[ https://issues.apache.org/jira/browse/COUCHDB-583?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12783744#action_12783744 ] 

Filipe Manana commented on COUCHDB-583:
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Using a filter like mime type for deciding whether or not to store the attachments in the "gziped" form seems a good idea to me. mod_gzip has that kind of filter for deciding whether a response is gziped or not. An example of mod_gzip config:

mod_gzip_item_include         mime       ^text/html$
mod_gzip_item_include         mime       ^text/plain$
mod_gzip_item_include         mime       ^httpd/unix-directory$

I do like that approach, but since I am a newbie with Couch's code/implementation, I don't know if storing the attachments directly in gziped form will break anything else. couch_stream could compress attachment data when receiving it and write each compressed block to disk.

Then, when requesting an attachment download, the client would have to send the header "Accept-Encoding: gzip" in order to get the compressed data. If it doesn't send that header, we would send him the attachment in the uncompressed form.

I volunteer for implementing it if you agree. 

cheers


> adding ?compression=(gzip|deflate) optional parameter to the attachment download API
> ------------------------------------------------------------------------------------
>
>                 Key: COUCHDB-583
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-583
>             Project: CouchDB
>          Issue Type: New Feature
>          Components: HTTP Interface
>         Environment: CouchDB trunk revision 885240
>            Reporter: Filipe Manana
>         Attachments: jira-couchdb-583-1st-try-trunk.patch, jira-couchdb-583-2nd-try-trunk.patch
>
>   Original Estimate: 24h
>  Remaining Estimate: 24h
>
> The following new feature is added in the patch following this ticket creation.
> A new optional http query parameter "compression" is added to the attachments API.
> This parameter can have one of the values:  "gzip" or "deflate".
> When asking for an attachment (GET http request), if the query parameter "compression" is found, CouchDB will send the attachment compressed to the client (and sets the header Content-Encoding with gzip or deflate).
> Further, it adds a new config option "treshold_for_chunking_comp_responses" (httpd section) that specifies an attachment length threshold. If an attachment has a length >= than this threshold, the http response will be chunked (besides compressed).
> Note that using non chunked compressed  body responses requires storing all the compressed blocks in memory and then sending each one to the client. This is a necessary "evil", as we only know the length of the compressed body after compressing all the body, and we need to set the "Content-Length" header for non chunked responses. By sending chunked responses, we can send each compressed block immediately, without accumulating all of them in memory.
> Examples:
> $ curl http://localhost:5984/testdb/testdoc1/readme.txt?compression=gzip
> $ curl http://localhost:5984/testdb/testdoc1/readme.txt?compression=deflate
> $ curl http://localhost:5984/testdb/testdoc1/readme.txt   # attachment will not be compressed
> $ curl http://localhost:5984/testdb/testdoc1/readme.txt?compression=rar   # will give a 500 error code
> Etap test case included.
> Feedback would be very welcome.
> cheers

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