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Knowledgebase:
Database available without a quorum?
23 January 2015 09:29 AM

Introduction

In the Scalability, Availabilty & Failover Guide, the node communication section describes a quorum as >50% of the nodes in a cluster. Is it possible for a database to be available for reads and writes, even if a quorum of nodes is not available in the cluster?

The answer is yes, there are configurations and sequences of events that can lead to forests remaining online when there are fewer than 50% of the hosts being online.

 

Details

If a single forest in a database is not available, the database is not be accessible.  It is also true that as long as all of a database's forests are available in the cluster, the database will be available for reads and writes regardless of any quorum issues. 

Of course, the Security database must also be available in the cluster for the cluster to function.

Forest Availability

In the simplest case, if you have a forest that is not configured with either local disk failover or shared disk failover and as long as the forest's host is online and exists in the cluster, the forest will be available regardless of any quorum issues. 

For forests configured for local disk failover, the sequence of events is important:

In response to a host failure that makes an "open" forest inaccessible, the forest will failover to the configured forest replica as long as a quorum exists and the configured replica forest was in the "sync replicating" state.  In this case, the configured replica forest will transition to the "open" state; the configured replica forest becomes the acting master forest and is available to the database for both reads and writes.

Additionally, an "open" forest will not go offline in response to another host being evicted from the cluster.

However, once a quorum is lost, forest failovers will no longer occur.

Conclusion

Depending on how your forests are distributed in the cluster and depending of the order of host failures, it is possible that a database can remain online even when there is no longer a quorum of hosts in the cluster.

Of course, databases with many forests spread across many hosts typically can't stay online if you lose quorum because some forest(s) will become unavailable.

Recommendation

Even though it is possible to have a functioning cluster with less than a quorum of hosts online, you should not architect your high availability solution to depend on it.

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