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About append_transform.c
This example is based on null_transform.c. It appends text to the body
of an HTML text response document on its way from the cache to the
client.
The plugin first makes sure that it has received a "200 OK"
response from the origin server. It then verifies that the returned
document is of type "text/html". It "transforms" the document by
appending text to the file. To read and write to the body of the
document, the plugin uses functions similar to those in null_transform.c.
You place the text to be appended in a file, and you must provide the
path to the file in plugin.config. In other words, plugin.config must
have a line similar to the following:
append_transform.so path/to/file
Specify an absolute path to the text file, or a relative path as
described in the file-plugin README.
TSPluginInit does the following:
- makes sure that there is a text file specified in plugin.config
(if not, it returns an error message)
- calls the function load to load the contents of the file into
a buffer to be appended to HTML response bodies
- sets up the global hook to call back the plugin:
TSHttpHookAdd(TS_HTTP_READ_RESPONSE_HDR_HOOK,
TSContCreate (transform_plugin, NULL));
The load function does the following (similar to file-1.c):
- opens the file specified in plugin.config using
TSfopen
- creates a buffer for the text to be appended using
TSIOBufferCreate
- creates a buffer reader for the text to be appended using
TSIOBufferReaderAlloc
- reads the contents of the file in to the buffer using:
TSIOBufferStart
TSIOBufferBlockWriteStart
TSfread
TSIOBufferProduce
TSIOBufferReaderAvail
- closes the file using
TSfclose
The transform_plugin function does the following:
- tests the response body to make sure it is text/html, using
the function "transformable". The transformable function
uses the following API calls:
TSHttpTxnServerRespGet
TSHttpHdrStatusGet
TSMimeHdrFieldFind
TSMimeHdrFieldValueStringGet
- if the response body is deemed transformable, transform_plugin calls
transform_add
- continues the HTTP transaction using
TSHttpTxnReenable
The transform_add function does the following:
- creates a continuation for the append transform, using
TSTransformCreate (append_transform, txnp);
The handler function for this continuation is
append_transform.
- adds a transaction hook for the append transform, using
TSHttpTxnHookAdd(txnp, TS_HTTP_RESPONSE_TRANSFORM_HOOK, connp);
This transaction hook sets up a callback during transactions.
When the event TS_HTTP_RESPONSE_TRANSFORM_HOOK happens,
the append_transform function is called back.
The remaining functions in the plugin, append_transform and
handle_transform, are similar to null_transform and
handle_transform in the null_transform.c plugin.
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