OCLC uses ETags (RFC2616) for handling caching and concurrency control of resources.
You may need to refer to a specific version of a particular entry. This is important in two cases in particular:
The OCLC Web Services provide ETags in two places:
Example - Header ETag: "S0wCTlpIIip7ImA0X0QI"
Example - Feed <feed xmlns='http://www.w3.org/2005/Atom' xmlns:gd="http://schemas.google.com/g/2005" gd:etag='W/"C0QBRXcycSp7ImA9WxRVFUk."'>
Example - Entry <entry gd:etag='"CUUEQX47eCp7ImA9WxRVEkQ."'>
An ETag is usually a string of letters and numbers, sometimes including hyphens and periods. The string is usually enclosed in quotation marks (the quotation marks are part of the ETag).
There are two kinds of ETags: strong and weak.
Example - Weak ETag ETag: W/"D08FQn8-eil7ImA9WxZbFEw."
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