Toolkit - Cache Invalidation

This post if part of a series about the File System Toolkit - a custom content delivery API for SDL Tridion.

In previous post, I explained the use of a Cache Factory. This post describes a very simple cache invalidation mechanism the Toolkit uses in order to make sure it does not serve stale values (for a long time).

Given the nature of the Toolkit storage, i.e. files on a file-system, it is very easy to check when they were last published. By simply looking at the JSON model file last modified attribute, one can see the very moment that model was created/updated. This is the last publish time-stamp as well.

When a CacheElement is placed in cache, it is also given a last-check timestamp, which initially is set to now. This last-check will be greater than the JSON file last-modified attribute. The moment the JSON model is republished, the file last-modified file attribute will be greater than the last-check. When this happens, we know the CacheElement is stale and we remove it from cache.

The stale logic also checks for the existence of a JSON model file. An unpublished (a.k.a. missing) file will immediately be considered stale, and as such removed from cache.

In order to make the cache invalidation check more performant and to minimize I/O operations with the file system, we only check for stale elements on (a) get from cache operation and (b) when a certain interval has passed since the previous stale check. This interval is configurable in Toolkit configuration under property name cacheMonitorSeconds. This value represents a period of time that is acceptable to retrieve potentially stale objects. Typically this value should be low - i.e. 5, 10 or 30 seconds or as long as you're comfortable with. A value of 0 will perform the stale check on all calls to the cache.get() method.

More information about possible values in the toolkit.properties file is available in post Installation and Configuration.

private boolean isStale(Element element) {
    CacheElement<Object> cacheElement = (CacheElement<Object>) element.getObjectValue();
    long lastCheck = cacheElement.getLastCheck();
    long now = System.currentTimeMillis();

    if (now - lastCheck > cacheMonitorInterval) {
        cacheElement.setLastCheck(now);
        Object value = cacheElement.getPayload();

        if (value instanceof IdentifiableObject) {
            IdentifiableObject identifiableObject = (IdentifiableObject) value;
            TcmUri tcmUri = identifiableObject.getTcmUri();
            PathMapper pathMapper = new PathMapper();
            File file = new File(pathMapper.getModelAbsolutePath(tcmUri));

            return !file.exists() || file.lastModified() > lastCheck;
        }
    }

    return false;
}