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.. include:: ../../../common.defs

.. _developer-plugins-hooks-http-transactions:

HTTP Transactions
*****************

The HTTP transaction functions enable you to set up plugin callbacks to
HTTP transactions and obtain/modify information about particular HTTP
transactions.

As described in the section on HTTP sessions, an **HTTP transaction** is
an object defined for the lifetime of a single request from a client and
the corresponding response from Traffic Server. The **``TSHttpTxn``**
structure is the main handle given to a plugin for manipulating a
transaction's internal state. Additionally, an HTTP transaction has a
reference back to the HTTP session that created it.

The sample code below illustrates how to register locally to a
transaction and associate data to the transaction.

.. code-block:: c

    /*
    * Simple plugin that illustrates:
    * - how to register locally to a transaction
    * - how to deal with data that's associated with a tranaction
    *
    * Note: for readability, error checking is omitted
    */

    #include <ts/ts.h>

    #define DBG_TAG "txn"

    /* Structure to be associated to txns */
    typedef struct {
       int i;
       float f;
       char *s;
    } TxnData;

    /* Allocate memory and init a TxnData structure */
    TxnData *
    txn_data_alloc()
    {
       TxnData *data;
       data = TSmalloc(sizeof(TxnData));

       data->i = 1;
       data->f = 0.5;
       data->s = "Constant String";
       return data;
    }

    /* Free up a TxnData structure */
    void
    txn_data_free(TxnData *data)
    {
       TSfree(data);
    }

    /* Handler for event READ_REQUEST and TXN_CLOSE */
    static int
    local_hook_handler (TSCont contp, TSEvent event, void *edata)
    {
       TSHttpTxn txnp = (TSHttpTxn) edata;
       TxnData *txn_data = TSContDataGet(contp);
       switch (event) {
       case TS_EVENT_HTTP_READ_REQUEST_HDR:
          /* Modify values of txn data */
          txn_data->i = 2;
          txn_data->f = 3.5;
          txn_data->s = "Constant String 2";
          break;

       case TS_EVENT_HTTP_TXN_CLOSE:
          /* Print txn data values */
          TSDebug(DBG_TAG, "Txn data i=%d f=%f s=%s", txn_data->i, txn_data->f, txn_data->s);

          /* Then destroy the txn cont and its data */
          txn_data_free(txn_data);
          TSContDestroy(contp);
          break;

       default:
           TSAssert(!"Unexpected event");
           break;
       }

       TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE);
       return 1;
    }

    /* Handler for event TXN_START */
    static int
    global_hook_handler (TSCont contp, TSEvent event, void *edata)
    {
       TSHttpTxn txnp = (TSHttpTxn) edata;
       TSCont txn_contp;
       TxnData *txn_data;

       switch (event) {
       case TS_EVENT_HTTP_TXN_START:
          /* Create a new continuation for this txn and associate data to it */
          txn_contp = TSContCreate(local_hook_handler, TSMutexCreate());
          txn_data = txn_data_alloc();
          TSContDataSet(txn_contp, txn_data);

          /* Registers locally to hook READ_REQUEST and TXN_CLOSE */
          TSHttpTxnHookAdd(txnp, TS_HTTP_READ_REQUEST_HDR_HOOK, txn_contp);
          TSHttpTxnHookAdd(txnp, TS_HTTP_TXN_CLOSE_HOOK, txn_contp);
          break;

       default:
          TSAssert(!"Unexpected event");
          break;
       }

       TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE);
       return 1;
    }


    void
    TSPluginInit (int argc, const char *argv[])
    {
       TSCont contp;

       /* Note that we do not need a mutex for this txn since it registers globally
          and doesn't have any data associated with it */
       contp = TSContCreate(global_hook_handler, NULL);

       /* Register gloabally */
       TSHttpHookAdd(TS_HTTP_TXN_START_HOOK, contp);
    }

See :ref:`developer-plugins-hooks-adding` for background on HTTP transactions
and HTTP hooks, as well as :ref:`developer-plugins-hooks-and-transactions`. See
also the :ref:`HTTP Transaction State Diagram <http-txn-state-diagram>` for an
illustration of the steps involved in a typical HTTP transaction.

The HTTP transaction functions are:

-  :c:func:`TSHttpTxnCacheLookupStatusGet`

-  :c:func:`TSHttpTxnCachedReqGet`
   - Note that it is an error to modify cached headers.

-  :c:func:`TSHttpTxnCachedRespGet`
   - Note that it is an error to modify cached headers.

-  :c:func:`TSHttpTxnClientReqGet`
   - Plugins that must read client request headers use this call to
   retrieve the HTTP header.

-  :c:func:`TSHttpTxnClientRespGet`

-  :c:func:`TSHttpTxnErrorBodySet`

-  :c:func:`TSHttpTxnHookAdd`

-  :c:func:`TSHttpTxnNextHopAddrGet`

-  :c:func:`TSHttpTxnParentProxySet`

-  :c:func:`TSHttpTxnReenable`

-  :c:func:`TSHttpTxnServerAddrGet`

-  :c:func:`TSHttpTxnServerAddrSet`

-  :c:func:`TSHttpTxnServerReqGet`

-  :c:func:`TSHttpTxnServerRespGet`

-  :c:func:`TSHttpTxnSsnGet`

-  :c:func:`TSHttpTxnTransformedRespCache`

-  :c:func:`TSHttpTxnTransformRespGet`

-  :c:func:`TSHttpTxnUntransformedRespCache`


