> On 16. Nov 2017, at 03:09, Adam Kocoloski <kocol...@apache.org> wrote: > > Oh also, meant to say - nice work on Spiegel :)
+1, nice work Geoff, Adam summed it all up nicely. Best Jan -- > >> On Nov 15, 2017, at 9:00 PM, Adam Kocoloski <kocol...@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 <redge...@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 <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 >>>> >>>> >>>> >> > -- Professional Support for Apache CouchDB: https://neighbourhood.ie/couchdb-support/
