Module Config File: what are resource connection definitions for

Question

I searched for that information quite a while but did not found any explanation.

Many examples and tutorials for custom modules with custom database tables suggest the following for the config.xml file of corresponding module:

<resources>
    <[modulename]_setup>
        <setup>
            <module>[NAMESPACE]_[MODULENAME]</module>
            <class>Mage_Eav_Model_Entity_Setup</class>
        </setup>
        <connection>
            <use>core_setup</use>
        </connection>
    </[modulename]_setup>
    <[modulename]_write>
        <connection>
            <use>core_write</use>
        </connection>
    </[modulename]_write>
    <[modulename]_read>
        <connection>
            <use>core_read</use>
        </connection>
    </[modulename]_read>
</resources>

So my questions are:

1. What are [modulename]_write and [modulename]_read for?
2. What is the connection definition in [modulename]_setup for?
3. What is <class>Mage_Eav_Model_Entity_Setup</class> for?

I ask this, because it works well with just

<resources>
    <[modulename]_setup>
        <setup>
            <module>[NAMESPACE]_[MODULENAME]</module>
        </setup>
    </[modulename]_setup>
</resources>

Answer 1

The <[modulename]_write> and <[modulename]_read> db resources are no longer required since Magento 1.4. Configuring them does no harm however, the core/resource class automatically falls back to the default_* set of db resources.
I guess most people still put them there because they follow old blog posts. The only valid reason would be if indeed a custom db connection would be configured, different from the default one, or to have your module support ancient Magento instances.

The <[modulename]_setup> resource is for using setup scripts with your module. The <class> node defines the PHP class that will execute the setup scripts found in sql/ or data/. If the <class> node is omitted, the default setup class Mage_Core_Model_Resource_Setup is used.