In a distributed, heterogeneous, proxy-based digital library, 
autonomous services and collections are accessed indirectly 
via proxies.  To facilitate metadata compatibility and interoperability 
in such a digital library, we have designed a metadata architecture 
that includes four basic component classes: attribute model proxies, 
attribute model translators, metadata facilities for search proxies, 
and metadata repositories.  Attribute model proxies elevate both
attribute sets and the attributes they define to first-class objects. 
They also allow relationships among attributes to be captured.  
Attribute model translators map attributes and attribute values 
from one attribute model to another (where possible).  Metadata 
facilities for search proxies provide structured descriptions both 
of the collections to which the search proxies provide access and 
of the search capabilities of the proxies.  Finally, metadata 
repositories accumulate selected metadata from local instances of 
the other three component classes in order to facilitate global 
metadata queries and local metadata caching.  In this paper, 
we outline further the roles of these component classes, discuss 
our design rationale, and analyze related work.