Hello,
I would like to discuss making ServletConnection cache-aware. Daniel
suggested earlier to utilize standard HTTP protocol concepts. I totally
agree with his opinion and would like to propose solution, but first
let's discuss requirements.
Requirements
============
Requirements that ServletConnection must meet are really simple:
1. ServletConnection should provide data that can be used for
constructing small validation object.
2. ServletConnection should expose functionality for checking if
previous response is still valid taking as input validation object only.
3. We would like ServletConnection to make as few as possible round
trips in every situation it encounters.
To satisfy these requirements I propose to use concept of HTTP
conditional gets[1], more precisely If-Modified-Since request-header[2]
field. This way we have following cases:
* ServletConnection does not have information needed to create
If-Modified-Since header, but response includes Last-Modified header and
full content. Validity object can be created.
* ServletConnection does not have information needed to create
If-Modified-Since header and response does not include Last-Modified
header but includes full content. Validity object cannot be created.
* ServletConnection does have information needed to create
If-Modified-Since. Resource has not been modified so 302 status code is
returned as response and response does not include full content. Thus
ServletConnection can just tell that content is still valid and can be
fetched from cache.
* ServletConnection does have information needed to create
If-Modified-Since. Resource has been modified so 200 status code is
returned as response and response includes full content.
ServletConnection tells that cached content is invalid and returns fresh
content.
Requirements are satisfied:
1. Last-Modified header can be used to construct validation object.
2. Taking date from validation object enables ServletConnection to
formulate conditional GET and then response HTTP code settles if
resource is still valid.
3. In every case we have only one round trip.
Implementation proposal
=======================
We should start from making pipelines more HTTP-compliant. This demands
taking If-Modified-Since headers into account and returning appropriate
status code when caching pipeline is processed. Behavior of non-caching
pipelines should not change.
Then we should implement setIfModifiedSince and getIfModifiedSince from
java.net.URLConnection and construct requests according to value of that
property. Also getResponseCode method should be implemented.
All changes proposed above will enable us to implement source validation
of ServletSource very easily.
Comments? Thoughts?
I can start implementing this as soon as we came with agreement on this.
However, I would like to point out that I'll need some support to make
changes in pipeline stuff. I've taken a look on code and not everything
seems to be clear. Any volunteer on the board? ;-)
Last remark. I know that my English is quite poor and it could be that I
do not express my thoughts clearly enough. I'm really working on it and
you should not hesitate to ask when something is hard to understand.
[1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.3
[2] http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25
--
Grzegorz Kossakowski
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