Wicket cluster session store, page store, data store

Question

I am dealing with a custom implementation for wicket session store, data store, page store. I have cu cluster wicket and make it work in the following situation:

There are 2 nodes in the cluster, node one fails and the user should be able to continue the flow without noticing, the pages a statefull, with a lot of ajax requests. For now I'm storing the wicket session in a custom storage over rmi, and I'm trying to extend the DiskPageStore. The new challenge is SessionEntry inner class, it is still hold by a ConcurrentMap.

My question is: Has anyone done this before? Do you have any suggestions on how to accomplish this?

Answer 1

My suggestion is forget about DiskPageStore and SessionEntry in your situation. The ConcurrentMap you mentioned is held in the heap locally. Once one of the nodes fails, there is no way to get access to its ConcurrentMap, and Wicket resources referred to from the ConcurrentMap will be impossible to be released.

Therefore, in a clustered environment, you need to cluster the Wicket page store. Page versions can be expired based on certain policy, or deliberately removed when their corresponding session expires.

I've enabled web session and data store clustering for Apache Wicket used in an enterprise web application in production, and it has been working very well. The software I use are:

• JDK 1.8.0_60
• Apache Tomcat 8.0.33 (Tomcat 7 works too)
• Wicket 6.16 (versions 6.22.0 and 7.2.0 should also work)
• Apache Ignite 1.7.0
• Load balancer: Crossroads
• Ubuntu 14.04.1

The idea is use Apache Ignite for web session clustering, and it is pretty straightforward following its instructions for Web Session Clustering.

Once I got the web session clustered, I then put the data store (which includes the page store already) into the Ignite distributed data grid, while at the same time I disabled the Wicket application scoped cache (so as to make sure all data is clustered). Take a look at the documentation on Wicket's page store to find out how to configure the data store.

Alternatively you should be able to use Wicket HttpSessionDataStore to put the data store into the session. As the session is clustered, the data store is clustered automatically. But this approach does not work compatibly with Apache Ignite for me. So I use my own implementation of the IDataStore interface, which puts the data store into the Ignite distributed data grid. See below the implementation.

import java.util.concurrent.TimeUnit;
import javax.cache.expiry.Duration;
import javax.cache.expiry.TouchedExpiryPolicy;
import org.apache.ignite.Ignite;
import org.apache.ignite.IgniteCache;
import org.apache.ignite.Ignition;
import org.apache.ignite.cache.CacheMemoryMode;
import org.apache.ignite.cache.CacheMode;
import org.apache.ignite.cache.eviction.lru.LruEvictionPolicy;
import org.apache.ignite.configuration.CacheConfiguration;
import org.apache.wicket.pageStore.IDataStore;
import org.apache.wicket.pageStore.memory.IDataStoreEvictionStrategy;
import org.apache.wicket.pageStore.memory.PageTable;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class IgniteDataStore implements IDataStore {

    private static final Logger log = LoggerFactory.getLogger(IgniteDataStore.class);

    private final IDataStoreEvictionStrategy evictionStrategy;
    private Ignite ignite;
    IgniteCache<String, PageTable> igniteCache;

    public IgniteDataStore(IDataStoreEvictionStrategy evictionStrategy) {
        this.evictionStrategy = evictionStrategy;

        CacheConfiguration<String, PageTable> cacheCfg = new CacheConfiguration<String, PageTable>("wicket-data-store");
        cacheCfg.setCacheMode(CacheMode.PARTITIONED);
        cacheCfg.setBackups(1);

        cacheCfg.setMemoryMode(CacheMemoryMode.OFFHEAP_VALUES);
        cacheCfg.setOffHeapMaxMemory(2 * 1024L * 1024L * 1024L); // 2 Gigabytes.

        cacheCfg.setEvictionPolicy(new LruEvictionPolicy<String, PageTable>(10000));

        cacheCfg.setExpiryPolicyFactory(TouchedExpiryPolicy.factoryOf(new Duration(TimeUnit.SECONDS, 14400)));
        log.info("IgniteDataStore timeout is set to 14400 seconds.");

        ignite = Ignition.ignite();
        igniteCache = ignite.getOrCreateCache(cacheCfg);
    }

    @Override
    public synchronized byte[] getData(String sessionId, int id) {
        PageTable pageTable = getPageTable(sessionId, false);
        byte[] pageAsBytes = null;
        if (pageTable != null) {
            pageAsBytes = pageTable.getPage(id);
        }
        return pageAsBytes;
    }

    @Override
    public synchronized void removeData(String sessionId, int id) {
        PageTable pageTable = getPageTable(sessionId, false);
        if (pageTable != null) {
            pageTable.removePage(id);
        }
    }

    @Override
    public synchronized void removeData(String sessionId) {
        PageTable pageTable = getPageTable(sessionId, false);
        if (pageTable != null) {
            pageTable.clear();
        }
        igniteCache.remove(sessionId);
    }

    @Override
    public synchronized void storeData(String sessionId, int id, byte[] data) {
        PageTable pageTable = getPageTable(sessionId, true);
        if (pageTable != null) {
            pageTable.storePage(id, data);
            evictionStrategy.evict(pageTable);
            igniteCache.put(sessionId, pageTable);
        } else {
            log.error("Cannot store the data for page with id '{}' in session with id '{}'", id, sessionId);
        }
    }

    @Override
    public synchronized void destroy() {
        igniteCache.clear();
    }

    @Override
    public boolean isReplicated() {
        return true;
    }

    @Override
    public boolean canBeAsynchronous() {
        return false;
    }

    private PageTable getPageTable(String sessionId, boolean create) {  
        if (igniteCache.containsKey(sessionId)) {
            return igniteCache.get(sessionId);
        }

        if (!create) {
            return null;
        }

        PageTable pageTable = new PageTable();
        igniteCache.put(sessionId, pageTable);
        return pageTable;
    }
}

Hope it helps.