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Caché High Availability Solutions
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High availability (HA) refers to the goal of keeping a system or application operational and available to users a very high percentage of the time, minimizing both planned and unplanned down time. Caché provides its own HA solution, and easily integrates with common HA solutions supplied by operating system providers.
The primary mechanism for maintaining high system availability is called failover. Under this approach, a failed primary system is replaced by a backup system; that is, production fails over to the backup system. Many HA configurations also provide mechanisms for disaster recovery (DR), which is the resumption of system availability when HA mechanisms have been unable to keep the system available.
This article briefly discusses these general strategies for achieving HA with Caché-based applications:
This article also discusses issues in Caché HA solutions, provides an HA solutions feature comparison, and discusses using Enterprise Cache Protocol (ECP) with a failover strategy.
Issues in Caché HA Solutions
Bear in mind the following two significant issues when evaluating potential HA solutions for your Caché systems:
No HA Solution
The structural and logical integrity of your Caché database is always protected from production system failure by the built-in features described in the Introduction to Data Integrity chapter of the Caché Data Integrity Guide: write image journaling, database journaling, and transaction processing. With no HA solution in place, however, a failure can result in significant down time, depending on the cause of the failure and your ability to isolate and resolve it. For many applications that are not business-critical, this risk may be acceptable.
Customers that adopt this approach share the following traits:
OS-Level Cluster HA
A common HA solution provided at the operating system level is the failover cluster, in which the primary production system is supplemented by a (typically identical) standby system, with shared storage and a cluster IP address that follows the active member. In the event of a production system failure, the standby assumes the production workload, taking over the programs and services formerly running on the failed primary. The standby must be capable of handling normal production workloads for as long as it may take to restore the failed primary. Optionally, the standby can become the primary, with the failed primary becoming the standby once it is restored.
Caché is designed to easily integrate with the following failover cluster technologies, as detailed in the appendixes of the Caché High Availability Guide:
A Caché instance is installed on the cluster’s shared storage device so that both cluster members recognize it, then added to the cluster configuration so it will be restarted automatically on the standby as part of failover. On restart after failover, the system automatically performs the normal startup recovery, maintaining structural and logical integrity exactly as if Caché had been restarted on the failed system. Multiple Caché instances can be installed on a single cluster if desired.
Failover Cluster Configuration
Virtualization Platform HA
Virtualization platforms generally provide HA capabilities, which typically monitor the status of both the guest operating system and the hardware it is running on. On the failure of either, the virtualization platform automatically restarts the failed virtual machine, on alternate hardware as needed. When the Caché instance restarts, it automatically performs the normal startup recovery, maintaining structural and logical integrity as if Caché had been restarted on a physical server.
Failover in a Virtual Environment
Virtualization HA has the advantage of being built into the virtualization platform infrastructure, and thus can require very little effort to configure, in some cases none at all. In addition, virtualization platforms allow the planned relocation of virtual machines to alternate hardware for maintenance purposes, enabling upgrade of physical servers, for example, without any down time.
Caché Mirroring
Caché mirroring with automatic failover takes a different approach to HA, relying on logical data replication between fully independent systems to avoid the single point of failure risk ofy shared storage and ensure that production can immediately fail over to an alternate Caché instance in almost all failure scenarios—system, storage, and network.
In a Caché mirror, one Caché instance, called the primary failover member, provides access to the production databases. Another instance on a separate host, called the backup failover member, communicates synchronously with the primary, retrieving its journal records, acknowledging their receipt, and applying them to its own copies of the same databases. In this way, both the primary and the backup always know whether the backup has the most recent journal files from the primary, and can therefore precisely synchronize its databases with those on the primary.
When this is the case, the mirror can quickly and automatically fail over to the backup in the event of a primary outage with no loss of data. A third system, the arbiter, helps the backup determine whether it should take over when the primary becomes unresponsive. A mechanism such as a virtual IP address shared by the failover members or Caché ECP redirects application connections to the new primary. The failover process takes just seconds; many users won’t even notice it happening. And because the backup has its own copies of the databases, even a total failure of the primary and its storage does not render the databases unavailable. In fact, even when the backup is missing the most recent journal data, the backup’s mirror agent can retrieve it from the primary’s host, if it is still online.
Once you restore the former primary to operation following failover, it becomes the backup and its databases quickly catch up with those on the new primary, returning the mirror to full HA capability. You can then return the systems to their original roles ior maintain the new arrangement.
Mirrors can also include disaster recovery (DR) async members, which are asynchronously maintained copies of the primary; a DR async can be promoted to failover member, for example to become backup when a failed primary cannot be restored to operation quickly, or (if physically separate) for disaster recovery when an outage, such as a data center failure, brings down both failover members. Finally, mirrors can contain reporting async members, which maintain asynchronous copies of the production databases for business intelligence and data warehousing purposes.
Caché Mirror
Mirroring can also be used together with virtualization platform HA to create a hybrid HA approach, under which the virtualization platform responds to unplanned system or OS-level outages while mirroring handles all planned outages and unplanned database outages (including both Caché outages and storage failures) through automatic failover.
For complete information about Caché mirroring, including its DR capabilities, see the Mirroring chapter of the Caché High Availability Guide.
HA Solutions Feature Comparison
The following table provides a very general feature comparison mirroring, clustering, and virtualization as HA solutions.
 
Caché Mirroring
OS-Level Clustering
Virtualization Platform HA
Failover after machine power loss or crash
Handles machine failure seamlessly in Caché version 2015.1 or later. Prior versions did not fail over automatically in this scenario; alternatives required careful planning.
Handles machine failure seamlessly.
Handles physical and virtual machine failures seamlessly.
Protection from storage failure and data corruption
Built-in replication protects against storage failure; logical replication avoids carrying forward most types of corruption.
Relies on shared storage device, so failure is disastrous; storage-level redundancy optional, but can carry forward some types of corruption
Relies on shared storage device, so failure is disastrous; storage-level redundancy optional, but can carry forward some types of corruption.
Failover after Caché shutdown, hang, or crash
Rapid detection and failover is built in.
Can be configured to fail over after Caché outage.
Can be configured to fail over after Caché outage.
Caché upgrades
Allows for minimum- downtime Caché upgrades.*
Caché upgrades require downtime.
Caché upgrades require downtime.
Application mean time to recovery
Failover time is typically seconds.
Failover time can be minutes.
Failover time can be minutes.
External file synchronization
Only databases are replicated; external files need external solution.
All files are available to both nodes.
All files available after failover.
* Requires a configuration in which application code, classes, and routines are kept in databases separate from those that contain application data
Using ECP with a Failover Strategy
Whatever approach you take to HA, Enterprise Cache Protocol (ECP) can be used to provide a layer of insulation between the users and the database server. Users remain connected to ECP application servers when the database server fails; user sessions and automatic transactions that are actively accessing the database during the outage will pause until the database server becomes available again through completion of failover or restart of the failed system.
Bear in mind, however, that adding ECP to your HA strategy can increase complexity and introduce additional points of failure.
For information about ECP features and implementation, see the ECP Failover chapter of the Caché High Availability Guide and the Caché Distributed Data Management Guide.