Registry Article Refer Method (Gnus Manual)

10.19.2 Fetching by Message-ID Using the Registry

The registry knows how to map each Message-ID to the group it’s in. This can be leveraged to enhance the “article refer method”, the thing that tells Gnus how to look up an article given its Message-ID (see Finding the Parent).

The nnregistry refer method does exactly that. It has the advantage that an article may be found regardless of the group it’s in—provided its Message-ID is known to the registry. It can be enabled by augmenting the start-up file with something along these lines:

;; Keep enough entries to have a good hit rate when referring to an
;; article using the registry. Use long group names so that Gnus
;; knows where the article is.
(setq gnus-registry-max-entries 2500)
(gnus-registry-initialize)
(setq gnus-refer-article-method
 '(current
 (nnregistry)
 (nnweb "google" (nnweb-type google))))

The example above instructs Gnus to first look up the article in the current group, or, alternatively, using the registry, and finally, if all else fails, using Google.