The Append-Transform Plugin

The append-transform plugin appends text to the body of an HTTP response. It obtains this text from a file; the name of the file containing the append text is a parameter you specify in plugin.config, as follows:

append-transform.so path/to/file

The append-transform plugin is based on null-transform.c. The only difference is that after the plugin feeds the document through the transformation, it adds text to the response.

Below is a list of the functions in append-transform.c, in the order they appear in the source code. Below each entry is a description of what the function does:

• ``my_data_alloc``

  Allocates and initializes a MyData structure. The plugin defines a struct, MyData, as follows:

  :::c
  typedef struct {
      TSVIO output_vio;
      TSIOBuffer output_buffer;
      TSIOBufferReader output_reader;
      int append_needed;
  } MyData;

  The MyData structure is used to represent data that the transformation (vconnection) needs. The transformation’s data pointer is set to a MyData pointer using TSContDataSet in the handle_transform routine.

• ``my_data_destroy``

  Destroys objects of type MyData. To deallocate the transform’s data, the append_transform routine (see below) calls my_data_destroy when the transformation is complete.

• ``handle_transform``

  This function does the actual data transformation. The transformation is created in transform_add (see below). handle_transform is called by append_transform.

• ``append_transform``

  This is the handler function for the transformation vconnection created in transform_add. It is the implementation of the vconnection.

  • If the transformation vconnection has been closed, then append_transform calls my_data_destroy to destroy the vonnection.
  • If append_transform receives an error event, then it calls back the continuation to let it know it has completed the write operation.
  • If it receives a WRITE_COMPLETE event, then it shuts down the write portion of its vconnection.
  • If it receives a WRITE_READY or any other event (such as TS_HTTP_RESPONSE_TRANSFORM_HOOK), then it calls handle_transform to attempt to transform more data.

• ``transformable``

  The plugin transforms only documents that have a content type of text/html. This function examines the Content-Type MIME header field in the response header. If the value of the MIME field is text/html, then the function returns 1; otherwise, it returns zero.

• ``transform_add``

  Creates the transformation for the current transaction and sets up a transformation hook. The handler function for the transformation is append_transform.

• ``transform_plugin``

  This is the handler function for the main continuation for the plugin. Traffic Server calls this function whenever it reads an HTTP response header. transform_plugin does the following:

  • Gets a handle to the HTTP transaction being processed
  • Calls transformable to determine whether the response document content is of type text/html
  • If the content is transformable, then it calls transform_add to create the transformation.
  • Calls TSHttpTxnReenable to continue the transaction

• ``load``

  Opens the file containing the text to be appended and loads the contents of the file into an TSIOBuffer called append_buffer.

• ``TSPluginInit``

  Does the following:

  • Checks to make sure that the required configuration information (the append text filename) is entered in plugin.config correctly.
  • If there is a filename, then TSPluginInit calls load to load the text.
  • Creates a continuation for the plugin. The handler for this continuation is transform_plugin.
  • Adds the plugin’s continuation to TS_HTTP_READ_RESPONSE_HDR_HOOK. In other words, it sets up a callback of the plugin’s continuation when Traffic Server reads HTTP response headers.