How filters work in Apache 2.0
This is a cut 'n paste job from an email
(<022501c1c529$f63a9550$7f00000a@KOJ>) and only reformatted for
better readability. It's not up to date but may be a good start for
There are three basic filter types (each of these is actually broken
down into two categories, but that comes later).
- Filters of this type are valid for the lifetime of this connection.
- Filters of this type are valid for the lifetime of this request from
the point of view of the client, this means that the request is valid
from the time that the request is sent until the time that the response
is received. (
- Filters of this type are valid for the time that this content is used
to satisfy a request. For simple requests, this is identical to
PROTOCOL, but internal redirects and sub-requests can change
the content without ending the request. (
It is important to make the distinction between a protocol and a
resource filter. A resource filter is tied to a specific resource, it
may also be tied to header information, but the main binding is to a
resource. If you are writing a filter and you want to know if it is
resource or protocol, the correct question to ask is: "Can this filter
be removed if the request is redirected to a different resource?" If
the answer is yes, then it is a resource filter. If it is no, then it
is most likely a protocol or connection filter. I won't go into
connection filters, because they seem to be well understood. With this
definition, a few examples might help:
- We have coded it to be inserted for all requests, and it is removed
if not used. Because this filter is active at the beginning of all
requests, it can not be removed if it is redirected, so this is a
- This filter actually writes the headers to the network. This is
obviously a required filter (except in the asis case which is special
and will be dealt with below) and so it is a protocol filter.
- The administrator configures this filter based on which file has been
requested. If we do an internal redirect from an autoindex page to an
index.html page, the deflate filter may be added or removed based on
config, so this is a resource filter.
The further breakdown of each category into two more filter types is
strictly for ordering. We could remove it, and only allow for one
filter type, but the order would tend to be wrong, and we would need to
hack things to make it work. Currently, the
only have one filter type, but that should change.
This is actually rather simple in theory, but the code is
complex. First of all, it is important that everybody realize that
there are three filter lists for each request, but they are all
concatenated together. So, the first list is
r->connection->output_filters. These correspond
CONNECTION filters respectively. The problem previously, was
that we used a singly linked list to create the filter stack, and we
started from the "correct" location. This means that if I had a
RESOURCE filter on the stack, and I added a
CONNECTION filter, the
CONNECTION filter would
be ignored. This should make sense, because we would insert the connection
filter at the top of the
c->output_filters list, but the end
r->output_filters pointed to the filter that used to be
at the front of
c->output_filters. This is obviously wrong.
The new insertion code uses a doubly linked list. This has the advantage
that we never lose a filter that has been inserted. Unfortunately, it comes
with a separate set of headaches.
The problem is that we have two different cases were we use subrequests.
The first is to insert more data into a response. The second is to
replace the existing response with an internal redirect. These are two
different cases and need to be treated as such.
In the first case, we are creating the subrequest from within a handler
or filter. This means that the next filter should be passed to
make_sub_request function, and the last resource filter in the
sub-request will point to the next filter in the main request. This
makes sense, because the sub-request's data needs to flow through the
same set of filters as the main request. A graphical representation
Default_handler --> includes_filter --> byterange --> ...
If the includes filter creates a sub request, then we don't want the
data from that sub-request to go through the includes filter, because it
might not be SSI data. So, the subrequest adds the following:
Default_handler --> includes_filter -/-> byterange --> ...
Default_handler --> sub_request_core
What happens if the subrequest is SSI data? Well, that's easy, the
includes_filter is a resource filter, so it will be added to
the sub request in between the
Default_handler and the
The second case for sub-requests is when one sub-request is going to
become the real request. This happens whenever a sub-request is created
outside of a handler or filter, and NULL is passed as the next filter to
In this case, the resource filters no longer make sense for the new
request, because the resource has changed. So, instead of starting from
scratch, we simply point the front of the resource filters for the
sub-request to the front of the protocol filters for the old request.
This means that we won't lose any of the protocol filters, neither will
we try to send this data through a filter that shouldn't see it.
The problem is that we are using a doubly-linked list for our filter
stacks now. But, you should notice that it is possible for two lists to
intersect in this model. So, you do you handle the previous pointer?
This is a very difficult question to answer, because there is no "right"
answer, either method is equally valid. I looked at why we use the
previous pointer. The only reason for it is to allow for easier
addition of new servers. With that being said, the solution I chose was
to make the previous pointer always stay on the original request.
This causes some more complex logic, but it works for all cases. My
concern in having it move to the sub-request, is that for the more
common case (where a sub-request is used to add data to a response), the
main filter chain would be wrong. That didn't seem like a good idea to
The final topic. :-) Mod_Asis is a bit of a hack, but the
handler needs to remove all filters except for connection filters, and
send the data. If you are using
mod_asis, all other
bets are off.
The absolutely last point is that the reason this code was so hard to
get right, was because we had hacked so much to force it to work. I
wrote most of the hacks originally, so I am very much to blame.
However, now that the code is right, I have started to remove some
hacks. Most people should have seen that the
add_required_filters functions are gone. Those inserted
protocol level filters for error conditions, in fact, both functions did
the same thing, one after the other, it was really strange. Because we
don't lose protocol filters for error cases any more, those hacks went away.
Byterange filters are all added in the
insert_filters phase, because if they were added earlier, we
had some interesting interactions. Now, those could all be moved to be
inserted with the
CORE_IN filters. That would make the code easier to