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From Geoffrey Cox <redge...@gmail.com>
Subject Re: First Demo/Draft of _access / per document permissions
Date Wed, 15 Nov 2017 22:30:37 GMT
Hey Jan,

I've been trying to solve a similar problem from a different angle using
efficient and scalable replication via spiegel
<https://github.com/redgeoff/spiegel>. I'm super excited that you are
drafting this level of access, but my major concern is on performance. From
what I gather, if you combine all the db-per-user docs into a single DB
then you'll have a massive DB. I know CouchDB is good at sharding, but
isn't there a significant performance implication when a user's docs are
being pulled from multiple shards on different servers? What about the
added overhead of calculating cross-server views, etc...

When I think about how big companies, e.g. Facebook, solve these types of
problems, I imagine that they create a denormalized DB per user. Among
other things, this design allows the set of data that a user needs to be
relatively small and live on less servers per user. Doesn't this lead to
better performance?

Even if this new level of access doesn't solve the db-per-user case
entirely, it will still be a useful addition as it would allow for more
data to be shared and less of a create a DB-per-role setup. So, I'm all for
it!

I'll take a closer look at these notes when I have some time, but I just
wanted to get you my high-level thoughts now. I'm sorry if any of this has
been based on some wild assumptions :)

Exciting stuff!

Geoff

On Wed, Nov 15, 2017 at 1:35 PM Jan Lehnardt <jan@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
>
>
>

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