Solr -> Xapian ?

[Joan Moreau]

I managed to deal with the namespace issue (updated makefile.am)

However, I reach : 

../../../src/lib/compat.h:207:19: error: conflicting declaration of
'ssize_t i_my_pread(int, void*, size_t, __off_t)' with 'C' linkage
# define pread i_my_pread
^~~~~~~~~~
../../../src/lib/compat.h:210:9: note: previous declaration with 'C++'
linkage
ssize_t i_my_pread(int fd, void *buf, size_t count, off_t offset);
^~~~~~~~~~
../../../src/lib/compat.h:208:20: error: conflicting declaration of
'ssize_t i_my_pwrite(int, const void*, size_t, __off_t)' with 'C'
linkage
# define pwrite i_my_pwrite 

Any help welcome 

Hi, 

I figured out the "namespace" issue 

Remaining questions are : 

1 - WHat does represent "subargs" in mail_search_args 

2 - for rescan : who is responsible for passing again the new email ? Is
the Dovecot core sending again all the emails to index ? or the fts
shall somehow access the mailbox and read all emails ? Wouldn't just be
saying "delete all index and get_last_uid is now 0" the easy way ? or
the fts must process all emails (and block the current thread as a
mailbx maybe quite large) 

3 - for get_last_uid : this uncertainity is very unclear. "If there is a
gap, then indexer first indexes all the missing" -> this mean at a
certain point, indexer maybe rebuilding a previous email, so *last* uid
is something different than max. And how indexer does know whther there
is a gap wihtout callong the fts backend (whch it does not as there are
no function for that) ?

4 - How to update configure.ac & additional files to add the
"--with-xapian" wichi will test for libxapian presence and add it to the
build ? 

Thank you 

On 2019-01-08 04:24, Timo Sirainen wrote:

On 7 Jan 2019, at 16.05, Joan Moreau via dovecot <dovecot at dovecot.org>
wrote:
Hi

ANyone to answer specifically ?

Q1 : get_last_uid -> Is this the last UID indexed (which may be not the
greatest value), or the gratest value (which may not be the latest) (the
code of existing plugins is unclear about this, Solr looks for the
greatest for insance)
All the mails are always supposed to be indexed from the beginning to
the last indexed mail. If there's a gap, indexer first indexes all the
missing mails. So the latest UID is supposed to be the greatest UID.
(Supporting out-of-order indexing would be rather difficult to keep
track of.)

Q2 : WHen Indexing an email, the data is not passed by "build_key". Why
so ? What is the link with "build_more" ?
The idea is that it calls something like:

 - build_key(type=hdr, hdr_name=From)
 - build_more("tss at iki.fi")
 - build_key(type=hdr, hdr_name=Subject)
 - build_more("Re: Solr -> Xapian ?")
 - build_key(type=body_part)
 - build_more("message body piece")
 - build_more("message body piece2")
 ...

Q3 : Searching/Lookup : THe fheader in which to llok for (must be a
least among "cc, to, from, subject, body") is not appearing in the
'struct' data. WHere to find it ?
lookup() gets struct mail_search_arg *args, which contains the entire
IMAP SEARCH query. This could be used for more or less complex query
builders.

In case of a single header search, you should have
args->args->hdr_field_name contain the header name and
args->args->value.str contain the content you're searching for.

Q4 : Refresh : this is very unclear. How come there would not be the
"latest" view on index. What is the real meaning of this function ?
In case of Xapian it might not matter if it automatically refreshes its
indexes between each query. But with some other indexes this could
happen:

 - IMAP session is opened
 - IMAP SEARCH is run, which opens and searches the index
 - a new mail is delivered to the mailbox and indexed
 - IMAP SEARCH is run. Without refresh() it doesn't see the newly
indexed mail and doesn't include it in the search results.

Q5 : Rescan : is it just a bout remonving all indexes for a specific
mailbox ?
It's run when "doveadm fts rescan" is run manually. Usually that's only
run manually to fix up some brokenness. So it's intended to verify that
the current mailbox contents match the FTS indexes:
 - If there are any mails in FTS index that no longer exist in the
actual mailbox, delete those mails from FTS
 - If FTS is missing any mails in the middle of the mailbox, make sure
that the next mailbox indexing will index those missing mails. I think
currently this basically means reindexing all the mails since the first
missing mail, even the mails that are already in the index.

fts-lucene implements this, but other FTS backends are lazy and simply
rebuild all mails. Actually fts-solr is bad because it doesn't even
delete the extra mails.

Q6 : lokkup_multi : isn't the function the same for all plugnins (see
below) ?and finally , for fts_backend_xxxx_lookup_multi, why is that
backend dependent ?
This function is called only when searching in virtual folders. So for
example the virtual "All mails" folder, which would contain all mails in
all folders. In that case the boxes[] would contain a list of user's all
folders, except Trash and Spam. If lookup_multi() isn't implemented
(left to NULL), the search is run separately via lookup() for each
folder. With lookup_multi() there can be just one lookup, and the
backend can filter only the wanted folders and return them directly. So
it's an optimization for FTS indexes that support user-global searches
rather than only per-folder searches.

static int fts_backend_xapian_lookup_multi(struct fts_backend *_backend,
struct mailbox *const boxes[], struct mail_search_arg *args, enum
fts_lookup_flags flags, struct fts_multi_result *result)
{
struct xapian_fts_backend_update_context *ctx =
(struct xapian_fts_backend_update_context *)_ctx;

int i=0;

while(boxes[i]!=NULL)
{
if(fts_backend_xapian_lookup(backend,box[i],args,flags,result->box_results[i])<0)
return -1;
i++;
}
return 0;
}
See fts_backend_lookup_multi() - if you leave lookup_multi=NULL it
basically does this.

For "rescan " and "optimize", wouldn't it be the dovecot core who
indicate which are to be dismissed (expunged), or re-ask for indexing a
particular (or all) uid ? WHy would the backend be aware of the
transactions on the mailbox ???
rescan() is about fixing up a more or less broken index, or simply to
verify that it's all ok. So core doesn't know what messages exist in the
FTS index and can't request specific reindexing or expunging. I guess an
alternative API could have been to have functions that iterate through
all mails in the index, and use that to implement rescan in core. Now
thinking about it, that sounds like a simpler and better way.

optimize() is currently done only when explicitly running "doveadm fts
optimize", which requests running a slower index optimization. Depends
on the FTS backend whether this is useful or not.

There is alredy "fts_backend_xxx_update_expunge", so I beleive the
management of the expunged messages is *NOT* in the backend, right ?
Normally when mails are expunged, update_expunge() is called to notify
FTS backend that it should delete the mail also from FTS index.

.flags = FTS_BACKEND_FLAG_NORMALIZE_INPUT,*-> what other flags ?*
You probably want to use FTS_BACKEND_FLAG_FUZZY_SEARCH only like Solr.
See enum fts_backend_flags in fts-api-private.h
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