Hi Thomas, Could you please explain how exactly Spring Session works ? This will help us both understand where the problem comes from. Also how exactly do you integrate it with Wicket ? Which extension points do you use ?
Since it is backed by Redis it means that it is distributed. But at the same time you try to use PerSessionPageStore which is in-memory store, i.e. it lives only in the memory of one of the web container nodes. Another node has its own instance of this class. Here is how it works in normal Servler environment: The HttpSession instances are managed by the web container, e.g. Tomcat. The container keeps them in memory! The container may persist them for failover/restart but usually the HttpSessions are kept in memory. 1) if there is just one web container node then Wicket asks the container for the javax.servler.HttpSession: httpServletRequest.getSession() Then Wicket extracts the org.apache.wicket.Session from the HttpSession attributes. The Wicket Session class has a member instance of PageAccessSynchronizer that is specific for the current instance of the Session. PageAccessSynchronizer has a Map<Integer, PageLock>, i.e. pageId -> PageLock. Whenever a request needs to use a specific Wicket Page then the current session's PageAccessSynchronizer is used to acquire a lock on it. This makes sure that a specific page instance is used by at most one request in the *current* server node. 2) if there is session replication in place, i.e. more than one server nodes, then: The web container fetches the HttpSessions for its backend. In case of Tomcat - it keeps the HttpSessions in memory but there is a ClusterManager that replicates the HttpSessions' which have been modified, i.e. their #setAttibute() has been called. In cluster mode there will be one HttpSession per server node, respectfully one Wicket Session instance per node, and one PageAccessSynchronizer per session per node. Using Wicket or not if you don't use sticky sessions you may face timing related issues in such environment. Every distributed software has this problem. On Wed, Feb 26, 2020 at 9:17 PM Thomas Heigl <tho...@umschalt.com> wrote: > 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 > This is called a distributed lock. It will reduce the problems you face but will make things slower too. > 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. > I don't see how application-scope would help at all. As I explained earlier PageAccessSynchronizer is per o.a.w.Session instance. Maing it application-scoped will only add more concurrency issues and you will have to add locks on session level. At the moment it is lock-free. > 3. If we want to use non-sticky session we could use Redis locks for > implementing a global PageAccessSynchronizer > Sticky sessions is usually good enough for most of the cases. Distributed locks will make your application much slower. If you use single instance of Redis you will have single point of failure. If you use Redis replication then the distributed locking will become even more complex and slow. > > 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? > Let us first hear back how this will solve the issue and then we will discuss such change. > > 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 > > >> > > >
