Hi Sven, Thanks for the link but I'm not using asynchronous serialization.
I thought some more about the issue and I think I figured it out. My setup looks like this: 1. Spring Session Redis 2. [Session Cache] (Not used because it is transient and stored with writeObject/readObject and does not get serialized into Redis as we do not use Java serialization) 3. PerSessionPageStore (Application-level cache held in memory) 4. RedisDataStore (Synchronous) Observations: 1. If i disable second-level cache or use the serializing second-level cache provided by the DefaultPageManager, there are no issues 2. As soon as I enable the PerSessionPageStore we run into concurrency issues Analysis: I first thought that there were some thread-safety issues with PerSessionPageStore but that is not the case because even a fully synchronized version shows these problems. The reason why disabling the 2nd-level cache or using a serializing variant works, is because they do not operate on the same *instance* of the page. Each thread gets their own instance because the page is deserialized before being accessed. PerSessionPageStore stores the page in memory without serializing it, thus all threads share the same instance. This is also the case when using the session cache or the session-based stores, but the PageAccessSynchronizer bound to the session takes care of ensuring that only single request can manipulate the page at any given time. So how does the synchronizer work? It keeps a Map<Integer, PageLock> in the session and checks whether the page is locked on every request. In a non-replicated environment this works as expected as the session object lives in the servlet container and is the same for each concurrent request. In my case, the session is not provided by the servlet container, but fetched from Redis by Spring Session on every request. So each concurrent thread has *their own version* of the session and the locks are *not shared between threads* because the session is only saved back to Redis after the request has finished. So the problematic flow looks like this 1. A request comes in, we fetch the session from Redis, the request acquires the session-scoped lock and starts processing 2. Before the request is finished, another request comes in, fetches the session from Redis, the lock map is empty because the state of request #1 has not been persisted to Redis 3. Now both requests can modify the page and we run into concurrency issues Summary: PageAccessSynchronizer does not work with Spring Session or other solutions that replace the servlet container session. Possible solutions: 1. We could ensure that session locks are updated in Redis immediately but that still leaves a couple of milliseconds for race conditions and adds a lot of overhead 2. We could use an application-scoped PageAccessSynchronizer. This solves the problem as long as sessions are sticky and all concurrent requests are sent to the same server. 3. If we want to use non-sticky session we could use Redis locks for implementing a global PageAccessSynchronizer I would like to go with solution #2 for now. The problem is that PageAccessSynchronizer is not an interface. Would it be possible to extract an interface so I can easily implement access synchronizers with different scopes? Best regards, Thomas On Wed, Feb 26, 2020 at 12:24 PM Sven Meier <s...@meiers.net> wrote: > Hi Thomas, > > Im wondering whether you're running into > https://issues.apache.org/jira/browse/WICKET-6702 > > I've been working on a solution to that problem, which is caused by pages > being asynchronously serialized while another request is already coming in. > > Or maybe it is something different. > Could you create a quickstart? > > Sven > > Am 25. Februar 2020 22:12:46 MEZ schrieb Thomas Heigl <tho...@umschalt.com > >: > >Hi again, > > > >I investigated a bit and it does not seem to have anything to do with > >the > >PerSessionPageStore. I implemented a completely synchronized version of > >it > >and the problems still exist. > > > >If I switch to the default second-level cache that stores serialized > >pages > >in application scope, everything works as expected. Only the > >non-serialized > >pages in PerSessionPageStore seem to be affected by concurrent ajax > >modifications. > > > >What is the difference between keeping pages in the session and keeping > >the > >same pages in the PerSessionPageStore? Is there some additional locking > >done for pages in the session? > > > >Best, > > > >Thomas > > > >On Tue, Feb 25, 2020 at 8:25 PM Thomas Heigl <tho...@umschalt.com> > >wrote: > > > >> Hi all, > >> > >> I'm currently experimenting with PerSessionPageStore as a > >second-level > >> cache. We are moving our page store from memory (i.e. session) to > >Redis and > >> keeping 1-2 pages per session in memory speeds up ajax requests quite > >a bit > >> because network roundtrips and (de)serialization can be skipped for > >cached > >> pages. > >> > >> Our application is very ajax heavy (it is basically a single page > >> application with lots of lazy-loading). While rapidly clicking around > >and > >> firing as many parallel ajax requests as possible, I noticed that it > >is > >> quite easy to trigger exceptions that I have never seen before. > >> ConcurrentModificationExceptions during serialization, > >> MarkupNotFoundExceptions, exceptions about components already > >dequeuing etc. > >> > >> So I had a look at the implementation of PerSessionPageStore and > >noticed > >> that is does not do any kind of synchronization and does not use > >atomic > >> operations when updating the cache. It seems to me that the > >second-level > >> cache is not really usable in a concurrent ajax environment. > >> > >> I think that writing pages to the second level cache store should > >either > >> synchronize on sessionId+pageId or attempt to use atomic operations > >> provided by ConcurrentHashMap. > >> > >> Did anyone else ever run into these issues? The code > >> of PerSessionPageStore is quite complex because of soft references, > >> skip-list maps etc. so I'm not sure what the right approach to > >address > >> these problems would be. > >> > >> Best regards, > >> > >> Thomas > >> >
