Reference: Continuity Engine Product Architecture

Reference: Continuity Engine Product Architecture


This Quick Reference provides an overview of the key concepts and components of Neverfail Continuity Engine product architecture:

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Key Concepts and Components

Component / Concept


Active-Passive Server Pair

Neverfail Continuity Engine employs an active-passive server pair methodology to provide a fully redundant failover solution.  If the active, production server fails for any reason, the fully redundant passive server quickly takes over all application processing, keeping end-users continuously connected to a working application.


Neverfail Continuity Engine provides flexibility in that it can be distributed on a LAN for High Availability and can also be distributed remotely over a WAN environment to provide the additional benefit of Disaster Recovery.

Hardware Agnostic

Neverfail Continuity Engine is hardware agnostic, meaning that the active and passive servers can be different in terms of manufacturer, model, and even processing power.  This simplifies deployment and can lower the total cost of ownership.

Hardware Differences  Hardware should not be so different that significant differences in overall performance exist. Otherwise, it will be difficult for the slower server to keep up with replication and adequately handle failover events.  Remember that either server can be active and providing application service to clients at any given time.

Disk Subsystem Performance Disk subsystem performance on each server should be of similar magnitude. Significant differences between the Primary and Secondary server disk subsystems could bottleneck the server pair and impact performance.

Shared Nothing

Neverfail Continuity Engine employs "shared nothing" architecture. Typical clusters share a quorum, holding the configuration on shared data drives (typically implemented on a Storage Area Network (SAN)). This introduces a single point of failure. Neverfail Continuity Engine does not use any shared hardware resulting in no single point of failure. Instead, all data changes that occur on the active server replicate to the passive server so that two separate copies of the data exist.

Engine Management Service

Neverfail IT Continuity Engine provides a flexible solution that can be adapted to meet most business requirements for deployment and management of critical business systems. Capitalizing on VMware vCenter Server's ability to manage virtual infrastructure assets combined with Neverfail's application-aware continuous availability technology, Neverfail IT Continuity Engine brings a best in class solution for protecting critical business systems.

Neverfail IT Continuity Engine consists of the Engine Management Service that is used to deploy and manage the Neverfail Engine service that provides for application-aware continuous availability used for protecting critical business systems.

Using Engine Management Service, users can deploy and manage Neverfail Engine with the ability to view Neverfail Engine status and perform most routine Neverfail Engine operations from a single pane of glass.

Cloned Servers

Neverfail Continuity Engine employs a unique installation process which automatically clones the servers so they appear to be identical. This includes:

·         All protected applications

·         All configuration settings

·         Machine name

·         Security Identifier or SID

·         IP address (when installed in a LAN environment)

Neverfail Continuity Engine’s unique approach offers seamless failover to users without complex updates to Windows Active Directory and presents a single server presence to the Windows network.  It also simplifies the installation process, removing the need to pre-configure the Secondary server with the same applications and configuration settings as the Primary server, thus saving time and eliminating the chance of errors at this critical setup stage.

Identity and Role

Neverfail Continuity Engine uses a server pair to provide High Availability and Disaster Recovery. Each server in the pair has both an identity and a role. The identity of the servers in the pair does not usually change unless you replace hardware. The role of the server changes if Neverfail Continuity Engine carries out a switchover or failover. A clear understanding of the difference between a server’s identity and its role is key to administering and maintaining the Neverfail Continuity Engine server pair. 

Identity  A server’s identity refers to its physical hardware and can be either Primary or Secondary. The terms Primary or Secondary identify an actual physical or virtual server machine, and these will never change during any operations. The Primary server is usually the principal server in the pair, often referred to as the production server before you install Neverfail Continuity Engine. The Secondary server is the opposing server to the Primary in the pair.  Refer to identity when you want to discuss the physical equipment. For example, you might describe the Primary and Secondary servers by location, color, manufacturer:

·         The Primary server is an Acme with a gray case, located in the basement of the corporate headquarters

·         The Secondary is an ABC server with a black case located on the second floor in the satellite office.

Role  A server’s role refers to what the server is doing currently. The role can be active or passive.  When you read Neverfail Continuity Engine literature, including the Administrator Guide and the Installation Guide, or communicate to other employees, Partners, or the Customer Support team, refer to the server by its role when you want  to discuss what a server is doing or what it’s expected to do.

·         Active role. The active server is the server that is visible to clients through the network. The active server is running protected applications and is servicing client requests.

·         Passive role. The passive server is the standby server in the pair. The passive is receiving replicated data across the replication channel. The passive is not delivering service to clients; it’s hidden from the rest of the network. Remember, either of the Neverfail Continuity Engine servers can assume the role of passive or active.

Neverfail Channel

Neverfail Continuity Engine communicates between servers using the Neverfail Channel. The channel is used for data replication and heartbeat messages which confirm to each server that the other is alive. The Neverfail Channel consists of at least one dedicated Network Interface Card or NIC added to each server, typically connected with either a crossover cable or a dedicated switch. This calls for a dedicated network subnet providing communications separate from the LAN. It is recommended that an additional, second channel NIC in each server to provide redundancy and to avoid a single point of failure. All replication and management communications occur across the channel using pre-defined IP ports, which can be changed.

IMPORTANT:  Firewalls must allow traffic between these servers on the pre-defined ports. Since the Neverfail Channel is a private network segment, data is not encrypted unless you use external encryption such as a Virtual Private Network or VPN in a WAN environment.

Windows Filtering Platform

To hide the passive server from the network and ensure that name and IP address conflicts don’t occur, Neverfail Continuity Engine incorporates use of Windows Filtering Platform on the Neverfail servers that clients use to access the active server. Windows Filtering Platform allows the active server to be visible to the local network while the passive server is hidden. During a switchover or failover, Windows Filtering Platform manages network visibility, allowing the active server to become passive and hidden from the local network while the passive server becomes active and visible to the users. You can configure both servers with multiple local network IP addresses, allowing you to manage either server even when they might be in a passive mode.

Neverfail Continuity Engine and Plug-ins

Neverfail Continuity Engine  Neverfail Continuity Engine is the real plumbing of the Neverfail Continuity Engine product. It provides data replication, server monitoring, network monitoring, and data rollback capabilities.

Neverfail Plug-ins On the other hand, the many Plug-ins available define data replication needs for a given application. They define services to monitor and start and stop during a switchover or failover process for that application, and monitoring needs for application-specific performance counters. Plug-ins also perform the actual application and performance monitoring.

Data Replication

Data replication is a critical function of the Neverfail Continuity Engine product and keeps your data safe from loss. Neverfail Continuity Engine replicates critical files used to store application data, and important registry settings to ensure that changes to the application configuration remain synchronized between servers. Neverfail Continuity Engine will auto discover any data that needs protection in real time, without user intervention, for instance, when new databases or file shares are created. You can also adjust these filters manually, using the Neverfail Advanced Client.

Replication Queues

Neverfail Continuity Engine performs replication asynchronously to ensure that the active server never slows down because it’s waiting to replicate data to the passive server. To do this, it queues all replication traffic. Neverfail uses two types of queues: the send queue and the receive queue. Since either server can assume the active or passive role, each server will have both a send and receive queue.

Send Queue Neverfail Continuity Engine’s send queue holds data on the active server that has not yet crossed the channel to the passive server. If the active server goes down or fails suddenly, the data in the send queue will be lost.

Receive Queue The receive queue holds data on the passive server that has crossed the Channel but has not been written to disk.  In the event of a failover, data stored in the receive queue will not be lost. Data builds up in the receive queue when the passive server is unable to apply updates at the same rate the active server intercepts them.

Replication Considerations Typically in a LAN environment, replication queues stay relatively empty (except under extremely high load conditions), and therefore these queues are stored in RAM, since RAM is fast. Any data in the send queue will be lost in the event of a failover, typically a critical unexpected outage or crash. However, any information in the send queue will NOT be lost in the event of a switchover, which is a planned, graceful event.

Wide Area Network (WAN) Environment

Another feature of the Neverfail Continuity Engine product is its ability to work in a Wide Area Network providing the same level of High Availability experienced in a LAN environment. Key differences in WAN operations include servers in a different subnet and large application loads. Servers operating in different subnets require unique IP addresses. Neverfail Continuity Engine overcomes this by automatically updating domain name servers, redirecting users to the currently active server. Low time-to-live or TTL values ensure that the DNS caches on client machines expire quickly, resulting in faster, seamless failover times.

WAN Acceleration

Under large application loads in WAN environments, it's normal to experience a buildup of data in the send queue due to lower bandwidth typically found in these environments. In these cases, Neverfail Continuity Engine offers a WAN Acceleration option. WAN Acceleration compresses data and performs de-duplication before transmitting to reduce the amount traffic across the WAN. Additionally, it makes use of disks rather than RAM for the replication queues to prevent exceeding RAM capacity.


Both the Primary and Secondary servers can run on physical hardware or within a virtual machine. By using hardware virtualization technology, you can fail multiple physical servers over to one physical server hosting multiple virtual machines, resulting in a many-to-one relationship. It’s important to remember that when you use any virtualization technology, it needs to be sized appropriately so that replication and failover processing runs seamlessly. A Secondary virtual server must be just as powerful as a Secondary physical server would have been.


Neverfail Continuity Engine can be configured to operate in both a LAN and a WAN simultaneously, providing both High Availability and Disaster Recovery using a third server. In a Trio configuration, the Primary and Secondary servers perform High Availability operations in a LAN while the Secondary and Tertiary servers perform Disaster Recovery operations in a WAN. Within the trio configuration, Neverfail Continuity Engine uses one active server, and two passive servers (the 1st passive server, and the 2nd passive server).

Trio Benefits In a trio configuration, any of the three servers can perform an active or passive role providing flexibility for maintenance. This configuration provides High Availability concurrently with Disaster Recovery.

Replication Chain In the trio configuration, the active server replicates to the 1st passive server and the 1st passive server replicates to the 2nd passive server.

Trio Considerations When configured in a trio, the following should be considered:

·         Each server requires a minimum of two channels, resulting in two separate NICs for channel communications per server.

·         The tertiary server will not operate in the middle of the replication chain, thereby preventing replication across the WAN link in both directions.

·         The communications between servers in the trio is a closed system and all channel connections must be functional to maintain both High Availability and Disaster Recovery.

Applies To

Neverfail Continuity Engine v8.0 and Later

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