Other replication solutions and Data Guard

There are many options to replicate an Oracle database data to a remote system. In the scope of disaster recovery, Oracle Data Guard and storage-based replication solutions such as EMC Symmetrix Remote Data Facility (SRDF), HP Continuous Access, Hitachi Universal Replicator and TrueCopy, IBM Global Mirror, and Metro Mirror are the main players in the market. When talking about Oracle database replication we also have to mention Oracle's well-known replication technologies GoldenGate and Streams. However, these products were not developed for disaster recovery fundamentally. Their primary aim is replication for ETL and data warehouse.

There are also some third-party tools capable of replicating Oracle database data, but here we'll mention about the most commonly-used technologies: Data Guard, storage-based replication solutions, GoldenGate, and Streams.

Storage-based replication solutions

Storage-base replication solutions technologies are based upon the storage-array based replication of data. Thus, the source of data does not matter. All kinds of application and database data can be replicated to a remote location, where Data Guard is only able to replicate Oracle databases.

In general there are two kinds of storage-based replication: synchronous and asynchronous replication. Synchronous replication means that each update to the source storage unit must also be updated in the target storage unit before another update can process. This guarantees zero data loss in the case of primary site failure. However, synchronous replication affects the I/O respond performance of the primary system depending on the distance between sites and network capacity. Therefore, this technology is distance limited. Synchronous replication technologies support up to 300 km distance between sites in the current technology level.

Asynchronous replication provides a long-distance replication solution with minimal impact on performance. In some products, the main problem with the asynchronous mode is the data consistency on the secondary site. The primary site sends a periodic, incremental copy of updates to the secondary site instead of a constant stream of updates. So there is no guarantee that dependent write operations on the primary site are transferred and applied to the remote destination in the same sequence.

Using storage-based replication solutions, it's not possible to start an Oracle instance and query database on the secondary site using the disks with the replicated data because of the data inconsistency issue. However Data Guard offers Active Data Guard, which enables users to query the standby database while replication is on the go. Some other advantages of Data Guard over storage-based replication solutions are enhanced corruption detection and prevention, automated database failover (fast-start failover), and RMAN backup offloading features that may not benefit from the use of storage-based replication solutions.

GoldenGate and Streams

GoldenGate is a data replication and integration tool for heterogeneous environments. It provides real-time capture, transformation, routing, and delivery of database transactions across heterogeneous systems (Oracle, DB2, MySQL, SQL Server, Sybase, Teradata, Netezza, and so on). Oracle agreed to acquire GoldenGate software in 2009 and then released 10.4, 11.1, and 11.2 versions with new enhancements. On the other hand, Streams is a built-in feature of the Oracle database that was first announced with database Version 9.2 and allows information sharing within an Oracle database or between Oracle databases.

Their common property is their capability of capturing, propagating, and applying data changes between Oracle databases.

On the other hand their main differences are:

• The heterogeneous platforms and data integration support of GoldenGate is different from that of Streams

• License conditions for Streams is included in the Oracle Enterprise Edition license and GoldenGate is a self-licensed product

Because of the GoldenGate's wider technology infrastructure and flexibility over Streams, Oracle announced that Oracle Streams will continue to be supported, but will not be actively enhanced and the best elements of Oracle Streams will be evaluated for inclusion with Oracle GoldenGate. It was also indicated that GoldenGate is the strategic product of Oracle on data distribution and data integration.

Tip

Oracle recommends Data Guard for full Oracle database protection with the high availability and disaster recovery purpose and recommends GoldenGate for information distribution and consolidation, application upgrades, changes, and also applications desiring flexible high availability needs.

An important feature of GoldenGate that makes the product different from its counterparts is the bidirectional replication capability, which is also called active-active replication. With this feature the primary and standby concepts are replaced by two active primary sites. Updates on site A are replicated to site B, and updates on site B are replicated to site A. The main challenges here are conflict handling and loop detection. A conflict is likely to occur in a bi-directional environment, where the same row or field is being updated at both sides and the changes are replicated. In this situation, a decision needs to be made if both transactions fail, or one transaction overwrites the other. The other key point is loop detection. If an update is replicated from site A to site B and then the same update from site B to site A, and so on, this loop needs to be detected and solved. The following diagram shows the general structure of an active-active GoldenGate configuration:

GoldenGate is a preferred solution to extract data from production databases in order to feed the data warehouse. It offers much flexibility to select specific data on the database and if needed transform the data before it hits the target.

The replication market's leaders, namely, Data Guard, storage-based replication products, and GoldenGate are compared in the following table with their most important features. Streams is out of this comparison because of the strategy mentioned by Oracle on its replication products:

	Data Guard	Storage-based replication	GoldenGate
Hardware independency	Supported. Possible to choose different server/storage vendors for primary and standby.	Not Supported. Must use the same storage vendor on both sides.	Supported. Possible to choose different server/storage vendors for primary and standby.
Software independency	Not supported. Only Oracle database replication.	Supported. All kinds of database and application data can be replicated.	Limited support. Different database products can be replicated.
Zero data loss capability	Supported with Maximum Protection mode.	Limited support with synchronous replication (distance limitation about 300 km).	Not supported.
Corruption detection and prevention	Supported.	Not supported.	Not supported.
Bidirectional replication within one database	Not supported.	Not supported.	Supported. Two active sites may send updates to each other.
Query standby data	Supported with Active Data Guard and Snapshot standby features.	Not supported.	Supported with continuously read/write accessible target databases.
Inside database selective replication	Limited support with logical standby databases.	Not supported.	Supported. Data may be selected and transformed before it hits the target.
Automatic database failover	Supported with fast-start failover feature.	Not supported.	Not supported.
GUI based management	Supported.	Supported.	Supported.
RMAN backup offload	Supported. The primary database RMAN backups can be offloaded to a physical standby and backups will physically be the same.	Not supported.	Supported. In a full replication of primary, RMAN backups may be offloaded but backups will only logically be the same, not physically.
Cascaded destinations for replication	Supported.	Supported.	Supported.
License	License required only for Active Data Guard. Otherwise no extra license required.	License required for storage replication software.	License required for GoldenGate software.

Note

The information on this table reflects the general characteristics of the storage-based replication products. All vendor products don't offer the exact same features; also the features for the same objective may have different capabilities and restrictions.

After reviewing the comparison table, it's obvious that Data Guard has better properties for high availability and disaster recovery purposed Oracle database replication. Storage-based replication products offer disaster recovery solution for the complete IT infrastructure data; however, when the case is Oracle databases, they cannot offer the Oracle integrated, flexible, and automatized features as in Data Guard. On the other side, we can see that GoldenGate was positioned especially for ETL and data integration requirements and it has great flexibility in this field. However, it also cannot reach Data Guard standards on data protection and disaster recovery.