Dual Data Center solutions

This article describes how to ensure business continuity using dual data centers.

To ensure database availability in case of a data center outage, you can implement two data centers in order to reduce downtime and protect your data. The data centers can, for example, be located in two different fire zones in the same building. This article describes different approaches for a dual data center solution.

ETL based solution

With an ETL based solution, each data center has an autonomous cluster with an autonomous database. Both databases can be used simultaneously, and business critical data is replicated regularly between them using an ETL approach. If one of the data centers has an outage, the connections are redirected to the database in the other data center.

• RTO (recovery time objective) is less than one hour.

• RPO (recovery point objective) depends on the ETL frequency – everything changed after the last ETL replication will be lost.

Backup based solutions

In backup based dual data center solutions, backups of the production database made in one data center are available for the second data center in case the first data center has an outage. A restore on the second site makes the production database available again.

Backup based solutions require that the cluster on the secondary database has the same number of nodes as the cluster on the production database.

• RTO is less than one hour.

• RPO depends on the backup frequency – everything changed after the last backup is lost.

A backup based solution can be configured in several ways:

2. Remote mirroring

The backups of the production database in DC 1 are mirrored by the backup storage layer to a remote archive volume in DC 2. The mirroring is transparent to the backup process on the production database. The backup storage in DC 2 cannot be used while the remote mirroring is in progress.

The mirroring solution must be provided by the backup storage vendor, it is not implemented in Exasol.

3. Stretched archive

The archive volume of the production database is stretched across both data centers (DC 1 and DC 2), while the data volume remains local to DC 1. The archive volume in DC 2 cannot be used while mirroring is in progress.

This solution is fully implemented in Exasol and does not require any external services.

Synchronous dual data center (SDDC)

A synchronous dual data center solution (SDDC) has one cluster stretched across two separate sites (data centers). Each data center has a database instance with the exact same number of nodes. Since the instances share the same data volume, only one of them can be running. The production database is normally running on the primary site, while data blocks in the master segments there are mirrored to redundancy segments on the secondary site.

This solution provides business continuity with minimum downtime in case of both node failure or a complete outage on the primary site. Since communication only happens on the storage layer during normal operation, there is no risk of performance or syncing issues due to network latency.

Fore more details about this solution, see Synchronous Dual Data Center (SDDC).

Comparison of DDC solutions

This table provides an overview of the advantages and drawbacks of different dual data center solutions and how they affect RTO (recovery point objective) and RPO (recovery time objective). To learn more about the RTO and RPO concepts, see Terminology and timeline.

Solution	RTO	RPO	Advantages	Drawbacks
ETL based	~1 h	significant	Second database can be used autonomously Sites can have different cluster configurations	The appropriate ETL mechanism must be developed and maintained
Test environment as standby	~1 h	significant	Second site can be used for testing	Additional backups must be taken to the second site
Remote mirroring	~1 h	significant	Backup methods used on production can be used as before	Storage vendor technology may lead to additional costs
Stretched archive	~1 h	significant	Backup methods used on production can be used as before Uses Exasol technology, no additional cost	Quality of network connection between sites can impact backup performance
Synchronous dual data center (SDDC)	~60s	none	Minimal downtime without data loss	Performance impact on the production database is proportional to the distance to the second site