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From Paul Davis <paul.joseph.da...@gmail.com>
Subject Re: [VOTE] Merge BigCouch
Date Tue, 07 May 2013 21:02:09 GMT
+1

On Tue, May 7, 2013 at 3:52 PM, Russell Branca <chewbranca@gmail.com> wrote:
> +1
>
>
> Very excited to see this! Great work!
>
>
> -Russell
>
>
> On Tue, May 7, 2013 at 1:44 PM, Robert Newson <rnewson@apache.org> wrote:
>
>> FYI: A zip of this work is available at
>> http://people.apache.org/~rnewson/dist/nebraska-merge-candidate.zip
>> made by 'git archive -o nebraska-merge-candidate.zip
>> nebraska-merge-candidate'
>>
>> On 7 May 2013 21:34, Robert Newson <rnewson@apache.org> wrote:
>> > Hi All,
>> >
>> > I propose to merge in the following work,
>> > https://github.com/rnewson/couchdb/tree/nebraska-merge-candidate to
>> > the official Apache CouchDB repository to a new branch (i.e, *not*
>> > master). Once there, the full CouchDB developer community can begin
>> > the work to incorporate the code here into an official release.
>> >
>> > You do not need to respond if you are in agreement. If there is no
>> > response in 72 hours, I will assume lazy consensus. If we reach
>> > consensus, I will start the IP clearance process and then the merge.
>> >
>> > As most of you know, Paul Davis and I recently sequestered ourselves
>> > away from society (in a place called Nebraska) to make this merge
>> > happen. I want to clarify that this work is not the BigCouch code you
>> > can see on github.com/cloudant/bigcouch but the Cloudant platform from
>> > which BigCouch was made. This means it is bang up to date with all the
>> > bug fixes and feature enhancements we've made in the last eighteen
>> > months or more. With that clarification made, here are our notes about
>> > what we achieved, what it means to the project and what isn't yet
>> > done;
>> >
>> > Nebraska Merge Roundup
>> >
>> >
>> > Stats:
>> >
>> >
>> > 1402 - total new commits
>> >
>> > 312 - commits written during the merge (will be reduced substantially
>> > by squashing)
>> >
>> > 408 - number of files changed
>> >
>> > 21,897 - number of lines added
>> >
>> > 4,277 - number of lines removed
>> >
>> > A retrospective:
>> >
>> > Bob Newson and I have come to the end of our merge sprint on getting
>> > BigCouch merged into Apache CouchDB. Its been a productive ten days
>> > here in the midwest. I managed to get Bob out to a bowling alley and
>> > he managed to get me to a sushi restaurant. In between the cultural
>> > exchanges we’ve also managed to get a significant amount of work done
>> > on the merging as well.
>> >
>> >
>> > The current status of the merge is that we’ve managed to resolve the
>> > differences in the single node execution of CouchDB. Both the
>> > JavaScript and Erlang test suites run with only one failure in the
>> > Erlang test suite due to a (deliberately) missing constraint on the
>> > number of operating system processes. This should be a relatively
>> > straightforward fix but was not prioritized during our limited time to
>> > work on the larger issues.
>> >
>> >
>> > We merged a large number of performance and stability enhancements
>> > back into single node CouchDB as well as a number of pure bug fixes.
>> > The biggest highlight is a brand new compactor that is both faster and
>> > creates smaller and better organized post-compaction databases.
>> >
>> >
>> > The current status of the merge is that single node operations should
>> > be completely unaffected as demonstrated by the test suite passing. On
>> > the other hand we haven’t yet finished getting the clustered code
>> > merged to use some of the new changes in single node CouchDB. The
>> > single most significant portion of this work involves updates to the
>> > internal cluster API for views to use the recently rewritten indexer
>> > APIs. This should be a relatively straightforward bit of work that
>> > we’ll be finishing over the next few weeks.
>> >
>> >
>> > All in all the merge work done so far has been quite successful. We’ve
>> > met our primary goal of getting the code merged in a fashion that does
>> > not affect single node operation while providing a starting point for
>> > the larger community to start reviewing the more significant changes
>> > made. Given the size of the diff between the two code bases we never
>> > expected to have a fully working clustered solution after ten days of
>> > work but we have succeeded in providing a base of work that will allow
>> > us and new contributors to get up to speed quickly.
>> >
>> >
>> > This work, coupled with work by Dave Cottlehuber and Benoît Chesneau
>> > on updating the build system and various other internal updates, will
>> > provide a solid foundation for work going forward. Its an exciting
>> > time for CouchDB and anyone interested should keep an eye on the next
>> > few releases as we ramp up work on various core aspects of the
>> > database.
>> >
>> >
>> > We’ve had an exciting few days working to prepare the road for an
>> > exciting next twelve to eighteen months. We hope that everyone will
>> > feel as excited as we do about the next twelve to eighteen months for
>> > Apache CouchDB. It should be an exciting ride.
>> >
>> >
>> >
>> > Things we got done
>> >
>> >
>> > * Large update to the source tree layout for Erlang applications. Each
>> > application now has a src/appname/(c_src|ebin|priv|src) structure. The
>> > build system has been updated.
>> >
>> > * Renamed src/couchdb to src/couch to match the Erlang convention of
>> > the top directory name matching the Erlang application name.
>> >
>> > * Imported Cloudant Erlang applications for clustered CouchDB. These
>> > are imported with their history by using git subtree and merging the
>> > top level commit. These are not external deps, development will happen
>> > within the CouchDB tree. The imported apps are:
>> >
>> >
>> >    * config - A couch_config replacement (Behavior is mostly identical
>> > to couch_config except how we listen for configuration changes
>> > internally to allow for smooth hot code upgrade).
>> >
>> >    * twig - An rsyslog source replacement for couch_log.
>> >
>> >    * rexi - An RPC library. Replaces Erlang’s built-in rex application
>> > to avoid costly safety measures in the interest of performance and
>> > throughput.
>> >
>> >    * mem3 - The “Dynamo” part of BigCouch responsible for managing
>> cluster state
>> >
>> >    * fabric - The internal cluster-aware CouachDB API
>> >
>> >    * ets_lru - A small library application that provides an LRU
>> > implementation using a couple ets tables.
>> >
>> >    * ddoc_cache - Caches design documents on each node for use in
>> > design handler functions. This uses an ets_lru cache with a very short
>> > TTL.
>> >
>> >    * chttpd - The cluster aware HTTP layer
>> >
>> >
>> > Each imported app also had its build system updated to use Autotools
>> > along with the necessary updates noted above for the new application
>> > layouts for existing CouchDB erlang apps.
>> >
>> >
>> > * Merged a large amount of updates and fixes to couch_replicator based
>> > on work done internally at Cloudant. Unfortunately due to an error
>> > when we created our internal clone we lost a bit of history in some of
>> > the initial merge and have a big commit that affects
>> > couch_replicator_manager mostly. There are a number of other commits
>> > related to couch_replicator that resolve the single node vs. clustered
>> > differences. Some noticeable couch_replicator features:
>> >
>> >
>> >    * Optionally disable checkpoints so that replication can work when
>> > a source is read only. This should only be used for smaller databases
>> > as each replication call has to scan the entire source database on
>> > each invocation.
>> >
>> >    * A new changes_pending field in the _active_tasks output
>> >
>> >    * A fix to the continuous replication to automatically reconnect to
>> > a continuous changes feed when it sees a last_seq value. This allows
>> > for the source to selectively recycle the HTTP connections used which
>> > can be quite useful for “permanent” replications.
>> >
>> >    * A multitude of smaller bug fix and stability enhancements.
>> >
>> >
>> > Updates to single node couch:
>> >
>> >
>> >  * We changed the by_seq tree to store a copy of the #full_doc_info{}
>> > record instead of the #doc_info{} record. This gives significant speed
>> > improvements for compaction and replication and generally anything
>> > that needs to walk the by_seq tree and access document bodies
>> > internally.
>> >
>> >  * We rewrote the compactor to be significantly faster as well as
>> > provides significantly better compacted databases. The two main halves
>> > are to use a temp file and replace the use of btrees in the temp file.
>> > The temp file only contains a temporary copy of the document ids. At
>> > the end of a compaction run we then rebuild the by_id btree in the
>> > compaction file from this temp file. The reason this helps so much is
>> > that the compaction is based on the update_seq btree, which for most
>> > cases means that the id tree is updated in roughly random order which
>> > is very bad for our append only btrees. By using the tmp file we can
>> > stream it in order back into the compacted db file at the end of
>> > compacting, generating a minimum amount of garbage in the process. The
>> > other upgrade was to implement an external merge sort module
>> > (couch_emsort) that is used with this temporary file.
>> >
>> >  * Reject updates to design docs that introduce updates that break
>> > compilation for source code. Currently we only check map and reduce
>> > calls as the other should provide user visible errors instead of
>> > inexplicably empty views.
>> >
>> > because my OCD kicked in and I was unable to resist.
>> >
>> >  * Reverted a change made a long time ago that uses two file
>> > descriptors for each database. See the todo list.
>> >
>> >  * The reason to remove the second fd is so that we can rewrite ref
>> > counting. Better ref counting makes everyone happy, but the real
>> > reason is for this next bullet point:
>> >
>> >  * Optimize couch_server to not require a round trip message pass for
>> > opening a database that’s in the LRU. This is a significant
>> > performance boost for high concurrency access. We also optimized
>> > couch_server internals to not blow up when it’s under load.
>> >
>> >  * Introduce a #leaf{} record into the revision trees. This is never
>> > written to disk but makes internal code a lot cleaner when dealing
>> > with multiple versions of rev tree values.
>> >
>> >  * Some changes to couch_changes to enable clustered access. Also some
>> > general cleanup
>> >
>> >  * Internal changes to how CouchDB is booted in Erlang land. Not very
>> > sexy but this removes a lot of complicated un-Erlangy bits. We still
>> > have a bit of work left here.
>> >
>> >  * btree chunk sizes are now configurable which can allow people to
>> > adjust the RAM/speed tradeoffs a bit more.
>> >
>> >  * We now load update validation functions on the first write. This is
>> > a cluster-motivated change because the clustered version of this call
>> > is expensive and can lead to race conditions when opening a bunch of
>> > db shards simultaneously. This should be invisible to external
>> > clients.
>> >
>> >  * Disabled conflict detection for local docs. They don’t replicate so
>> > there’s no point. This just led to clusters getting stuck and confused
>> > when there were lots of replications happening.
>> >
>> >  * Changes to the multipart/mime parsing code. Necessary for clustered
>> > attachment uploads to split the incoming data  stream into N copies.
>> >
>> >  * Don’t use init:restart/0 when reloading the ICU driver. I think
>> > this has a bug. But we should rewrite this driver to be a NIF anyway.
>> >
>> >  * New couch OS process manager. Significantly faster access to OS
>> > processes under heavy load. This replaces the hard limit with a soft
>> > limit. Process spawned over the soft limit will be used until they’ve
>> > sat idle for a few minutes and then be closed. We have a todo item to
>> > add the hard ceiling back in (while keeping the soft ceiling).
>> >
>> >  * Automatically replace some easily identifiable JS reductions with
>> > their builtin counterparts. Uses a regex to do the detection so its
>> > not too smart.
>> >
>> >  * Improved view updater write batch.
>> >
>> >  * Updates to couchjs’ views.js to improve index update speeds
>> >
>> >  * Updates to the _stats bultin reduce to allow reduces to work over
>> > emitted stats objects. Sometimes clients have summary data in a doc,
>> > and this allows them to combine stats if they follow the same pattern
>> > as the builtin expects.
>> >
>> >  * Added a config:reload() that is accessible by POST’ing to
>> > _config/_reload. Used by the JS tests to reset the config to what's on
>> > disk. This should prevent those test run failures where a test fails
>> > leaving the config in a bad state causing all subsequent tests to
>> > fail. I think. Maybe.
>> >
>> >  * Databases are deleted synchronously in the test suite. We may need
>> > to address this on Windows. But it does seem to reduce the number of
>> > “{error, file_exists}” failures.
>> >
>> >  * I reimplemented the JS restartServer() function. There’s a new
>> > _restart/token URL that will given a unique value for each instance of
>> > the Erlang VM. To run a restart we grab the current token value, hit
>> > _restart, then wait till we get a successful response with a different
>> > token. This appears to have made the restart strategy more robust.
>> >
>> >
>> >
>> > Things that need doing
>> >
>> >
>> > IP Clearance -
>> >
>> >
>> > We’ll need to track down if we have the CCLA as well as look at each
>> > source file added to make sure each one is strictly from Cloudant or
>> > has an amenable license. I’m pretty sure that the only one of interest
>> > is trunc_io.erl but we need to be thorough.
>> >
>> > documentation -
>> >
>> >
>> > There shouldn’t be much here since the entire point of this merge was
>> > to not change the visible behavior of single node couch. A few things
>> > to add about the testing endpoints. Maybe an update to the compaction
>> > section mention the two new file names used.
>> >
>> >
>> > Copyright notices -
>> >
>> >
>> > We need to strip out copyright notices from individual files and make
>> > sure all files have a standard Apache License v2 header.
>> >
>> >
>> > clustered vhosts -
>> >
>> >
>> > We’ve never implemented this at Cloudant. We either need to write a
>> > cluster or go back and tell people to use HAProxy (or similar) for
>> > such things.
>> >
>> >
>> > twig -
>> >
>> >
>> > We need to add another output type to twig that is configurable in
>> > some manner. Right now we spit out entire rsyslog records which isn’t
>> > useful for most people. We’ll need to implement the file writer from
>> > couch_log as well as update the _log HTTP handler to know when it can
>> > and can’t expect to find data on disk.
>> >
>> >
>> > fabric -
>> >
>> >
>> > This is going to need a lot of work. Specifically view access is going
>> > to need to be updated to work with couch_mrview and friends.
>> >
>> >
>> > Boot a dev cluster -
>> >
>> >
>> > Once we fix up the clustering code we’ll need to write instructions
>> > and scripts for pulling up a dev cluster.
>> >
>> >
>> > OTP stuff -
>> >
>> >
>> > We’ve updated each app but we still need to pull some parts out of
>> > couchdb into their own application. Specifically the HTTP layer needs
>> > its own app. We could probably pull out the os process/query_servers
>> > as well as the os daemons and friends. Once done we need to update the
>> > supervision trees so we don’t have things like couch starting and
>> > managing the replication manager process.
>> >
>> >
>> > ddoc_cache -
>> >
>> >
>> > Wire this up in couch_httpd_db to actually be used. Right now its only
>> > used in chttpd.
>> >
>> >
>> > couch_file upgrade -
>> >
>> >
>> > The revert to remove the second updater_fd from each #db{} record
>> > means that we’re back in the original position of files appearing to
>> > slow down significantly under load. Since the initial hammer approach
>> > of just adding a second fd we’ve since discovered that the underlying
>> > bug is due to the way that message passing works combined with
>> > Erlang’s file io. Significantly though is the fact that the fix is
>> > rather simple to implement. A first draft of this work is on an old
>> > branch of mine here:
>> >
>> >
>> >    https://github.com/davisp/couchdb/commit/d856878
>> >
>> >
>> > finish the size calculating changes -
>> >
>> >
>> > The #leaf{} record change is to enable us to add more data size
>> > calculations. CouchDB master calculates a data size that account for
>> > all bytes that are active in a .couch file. Cloudant is interested in
>> > the total size of uncompressed docs and attachments minus the internal
>> > overhead of btrees. And there’s a fourth number to calculate based on
>> > the compression level used. Having each of these numbers will be
>> > useful as well as the calculations they’ll enable (ie, dead bytes in
>> > file, bytes used for overhead, compression ratio achieved, etc).
>> >
>> >
>> > couch_proc_manager -
>> >
>> >
>> > We need to implement the hard ceiling for capping the number of OS
>> > processes. We’ve started seeing a need for this at Cloudant with some
>> > work loads so motivation to fix this is high. The only failing etap is
>> > the assertion of this ceiling.
>> >
>> >
>> > Synchronous db delete on Windows -
>> >
>> >
>> > I did this because running the test suite was driving me bonkers. I
>> > need to ask Dave about how this behaves on Windows (my guess is not
>> > well) but I think we can close things up so that it works better than
>> > the status quo.
>>

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