Continuous data protection (CDP) is a form of data replication that copies data from a source to a target. True CDP does this every time a change is made, while so-called near-CDP does this at pre-set time intervals. Near-CDP is effectively the same as snapshotting.
Because true CDP copies all delta changes, a system can be restored to any point in time required. This can be especially useful if you need to roll back to a point before a corruption event took place, for example.

Because they depend on fixed-interval copies, near-CDP/snapshots only allow you to roll back to a given point in time. For this reason, true CDP offers a recovery point objective (RPO) of zero, while the equivalent for near-CDP/snapshots is the last time a copy took place.

Continuous data protection can be implemented in two main forms: software based with agents running on designated servers, or as a hardware-based appliance that can be in-band (in the data path) or out-of-band (outside the data path).
How does CDP work?

Continuous data protection works by copying changed data from the source system to a target. Often, this will be disk at the same site as the source, and this provides a method of effecting very quick recovery of data.

However, it is possible to replicate off-site and this clearly provides much greater protection and potential disaster recovery provision. With some CDP products you can replicate to two locations — one on-site for rapid recovery and a disaster recovery site further afield.

True CDP systems record every write and copy them to the target where all changes are stored in a log. In case of a physical system failure, the continuous data protection system will have kept all changes up to the last write before failure, and you can restore to that point or to the last point before any corruption occurred.
True CDP systems will handle replication of files or applications with equal ease as they record every change.

By contrast, near-CDP/snapshot systems copy files in a straightforward manner but require applications to be quiesced and made ready for backup, either via the application’s backup mode or using, for example, Microsoft’s Volume Shadow Copy Services (VSS).