Is the requested state compelled to provide assistance and if not, what are the bases for refusal?
In general, no. However, international conventions and agreements call for states to afford one another the highest possible assistance in criminal matters. If there are no substantial reasons to refuse MLA requests, states should provide the widest extent of assistance, in accordance with their international commitments and their responsibilities in the fight against crime.
When it comes to what amounts to the bases of refusal, in most international agreements it is provided in numerus clausus. Requests of assistance relating to political crimes, military offences, or fiscal offences may be refused.
However, there are exceptions where these reasons cannot amount to a basis for refusal. For example, acts of murder or attempted murder of a head of state are not accepted as political offences and therefore, in such a case, requests cannot be refused on the basis that it relates to a political offence.
For military offences, in cases where a military offence also constitutes an offence under civilian criminal law (i.e. genocide, crimes against humanity, voluntary manslaughter etc.), same thing can be said.
And for fiscal offences, we can say that there is a growing tendency to remove them from the bases of refusal as fiscal offences can be easily related to grave predicate offences and fiscal transactions can be used for money laundering purposes.
Lastly, countries may refuse MLA requests if they consider the execution of the request is likely to prejudice their sovereignty, security or ‘ordre public’. What amounts to a prejudice to sovereignty or security can be clear in most cases. However, ordre public is a vague term and needs to be elaborated.
Ordre public has two aspects. One relates to the conditions of human rights in requesting party and the other relates to the observance and respect of the fundamental rights which relate to criminal and criminal procedure law of the requested party. 
For example, if a person faces the danger of inhumane treatment or torture in the requesting country, the requesting country should refuse the request based on ordre public. Also, if the request is against the provisions of criminal procedure of the requested state and in contradiction to the rights and duties it provides, the request will be refused based on ordre public.
 GILLMEISTER, F., Auslieferung und Auslieferungshaft – mit Berücksichtigung des Betäubungsmittelrechts, NJW 1991, p. 2249.