<config>
    <cacheObserver>
        <!--
            The observer id should be a unique key. Note that all observers will be sorted alphabetically by the
            observer key.
            You may make use of Magento's config.xml merging to overwrite another module's configs if needed.
        -->
        <module_name_observer_id>
            <!--
                `model` is a standard model alias to tell the model
            -->
            <model>module_name/model</model>
            <method>myCustomCacheObserverMethod</method>
            <classes>
                <!-- A list of classes to
                <Some_Module_Block_To_Cache/>
            </classes>
        </module_name_observ_id>
    </cacheObserver>
</config>