back an
event that it had previously acknowledged, or else the source of
the 'Diff' moved the infimum forward without a full acknowledgement
from all participants. Both of these conditions should be regarded
as serious bugs.
-}
| DiffTooSparse (EventFold o p e) (Diff o p e)
{- ^
The 'Diff' assumes we know about events that we do not in fact know
about. This is only possible if we rolled back our copy of the state
somehow and "forgot" about state that we had previous acknowledged,
or else some other participant erroneously acknowledged some events
on our behalf.
-}
deriving stock instance
( Show (Output e)
, Show o
, Show p
, Show e
, Show (State e)
)
=>
Show (MergeError o p e)
{- | `Delta` is how we represent mutations to the event fold state. -}
data Delta p e
= Join p
| UnJoin p
| Event e
| Error (Output e) (Set p)
deriving stock (Generic)
deriving stock instance (Eq p, Eq e, Eq (Output e)) => Eq (Delta p e)
deriving stock instance (Show p, Show e, Show (Output e)) => Show (Delta p e)
instance (Binary p, Binary e, Binary (Output e)) => Binary (Delta p e)
{- |
Instances of this class define the particular "events" being "folded"
over in a distributed fashion. In addition to the event type itself,
there are a couple of type families which define the 'State' into which
folded events are accumulated, and the 'Output' which application of
a particular event can generate.
TL;DR: This is how users define their own custom operations.
-}
class Event e where
type Output e
type State e
{- | Apply an event to a state value. **This function MUST be total!!!** -}
apply :: e -> State e -> EventResult e
{- | The most trivial event type. -}
instance Event () where
type Output () = ()
type State () = ()
apply () () = Pure () ()
{- | The union of two event types. -}
instance (Event a, Event b) => Event (Either a b) where
type Output (Either a b) = Either (Output a) (Output b)
type State (Either a b) = (State a, State b)
apply (Left e) (a, b) =
case apply e a of
SystemError o -> SystemError (Left o)
Pure o s -> Pure (Left o) (s, b)
apply (Right e) (a, b) =
case apply e b of
SystemError o -> SystemError (Right o)
Pure o s -> Pure (Right o) (a, s)
{- |
The result of applying an event.
Morally speaking, events are always pure functions. However, mundane
issues like finite memory constraints and finite execution time can
cause referentially opaque behavior. In a normal Haskell program, this
usually leads to a crash or an exception, and the crash or exception
can itself, in a way, be thought of as being referentially transparent,
because there is no way for it to both happen and, simultaneously,
not happen.
However, in our case we are replicating computations across many
different pieces of hardware, so there most definitely is a way
for these aberrant system failures to both happen and not happen
simultaneously. What happens if the computation of the event runs out
of memory on one machine, but not on another?
There exists a strategy for dealing with these problems: if the
computation of an event experiences a failure on every participant, then
the event is pushed into the infimum as a failure (i.e. a no-op), but if
any single participant successfully computes the event then all other
participants can (somehow) request a "Full Merge" from the successful
participant. The Full Merge will include the infimum __value__ computed
by the successful participant, which will include the successful
application of the problematic event. The error participants can thus
bypass computation of the problem event altogether, and can simply
overwrite their infimum with the infimum provided by the Full Merge.
Doing a full merge can be much more expensive than doing a simple
'Diff' merge, because it requires transmitting the full value of the
'EventFold' instead of just the outstanding operations.
This type represents how computation of the event finished; with either a
pure result, or some kind of system error.
In general 'SystemError' is probably only ever useful for when your
event type somehow executes untrusted code (for instance when your event
type is a Turing-complete DSL that allows users to submit their own
custom-programmed "events") and you want to limit the resources that can
be consumed by such user-generated code. It is much less useful when
you are encoding some well defined business logic directly in Haskell.
-}
data EventResult e
= SystemError (Output e)
| Pure (Output e) (State e)
{- |
Construct a new 'EventFold' with the given origin and initial
participant.
-}
new
:: (Default (State e), Ord p)
=> o {- ^ The "origin", identifying the historical lineage of this CRDT. -}
-> p {- ^ The initial participant. -}
-> EventFold o p e
new o participant =
EventFold
EventFoldF {
psOrigin = o,
psInfimum = Infimum {
eventId = def,
participants = Set.singleton participant,
stateValue = def
},
psEvents = mempty
}
{- |
Get the outstanding events that need to be propagated to a particular
participant.
-}
events :: (Ord p) => p -> EventFold o p e -> Diff o p e
events peer (EventFold ef) =
Diff {
diffEvents = omitAcknowledged <$> psEvents ef,
diffOrigin = psOrigin ef,
diffInfimum = eventId (psInfimum ef)
}
where
{- |
Don't send the event data to participants which have already
acknowledged it, saving network and cpu resources.
-}
omitAcknowledged (d, acks) =
(
case (d, peer `member` acks) of
(Identity Error {}, _) -> Just (runIdentity d)
(_, False) -> Just (runIdentity d)
_ -> Nothing,
acks
)
{- | A package containing events that can be merged into an event fold. -}
data Diff o p e = Diff {
diffEvents :: Map (EventId p) (Maybe (Delta p e), Set p),
diffOrigin :: o,
diffInfimum :: EventId p
}
deriving stock (Generic)
deriving stock instance (
Show o, Show p, Show e, Show (Output e)
) =>
Show (Diff o p e)
instance (
Binary o, Binary p, Binary e, Binary (Output e)
) =>
Binary (Diff o p e)
{- |
Like `fullMerge`, but merge a remote 'Diff' instead of a full remote
'EventFold'.
-}
diffMerge
:: ( Eq (Output e)
, Eq e
, Eq o
, Event e
, Ord p
)
=> p
-> EventFold o p e
-> Diff o p e
-> Either
(MergeError o p e)
(UpdateResult o p e)
diffMerge
_
(EventFold EventFoldF {psOrigin = o1})
Diff {diffOrigin = o2}
| o1 /= o2 =
Left (DifferentOrigins o1 o2)
diffMerge _ ef pak | tooNew =
Left (DiffTooNew ef pak)
where
maxState =
maximum
. Set.insert (eventId . psInfimum . unEventFold $ ef)
. Map.keysSet
. psEvents
. unEventFold
$ ef
tooNew :: Bool
tooNew = maxState < diffInfimum pak
diffMerge
participant
orig@(EventFold (EventFoldF o infimum d1))
ep@(Diff d2 _ i2)
=
case
reduce
i2
EventFoldF {
psOrigin = o,
psInfimum = infimum,
psEvents =
Map.Merge.merge
(Map.Merge.mapMissing (const (first Just)))
Map.Merge.preserveMissing
(Map.Merge.zipWithMatched (const mergeAcks))
(first runIdentity <$> d1)
d2
}
of
Nothing -> Left (DiffTooSparse orig ep)
Just (ef1, outputs1) ->
let (ef2, outputs2) = acknowledge participant ef1
in
Right (
UpdateResult
(EventFold ef2)
(Map.union outputs1 outputs2)
(
i2 /= eventId infimum
|| not (Map.null d2)
|| ef2 /= unEventFold orig
)
)
where
mergeAcks :: (Ord p)
=> (Delta p e, Set p)
-> (Maybe (Delta p e), Set p)
-> (Maybe (Delta p e), Set p)
mergeAcks
(Error output eacks1, acks1)
(Just (Error _ eacks2), acks2)
=
(Just (Error output (eacks1 `union` eacks2)), acks1 `union` acks2)
mergeAcks
(Error {}, acks1)
(d, acks2)
=
(d, acks1 `union` acks2)
mergeAcks
(d, acks1)
(Just _, acks2)
=
(Just d, acks1 `union` acks2)
mergeAcks
(d, acks1)
(Nothing, acks2)
=
(Just d, acks1 `union` acks2)
{- |
Monotonically merge the information in two 'EventFold's. The resulting
'EventFold' may have a higher infimum value, but it will never have
a lower one. Only 'EventFold's that originated from the same 'new'
call can be merged. If the origins are mismatched, or if there is some
other programming error detected, then an error will be returned.
Returns the new 'EventFold' value, along with the output for all of
the events that can now be considered "fully consistent".
-}
fullMerge
:: ( Eq (Output e)
, Eq e
, Eq o
, Event e
, Ord p
)
=> p {- ^ The participant doing the merge. -}
-> EventFold o p e
-> EventFold o p e
-> Either (MergeError o p e) (UpdateResult o p e)
fullMerge participant (EventFold left) (EventFold right@(EventFoldF o2 i2 d2)) =
case
diffMerge
participant
(
EventFold
left {
psInfimum = max (psInfimum left) i2
}
)
Diff {
diffOrigin = o2,
diffEvents = first (Just . runIdentity) <$> d2,
diffInfimum = eventId i2
}
of
Left err -> Left err
Right (UpdateResult ef outputs _prop) ->
let (ef2, outputs2) = acknowledge participant (unEventFold ef)
in
Right (
UpdateResult
(EventFold ef2)
(Map.union outputs outputs2)
(ef2 /= left || ef2 /= right)
)
{- |
The result updating the 'EventFold', which is some information
containing the new 'EventFold' value, the outputs of events that have
reached the infimum as a result of update (i.e. "totally consistent
outputs"), and a flag indicating whether the other participants need
to hear about the changes.
-}
data UpdateResult o p e =
UpdateResult {
urEventFold :: EventFold o p e,
{- ^ The new 'EventFold' value -}
urOutputs :: Map (EventId p) (Output e),
{- ^
Any consistent outputs resulting from
the merge.
-}
urNeedsPropagation :: Bool
{- ^
'True' if any new information was added to
the 'EventFold' which might need propagating
to other participants.
-}
}
{- |
Record the fact that the participant acknowledges the information
contained in the 'EventFold'. The implication is that the participant
__must__ base all future operations on the result of this function.
Returns the new 'EventFold' value, along with the output for all of
the events that can now be considered "fully consistent".
-}
acknowledge :: (Event e, Ord p)
=> p
-> EventFoldF o p e Identity
-> (EventFoldF o p e Identity, Map (EventId p) (Output e))
acknowledge p ef =
{-
First do a normal reduction, then do a special acknowledgement of the
reduction error, if any.
-}
let
(ps2, outputs) =
runIdentity $
reduce
(eventId (psInfimum ef))
ef {psEvents = fmap ackOne (psEvents ef)}
(ps3, outputs2) = ackErr p ps2
in
(ps3, outputs <> outputs2)
where
ackOne (e, acks) = (e, Set.insert p acks)
{- | Acknowledge the reduction error, if one exists. -}
ackErr :: (Event e, Ord p)
=> p
-> EventFoldF o p e Identity
-> (EventFoldF o p e Identity, Map (EventId p) (Output e))
ackErr p ef =
runIdentity $
reduce
(eventId (psInfimum ef))
ef {
psEvents =
case Map.minViewWithKey (psEvents ef) of
Just ((eid, (Identity (Error o eacks), acks)), deltas) ->
Map.insert
eid
(Identity (Error o (Set.insert p eacks)), acks)
deltas
_ -> psEvents ef
}
{- |
Allow a participant to join in the distributed nature of the
'EventFold'. Return the 'EventId' at which the participation is
recorded, and the resulting 'EventFold'. The purpose of returning the
'EventId' is so that you can use it to tell when the participation
event has reached the infimum. See also: 'infimumId'
-}
participate :: forall o p e. (Ord p, Event e)
=> p {- ^ The local participant. -}
-> p {- ^ The participant being added. -}
-> EventFold o p e
-> (EventId p, UpdateResult o p e)
participate self peer (EventFold ef) =
(
eid,
let
(ef2, outputs1) =
acknowledge
self
ef {
psEvents =
Map.insert
eid
(Identity (Join peer), mempty)
(psEvents ef)
}
(ef3, outputs2) = acknowledge peer ef2
in
UpdateResult {
urEventFold = EventFold ef3,
urOutputs = Map.union outputs1 outputs2,
{-
By definition, we have added some new information that
needs propagating.
-}
urNeedsPropagation = True
}
)
where
eid :: EventId p
eid = nextId self ef
{- |
Indicate that a participant is removing itself from participating in
the distributed 'EventFold'.
-}
disassociate :: forall o p e. (Event e, Ord p)
=> p {- ^ The peer removing itself from participation. -}
-> EventFold o p e
-> (EventId p, UpdateResult o p e)
disassociate peer (EventFold ef) =
let
(ef2, outputs) =
acknowledge
peer
ef {
psEvents =
Map.insert
eid
(Identity (UnJoin peer), mempty)
(psEvents ef)
}
in
(
eid,
UpdateResult {
urEventFold = EventFold ef2,
urOutputs = outputs,
{-
By definition, we have added some new information that
needs propagating.
-}
urNeedsPropagation = True
}
)
where
eid :: EventId p
eid = nextId peer ef
{- |
Introduce a change to the EventFold on behalf of the participant.
Return the new 'EventFold', along with the projected output of the
event, along with an 'EventId' which can be used to get the fully
consistent event output at a later time.
-}
event :: (Ord p, Event e)
=> p
-> e
-> EventFold o p e
-> (Output e, EventId p, UpdateResult o p e)
event p e ef =
let
eid = nextId p (unEventFold ef)
in
(
case apply e (projectedValue ef) of
Pure output _ -> output
SystemError output -> output,
eid,
let
(ef2, outputs) =
acknowledge
p
(
(unEventFold ef) {
psEvents =
Map.insert
eid
(Identity (Event e), mempty)
(psEvents (unEventFold ef))
}
)
in
UpdateResult {
urEventFold = EventFold ef2,
urOutputs = outputs,
urNeedsPropagation =
{-
An event is, by definition, adding information to the 'EventFold',
and the only time we might not need to propagate this this
information is if the local participant is the only participant.
-}
allParticipants (EventFold ef2) /= Set.singleton p
}
)
{- | Return the current projected value of the 'EventFold'. -}
projectedValue :: (Event e) => EventFold o p e -> State e
projectedValue
(
EventFold
EventFoldF {
psInfimum = Infimum {stateValue},
psEvents
}
)
=
foldr
(\ e s ->
case apply e s of
Pure _ newState -> newState
SystemError _ -> s
)
stateValue
changes
where
changes = foldMap getDelta (toDescList psEvents)
getDelta :: (EventId p, (Identity (Delta p e), Set p)) -> [e]
getDelta (_, (Identity (Event e), _)) = [e]
getDelta _ = mempty
{- | Return the current infimum value of the 'EventFold'. -}
infimumValue :: EventFold o p e -> State e
infimumValue (EventFold EventFoldF {psInfimum = Infimum {stateValue}}) =
stateValue
{- | Return the 'EventId' of the infimum value. -}
infimumId :: EventFold o p e -> EventId p
infimumId = eventId . psInfimum . unEventFold
{- |
Gets the known participants at the infimum.
-}
infimumParticipants :: EventFold o p e -> Set p
infimumParticipants
(
EventFold
EventFoldF {
psInfimum = Infimum {participants}
}
)
=
participants
{- |
Get all known participants. This includes participants that are
projected for removal.
-}
allParticipants :: (Ord p) => EventFold o p e -> Set p
allParticipants
(
EventFold
EventFoldF {
psInfimum = Infimum {participants},
psEvents
}
)
=
foldr updateParticipants participants (toDescList psEvents)
where
updateParticipants :: (Ord p)
=> (EventId p, (Identity (Delta p e), Set p))
-> Set p
-> Set p
updateParticipants (_, (Identity (Join p), _)) = Set.insert p
updateParticipants _ = id
{- |
Get all the projected participants. This does not include participants that
are projected for removal.
-}
projParticipants :: (Ord p) => EventFold o p e -> Set p
projParticipants
(
EventFold
EventFoldF {
psInfimum = Infimum {participants},
psEvents
}
)
=
foldr updateParticipants participants (toDescList psEvents)
where
updateParticipants :: (Ord p)
=> (EventId p, (Identity (Delta p e), Set p))
-> Set p
-> Set p
updateParticipants (_, (Identity (Join p), _)) = Set.insert p
updateParticipants (_, (Identity (UnJoin p), _)) = Set.delete p
updateParticipants _ = id
{- |
Returns the participants that we think might be diverging. In
this context, a participant is "diverging" if there is an event
that the participant has not acknowledged but we are expecting it
to acknowledge. Along with the participant, return the last known
`EventId` which that participant has acknowledged.
-}
divergent :: forall o p e. (Ord p) => EventFold o p e -> Map p (EventId p)
divergent
(
EventFold
EventFoldF {
psInfimum = Infimum {participants, eventId},
psEvents
}
)
=
let (byParticipant, maxEid) = eidByParticipant
in Map.filter (< maxEid) byParticipant
where
eidByParticipant :: (Map p (EventId p), EventId p)
eidByParticipant =
foldr
accum
(Map.fromList [(p, eventId) | p <- Set.toList participants], eventId)
(
let flatten (a, (Identity b, c)) = (a, b, c)
in (flatten <$> toAscList psEvents)
)
accum
:: (EventId p, Delta p e, Set p)
-> (Map p (EventId p), EventId p)
-> (Map p (EventId p), EventId p)
accum (eid, Join p, acks) (acc, maxEid) =
(
unionWith
max
(Map.insert p eid acc)
(Map.fromList [(a, eid) | a <- Set.toList acks]),
max maxEid eid
)
accum (eid, _, acks) (acc, maxEid) =
(
unionWith
max
acc
(Map.fromList [(a, eid) | a <- Set.toList acks]),
max maxEid eid
)
{- | Return the origin value of the 'EventFold'. -}
origin :: EventFold o p e -> o
origin = psOrigin . unEventFold
{- |
This helper function is responsible for figuring out if the 'EventFold'
has enough information to derive a new infimum value. In other words,
this is where garbage collection happens.
-}
reduce
:: forall o p e f.
( Event e
, Monad f
, Ord p
)
=> EventId p
{- ^
The infimum 'EventId' as known by some node in the cluster. "Some
node" can be different than "this node" in the case where another
node advanced the infimum before we did (because it knew about our
acknowledgement, but we didn't know about its acknowledgement)
and sent us an 'Diff' with this value of the infimum. In this
case, this infimum value acts as a universal acknowledgement of
all events coming before it.
-}
-> EventFoldF o p e f
-> f (EventFoldF o p e Identity, Map (EventId p) (Output e))
reduce
infState
ef@EventFoldF {
psInfimum = infimum@Infimum {participants, stateValue},
psEvents
}
=
case Map.minViewWithKey psEvents of
Nothing ->
pure
(
EventFoldF {
psOrigin = psOrigin ef,
psInfimum = psInfimum ef,
psEvents = mempty
},
mempty
)
Just ((eid, (getUpdate, acks)), newDeltas)
| eid <= eventId infimum -> {- The event is obsolete. Ignore it. -}
reduce infState ef {
psEvents = newDeltas
}
| isRenegade eid -> {- This is a renegade event. Ignore it. -}
reduce infState ef {
psEvents = newDeltas
}
| otherwise -> do
implicitAcks <- unjoins eid
update <- getUpdate
let
{- |
Join events must be acknowledged by the joining
participant before moving into the infimum.
-}
joining =
case update of
Join p -> Set.singleton p
_ -> mempty
if
Set.null (((participants `union` joining) \\ acks) \\ implicitAcks)
|| eid <= infState
then
case update of
Join p ->
reduce infState ef {
psInfimum = infimum {
eventId = eid,
participants = Set.insert p participants
},
psEvents = newDeltas
}
UnJoin p ->
reduce infState ef {
psInfimum = infimum {
eventId = eid,
participants = Set.delete p participants
},
psEvents = newDeltas
}
Error output eacks
| Set.null (participants \\ eacks) -> do
(ps2, outputs) <-
reduce infState ef {
psInfimum = infimum {
eventId = eid
}
}
pure (ps2, Map.insert eid output outputs)
| otherwise -> do
events_ <- runEvents psEvents
pure
(
EventFoldF {
psOrigin = psOrigin ef,
psInfimum = psInfimum ef,
psEvents = events_
},
mempty
)
Event e ->
case apply e stateValue of
SystemError output -> do
events_ <- runEvents newDeltas
pure
(
EventFoldF {
psOrigin = psOrigin ef,
psInfimum = infimum,
psEvents =
Map.insert
eid
(Identity (Error output mempty), acks)
events_
},
mempty
)
Pure output newState -> do
(ps2, outputs) <-
reduce infState ef {
psInfimum = infimum {
eventId = eid,
stateValue = newState
},
psEvents = newDeltas
}
pure (ps2, Map.insert eid output outputs)
else do
events_ <- runEvents psEvents
pure
(
EventFoldF {
psOrigin = psOrigin ef,
psInfimum = psInfimum ef,
psEvents = events_
},
mempty
)
where
{- | Unwrap the events from their monad. -}
runEvents
:: Map (EventId p) (f (Delta p e), Set p)
-> f (Map (EventId p) (Identity (Delta p e), Set p))
runEvents events_ =
Map.fromList <$> sequence [
do
d <- fd
pure (eid, (Identity d, acks))
| (eid, (fd, acks)) <- Map.toList events_
]
{- | Figure out which nodes have upcoming unjoins. -}
unjoins
:: EventId p
{- ^
The even under consideration, unjoins only after which we
are interested.
-}
-> f (Set p)
unjoins eid =
Set.fromList
. Map.elems
. Map.filterWithKey (\k _ -> eid <= k)
<$> unjoinMap
{- | The static map of unjoins. -}
unjoinMap :: f (Map (EventId p) p)
unjoinMap =
Map.fromList . catMaybes <$> sequence [
update >>= \case
UnJoin p -> pure (Just (eid, p))
_ -> pure Nothing
| (eid, (update, _acks)) <- Map.toList psEvents
]
{- |
Renegade events are events that originate from a non-participating
peer. This might happen in a network partition situation, where
the cluster ejected a peer that later reappears on the network,
broadcasting updates.
-}
isRenegade BottomEid = False
isRenegade (Eid _ p) = not (p `member` participants)
{- |
A utility function that constructs the next `EventId` on behalf of
a participant.
-}
nextId :: (Ord p) => p -> EventFoldF o p e f -> EventId p
nextId p EventFoldF {psInfimum = Infimum {eventId}, psEvents} =
case maximum (eventId:keys psEvents) of
BottomEid -> Eid 0 p
Eid ord _ -> Eid (succ ord) p
{- |
Return 'True' if progress on the 'EventFold' is blocked on a
'SystemError'.
The implication here is that if the local copy is blocked on a
'SystemError', it needs to somehow arrange for remote copies to send
full 'EventFold's, not just 'Diff's. A 'diffMerge' is not sufficient
to get past the block. Only a 'fullMerge' will suffice.
If your system is not using 'SystemError' or else not using 'Diff's,
then you don't ever need to worry about this function.
-}
isBlockedOnError :: EventFold o p e -> Bool
isBlockedOnError (EventFold ef) =
case Map.minView (psEvents ef) of
Just ((Identity (Error _ _), _), _) -> True
_ -> False