One is referred to as failover and the other as switchover. Users should detect no service disruption when either occurs.

Failover

Neverfail Continuity Engine initiates failover when a hardware failure, power outage, or similar condition causes a catastrophic failure of the active server. With the active server unavailable, the failover process changes the passive server to the active role. The new active server immediately begins servicing clients. A switchover gracefully swaps the roles of the two servers. The active server becomes passive, and the passive server becomes active. Neverfail Continuity Engine can initiate an automatic switchover when a less critical issue occurs, such as networking, application, or performance problems.

Switchover 

You can also initiate a switchover manually when desired. Because switchovers are quick and seamless to end-users, you can perform certain types of maintenance on your Primary server without any outage to the end-user, for example, hardware upgrades. All you need to do is switchover to the Secondary server, perform your maintenance on the Primary, and then switchback, outage free.

Continuous Availability
Due to Neverfail Continuity Engine's unique, cloned server architecture, all failover and switchover operations are seamless to the end-user. During a failover or switchover, users don't normally need to restart any applications, logout and login to Windows, nor make any configuration changes. Other benefits for the IT Operations environment include the following:

• Neverfail Continuity Engine switchovers and failovers require no changes to Active Directory (AD), and consequently carry considerably less risk than solutions that do require these types of changes.
  
• Neverfail Continuity Engine doesn't need to wait for AD replication before completing a failover or switchover.
  
• Neverfail Continuity Engine failovers and switchovers are fast.
  

Replication

Neverfail Continuity Engine keeps the active and passive servers synchronized in near real time through the use of a proprietary replication engine. Neverfail Continuity Engine replicates data changes at the byte level, only changed data, and both file and registry updates from protected applications. 

Benefits of Data Replication

Benefits to Neverfail's approach to data replication include the following:

• The process is efficient because Neverfail replicates only changed data at the byte level, for example, writes, not reads.
  
• The active and passive servers remain in sync at all times.
  
• Because Neverfail Continuity Engine replicates both file and registry updates from protected applications, even configuration changes to the protected application are synchronized and consequently no manual intervention is required on the Secondary server if the configuration of the application on the Primary server ever changes.
  

Data Rollback

With all replication technologies, if data corruption occurs on the source server, the corruption replicates to the target server. Neverfail Continuity Engine developed the Data Rollback Module to handle this situation if it occurs when Neverfail Continuity Engine installed. You can configure the Data Rollback Module to take snapshots of the protected data set at configured intervals or on demand. If corruption does occur, you can roll the application back to a pre-corrupted state.

Neverfail Continuity Engine provides several types of continuous monitoring. 

Hardware and System Monitoring

Neverfail Continuity Engine continuously monitors the active server to ensure it's still working and available by sending a heartbeat message from the passive server to the active. The message is more than a simple ping, as a crashed system with a functional network card can still respond to a ping, even if the server is blue screened. The heartbeat message asks, “Are you alive?”; to which the active server must respond, ”Yes, I am.” If the active server fails to respond within the allotted time, Neverfail Continuity Engine can notify a System Administrator and optionally perform an automatic failover.

Network Monitoring

Neverfail Continuity Engine can also monitor up to three IP addresses elsewhere on the network to ensure that the server is still connected to the network, and the network is still visible.  If the active server can't successfully contact all three IP addresses, suggesting the network is down, Neverfail Continuity Engine can initiate an automatic switchover.

Performance Monitoring

Neverfail Continuity Engine monitors key performance attributes of the protected application, as well as the server hardware itself. You can configure Neverfail Continuity Engine to take preemptive action to fix the problem - such as restarting the service or running any predefined set of commands - if monitoring identifies any performance problems or bottlenecks; and if it can't correct the problem, an automatic switchover can be initiated.

Application Monitoring and Protection

Neverfail Continuity Engine monitors and protects applications such as Microsoft Exchange or SQL Server through the use of Neverfail Plug-ins. Specifically, Neverfail Continuity Engine monitors the state of the protected application. If monitoring detects that application services are not running, Neverfail Continuity Engine can take preemptive action to fix the problem, such as restarting the service or running any predefined set of commands. If Neverfail Continuity Engine can't fix the problem, it can initiate an automatic switchover.

LAN - network environments are ideally suited for High Availability purposes due to the abundance of available bandwidth. Bandwidth in excess of 100 Mbit/s allows the transfer of data to the passive server at nearly the same rate that the active server generates it.  

WAN - You can achieve Disaster Recovery protection by placing the secondary server at on offsite facility and replicating the data through a WAN link. Neverfail Continuity Engine has an optional WAN Acceleration capability which provides data compression to reduce bandwidth needs. It also optimizes replication using data de-duplication, making the process ideally suited to low bandwidth connections.

Alerting

Neverfail Continuity Engine provides the ability to send an alert by way of email via any SMTP server, or any other command-line means, for example, a netsend message, or a pager. 

Protection of Auxiliary Applications

Neverfail Plug-ins provide protection for business-critical applications. Neverfail Continuity Engine protects these applications through the use of Plug-ins, for the following reasons: if you ignore the auxiliary applications running on a server, your primary application may not operate correctly, and in order to provide true High Availability, you need to protect the entire environment including these auxiliary applications. Neverfail offers many Plug-ins out of the box, requiring no custom scripting.