Setting a response header in a CQ5 publishing instance - aem

Question

I want to disable caching in a CQ component and I have the following line in my jsp (documentation):
response.setHeader("Dispatcher", "no-cache");
If I insert the component in a page and load the page in an authoring instance everything works as expected and I get an HTTP header named Dispatcher with the content no-cache.
Now if I do the same on a publishing instance (same configuration with CQ_RUNMODE='publish' and same content) the component works but for setting the HTTP header.
Any idea on why the two instances could behave differently?
Update
I tried to set other headers and the instance behaves in the same way: in the authoring mode the headers are generated in the publishing mode not (same configuration but for the CQ_RUNMODE)
Update 2
I was trying to reduce my example by removing everything that is unnecessary from the page (layout, code for headers, footer, ...) and I noticed that after a certain size threshold my header is correctly generated.
In other words by removing stuff from the page (even simple HTML) I reach a certain point where the header appears (if the page is small enough).
Any idea on why CQ is only generating the header for very small pages?

Answer 1

If you're trying to set the header in a component far down on the page, you could be getting the issue that you're trying to write it after the response has been committed.
If you need to flag the page as not cached & you cannot avoid placing the code higher in the buffer, you could instead write in a check for this node type at the start of the JSP (using node.listChildren() for example), or provide a page property that let's the editor control if the page is cached or not.

Answer 2

You didn't indicate which version of CQ5 you're using - I just tested with a minimal JSP script on a CQ 5.5 GA publish instance, and the header is correctly set:
$ curl -u admin:admin http://localhost:4503/tmp/x.tidy.json
{
"sling:resourceType": "x",
...
}
$ curl -u admin:admin http://localhost:4503/apps/x/x.jsp
<%
response.setHeader("Dispatcher","no-cache");
%>
Here's the content.
$ curl -D - -u admin:admin http://localhost:4503/tmp/x.html
HTTP/1.1 200 OK
Connection: Keep-Alive
...
Dispatcher: no-cache
Here's the content.
You might want to start with this minimal test and compare with what you're doing.