Starting from OrientDB Enterprise Edition v2.2.4, you can define how your
servers are deployed in multiple Data Centers. All you need is using the
tag "dataCenters" in your default-distributed-config.json configuration
file. This is the format:
"dataCenters": {
"<data-center1-name>": {
"writeQuorum": "<data-center1-quorum>",
"servers": [ "<data-center1-server1>", "<data-center1-server2>",
"<data-center1-serverN>" ]
},
"<data-center2-name>": {
"writeQuorum": "<data-center2-quorum>",
"servers": [ "<data-center2-server1>", "<data-center2-server2>",
"<data-center2-serverN>" ]
},
},
NOTE: This feature is available only in the OrientDB Enterprise Edition. If
you are interested in a commercial license look at OrientDB Subscription
Packages <http://orientdb.com/support/>.
*Write Quorum*
The most common reason why defining data centers is a good idea is the
possibility of defining the consistency at data center level. A typical
scenario is having a synchronous replication between the servers in the
same data center where the coordinator server is located and then propagate
changes to the other data centers in asynchronous way. In this way you can
avoid the cost of the latency of the replication to the servers located on
different data centers. In order to activate this mode, set the
global"writeQuorum": "localDataCenter" and then specify a writeQuorum per
data center.
For example, if a write operation is executed by a server where its data
center's write quorum setting ismajority, then the used quorum will be
majority between only the servers located in the same data center.
Example about the configuration of 2 data centers, "rome" and "austin",
with rispectively 3 and 2 servers.
{
"autoDeploy": true,
"readQuorum": 1,
"writeQuorum": "localDataCenter",
"readYourWrites": true,
"dataCenters": {
"rome": {
"writeQuorum": "majority",
"servers": [ "europe-0", "europe-1", "europe-2" ]
},
"austin": {
"writeQuorum": "majority",
"servers": [ "usa-0", "usa-1" ]
}
},
"servers": { "*": "master" },
"clusters": {
"internal": {},
"*": { "servers": [ "<NEW_NODE>" ] }
}
}
If a write operation is executed on the server "europe-0", the quorum used
will be majority, but only between the servers locates in the same data
center: namely "europe-0" (the coordinator), "europe-1" and "europe-2".
Since the coordinator writes in the database before to distribute the
operation, the write operation succeed as soon as at least one between the
"europe-1" and "europe-2" servers provide the same result as with
"europe-0". The rest of the replication will be executed in background in
asycnhronous way.
*Consistency*
Since multiple data centers can have a local quorum, it is possible having
an Eventual Consistencybetween them. It's always suggested to keep the
number of servers odd, so you can, eventually, always be consistent.
Example of 2 data centers with equal number of servers:
"dataCenters": {
"rome": {
"writeQuorum": "all",
"servers": [ "europe-0", "europe-1", "europe-2" ]
},
"austin": {
"writeQuorum": "all",
"servers": [ "usa-0", "usa-1", "usa-2" ]
}
In this case if an UPDATE operation is executed by the server "usa-0" (the
coordinator), it will reach the quorum only if all the servers in the
"austin" data center provide the same result. Let's say the result for all
these 2 servers was 5 (in the UPDATE operation the result is the updated
version of the record). But what happens if all the other 3 servers in
"rome" return the version 6? You have no majority in this case (3 against
3), so the coordinator cannot establish who is the winner. The database
become inconsistent.
In order to automatically manage conflicts, the suggested configuration is
always to keep an odd number of servers if you have or not data centers.
*Conflict Resolution Policy*
In case of even number of servers, a Conflict Resolution Policy has to be
defined. This is not yet available, but it's in our roadmap.
Any question/comment/idea/improvement on this?
Best Regards,
Luca Garulli
Founder & CEO
OrientDB LTD <http://orientdb.com/>
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