Adding Hooks

There are several ways to add hooks to your plugin.

Global HTTP hooks

HTTP transaction hooks are set on a global basis using the function INKHttpHookAdd. This means that the continuation specified as the parameter to INKHttpHookAdd is called for every transaction. INKHttpHookAdd must be used in INKPluginInit.
Transaction hooks

Transaction hooks can be used to call plugins back for a specific HTTP transaction. You cannot add transaction hooks in INKPluginInit; you first need a handle to a transaction. See Accessing the Transaction Being Processed.
Transformation hooks

Transformation hooks are a special case of transaction hooks. See INKVConnCacheObjectSizeGet for more information about transformation hooks. You add a transformation hook using INKHttpTxnHookAdd, as described in HTTP Transactions.
Session hooks

An HTTP session starts when a client opens a connection to Traffic Server and ends when the connection closes. A session can consist of several transactions. Session hooks enable you to hook your plugin to a particular point in every transaction within a specified session (see HTTP Sessions). Session hooks are added in a manner similar to transaction hooks (ie, you first need a handle to an HTTP session).
HTTP select alternate hook

Alternate selection hooks enable you to hook on to the alternate selection state. These hooks must be added globally, since Traffic Server does not have a handle to a transaction or session when alternate selection is taking place. See HTTP Alternate Selection for information on the alternate selection mechanism.

All of the hook addition functions (INKHttpHookAdd, INKHttpSsnHookAdd, INKHttpSsnReenable) take INKHttpHookID (identifies the hook to add on to) and INKCont (the basic callback mechanism in Traffic Server). A single INKCont can be added to any number of hooks at a time.

An HTTP hook is identified by the enumerated type INKHttpHookID. The values for INKHttpHookID are:

Table 8.1. `INKHttpHookID` Values
Values for INKHttpHookID	Description
`INK_HTTP_READ_REQUEST_HDR_HOOK`	Called immediately after the request header is read from the client. Corresponds to the event `INK_EVENT_HTTP_READ_REQUEST_HDR`.
`INK_HTTP_OS_DNS_HOOK`	Called immediately after the HTTP state machine has completed a DNS lookup of the origin server. The HTTP state machine will know the origin server's IP address at this point, which is useful for performing both authentication and blacklisting. Corresponds to the event `INK_EVENT_HTTP_OS_DNS`.
`INK_HTTP_SEND_REQUEST_HDR_HOOK`	Called immediately before the proxy's request header is sent to the origin server or the parent proxy. This hook is not called if the document is being served from cache. This hook is usually used for modifying the proxy's request header before it is sent to the origin server or parent proxy.
`INK_HTTP_READ_CACHE_HDR_HOOK`	Called immediately after the request and response header of a previously-cached object is read from cache. This hook is only called if the document is being served from cache. Corresponds to the event `INK_EVENT_HTTP_READ_CACHE_HDR`.
`INK_HTTP_READ_RESPONSE_HDR_HOOK`	Called immediately after the response header is read from the origin server or parent proxy. Corresponds to the event `INK_EVENT_HTTP_READ_RESPONSE_HDR`.
`INK_HTTP_SEND_RESPONSE_HDR_HOOK`	Called immediately before the proxy's response header is written to the client; this hook is usually used for modifying the response header. Corresponds to the event `INK_EVENT_HTTP_SEND_RESPONSE_HDR`.
`INK_HTTP_REQUEST_TRANSFORM_HOOK`	See“Transformations” for information about transformation hooks.
`INK_HTTP_RESPONSE_TRANSFORM_HOOK`	See ““Transformations” for information about transformation hooks.
`INK_HTTP_TXN_START_HOOK`	Called when an HTTP transaction is started. A transaction starts when either a client connects to Traffic Server and data is available on the connection, or a previous client connection that was left open for keep alive has new data available.
`INK_HTTP_TXN_CLOSE_HOOK`	Called when an HTTP transaction ends.
`INK_HTTP_SELECT_ALT_HOOK`	See “HTTP Alternate Selection” for information about the alternate selection mechanism.
`INK_HTTP_SSN_START_HOOK`	Called when an HTTP session is started. A session starts when a client connects to Traffic Server. You can only add this hook as a global hook.
`INK_HTTP_SSN_CLOSE_HOOK`	Called when an HTTP session ends. A session ends when the client connection is closed. You can only add this hook as a global hook
`INK_HTTP_CACHE_LOOKUP_COMPLETE_HOOK`	Called after the HTTP state machine has commpleted the cache lookup for the document requested in the ongoing transaction. Register this hook via `INKHttpTxnHookAdd` or `INKHttpHookAdd`. Corresponds to the event `INK_EVENT_HTTP_CACHE_LOOKUP_COMPLETE`.

The function you use to add a global HTTP hook is INKHttpHookAdd.

Traffic Server Software Developers Kit

Adding Hooks