How filters work in Apache 2.0

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Warning

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 + further research.

Filter Types
How are filters inserted?
Asis
Explanations

Filter Types

There are three basic filter types (each of these is actually broken + down into two categories, but that comes later).

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CONNECTION: Filters of this type are valid for the lifetime of this connection. + (AP_FTYPE_CONNECTION, AP_FTYPE_NETWORK)
PROTOCOL: 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. (AP_FTYPE_PROTOCOL, + AP_FTYPE_TRANSCODE)
RESOURCE: 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. (AP_FTYPE_RESOURCE, + AP_FTYPE_CONTENT_SET)

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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:

+ +

Byterange: 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 + protocol filter.
http_header: 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.
Deflate: 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.

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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 RESOURCE filters + only have one filter type, but that should change.

How are filters inserted?

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:

r->output_filters (corresponds to RESOURCE)
r->proto_output_filters (corresponds to PROTOCOL)
r->connection->output_filters (corresponds to CONNECTION)

+ +

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 + of 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.

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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 + might help:

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Default_handler --> includes_filter --> byterange --> ...

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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 + sub_request_core filter.

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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 + the make_sub_request function.

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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 + me.

Asis

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.

Explanations

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 reset_filters + and 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. + The HTTP_HEADER, Content-length, and + 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 HTTP_IN, CORE, and + CORE_IN filters. That would make the code easier to + follow.