flow-development-collection: neos/cache 5.3.25...5.3.26 Cannot add or modify cache entry because the affected keys are being modified by another process ("Flow_Session_Storage")

Description

Since neos/cache upgrade from 5.3.25 to 5.3.26 we (randomly?) got many of the following exception:

Neos\Cache\Exception: Cannot add or modify cache entry because the affected keys are being modified by another process ("Flow_Session_Storage") (1594725688)

#0 Packages/Libraries/neos/cache/Classes/Frontend/VariableFrontend.php(68): Neos\Cache\Backend\RedisBackend->set('...', '...', Array, 0)
#1 Data/Temporary/Production/Cache/Code/Flow_Object_Classes/Neos_Flow_Session_Session.php(479): Neos\Cache\Frontend\VariableFrontend->set('...', Array, Array, 0)
#2 Data/Temporary/Production/Cache/Code/Flow_Object_Classes/Neos_Flow_Session_Session.php(751): Neos\Flow\Session\Session_Original->putData('Neos_Flow_Secur...', Array)
#3 Data/Temporary/Production/Cache/Code/Flow_Object_Classes/Neos_Flow_Session_Session.php(689): Neos\Flow\Session\Session_Original->storeAuthenticatedAccountsInfo(Array)
#4 Packages/Framework/Neos.Flow/Classes/ObjectManagement/ObjectManager.php(561): Neos\Flow\Session\Session_Original->shutdownObject()
#5 Packages/Framework/Neos.Flow/Classes/ObjectManagement/ObjectManager.php(464): Neos\Flow\ObjectManagement\ObjectManager->callShutdownMethods(Object(SplObjectStorage))
#6 Data/Temporary/Production/Cache/Code/Flow_Object_Classes/Neos_Flow_Security_Context.php(219): Neos\Flow\ObjectManagement\ObjectManager->Neos\Flow\ObjectManagement\{closure}()
#7 Data/Temporary/Production/Cache/Code/Flow_Object_Classes/Neos_Flow_Security_Context.php(219): Closure->__invoke()
#8 Packages/Framework/Neos.Flow/Classes/ObjectManagement/ObjectManager.php(465): Neos\Flow\Security\Context_Original->withoutAuthorizationChecks(Object(Closure))
#9 [internal function]: Neos\Flow\ObjectManagement\ObjectManager->shutdown('Runtime', 'Neos\\Flow\\Core\\...')
#10 Packages/Framework/Neos.Flow/Classes/SignalSlot/Dispatcher.php(140): call_user_func_array(Array, Array)
#11 Packages/Framework/Neos.Flow/Classes/Core/Bootstrap.php(467): Neos\Flow\SignalSlot\Dispatcher->dispatch('Neos\\Flow\\Core\\...', 'bootstrapShutti...', Array)
#12 Packages/Framework/Neos.Flow/Classes/Core/Bootstrap.php(137): Neos\Flow\Core\Bootstrap->emitBootstrapShuttingDown('Runtime')
#13 Packages/Framework/Neos.Flow/Classes/Http/RequestHandler.php(114): Neos\Flow\Core\Bootstrap->shutdown('Runtime')
#14 Packages/Framework/Neos.Flow/Classes/Core/Bootstrap.php(112): Neos\Flow\Http\RequestHandler->handleRequest()
#15 Web/index.php(27): Neos\Flow\Core\Bootstrap->run()
#16 {main}

We can’t relate the exception with a specific action, it seems to happen across multiple different controllers. Our Caches.yaml sets the flow session caches to redis as follows:

Flow_Session_MetaData:
  persistent: true
  backend: 'Neos\Cache\Backend\RedisBackend'
  backendOptions:
    defaultLifetime: 0
    hostname: '%env:REDIS_HOST%'
    password: '%env:REDIS_PASSWORD%'
    port: '%env:REDIS_PORT%'
    database: 0

Flow_Session_Storage:
  persistent: true
  backend: 'Neos\Cache\Backend\RedisBackend'
  backendOptions:
    defaultLifetime: 0
    hostname: '%env:REDIS_HOST%'
    password: '%env:REDIS_PASSWORD%'
    port: '%env:REDIS_PORT%'
    database: 0 

The upgrade from neos/cache 5.3.25 to 5.3.26 also seems to increase the number of requests to redis. I’ve attached a list of commands that redis sees on a single web request (opening the home page in our case) for the two versions:

5.3.25.txt 5.3.26.txt

5.3.25 causes 32 redis commands, with 5.3.26 it’s 56 (+3 WATCH, +10 TTL, +10 PERSIST). The number also seemed pretty high before, is this to be expected or are we doing something wrong?

Steps to Reproduce

We are not able to consistently reproduce the issue. What we can say is that we run multiple instances of the app using the same Redis backend. We’ve also seen the exception mostly on production systems with higher user traffic. Since we’ve downgraded neos/cache back to 5.3.25 we didn’t see the exception anymore.

Expected behavior

No exceptions

Actual behavior

Random exceptions

Affected Versions

neos/cache 5.3.26 (did not happen with 5.3.25) neos/flow 5.3.26