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 TSHttpHookAdd. This means that the continuation specified as the parameter to TSHttpHookAdd is called for every transaction. TSHttpHookAdd must be used in TSPluginInit.
Transaction hooks Transaction hooks can be used to call plugins back for a specific HTTP transaction. You cannot add transaction hooks in TSPluginInit; 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 TSVConnCacheObjectSizeGet() for more information about transformation hooks. You add a transformation hook using TSHttpTxnHookAdd, 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 (TSHttpHookAdd(), TSHttpSsnHookAdd(), TSHttpSsnReenable()) take TSHttpHookID (identifies the hook to add on to) and TSCont (the basic callback mechanism in Traffic Server). A single TSCont can be added to any number of hooks at a time.

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

Values for TSHttpHookID

TS_HTTP_CACHE_LOOKUP_COMPLETE_HOOK: Called after the HTTP state machine has completed the cache lookup for the document requested in the ongoing transaction. Register this hook via TSHttpTxnHookAdd or TSHttpHookAdd. Corresponds to the event TS_EVENT_HTTP_CACHE_LOOKUP_COMPLETE.
TS_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 denylisting. Corresponds to the event TS_EVENT_HTTP_OS_DNS.
TS_HTTP_POST_REMAP_HOOK: Called immediately after remapping occurs, before cache lookup. Corresponds to the event TS_EVENT_HTTP_POST_REMAP.
TS_HTTP_PRE_REMAP_HOOK: Called after the request header is read from the client, before any remapping of the headers occurs. Corresponds to the event TS_EVENT_HTTP_PRE_REMAP.
TS_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 TS_EVENT_HTTP_READ_CACHE_HDR.
TS_HTTP_READ_RESPONSE_HDR_HOOK: Called immediately after the response header is read from the origin server or parent proxy. Corresponds to the event TS_EVENT_HTTP_READ_RESPONSE_HDR.
TS_HTTP_RESPONSE_TRANSFORM_HOOK: See “Transformations” for information about transformation hooks.
TS_HTTP_READ_REQUEST_HDR_HOOK: Called immediately after the request header is read from the client. Corresponds to the event TS_EVENT_HTTP_READ_REQUEST_HDR.
TS_HTTP_REQUEST_TRANSFORM_HOOK: See “Transformations” for information about transformation hooks.
TS_HTTP_SELECT_ALT_HOOK: See “HTTP Alternate Selection” for information about the alternate selection mechanism.
TS_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 TS_EVENT_HTTP_SEND_RESPONSE_HDR.
TS_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.

Caution

TS_HTTP_SEND_REQUEST_HDR_HOOK may callback several times when the OS crashed. Be careful to use functions such as TSContDestroy in TS_HTTP_SEND_REQUEST_HDR_HOOK hook.

TS_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
TS_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.
TS_HTTP_TXN_CLOSE_HOOK: Called when an HTTP transaction ends.
TS_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.

The function you use to add a global HTTP hook is TSHttpHookAdd().