improve comment description

Author: rakeshkky

server/src-lib/Hasura/Eventing/ScheduledTrigger.hs

@@ -32,6 +32,10 @@ other. They can be split into different threads for a better performance.
 
 == Implementation
 
+The scheduled triggers eventing is being implemented in the metadata storage.
+All functions that make interaction to storage system are abstracted in
+the @'MonadMetadataStorage' class.
+
 During the startup, two threads are started:
 
 1. Generator: Fetches the list of scheduled triggers from cache and generates

server/src-lib/Hasura/Metadata/Class.hs

@@ -16,29 +16,13 @@ import           Hasura.RQL.Types
 
 import qualified Hasura.Tracing                         as Tracing
 
--- | Metadata storage transformer that enables raising @'QErr' exceptions.
-newtype MetadataStorageT m a
-  = MetadataStorageT {unMetadataStorageT :: ExceptT QErr m a}
-  deriving ( Functor, Applicative, Monad
-           , MonadError QErr
-           , MonadTrans
-           , MonadIO
-           , MFunctor
-           , Tracing.HasReporter
-           )
-
-runMetadataStorageT
-  :: MetadataStorageT m a -> m (Either QErr a)
-runMetadataStorageT =
-  runExceptT . unMetadataStorageT
-
 {- Note [Todo: Common interface for eventing sub-system]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Postgres tables' event triggers and scheduled event triggers are similar in the
-core logic. But currently, their implementation is completely isolated and do not share
-a common schema in Postgres. We're having a plan to simplify them via a common interface
-(maybe via a Postgres extension). This will potentially reduce number of interactions made
-to database and schema foot print.
+core logic. But currently, their implementation is completely isolated and do not
+share a common schema in Postgres. We're having a plan to simplify them via a
+common 'event storage and retrieval' interface (maybe via a Postgres extension?).
+This will potentially reduce number of interactions made to database and schema foot print.
 
 TODO: Reference to open issue or rfc?
 -}
@@ -52,24 +36,23 @@ TODO: Reference to open issue or rfc?
 --
 -- This class has functions broadly related to:
 --
--- 1. Metadata Management
+-- 1. Metadata Management (TODO: Need to be added to the type class)
 -- ----------------------
--- TODO
 -- Basic metadata management functions such as retrieving metadata from storage
 -- database and replacing the given metadata.
 --
 -- 2. Scheduled Triggers
 -- ---------------------
 -- Eventing sub-system for scheduled triggers is implemented via metadata storage.
--- All necessary functions are included in the type class.
--- TODO
--- This also includes functions to fetch events and their invocations so that the
--- console can show them in the UI.
 -- For more details, refer description in 'Hasura.Eventing.ScheduledTrigger' module.
 --
--- 3. Async Actions
+-- TODO: Functions need to be added to the type class
+-- - Retrieving invocation logs from storage (console requirement)
+-- - Deleting an scheduled event
+-- - Creating an one-off scheduled event
+--
+-- 3. Async Actions (TODO: Need to be added to the type class)
 -- ----------------
--- TODO
 -- Operations to implement async actions sub-system. This includes recording an
 -- async action event and retreiving the details of action delivery to the webhook.
 -- For more details see Note [Async action architecture] in 'Hasura.GraphQL.Execute.Action' module.
@@ -81,9 +64,12 @@ TODO: Reference to open issue or rfc?
 class (MonadError QErr m) => MonadMetadataStorage m where
 
   -- Scheduled triggers
-  -- By design, scheduled trigger eventing has many database interactions.
-  -- Hence we have many functions. We can reduce the functions by having
-  -- a common interface for eventing. See Note [Todo: Common interface for eventing sub-system]
+  -- TODO:-
+  -- Ideally we would've liked to avoid having functions that are specific to
+  -- scheduled/cron triggers and instead have functions that provide a generic
+  -- 'event storage and retrieval' interface but we'll have to change a lot of
+  -- existing code for scheduled and cron triggers. We can get to this after the
+  -- multi-source work is done. See Note [Todo: Common interface for eventing sub-system]
   getDeprivedCronTriggerStats :: m [CronTriggerStats]
   getScheduledEventsForDelivery :: m ([CronEvent], [OneOffScheduledEvent])
   insertScheduledEvent :: ScheduledEventSeed -> m ()
@@ -109,3 +95,20 @@ instance (MonadMetadataStorage m) => MonadMetadataStorage (Tracing.TraceT m) whe
   setScheduledEventOp a b c          = lift $ setScheduledEventOp a b c
   unlockScheduledEvents a b          = lift $ unlockScheduledEvents a b
   unlockAllLockedScheduledEvents     = lift unlockAllLockedScheduledEvents
+
+-- | The 'MetadataStorageT' transformer adds ability to throw exceptions
+-- for monads deriving @'MonadMetadataStorage' instance.
+newtype MetadataStorageT m a
+  = MetadataStorageT {unMetadataStorageT :: ExceptT QErr m a}
+  deriving ( Functor, Applicative, Monad
+           , MonadError QErr
+           , MonadTrans
+           , MonadIO
+           , MFunctor
+           , Tracing.HasReporter
+           )
+
+runMetadataStorageT
+  :: MetadataStorageT m a -> m (Either QErr a)
+runMetadataStorageT =
+  runExceptT . unMetadataStorageT