With local-disk failover, replica forests will automatically take over
when hosts in the same cluster fail.
With database replication, there is no automatic failover when the
entire master cluster goes down. If you want automatic failover, you
have to build that up yourself since it involves more than just your
MarkLogic instance (i.e. a failed data center likely means non-MarkLogic
servers are also failing over).
The documentation discusses the issues you need to take into
consideration, but I'll provide a quick overview here.
Database replication is performed independently and asynchronously
between each master/replica forest pair (as opposed to synchronously for
local-disk failover that you are also using). As a result, when an
active master cluster fails you potentially have a situation where one
participant forest has replicated its portion of a committed transaction
but another participant forest has not.
MarkLogic server runs queries at a particular point in time (i.e commit
timestamp), and when running against a replica database it automatically
determines the most current timestamp a query can use based on how up to
date each forest is. Queries see a consistent replica database at a
slight lag behind the master database.
If you're just going to query the replica database when the master
cluster goes down, you don't need to do anything. The server will
automatically run queries at the most recent commit timestamp it can
until the master cluster becomes available again.
If you decide to do updates on the replica database, you run into the
issue of partially replicated transactions. You can use
xdmp:forest-status() to see how up to date each forest is, choose the
minimum commit timestamp across all forests, deconfigure replication on
the replica database, and do a rollback to that timestamp. This ensures
all forests in the database are consistent to that point in time, but
potentially drops a few transactions that had been committed on the
master but not yet fully replicated.
Later, when your failed master database comes back up, you configure it
as a replica of the database you promoted to master and just the
differences will be replicated back.
Wayne.
On 07/19/2012 09:54 AM, Danny Sinang wrote:
Hi Wayne,
If I am replicating the forests of US1 to US2 and EU2, what happens
when US1 goes down ?
Which server will take over and server the US1 forests ? US2 or EU2 ?
Regards,
Danny
On Wed, Jul 18, 2012 at 7:16 PM, Wayne Feick
<[email protected] <mailto:[email protected]>> wrote:
Yes, this should work fine.
You'll have a master US-Database in the US, and a master
UK-Database in the UK, with each replicating to the site.
Local-disk failover works fine at both ends for both master and
replica databases.
Extending to a third site should also be fine, just keep in mind
that a network brown out to either of the replica sites will
degrade your foreground performance in order to enforce the lag limit.
Wayne.
On 07/18/2012 12:40 PM, Danny Sinang wrote:
Hi,
We currently have a 3-node ML cluster here in the US (let's call
them US1, US2, US3), with forest replication and failover enabled.
Should we need to expand to Europe, would the setup below achieve :
1. Traffic Localization (during normal operations)
2. Continued ML availability (in the event we ever need to
bring one cluster down for hardware or software upgrades / fixes)
?
*Draft EU Expansion Plan*
1. Set up another 3-node ML cluster in Europe (EU1, EU2,
EU3), with forest replication and failover enabled.
2. Also replicate the forests of the US cluster to the EU
clusters and vice versa
3. Direct US customers (via some geo DNS) to US webservers
which use US1, US2, and US3
- This would save US customer data to the forests on
US1, US2, and US3
4. Direct EU customers to EU webservers which use EU1, EU2,
and EU3
- This would save EU customer data to the forests on
EU1, EU2, and EU3
5. Should the US ML Cluster ever go down, point the US
websevers to the EU ML Cluster
- I'm hoping this would activate and make available
the data from US1, US2, and US3 on EU1, EU2, and EU3. Am I
correct ?
- Same thing should happen the other way around
(i.e. if EU ML cluster goes down, point EU webservers to US
ML cluster)
Do you think this would work ?
Is there a better way to achieve our goals ?
How do we extend this model should the time come for us to expand
to Asia ?
Regards,
Danny
--
Wayne Feick
Principal Engineer
MarkLogic Corporation
[email protected] <mailto:[email protected]>
Phone:+1 650 655 2378 <tel:%2B1%20650%20655%202378>
www.marklogic.com <http://www.marklogic.com>
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Wayne Feick
Principal Engineer
MarkLogic Corporation
[email protected]
Phone: +1 650 655 2378
www.marklogic.com
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is intended solely for the use of the individual to whom it is addressed. Any
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notify us immediately by returning this message to the sender and delete all
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