Stéphane Alnet created COUCHDB-1893:
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Summary: Allow replication filters to meaningfully apply to
deleted documents
Key: COUCHDB-1893
URL: https://issues.apache.org/jira/browse/COUCHDB-1893
Project: CouchDB
Issue Type: Improvement
Components: JavaScript View Server
Reporter: Stéphane Alnet
A document that is deleted using the DELETE command will be presented to a
replication filter as an empty record with only a `_deleted:true` field. A
replication filter can then only use the document ID to decide whether or not
to propagate the deletion; in most cases this is not sufficient, and one may
have to pass along deletion documents for IDs that would not have been
replicated by the filter.
This might lead to document IDs being leaked to the target database, which
might be undesirable; more importantly if the goal of filtering was to build a
smaller subset of the source database (for example to replicate a very large
database to a device that has smaller storage space), those deletion documents
might overfill the database (they never get compacted).
I had somewhat documented this issue on the Wiki
(http://wiki.apache.org/couchdb/Replication#Filtered_Replication) a while back
but never got to add it to JIRA.
Dave Cottlehuber on the PouchDB list suggested to use PUT with a
`_deleted:true` field to work around the problem (the PUT body can then contain
data sufficient to enable the filter to work). However we're still stuck in
case DELETE was used instead.
My suggestion is to expand the replication filter API to add an optional third
argument
filter(doc,req,old_doc)
where old_doc if present references the version of the document that will get
deleted. It is then up to the filter to use the _deleted flag in `doc` and the
values in `old_doc`.
(It might be useful/meaningful/easier to add old_doc in all cases; at this
point I'm only suggesting to add it in the case doc contains a _deleted field.)
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