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From Jan Lehnardt <...@apache.org>
Subject Re: CouchDB Next
Date Tue, 27 Sep 2016 17:25:19 GMT

> On 27 Sep 2016, at 18:31, Johannes Jörg Schmidt <schmidt@netzmerk.com> wrote:
> 
> Woah, what an impressive list!
> 
> For the validation part - why not somehow use JSON Schema[1]? I have used
> it in several projects and it plays nicely with CouchDB documents. It
> covers most common validation needs like requiring certain fields, enum
> support, pattern matching etc.

This is best for when we have this in JIRA, but just a quick note: My thinking
is to not have too man “languages” in CouchDB. I know I’m already bending this
rule with suggesting JSON Pointers, but they could be snuck in as URL-JSON-paths
without a huge learning curve. Hence my thought of using Mango for validation,
because it’s just one thing to learn with queries.

I’m happy to be convinced otherwise, too :)

Best
Jan
-- 

> 
> Best,
> Johannes
> 
> [1] http://json-schema.org/
> 
> Am 27.09.2016 2:57 nachm. schrieb "Jan Lehnardt" <jan@apache.org>:
> 
>> Hi all,
>> 
>> apologies in advance, this is going to be a long email.
>> 
>> 
>> I’ve been holding this back intentionally in order to be able to focus on
>> shipping 2.0, but now that that’s out, I feel we should talk about what’s
>> next.
>> 
>> This email is separated into areas of work that I think CouchDB could
>> improve on, some with very concrete plans, some with rather vague ideas.
>> I’ve been collecting these over the past year or <strike>two</strike>five,
>> so it’s fairly wide, but I’m sure I’m missing things that other people find
>> important, so please add to this list.
>> 
>> After the initial discussion here, I’ll move all of the individual issues
>> to JIRA, so we can go down our usual process.
>> 
>> This is basically my wish list, and I’d like this to become everyone’s
>> wish list, so please add what I’ve been missing. :) — Note, this isn’t a
>> free-for-all, only suggest things that you are prepared to see through
>> being shipped, from design, implementation to docs.
>> 
>> I don’t have a specific order for these in mind, although I have a rough
>> idea of what we should be doing first. Putting all of this on a roadmap is
>> going to be a fun future exercise for us, though :)
>> 
>> One last note: this doesn’t include anything on documentation or testing.
>> I fully expect to step our game from here on out. This list is for the
>> technical aspects of the project.
>> 
>> * * *
>> 
>> These are the areas of work I’ve roughly come up with that my suggestions
>> fit into:
>> 
>> - API
>> - Storage
>> - Query
>> - Replication
>> - Cluster
>> - Fauxton
>> - Releases
>> - Performance
>> - Internals
>> - Builds
>> - Features
>> 
>> (I’m not claiming these are any good, but it’s what I’ve got)
>> 
>> 
>> Let’s go.
>> 
>> 
>> * * *
>> 
>> # API
>> 
>> ## HTTP2
>> 
>> I think this is an obvious first next step. Our HTTP Layer needs work, our
>> existing HTTP server library is not getting HTTP2 support, it’s time to
>> attack this head-first. I’m imagining a Cowboy[1]-based HTTP layer that
>> calls into a unified internals layer and everything will be rose-golden.
>> HTTP2 support for Cowboy is still in progress. Maybe we can help them
>> along, or we focus on the internals refactor first and drop Cowboy in later
>> (not sure how feasible this approach is, but we’ll figure this out.
>> 
>> In my head, we focus on this and call the result 3.0 in 6-12 months. That
>> doesn’t mean we *only* do this, but this will be the focus (more on this
>> later).
>> 
>> There are a few fun considerations, mainly of the “avoid Python
>> 2/3-chasm”-type. Do we re-implement the 2.0 API with all its
>> idiosyncrasies, or do we take the opportunity to clean things up while we
>> are at it? If yes, how and how long do we support the then old API? Do we
>> manage this via different ports? If yes, how can this me made to work for
>> hosting services like Cloudant? Etc. etc.
>> 
>> [1] https://github.com/ninenines/cowboy
>> 
>> 
>> ## Sub-Document Operations
>> 
>> Currently a doc update needs the whole doc body sent to the server. There
>> are some obvious performance improvements possible. For the longest time, I
>> wanted to see if we can model sub-document operations via JSON Pointers[2].
>> These would roughly allow pointing to a JSON value via a URL.
>> 
>> For example in this doc:
>> 
>> {
>>  "_id": "123abc",
>>  "_rev": "zyx987",
>>  "contact": {
>>    "name": "",
>>    "address": {
>>      "street": "Long Street",
>>      "nr": 123
>>      "zip": "12345"
>>    }
>> }
>> 
>> An update to the zip code could look like this:
>> 
>> curl -X POST $SERVER/db/123abc/_jsonpointer/contact/address/zip?rev=zyx987
>> -d '54321'
>> 
>> GET/DELETE accordingly. We could shortcut the `_jsonpointer` to just `_`
>> if we like the short magic.
>> 
>> JSONPointer can deal with nested objects and lists and works fairly well
>> for this type of stuff, and it is rather simple to implement (even I could
>> do it: https://github.com/janl/erl-jsonpointer/blob/master/src/
>> jsonpointer.erl — This idea is literally 5 years old, it looks like, no
>> need to use my code if there is anything better).
>> 
>> This is just a raw idea, and I’m happy to solve this any other way, if
>> somebody has a good approach.
>> 
>> [2] https://tools.ietf.org/html/rfc6901
>> 
>> 
>> ## HTTP PATCH / JSON Diff
>> 
>> Another stab at a similar problem are HTTP PATCH with JSON Diff, but with
>> the inherent problems of JSON normalisation, I’m leaning towards the
>> JSONPointer variant as simpler, but I’d be open for this as well, if
>> someone comes up with a good approach.
>> 
>> 
>> ## GraphQL[3]
>> 
>> It’s rather new, but getting good traction[4]. This would be a nice
>> addition to our API. Somebody might already be hacking on this ;)
>> 
>> [3]: http://graphql.org
>> [4]: http://githubengineering.com/the-github-graphql-api/
>> 
>> 
>> ## Mango for Document Validation
>> 
>> The only place where we absolutely require writing JS is
>> validate_doc_update functions. Some security behaviour can only be enforced
>> there. With their inherent performance problems, I’d like to get doc
>> validations out of the path of the query server and would love to find a
>> way to validate document updates through Mango.
>> 
>> 
>> ## Redesign Security System
>> 
>> Our security system is slowly grown and not coherently designed. We should
>> start over. I have many ideas and opinions, but they are out of scope for
>> this. I think everybody here agrees that we can do better. This *very
>> likely* will *not* include per-document ACLs as per the often stated issues
>> with that approach in our data model.
>> 
>> * * *
>> 
>> 
>> # Replication
>> 
>> This is our flagship feature of course, and there are a few things we can
>> do better.
>> 
>> 
>> ## Mobile-optimised extension or new version of the protocol
>> 
>> The original protocol design didn’t take mobile devices into account and
>> through PouchDB et.al. we are now learning that there are number of
>> downsides to our protocol. We’ve helped a lot with introducing
>> _bulk_get/_revs, but that’s more a bandaid than a considered strategy ;)
>> 
>> That new version could also be HTTP2-only, to take advantage of the new
>> connection semantics there.
>> 
>> 
>> ## Easy way to skip deletes on sync
>> 
>> This one is self-explanatory, mobile clients usually don’t need to sync
>> deletes from a year ago first. Mango filters might already get us there,
>> maybe we can do better.
>> 
>> 
>> ## Sync a rolling subset
>> 
>> Say you always want to keep the last 90 days of email on a mobile device
>> with optionally back-loading older documents on user-request. It is
>> something I could see getting a lot of traction.
>> 
>> Today, this can be built on 1.x with clever use of _purge, but that’s
>> hardly a good experience. I don’t know if it can be done in a cluster.
>> 
>> 
>> ## Selective Sync
>> 
>> There might be other criteria than “last 90 days”, so the more general
>> solution to this problem class would be arbitrary (e.g. client-directed)
>> selective sync, but this might be really hard as opposed to just very hard
>> of the “last 90 days” one, so happy to punt on this first. But filters are
>> generally not the answer, especially with large data sets. Maybe proper
>> sync from views _changes is the answer.
>> 
>> 
>> ## A _db_updates powered _replicator DB
>> 
>> Running thousands+ of replications on a server is not really resource
>> friendly today, we should teach the replicator to only run replication on
>> active databases via _db_updates. Somebody might already be looking into
>> this one.
>> 
>> * * *
>> 
>> 
>> # Storage
>> 
>> 
>> ## Pluggable Storage Engines
>> 
>> Paul Davis already showed some work on allowing multiple different storage
>> backends. I’d like to see this land.
>> 
>> ## Different Storage Backends
>> 
>> These don’t all have to be supported by the main project, but I’d really
>> like to see some experimentation with different backends like
>> LevelDB[5]/RocksDB[6], InnoDB[7], SQLite[8] a native-erlang one that is
>> optimised for space usage and not performance (I don’t want to budge on
>> safety). Similarly, it’d be fun to see if there is a compression format
>> that we can use as a storage backend directly, so we get full-DB
>> compression as opposed to just per-doc compression.
>> 
>> [5]: http://leveldb.org
>> [6]: http://rocksdb.org
>> [7]: https://en.wikipedia.org/wiki/InnoDB
>> [8]: https://www.sqlite.org
>> 
>> * * *
>> 
>> 
>> # Query
>> 
>> ## Teach Mango JOINs and result sorting
>> 
>> It’s the natural path for query languages. We should make these happen.
>> Once we have the basics, we might even be able to find a way to compile
>> basic SQL into Mango, it’s going to be glorious :)
>> 
>> 
>> ## “No-JavaScript”-mode
>> 
>> I’ve hinted at this above, but I’d really like a way for users to use
>> CouchDB productively without having to write a line of JavaScript. My main
>> motivation is the poor performance characteristics of the Query Server
>> (hello CGI[9]?). But even with one that is improved, it will always faster
>> to do any, say filtering or validation operations in native Erlang. I don’t
>> know if we can expand Mango to cover all this, and I’m not really concerned
>> about the specifics, as long as we get there.
>> 
>> Of course, for pro-users, the JS-variant will still be around.
>> 
>> [9]: https://en.wikipedia.org/wiki/Common_Gateway_Interface
>> 
>> 
>> ## Query Server V2
>> 
>> We need to revamp the Query Server. It is hardcoded to an out-of-date
>> version of SpiderMonkey and we are stuck with C-bindings that barely anyone
>> dares to look at, let alone iterate on.
>> 
>> I believe the way forward is re-vamping the query server protocol to use
>> streaming IO instead of blocking batches like we do now, and use JS-native
>> implementation of the JS-side instead of C-bindings.
>> 
>> I’m partial to doing this straight in Node, because there is a ton of
>> support for things we need already, and I believe we’ve solved the
>> isolation issues required for secure MapReduce, but I’m happy to use any
>> other thing as well, if it helps.
>> 
>> Other benefits would be support for emerging JS features that devs will
>> want to use.
>> 
>> And we can have two modes: standalone QS like now, and embedded QS where,
>> say, V8 is compiled into the Erlang VM. Not everybody will want to run
>> this, but it’ll be neat for those who do.
>> 
>> 
>> * * *
>> 
>> 
>> # Cluster
>> 
>> ## Rebalancing
>> 
>> With this we will be able to grow clusters one by one instead of hitting a
>> wall when eventually each shard lives on a single machine. E.g. when you
>> add a node to the cluster, all other nodes share 1/Nth of their data with
>> the new node, and everything can keep going. Same for removing a node and
>> shrinking the cluster.
>> 
>> Couchbase has this and it is really nice.
>> 
>> 
>> ## Setup
>> 
>> Even without rebalancing, we need a nice Fauxton UI to manage the cluster,
>> so far we only have a simple setup procedure (which is great don’t get me
>> wrong), but users will want to do more elaborate cluster management and we
>> should make that easy with a slick UI.
>> 
>> 
>> ## Cluster-Aware Clients
>> 
>> This might end up being not a good idea, but I’d like some experimentation
>> here. Say you’d have a CouchDB client that could be hooked into the cluster
>> topology so it’d know which nodes to query for which data, then we can save
>> a proxy-hop, and build clients that have lower-latency access to CouchDB.
>> Again, this is something that Couchbase does and I think is worth exploring.
>> 
>> 
>> 
>> * * *
>> 
>> 
>> # Fauxton
>> 
>> Fauxton is great, but it could be better too, I think. I’m mostly
>> concerned about number of clicks/taps required for more specialised actions
>> (like setting the group_level of a reduce query, it’s like 15 or so). More
>> cluster info would also be nice, and maybe a specialised dashboard for
>> db-per-user setups.
>> 
>> 
>> * * *
>> 
>> 
>> # Releases
>> 
>> 
>> ## Six-Week Release Trains
>> 
>> We need to get back to frequent releases and I propose to go back to our
>> six-week-release train plans from three years ago. Whatever lands within a
>> release train time frame goes out. The nature of the change dictates the
>> version number increment as per semver, and we just ship a new version
>> every six weeks, even if it only includes a single bug fix. We should
>> automate most of this infrastructure, so actual releases are cheap. We are
>> reasonably close with this, but we need some more folks to step up on using
>> and maintaining our CI systems.
>> 
>> 
>> ## One major feature per major version
>> 
>> I also propose to keep the scope of future major versions small, so we
>> don’t have to wait another 3-5 years for 3.0. In particular, I think we
>> should focus on a single major feature per major version and get that
>> shipped within 6-12 months tops. If anything needs more time, it needs to
>> be broken up. Of course we continue to add features and fix things while
>> this happens, but as a project, there is *one* major feature we push. For
>> example, for 3.0 I see our push be behind HTTP2 support. There is a lot of
>> subsequent work required to make that happen, so it’ll be a worthwhile 3.0,
>> but we can ship it in 6-12 months (hopefully).
>> 
>> Best case scenario, we have CouchDB 4.0 coming out 12 months from now with
>> two new major features. That would be amazing.
>> 
>> 
>> * * *
>> 
>> 
>> # Performance
>> 
>> ## Perf Team
>> 
>> We need a team to comprehensive look at CouchDB performance. There is a
>> lot of low-hanging fruit like Robert Kowalski showed a while back, we
>> should get back into this. I’m mostly inspired by SQLite who’ve done a
>> release a while back that only focussed on 1-2% performance improvements,
>> but got like 20-30 of those and made the thing a lot faster across the
>> board. I can’t remember where I read about this, but I’ll update this once
>> I find the link.
>> 
>> 
>> ## Benchmark Suite
>> 
>> We need a benchmark suite that tests a variety of different work loads.
>> The goal here is to run different versions of CouchDB against the same
>> suite on the same hardware, to see where are going. I’m imagining a
>> http://arewefastyet.com style dashboard where we can track this, and even
>> run this on Pull Requests and not allow them if they significantly impact
>> performance.
>> 
>> 
>> ## Synthetic Load Suite
>> 
>> This one is for end users. I’d like to be able to say: My app produces
>> mostly 10-20kb-sized docs, but millions of those in a single database, or
>> across 1000s of databases, with these views etc. and then run this on
>> target hardware so I’d know, e.g. how many nodes I need for a cluster with
>> my estimated workload. I know this can only be done in approximation, but I
>> think this could make a big difference in CouchDB adoption and feed back
>> into Perf Team mentioned above.
>> 
>> * * *
>> 
>> 
>> # Internals
>> 
>> ## Consolidate Repositories
>> 
>> With 2.0 we started to experiment with radically small modules for our
>> components and I think we’ve come to the conclusion that some consolidation
>> is better for us going forward. Obvious candidates for separate repos are
>> docs, Fauxton etc. but also some of the Erlang modules that other projects
>> reasonably would use.
>> 
>> 
>> ## Elixir
>> 
>> I’d like it very much if we elevate Elixir as a prime target language for
>> writing CouchDB internals. I believe this would get us an influx of new
>> developers that we badly need to get all the things I’m listing here done.
>> Somebody might be looking into the technical aspects of this already, but
>> we need to decide as a project if we are okay with that.
>> 
>> 
>> ## GitHub Issues
>> 
>> I hope we can transition to GitHub Issues soon.
>> 
>> * * *
>> 
>> 
>> # Builds
>> 
>> I’d like automated builds for source, Docker et.al., rpm, deb, brew,
>> ports, Mac Binary, etc with proper release channels for people to subscribe
>> to, all powered by CI for nightly builds, so people can test in-development
>> versions easily.
>> 
>> I’d also like builds that include popular community plugins like Geo or
>> Fulltext Search.
>> 
>> 
>> 
>> * * *
>> 
>> 
>> # Features
>> 
>> ## Better Support for db-per-user
>> 
>> I don’t know what this will look like, but this is a pattern, and we need
>> to support it better.
>> 
>> One approach could be “virtual dbs” that are backed by a single database,
>> but that’s usually at odds with views, so we could make this an XOR and
>> disable views on these dbs. Since this usually powers client-heavy apps,
>> querying usually happens there anyway.
>> 
>> Another approach would be better / easier cross-db aggregation or
>> querying. There are a few approaches, but nothing really slick.
>> 
>> 
>> ## Schema Extraction
>> 
>> I have half an (old) patch that extracts top level fields from a document
>> and stores them with a hash in an “attachment” to the database header. So
>> we only end up storing doc values and the schema hash. First of all this
>> trades storage for CPU time (I haven’t measured anything yet), but more
>> interestingly, we could use that schema data to do smart things like
>> auto-generating a validation function / mango expression based on the data
>> that is already in the database. And other fun things like easier schema
>> migration operations that are native in CouchDB and thus a lot faster than
>> external ones. For the curious ones, I’ve got the idea from V8’s property
>> access optimisation strategy[10].
>> 
>> [10]: https://github.com/v8/v8/wiki/Design%20Elements#fast-property-access
>> 
>> * * *
>> 
>> Alright, that’s it for now. Can’t wait for your feedback!
>> 
>> Best
>> Jan
>> --
>> Professional Support for Apache CouchDB:
>> https://neighbourhood.ie/couchdb-support/
>> 
>> 

-- 
Professional Support for Apache CouchDB:
https://neighbourhood.ie/couchdb-support/


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