Headers Transformation
=====================

Ocelot allows the user to transform headers pre and post downstream request. At the moment Ocelot only supports find and replace. This feature was requested `GitHub #190 <https://github.com/TomPallister/Ocelot/issues/190>`_ and I decided that it was going to be useful in various ways.

Syntax
^^^^^^

In order to transform a header first we specify the header key and then the type of transform we want e.g.

.. code-block:: json

    "Test": "http://www.bbc.co.uk/, http://ocelot.com/"

The key is "Test" and the value is "http://www.bbc.co.uk/, http://ocelot.com/". The value is saying replace http://www.bbc.co.uk/ with http://ocelot.com/. The syntax is {find}, {replace}. Hopefully pretty simple. There are examples below that explain more.

Pre Downstream Request
^^^^^^^^^^^^^^^^^^^^^^

Add the following to a ReRoute in configuration.json in order to replace http://www.bbc.co.uk/ with http://ocelot.com/. This header will be changed before the request downstream and will be sent to the downstream server.

.. code-block:: json

     "UpstreamHeaderTransform": {
        "Test": "http://www.bbc.co.uk/, http://ocelot.com/"
    },

Post Downstream Request
^^^^^^^^^^^^^^^^^^^^^^

Add the following to a ReRoute in configuration.json in order to replace http://www.bbc.co.uk/ with http://ocelot.com/. This transformation will take place after Ocelot has received the response from the downstream service.

.. code-block:: json

    "DownstreamHeaderTransform": {
        "Test": "http://www.bbc.co.uk/, http://ocelot.com/"
    },

Placeholders
^^^^^^^^^^^^

Ocelot allows placeholders that can be used in header transformation. At the moment there is only one placeholder.

{BaseUrl} - This will use Ocelot's base url e.g. http://localhost:5000 as its value.

Handling 302 Redirects
^^^^^^^^^^^^^^^^^^^^^^
Ocelot will by default automatically follow redirects however if you want to return the location header to the client you might want to change the location to be Ocelot not the downstream service. Ocelot allows this with the following configuration.

.. code-block:: json

    "DownstreamHeaderTransform": {
        "Location": "http://www.bbc.co.uk/, http://ocelot.com/"
    },
     "HttpHandlerOptions": {
        "AllowAutoRedirect": false,
    },

or you could use the BaseUrl placeholder.

.. code-block:: json

    "DownstreamHeaderTransform": {
        "Location": "http://localhost:6773, {BaseUrl}"
    },
     "HttpHandlerOptions": {
        "AllowAutoRedirect": false,
    },

Ocelot will not try and replace the location header returned by the downstream service with its own URL.