Hi Jan, I took a closer read and I do think you’re on the right path. I certainly agree with reusing the secondary index machinery to create the extra internal indexes.
On the by-access-seq index … did we ever discuss how to efficiently track and report the last observed sequences from the various ranges of the index to which a user has access? I suppose the single seq from each contributing shard could change to an array of seqs, one from each range. I do worry about the size of the merged sequence (I’m remembering the 2^n-1 possible role combinations granting access for a user possessing n roles). I didn’t see anything in the summit notes. Adam > On Nov 15, 2017, at 4:35 PM, Jan Lehnardt <j...@apache.org> wrote: > > Hi all, > > in the midst of handling the security stuff I had a moment of clarity how the > often requested per document permissions could be implemented. We had > discussed a potential approach extensively in the February Boston Developer > Summit (notes here: > https://lists.apache.org/thread.html/09a5686bca8049010b82796cc0fe99ef27aed4983a3f02fd6956259f@%3Cdev.couchdb.apache.org%3E) > > What was so alluring about this proposal was that it solves per doc access > control and per-user-db in one go. E.g. it would be able to share a single > database with multiple distrusting users, allow them to have their own set of > views, and even independently use their share of a single database as a > replication endpoint without interfering with any of the other users on that > database. > > I gave it a shot. Essentially, we need to build new indexes: by-access-id and > by-access-seq to make all that work. I’m just focussing on the core of this, > trying to re-use the existing couch_mrview/couch_index machinery as much as > possible. Strictly, for replication only by-access-seq would be required, but > by-update-id is a little easier to do, so I’ve done that first, and I believe > the results are encouraging. > > I’ve put a diff against master into a gist for your perusal: > > https://gist.github.com/janl/20b218a3f0eafbf963ee28780261f9fc > > > The core bits are: > > > https://gist.github.com/janl/20b218a3f0eafbf963ee28780261f9fc#file-by-access-id-diff-L189-L215 > > and > > > https://gist.github.com/janl/20b218a3f0eafbf963ee28780261f9fc#file-by-access-id-diff-L189-L215 > > Here’s an example Doc: > > { > "_id":"1fb94bf8c3d5a73745f3cc4f5f000a8d”, > "_rev":"4-bcbc975e61bdb80f3de1b87f6cad6a76”, > "_access":["b”] > } > > It shows up for user b: > > > curl b:b@127.0.0.1:15984/a/_all_docs > > {"total_rows”:2,"offset":0,"rows":[ > > {"id":"1fb94bf8c3d5a73745f3cc4f5f000a8d","key":["b","1fb94bf8c3d5a73745f3cc4f5f000a8d"],"value":"4-bcbc975e61bdb80f3de1b87f6cad6a76”} > ]} > > But not for user c: > > >> curl c:c@127.0.0.1:15984/a/_all_docs > > {"total_rows”:2,"offset":2,"rows":[ > > ]} > > > * * * > > > I’d like to get some general design feedback on this approach to find out if > it is worth pursuing further. See “Next Steps” way below for my thinking on > how to get by-access-seq going. > > The rest of this email are my notes from reading the source and trying to > explain my thinking as well as guide folks that might not be very familiar > with the CouchDB sources to follow along what is happening. > > I’d especially like to get some feedback about this from some of the folks > here who don’t spend their days in the main Erlang codebase :) > > Let me know what you think. > > Thanks! > Jan > > * * * > > CouchDB Access Notes > > Background: > https://lists.apache.org/thread.html/09a5686bca8049010b82796cc0fe99ef27aed4983a3f02fd6956259f@%3Cdev.couchdb.apache.org%3E > > # Overview > > To solve the problems with the db-per-user pattern, we want to introduce > document level access control. The result should be a single CouchDB database > that can be used by multiple mutually untrusting users while retaining > CouchDB’s full semantics. > > // TODO: link to appendix: problems with db-per-user > > We decided on an approach to define access control in documents with a new > property `_access` which is specified as an array of strings and arrays. > Strings represent usernames and roles, sub-arrays are used as logical AND, > elements in the top level array are used as logical OR. For example. an > _access field with the value [[‘management’, ‘senior’], ‘ceo-jane’] would > allow everyone with the roles ‘management’ AND ‘senior’, OR the user > ‘ceo-jane’ access to that doc. but not e.g. users with roles ‘development’, > ‘senior’, nor user ‘vp-jenn’. > > To achieve main CouchDB semantics, we need to introduce new behaviour for the > _all_docs and _changes endpoints. The plan is to special case-this based on > the authenticated user context (userCtx, e.g, username and associated roles, > after authentication). > > The existing by-id and by-seq indexes are not equipped to efficiently return > results per user, so we are introducing two new indexes (either can be > optionally configured, depending on the use-case and performance and storage > needs): by-access-id and by-access-seq. In contrast with by-id and by-seq, > these indexes are not stored in the main database file, but in a separate > file, ideally managed by the existing couch_index infrastructure. > > > # Development considerations > > This first spike is only concerned with getting per-access-id to work with > minimal effort. > > To get started, let’s look at how _all_docs works today using the by-id index. > > ## The Anatomy of a Clustered _all_docs Request > > CouchDB’s clustering layer consists of three main modules: chttpd, fabric and > refi. chttpd’s job is to handle everything HTTP and route requests to the > right place in the rest of the code. It’s a HTTP router, mapping URLs, > request methods and options to handler functions that do with the work the > requests are specified to fulfil. > > fabric’s job is to distribute a single request from the outside to multiple > nodes of the cluster. Some requests require only talking to the local node, > but that’s less important for the moment. fabric includes fabric_rpc, a > module that turns a request to the cluster into one or more requests to other > nodes in the cluster. > > rexi’s job is know about the cluster state: which nodes are in the cluster, > which of them are active/reachable/failed, which shards live on which nodes. > fabric uses rexi to know which nodes to contact for which shards. > > After a bit of indirection, we find ourselves at the first _all_docs-specific > function in chttpd_db.erl: all_docs_view/4: > > ``` > all_docs_view(Req, Db, Keys, OP) -> > Args0 = couch_mrview_http:parse_params(Req, Keys), > Args1 = Args0#mrargs{view_type=map}, > Args2 = couch_mrview_util:validate_args(Args1), > Args3 = set_namespace(OP, Args2), > Options = [{user_ctx, Req#httpd.user_ctx}], > Max = chttpd:chunked_response_buffer_size(), > VAcc = #vacc{db=Db, req=Req, threshold=Max}, > {ok, Resp} = fabric:all_docs(Db, Options, fun couch_mrview_http:view_cb/2, > VAcc, Args3), > {ok, Resp#vacc.resp}. > ``` > > The first five lines handle query options and request parameters or > arguments. The next three lines are the bulk of the job: start a response, > call fabric:all_docs/5 with a callback to handle rows. The last line returns > the accumulator that is returned by fabric:all_docs/5. > > fabric:all_docs/5 is a thin wrapper around fabric_view_all_docs:go/5. Before > we jump down, we notice that there is also a fabric_view_changes.erl, which > we should remember for the next iteration when we implement by-access-seq. > > go/5 comes in two variants and we’ll ignore the second here for the moment, > because it is a performance optimisation. The main work for go/5 is in the > top third of the function. First it gets all shards for the current database > from mem3, then it starts a fabric_rpc worker for each shard, and then waits > for the results to come back by calling go/6 with all workers. The bottom two > thirds are timeout and error handling. > > go/6 registers the handle_message/3 function as the callback for rexi_utils’ > recv/6 (read “receive”) function. > > handle_message/3 comes in a number of variants to handle rexi errors, > receiving metadata, receiving result rows and a notification “complete” about > all rows having been sent. > > Our next level down is looking into fabric_rpc and how it handles all_docs > requests. fabric_rpc/3 is again a short wrapper, this time around > couch_mrview:query_all_docs/4 which is the node-local function that handles > querying. > > couch_mrview includes a bunch of functions map/reduce views. It seems like a > natural place doing our distinction between a normal by-id request and a > by-access-id request. > > I’m skipping a step here, but with a little printf-debugging, I’ve found out > that the `Db` variable we get passed in, includes the authenticated userCtx > including username and any roles. We can use couch_db:is_admin/1 to get a > boolean back for the distinction we are going to have to make: > > ``` > query_all_docs(Db, Args0, Callback, Acc) -> > case couch_db:is_admin(Db) of > true -> query_all_docs_admin(Db, Args0, Callback, Acc); > false -> query_all_docs_access(Db, Args0, Callback, Acc) > end. > ``` > > query_all_docs_admin/4 is the existing query_all_docs/4 function and we’re > introducing query_all_docs_access/4, that we now have to fill out with > querying our view. > > Before we can do that, we need to understand how view work. > > > ## The Anatomy of a View Request > > Querying a view has three stages: > > 1. define the view > 2. build the view index > 3. query the view index > > A view definition is always in a design document. It can be one or JavaScript > map/reduce functions, Erlang map/reduce functions, or a mango index > definition. > > // TODO: link all these view definition options. > > Building the view index is an implicit step in CouchDB. View indexes are > refreshed at query time, but only if there were any changes in the database > since the last query. If no refresh is needed, the view result is returned > from the index directly. > > // TODO: explain query_server > > Querying indexes follows a similar path through chttpd, fabric, rexi, > fabric_rpc down to the per-node handlers in couch_mrview. Just a few lines > below couch_mrview:query_all_docs/4 we find query_view/5 which decides > between map and reduce requests. We care about map-only for now. query_view/5 > is preceded by query_view/6 which includes a call to > couch_mrview_util:get_view/4 which looks like it is where we want to look > next, as the map_fold/5 called by query_view/5 is about looping over rows. We > hope we can re-use all that logic, and maybe get_view/4 lets us find out how > we can have it return our new view. > > get_view/4 calls get_view_index_state/4 which in turn calls > get_view_index_pid/4 that finally calls into couch_index_server:get_index/4 > which looks like it returns the index for our request. Let’s have a look. > > get_index/4 will dive into get_index/2 eventually and that looks indeed like > where we need to look. In there, we look up view index in an ETS table (an > in-memory database), and if it can’t find it there, start a new one. Either > way, a view index is returned. The lookup is by DbName and Sig(nature), an > md5 hash over the `views` property in a design doc, that also corresponds to > the *.view filename of the view index. > > > ## Faking the index > > So how would we get this to return the index we want to query? We need to > create an index definition that matches the design doc `views` hash. Hm. > > It is relatively easy to produce a map function that behaves like we want: > > function (doc) { > var _access = doc.access > if (!_access) { return } > if (!isArray(_access) || _access,length === 0) { return } > _access.forEach( function (user_or_role) { > emit([user_or_role, doc._id], doc._rev) > }) > } > > At query time, we’d have to match the requesting username and roles against > the first element in the key-array and return the results, while replacing > the key-array with the second element (the doc _id). All this doesn’t sound > too hard. Good. > > One snag though: if we think ahead and try to see how we could implement > by-access-changes we get stuck: a view does not include rows for deleted > documents while _changes does. In addition, the update sequence for a > document is not available in a map function. So a regular view can not be > used here. > > The filtering of deleted docs from a view index happens in > couch_mrview:map_fold/3. So if we could augment that for our internal view > requests, that could get us a long way towards reusing the rest of the > couch_mrview/couch_index machinery. > > Note to self: make sure view compaction doesn’t remove deleted docs. But a > cursory glance at couch_mrview_compactor:compact_view_btree/5 suggests no > such thing, but we need to validate this, and if it doesn’t hold, change > view_compation to keep deleted entries. > > * * * > > We’ll start giving this a try by forking things off in > couch_mrview:query_all_docs/4 and pretending to call a view with a mocked > ddoc: > > { > “_id”: “_design/_access”, > “language”: “_access” > “views”: {} // if needed > } // TODO see which other fields it needs > > We’ll try this road to see if we get to the point where we get a “view index > not found” error, because we didn’t actually have a view index yet. We’ll > then try and see if we can produce one. We could try the other way around > too, building the index first and then trying to query, but the approach > doesn’t make much of a difference. > > First demo working: > https://gist.github.com/janl/20b218a3f0eafbf963ee28780261f9fc > > > Next Steps: > - make sure the startkey/endkey/descending argument handling is all correct > and complete > - add key un-munging, so the user/role prefix gets filtered out on reads > - handle roles: > - instead of querying the _access view once, we need to issue a > multi-query, probably via #mrags.multi_get, read up on how that is used > - then we could start thinking about by-access-seq: > - we need access to the update-seq in couch_access_native_proc:map_doc, > might require view protocol upgrade, or we have a post-process function that > tags on the update-seq, we’ll see. > - the admin/access split we’re doing in query_all_docs should probably > happen in couch_db:changes_since/5 > > > > > > > # More specification details > > > Documents with in databases with _access enabled are private/admin-only by > default, and can be made public with the special role _public > > TODO: shared id space or auto-prefix ids > >
