Mirror of Apache CouchDB

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  1. 0e9e15b last bit of notes by Jan Lehnardt · 7 years ago master
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Cassim is responsible for storing security information for clustered databases in a clustered database (cassim). This information is then heavily cached for reference at runtime.

Note: this is not yet used in CouchDB 2.x yet, but is planned to land in the future.



[22:12:49]  <+jan____>	chewbranca: davisp: what’s the state of cassim at Cloudant?
[22:13:12]  <chewbranca>	    heh... fabric:get_security is terrible and cassim would be a great replacement for it
[22:13:32]  <chewbranca>	    well, at least for the primary case of caching the data, you still need fabric:get_security behind cassim 
[22:15:13]  <chewbranca>	    cassim is not used right now because of the CouchDB security model, and Cloudant supports both. The Cloudant security model happens in chttpd so it's on the coordinator side and is perfectly fine for dealing with clustered databases. The CouchDB security model on the other hand operates on the shard level, which makes it really awkward to be used with cassim as cassim is powered by the clustered _metadata db, so you run into
[22:15:13]  <chewbranca>	    a chicken and egg situation with opening the shards 
[22:16:33]  <chewbranca>	    I've got it on my todo list to get the cassim conversation going again, because fabric:get_security really is bad and is a significant resource usage and is now called now once but *twice* per doc request, further negatively impacting performance 
[22:17:02]  <chewbranca>	    so that leaves the question with what to do about the CouchDB security model 
[22:17:43]  <chewbranca>	    we _could_ leave things as is and change cassim to be a node local clustered database like the nodes/dbs dbs, although I would prefer avoiding that 
[22:18:06]  <banjiewen>		    Why would you prefer to avoid that, chewbranca?
[22:18:09]  <chewbranca>	    the other option is we rip out the current security model and use a security model on the coordinator side 
[22:18:40]  <+Wohali>		    security on the coordinator side sounds like an advantage
[22:18:51]  <chewbranca>	    banjiewen: just seems clunky and conflict inducing, perhaps not though, it could be ok
[22:19:11] banjiewen		    nods
[22:19:23]  <banjiewen>		    Perhaps, but if it avoids a painful deprecation path maybe it's worth it
[22:19:36]  <chewbranca>	    Wohali: yeah, fwiw when you set the security properties on Cloudant, if you don't use the CouchDB style security then all security checks happen on the coordinator side
[22:19:55]  <chewbranca>	    banjiewen: eh we're just delaying the inevitable though, the security models of both Cloudant and CouchDB need to be rewritten 
[22:20:00]  <chewbranca>	    neither of them are sufficient 
[22:20:15]  <banjiewen>		    Sure, that's fair
[22:21:18]  <+Wohali>		    chewbranca: that rings a distant bell, though it's been a long time since i looked at dbcore/dbnext
[22:21:33]  <chewbranca>	    but yeah, RE Cassim, it turns out that fabric:get_security is a serious hog and calling it twice per doc request is brutal. We've seen on the order of a 30% throughput improvement by turning Cassim on 
[22:24:19]  <+Wohali>		    chewbranca: sounds like a great candidate for 2.2 ;)
[22:24:22]  <+Wohali>		    land that and PSE :D
[22:25:53]  <+Wohali>		    there was a good comment in #couchdb about improving the compaction daemon to preferentially pick shards that have the biggest potential gains to be had by compression
[22:26:40]  <+jan____>		    chewbranca: thanks, that’s lots of good context.
[22:26:58]  <chewbranca>	    Wohali: well either need to rewrite the security engine or rewrite the storage layer of cassim, either way there's a bit of work 
[22:27:46]  <chewbranca>	    jan____: np, I'm not sure why Cloudant security didn't land in the merge, probably just to keep things simple and not add in a second not quite redundant security model, but this has needed to be fixed for years now 
[22:28:18]  <+jan____>		    I haven’t put any thought into this, but would there be a way to model CouchDB security semantics on top of code that lives in chttpd?
[22:30:58]  <chewbranca>	    jan____: yeah that's exactly what the Cloudant security does, there's just a different implementation of https://github.com/apache/couchdb/blob/master/src/chttpd/src/chttpd_auth.erl#L44-L45
[22:32:04]  <+jan____>		    chewbranca: right, I mean, how hard would it be to lift couchdb security up into chttpd? 
[22:33:14]  <chewbranca>	    not hard at all, the plumbing is already there, the bigger issue is what to do about roles and what not, which _could_ be done orthogonally, but needs to be done at some point 
[22:36:31]  <chewbranca>	    huh... looks like it's even further along than I thought, it's basically just this but with more role checks and what not: https://github.com/apache/couchdb/blob/1deeac0adbc39546d7061ab2cb6847cb425f615b/src/chttpd/src/chttpd_auth_request.erl
[22:39:48]  <chewbranca>	    but yeah, the main difference is that on the shard level there's an extra security check here: https://github.com/apache/couchdb/blob/1deeac0adbc39546d7061ab2cb6847cb425f615b/src/couch/src/couch_db.erl#L94
[22:40:21]  <chewbranca>	    in the past Cloudant's auth model only has relevant values there for checking CouchDB style security, so Cloudant style security would skip that entirely 
[22:42:26]  <+jan____>		    chewbranca: so you’re saying by the time we’re hitting couch_db:open(), we should already know that the user doing this is allowed to do it?
[22:44:00]  <chewbranca>	    jan____: well kind of. What I'm saying is that Cloudant's security model does exactly that, and _only_ does the auth checks in chttpd, whereas the CouchDB security model does the additional check in couch_db:open, and it's that extra check in couch_db:open that causes the chicken and egg scenario of needing to load security properties on a clustered database while interacting with a local shard 
[22:44:12]  <chewbranca>	    for instance, think about what would happen if you do couch_db:open(cassim)
[22:44:31]  <chewbranca>	    you need to then open the cassim db to determine if you have access to the cassim db ;-)
[22:45:01]  <chewbranca>	    so cassim works in the clustered case, but is awkward for multiple reasons in the unclustered case 
[22:45:08]  <+jan____>		    chewbranca: so yeah, I think that check should not be in couch_db:open
[22:45:24]  <+jan____>		    like at all
[22:46:12]  <chewbranca>	    that's my general opinion on it, but I know rnewson (and maybe davisp) have had some reservations about removing that in the past, but given that's exactly what Cloudant does there's at the very least precedent for this being a reasonable approach
[22:46:34]  <+jan____>		    is_member code looks fairly portable to chttpd: https://github.com/apache/couchdb/blob/1deeac0adbc39546d7061ab2cb6847cb425f615b/src/couch/src/couch_db.erl#L428-L452
[22:47:40]  <chewbranca>	    jan____: fwiw there's already most of that logic in https://github.com/apache/couchdb/blob/master/src/chttpd/src/chttpd_auth_request.erl#L94-L126
[23:06:12]  <+rnewson>		    once chttpd is the only path, we should pull up the couch_db security checks
[23:06:18]  <+rnewson>		    should be in the fabric layer only imo
[23:07:53]  <chewbranca>	    rnewson: ok cool, glad to hear. I seemed to remember you expressing concerns about it in the past. I completely agree though and the awkwardness of not going through chttpd was concerning as well 
[23:08:34]  <chewbranca>	    the chttpd authentication goes through couch_httpd_auth, the chttpd authorization goes through chttpd_auth_request