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Today I was asked “what is StrongWildcard and WeakWildcard, and which one should I use?”. The piece of object model that my co-worker is asking about is:

Most bindings have a HostNameComparisonMode property on them. This property is used for service-side endpoints (IChannelListener), in conjunction with the ListenUri of that endpoint. The values are semantics similar to + and * for http.sys registrations. “Wildcard” vs. “Exact” refers to how we match the authority (hostname+port) piece of an incoming Uri. For “Wildcard”, anything matches. For “Exact”, we perform a case-insensitive string comparison to determine equality.

“Strong” vs. “Weak” has to do with the relative priorities of endpoints. We use three longest-prefix-match tables, one each for StrongWildcard, Exact, and WeakWildcard. When a Message comes into the system, we first try to match the Uri against our Strong table (which uses a wildcard comparison on hostname+port). If nothing matches, we try the Exact table (using a case-insensitive exact comparison on hostname+port). Lastly, if there was no match on Strong or Exact, we check the Weak table (again using a wildcard match for hostname+port).

For example, let’s say you have the following endpoints registered on your machine:

1. (http://foo.com/a/b/c/, StrongWildcard)
2. (http://foo.com/a/b/, Exact)
3. (http://bar.com/, Exact)
4. (http://foo.com/a/, WeakWildcard)

A request coming in for http://foo.com/a/b/c/d/ will match (1).

A request for http://foo.com/a/b/ will match (2) since the strong comparison fails (/a/b/c/ is not a prefix of /a/b/).

A request for http://foo.com/a/d/ will match (4). Even though “/” is a prefix match of “/a/d/” for (3), the hostnames differ and so the Exact match will fail.

Lastly, a request for http://bar.com/a/b/ will match (3). (2) is a prefix match but the hostnames differ, and (3) takes precedence over (4) in priority order.

Hopfully this helps demystify our endpoint URI matching process!

This entry was posted on Wednesday, March 29th, 2006 at 3:55 pm and is filed under Indigo. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.