fix: Add random delay in Listener.cacheReloader

postgrest.cabal

@@ -158,6 +158,7 @@ library
                     , stm-hamt                  >= 1.2 && < 2
                     , focus                     >= 1.0 && < 2
                     , some                      >= 1.0.4.1 && < 2
+                    , MonadRandom               >= 0.6.2 && < 0.7
                       -- -fno-spec-constr may help keep compile time memory use in check,
                       --   see https://gitlab.haskell.org/ghc/ghc/issues/16017#note_219304
                       -- -optP-Wno-nonportable-include-path

src/PostgREST/Listener.hs

@@ -18,6 +18,7 @@ import qualified PostgREST.AppState as AppState
 import qualified PostgREST.Config   as Config
 
 import           Control.Arrow              ((&&&))
+import           Control.Monad.Random
 import           Data.Bitraversable         (bisequence)
 import           Data.Either.Combinators    (whenRight)
 import qualified Database.PostgreSQL.LibPQ  as LibPQ
@@ -102,7 +103,13 @@ retryingListen appState = do
          | msg == "reload config" -> observer (DBListenerGotConfigMsg channel) >> AppState.readInDbConfig False appState
          | otherwise              -> pure () -- Do nothing if anything else than an empty message is sent
 
-    cacheReloader =
+      -- add a random delay between 0 and 1 second to avoid thundering herd problem
+      -- if multiple listeners receive the same notification at the same time
+      -- the cacheReloader is launched in a separate thread to avoid blocking the listener
+      -- during the random delay
+    cacheReloader = void $ forkIO $ do
+      delay <- getRandomR (0, 1000000)
+      threadDelay delay
       AppState.schemaCacheLoader appState
 
     releaseConnection = void . forkIO . handle (observer . DBListenerConnectionCleanupFail) . SQL.release

test/io/postgrest.py

@@ -18,7 +18,7 @@
 
 def sleep_until_postgrest_scache_reload():
     "Sleep until schema cache reload"
-    time.sleep(0.3)
+    time.sleep(1.3)
 
 
 def sleep_until_postgrest_config_reload():
@@ -28,7 +28,7 @@ def sleep_until_postgrest_config_reload():
 
 def sleep_until_postgrest_full_reload():
     "Sleep until schema cache plus config reload"
-    time.sleep(0.3)
+    time.sleep(1.3)
 
 
 class PostgrestTimedOut(Exception):
@@ -71,7 +71,7 @@ def read_stdout(self, nlines=1):
             time.sleep(0.1)
         return output
 
-    def wait_until_scache_starts_loading(self, max_seconds=1):
+    def wait_until_scache_starts_loading(self, max_seconds=2):
         "Wait for the admin /ready return a status of 503"
 
         wait_until_status_code(