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From Adam Kocoloski <kocol...@apache.org>
Subject Re: First Demo/Draft of _access / per document permissions
Date Thu, 16 Nov 2017 02:09:02 GMT
Oh also, meant to say - nice work on Spiegel :)

> On Nov 15, 2017, at 9:00 PM, Adam Kocoloski <kocolosk@apache.org> wrote:
> 
> Hiya Geoff,
> 
> You’re right, there is a non-trivial overhead in calculating view responses that need
to pull from every shard. On the other hand, maintaining a unique database file for every
user is quite problematic at scale. It works OK up to several thousand users, but eventually
you start running into a lot of operational headaches. Running a million databases in a cluster
is possible but painful.
> 
> I have some detailed thoughts about how we can improve the efficiency of queries scoped
to a single user in a large sharded database, but that’s a topic for another thread :)
> 
> Jan - wow, look at that! I’ll take a close look over the next couple of hours but a
quick scan is encouraging.
> 
> Adam
> 
>> On Nov 15, 2017, at 5:30 PM, Geoffrey Cox <redgeoff@gmail.com> wrote:
>> 
>> 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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