(әb-lig´ә-tor″e) obligate.
[adj] - required by obligation or compulsion or convention 2. [adj] - morally or legally constraining or binding
• (a.) Binding in law or conscience; imposing duty or obligation; requiring performance or forbearance of some act; -- often followed by on or upon; as, obedience is obligatory on a soldier.
adjective required by obligation or compulsion or convention; `he made all the obligatory apologies`
Ob'li·ga·to·ry adjective [ Latin obligatorius : confer French obligatoire .] Binding in law or conscience; imposing duty or obligation; requiring performance or forbearance of some act; -- often followed by on or upon ; as, obedience is obligatory on a soldier. &#x...
Not optional.
obligate or required. For example, an obligatory cleaner fish relies entirely on this feeding mode to obtain nutrients. 
obligatory 1. Required by obligation or compulsion or convention. 2. Morally or legally constraining or binding.