Which HTTP messages are considered for adaptation?

Short answer: Which HTTP messages are considered for adaptation and which eCAP service(s) are used for that adaptation depends on the host application configuration.

Explanation:

The adapter (the loadable module) registers Service objects. The eCAP service registration API does not specify how the objects are used by the host application, but the host application normally maps Service objects to various adaptation vectoring points. How that mapping is done, and which points are supported, depends on the host application. In general, REQMOD vectoring points receive HTTP requests and RESPMOD vectoring points receive HTTP responses (plus request headers as a constant "cause" of the response).

If you deal with an HTTP proxy, you may want to read RFC 3507 to learn more about the overall adaptation architecture in HTTP proxies. ICAP and eCAP share the same high-level adaptation principles.

For example, Squid maps Service objects to vectoring points using ecap_service directives in squid.conf. The mapping is done using Service URIs (the "uri" parameter of the ecap_service directive). IIRC, Squid requires that each ecap_service line has a unique service URI. Which HTTP messages are actually sent to a which service depends on the ACL-driven adaptation_access rules in squid.conf. If you have Squid configuration questions, please post them on Squid support forums and not here.