RPO and RTO
In Disaster Recovery, there are two concepts named RTO (Recovery Time Objective) and RPO (Recovery Point Object) which are helpful in defining our response to a disaster and they give a measure of how quickly and how much can we recover. Both of the parameters are defined based on the business needs and criticality of the system.
RPO is the amount of data loss expressed in terms of time, which a business can afford to lose when there is a disaster. Thus, RPO also indirectly specifies the minimum backup frequency. For example, in the above diagram, the backup is taken frequently at time points T1, T2 and disaster strikes at T3. Obviously, the data generated by the system between T2 and T3 is lost and that difference in time period (T3-T2) is the RPO.
If a backup is taken at 8 PM every day and a disaster happens the following day at 6 Am, then the data between 8 PM and 6 AM is lost. So RPO = 10 hours. Higher RPO indicates lower data recovery and lower RPO indicates higher data recovery.
Note that RPO is not defined by the backup frequency of the system, but it is other way round. i.e., based on the RPO defined by the business needs, the backup frequency is determined.
RTO is the time period within which a system should recover and get back to normal running state, after a disaster has struck. For example, in the above diagram, if the system is expected to come back online at T4, then RTO is the down time period (T4-T3).
If RTO= 0, then it means, the system cannot afford to go down. The system should be up and running 24X7X365. Lower RTO indicates higher availability.