## Managers - Intro

To use Managers in Torque, you need to generate your object model with the generator option `torque.om.useManagers = true`.

A manager is responsible for instantiating new objects, retrieving stored objects, and possibly caching these objects. Managers provide static accessors for much of its functionality, so usage examples are:

    Foo foo = FooManager.getInstance(); // gets a new Foo.
    // id is an ObjectKey that identifies the object in the db
    foo = FooManager.getInstance(id);
    // ids is a List of ObjectKey's that identifies objects in the db
    List foos = FooManager.getInstances(ids);

Note that the cache is global and is not Transaction-aware. This implies that isolation of different transactions will be difficult to achieve. For example, you might read uncommitted data out of the cache even if you database transaction isolation is set to `READ_COMMITTED`.

## Business Object Cache

If the no-arg constructor of FooManager calls setRegion(region) where the String region is the key used to determine the cache, the manager will cache instances of Foo retrieved via the getInstance(ObjectKey id) and getInstances(List ids) methods. One possibility for the region key is the fully qualified classname with dots replaced by underscores.

    public FooManager()
        throws TorqueException
    {
        setRegion("net_bar_om_Foo");
    }

The key given for the region is used in a JCS configuration file, cache.ccf, to set up a cache that the manager uses to store objects for which it is responsible. See the JCS documentation for details on configuring JCS. But here is a simple section that creates an in-memory only LRU cache for FooManager.

    jcs.region.net_bar_om_Foo=
    jcs.region.net_bar_om_Foo.cacheattributes=
        org.apache.commons.jcs.engine.CompositeCacheAttributes
    jcs.region.net_bar_om_Foo.cacheattributes.MaxObjects=1200
    jcs.region.net_bar_om_Foo.cacheattributes.MemoryCacheName=
        org.apache.commons.jcs.engine.memory.lru.LRUMemoryCache

It is a good idea to set a region for each manager, but this behavior is optional. There also will be no caching if JCS is not configured for the region given in the Constructor.

The generated object model classes have methods for getting objects that are related by foreign keys. If the FOO table contains an fk to the BAR table then Foo.getBar() will exist. This method uses BarManager.getInstance(bar_id) and therefore will return a cached Bar, if the Bar has been previously requested (and it still exists in the cache.)

## Method Result Cache

The above fk relationship will also generate a Bar.getFoos(Criteria). It would be preferable that repeated requests to this method returned cached results as opposed to hitting the db for each call. It could be possible to add such caching to the generated method, and Criteria implements an equals() method that would make this possible. But determining the equality of a Criteria is complex and possibly buggy (this is the perception of the author of this doc, there are no known bugs). Invalidating the results has also not been reduced to templated Java code. So whether to cache these kinds of results is left to the developer who is using torque.

It is a good practice to write methods within Bar that wrap the getFoos(Criteria) method. The conversion from application parameters to a Criteria is then implemented in a more maintainable manner. For example:

    public List getFoos(FooType type, boolean deleted)
    {
        List result = null;

        Criteria crit = new Criteria();
        crit.add(FooPeer.TYPE_ID, type.getId());
        crit.add(FooPeer.DELETED, deleted);
        result = getFoos(crit);

        return result;
    }

In the above code the database will be hit for every call to the method. BarManager provides some convenience code to add caching to the above method, so it can be rewritten as:

    public List getFoos(FooType type, boolean deleted)
    {
        List<Foo> result = null;
        Boolean b = (deleted ? Boolean.TRUE : Boolean.FALSE);
        Object obj = BarManager.getMethodResult().get(this, "getFoos", type, b);
        if ( obj == null )
        {
            Criteria crit = new Criteria();
            crit.add(FooPeer.TYPE_ID, type.getId());
            crit.add(FooPeer.DELETED, deleted);
            result = getFoos(crit);

            BarManager.getMethodResult().put(result, this, "getFoos", type, b);
        }
        else
        {
            result = (List<Foo>)obj;
        }
        return result;
    }

The getMethodResult() method returns a MethodResultCache object, which creates a key from the arguments given in the get method. All the arguments must be Serializable. The first object should be the business object on which the method was called. If the object is not Serializable or the method is static, a String as given by Object.toString() method or the className might be used. The second argument is the method name. The get method takes a variable number of additional arguments that will be the arguments to the method, or if they are not Serializable some Serializable proxy. Now the method will return cached results as long as the results remain in the cache. So there must be some way to invalidate these results, if the database changes in a way that is likely to affect the result that should be returned by the method.

## Invalidating the Cache

An event model exists for invalidating cached method results. Continuing the example from above, BarManager should register itself as a listener with the FooManager. Then FooManager will notify BarManager, if a foo.save() is called that might affect its cached results. The following code is added to BarManager.java which implements the CacheListener interface.

    /**
     * Method should be overridden to notify other managers with
     * relevant CacheEvents.
     */
    protected void registerAsListener()
    {
        FooManager.addCacheListener(this);
        XManager.addCacheListener(this);
        ...
    }

    // -------------------------------------------------------------------
    // CacheListener implementation

    public void addedObject(Persistent om)
    {
        if (om instanceof Foo)
        {
            getMethodResult().removeAll(om, "getFoos");
        }
        else if (om instanceof X)
        {
            getMethodResult().remove(om, GET_URLS);
            getMethodResult().removeAll(om, GET_COMMENTS);
        }
        ...
    }

    public void refreshedObject(Persistent om)
    {
        addedObject(om);
    }

    /** fields which interest us with respect to cache events */
    public List getInterestedFields()
    {
        List<Column> interestedCacheFields = new LinkedList<>();
        interestedCacheFields.add(FooPeer.BAR_ID);
        interestedCacheFields.add(XPeer.X_ID);
        ...
        return interestedCacheFields;
    }

When a foo which is of interest to BarManager is saved, the instance is passed to the appropriate listener method. This object may contain information that could result in no action or possibly more precise repair of the cached data. In the above examples the cache is just cleared of all data that is potentially invalid. Some code is also added to FooManager to support the invalidation.

    /**
     * Creates a new <code>FooManager</code> instance.
     */
    public FooManager()
        throws TorqueException
    {
        setRegion("net_bar_om_Foo");
        addValidField(FooPeer.BAR_ID);
    }

    protected Persistent putInstanceImpl(Persistent om)
        throws TorqueException
    {
        Persistent oldOm = super.putInstanceImpl(om);
        notifyListeners(FooPeer.BAR_ID, oldOm, om);
        return oldOm;
    }

Now FooManager will notify BarManager when foo's are modified.