Hi,
Quoting ellie timoney <el...@fastmail.com>:
- a single cyrus instance may be the primary server for some users
but a
replica server for other users
Are you sure about that?
I'm
not sure about any of this. But you make a good point: I thought I
could see a way that this was possible, but now I don't think I can.
I think I can, again, though I guess not in a murder.
But in a non-murder setup, I believe the only difference between a
primary vs replica is "which one the user interacts with"? Which, in a
non-murder setup, one is presumably managing in some way external to
cyrus (e.g. database, proxies, and some glue)...
So it seems like it should be plausible to have two (or more) cyrus
instances (hosted wherever), each running a sync_server plus a
channel/sync_client for each of the others, and whatever happens on any
replicates to the others. You'd be trusting your outer, non-cyrus layer
to make sure not to proxy any individual user to the wrong instance (at
least, not while there's pending replication transactions for them).
And I guess shared folders would be thorny. But I don't (yet) see a
reason from a replication standpoint why this wouldn't work.
One possible problem I am seeing is that after a split brain, or switch of
primary and replica for some users there might be some problems for renamed
folders, subsciptions and folder annotations. Cyrus is able to handle changes
on both sides in a mailbox (new mail, deleded mail, changed mail) because
of modseq, but this is missing for mailbox metadata.
So:
- a single cyrus instance may* be the primary server for some users but
a replica server for other users
[* with caveats and requiring custom tooling, and specifically not
in a murder]
On the other hand working with multiple instances on one host is working fine
(even in an murder setup).
See http://www.spinics.net/lists/info-cyrus/msg16035.html
ellie
On Fri, Jul 24, 2015, at 10:01 AM, ellie timoney wrote:
Okay, I'll bite. Here's what a bit of a sync_log looks like:
Thanks! So anything processing a sync_log (sync_client, squatter,
etc) needs to look at an actual copy of the user/mailbox in order to
determine its current state, and needs to look at both copies to work
out what to replicate between them.
- a single cyrus instance may be the primary server for some users
but a
replica server for other users
Are you sure about that?
I'm not sure about any of this. But you make a good point: I thought
I could see a way that this was possible, but now I don't think I can.
Cheers,
ellie
On Thu, Jul 23, 2015, at 11:42 PM, Nic Bernstein wrote:
On 07/23/2015 01:14 AM, ellie timoney wrote:
Is the file format of the sync log defined anywhere? I assume it
> correlates with a set of commands. (Not that this is important to
> a user: it may as well be opaque, but it made me wonder!)
I'm a bit confused about this myself. Each time I go digging
into the
code my understanding flips back the opposite way.
I think, either:
* the sync log contains all the information needed to reproduce what's
happened (e.g. if a message has arrived, the sync log will contain the
message itself); OR
* the sync log contains just enough to identify things that have changed
(e.g. if a message has arrived, the sync log contains a message id of
some sort), and the sync_client processing the log just uses the log
to discover which things to sync, but then uses the actual mailbox to
construct the changes to send to the replica.
Either way I haven't seen any documentation on the sync log format. I
suspect it's either the raw sync protocol or some subset thereof?
Okay, I'll bite. Here's what a bit of a sync_log looks like:
MAILBOX user.newjersey
MAILBOX user.support USER onlight USER nic USER admin USER randy MAILBOX
user.randy.Trash USER lynn MAILBOX user.lynn.Trash
It's basically a list of either users or mailboxes which have been
altered. When sync_log is enabled, all of the daemons which might
alter a mailbox or user will write a line to this log each time they
do so. That means the obvious suspects -- imapd, pop3d, timsieved,
lmtpd, etc. -- but also cyr_expire and friends (as in the
USER...MAILBOX couplets at the end of the sample).
- a single cyrus instance may be the primary server for some users
but a
replica server for other users
Are you sure about that? How does one specify the users for which
such an instance would play each role? A single HOST may run
separate instances, which may perform these different roles, but I
cannot fathom how to configure a single instance to do both at once
for different user cohorts.
This raises potential problems when one deploys replication within a
murder (Cyrus aggregation). Only one server may claim ownership of
any given mailbox, via a mupdate call, so an instance which is a
replica MAY NOT push updates to mupdate master, or mayhem will
ensue. Here's a commented section from /etc/cyrus.conf on a
replication master instance:
##
# Master sends mailbox updates to mupdate. Replication client runs on
# Master. Comment these 2 lines out on replicas
mupdatepush cmd="/usr/lib/cyrus/bin/ctl_mboxlist -m"
syncclient cmd="/usr/lib/cyrus/bin/sync_client -r"
A nice daydream of mine envisions a world wherein mailboxes.db keeps
track of replica locations, as well, which would allow for the dual-
role operation Ellie describes.
Cheers, -nic
--
Nic Bernstein n...@onlight.com Onlight llc.
www.onlight.com 219 N. Milwaukee St., Ste. 2A v.
414.272.4477 Milwaukee, Wisconsin 53202 f. 414.290.0335
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