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From Kyle Dunn <kd...@pivotal.io>
Subject Re: HAWQ standby master sync process
Date Mon, 12 Sep 2016 21:32:05 GMT
Hey Ming -

Am I understanding correctly that a standby master will register
automagically to the active master, based on the contents of hawq-site.xml?

What would happen if two different standby masters on different nodes both
tried registering with the same active master? I ask because this is the
exact situation that would be useful for having a passive DR site with HAWQ
installed, querying for new WALs in the same flow as a local standby.

As for "daisy chaining" masters, which I believe is what you described in
(2) above: Master -> WAL -> Standby -> DR node, I think this may be less
desirable than multiple "normal" standby client nodes, as losing the
standby node becomes a cascading failure into DR.

Anytime we can make use of the DFS available (I say DFS, rather than HDFS,
as the hope is eventually this would be S3, Azure blob, Ceph, etc) - we
should!  (unrelated to DR) In my mind, this includes propagating the system
catalog to segment nodes via the underlying DFS, rather than transmitting
as part of each query.

Thank you for the helpful insight and discussion!


-Kyle

On Thu, Sep 8, 2016 at 10:55 PM Ming Li <mli@pivotal.io> wrote:

> Hi Kyle,
>
> As for your question how to config standby host, when standby nodes(which
> is config in hawq-site.xml) started, it will auto registered it's info in
> the system table gp_segment_configuration(
> there is system table:
>
> http://hdb.docs.pivotal.io/20/reference/catalog/gp_segment_configuration.html
> ),
> so that hawq can use this info internally in catalog.  if you need more
> details about it, @wen lin can help you.
>
> Then standby will report the LSN of WALs it synched to master node, master
> node according to this LSN to test the gap between master and node is still
> in xlog file or it is overwritten (because xlog file recycled). If the gap
> is not in the xlog file, we cannot do further just report "out of sync",
> which need to manually run hawq init standby to recreate standby node; else
> we just push the WAL after this LSN to standby node, and redo them. All
> related standby script problem can ask @radar for help.
>
> In most cases the standby should be less workload than master, so I
> suggestion maybe we can implement it as:
> (1) Master push WAL to standby node, when standby received them, it firstly
> write to file, then report successfully to master so that no blocking
> transaction commit.
> (2) standby node redo them on this node, and at the same time, it need to
> guarantee that the WAL should be transferred to the remote DR node, we can
> set different sync policy (whether need to guarantee WAL transferred to
> remote node when transaction committed ) in case of different transaction
> commit latency and different data loss acceptance at remote node.
>
> More to discussed:
> (1) If standby "report out of sync" and gap is not available on master
> node, we need to reinit standby manually, which need to shutdown master
> node. We need to think an stronger policy for this scenario, e.g. just push
> WAL to other nodes, and write as duplicate file? or we can further to write
> into hdfs directly?
> (2) If multiple master feature implemented, maybe the design need to be
> changed. I don't take time on it.
>
> Any comments or suggestions are welcomed. Thanks.
>
>
> On Fri, Sep 9, 2016 at 1:22 AM, Kyle Dunn <kdunn@pivotal.io> wrote:
>
> > Ming -
> >
> > Thank you for the info, this is very helpful in understanding how WAL
> > shipment happens.
> >
> > One question I have is: if/where the destination host is configured in
> > walsendserver.c? Alternatively, does a standby master client initiate the
> > request rather than the active master pushing out WALs as they become
> > available? I ask because for a more robust DR solution than what I'm
> > currently working on would allow multiple standby targets (i.e. one
> > traditional standby, one DR mirror, etc.)
> >
> > At the moment I've opted for an approach that stops the active HAWQ
> master,
> > creates a tarball of the entire MASTER_DATA_DIRECTORY, archives it on
> HDFS,
> > then invokes distcp via Apache Falcon to mirror /hawq_default in HDFS to
> > the DR site. After a DR event there would be some manual process to
> restore
> > said archive and update the hostname / DFS references to reflect the
> actual
> > DR environment.
> >
> > This approach is a step in the right direction but the act of creating
> the
> > tarball necessitates a brief HAWQ master outage (currently ~1 minute when
> > excluding pg_log contents and not compressing), whereas extending the
> > walserver code could avoid any outage by allowing WAL replication to have
> > multiple destinations.
> >
> > The top-level code for orchestrating this process is currently written in
> > Python 2.6 compatible code - I'd like to have some review of it by the
> DEV
> > team, if possible, as a first step to a future PR for "HAWQ DR" via
> Falcon.
> >
> > Thoughts?
> >
> >
> > -Kyle
> >
> > On Mon, Sep 5, 2016 at 9:41 AM Ming Li <mli@pivotal.io> wrote:
> >
> > > Hi,
> > >
> > > The general idea please refer to PostgreSQL:
> > >
> > > https://www.pgcon.org/2008/schedule/attachments/61_
> > Synchronous%20Log%20Shipping%20Replication.pdf
> > >
> > >
> > > Here just share some info about standby code.
> > >
> > > The standby related code is here:
> > > src/backend/postmaster/walredoserver.c
> > > src/backend/postmaster/walsendserver.c
> > >
> > > Global pic:
> > > - Backend generate WAL and pass it to the forked process "WAL Sender",
> > the
> > > calling stack is: XLogQDMirrorWrite() => WalSendServerClientSendRequest
> > ()
> > >
> > > - "WAL sender" process will be forked up and loop for processing
> request
> > > and response, the calling stack is:
> > > walsendserver_forkexec() -> walsendserver_start() -> ServiceMain() ->
> > > ServiceListenLoop() -> ServiceProcessRequest() ->
> > > serviceConfig->ServiceRequest()
> > > -> WalSendServer_ServiceRequest()
> > >
> > > - "WAL Sender" send WAL to "WAL Receiver" which is on the standby node,
> > the
> > > calling stack is:
> > > WalSendServer_ServiceRequest() => WalSendServerDoRequest() =>
> > > disconnectMirrorQD_SendClose() => write_qd_sync() => PQsendQuery()
> > >
> > > - On the standby side, all API are similar,  e.g.
> > walredoserver_forkexec()
> > > vs walsendserver_forkexec()
> > >
> > > Hope it helps you! ~_~
> > >
> > >
> > >
> > > On Thu, Aug 11, 2016 at 1:09 AM, Kyle Dunn <kdunn@pivotal.io> wrote:
> > >
> > > > Hello,
> > > >
> > > > I'm investigating DR options for HAWQ and was curious about the
> > existing
> > > > master catalog synchronization process. My question is mainly around
> > what
> > > > this process does at a high level and where I might look in the code
> > base
> > > > or management tools to see about extending it for additional standby
> > > > masters (e.g. one in a geographically distant data center and/or
> > > different
> > > > logical HAWQ cluster). The assumption is the HDFS blocks would be
> > > > replicated by something like distcp via Falcon.
> > > >
> > > > I believe there are obvious things to address like DFS / namenode URI
> > > > parameters, FQDNs, and certainly failure scenarios / edge cases, but
> > I'm
> > > > mainly trying to get a dialog started to see what input, ideas, and
> > > > considerations others have. One thing I'm specifically interested in
> is
> > > > whether / how WAL can be used (@Keaton).
> > > >
> > > >
> > > > Thanks,
> > > > Kyle
> > > > --
> > > > *Kyle Dunn | Data Engineering | Pivotal*
> > > > Direct: 303.905.3171 <3039053171> | Email: kdunn@pivotal.io
> > > >
> > >
> > --
> > *Kyle Dunn | Data Engineering | Pivotal*
> > Direct: 303.905.3171 <3039053171> | Email: kdunn@pivotal.io
> >
>
-- 
*Kyle Dunn | Data Engineering | Pivotal*
Direct: 303.905.3171 <3039053171> | Email: kdunn@pivotal.io

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