# 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:

typedef struct {
TSVIO output_vio;
TSIOBuffer output_buffer;
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.