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Keith B. Evans Shadowbase
Product Management

Shadowbase Solutions and HP Pathway Domains — Perfect Together!

In today’s business world, access to real-time online transactional data is a potential competitive advantage. To realize the advantage, this data must be available at any time, from anywhere, and it must be current. The corollary to this advantage is that the inability to access or update this current data carries a significant business cost, possibly measured in many thousands of dollars per second. The Shadowbase suite of data replication software products provides solutions to meet the requirements of continuous data availability in the event of planned or unplanned outages.

So, Shadowbase software can provide all the capabilities necessary to ensure data remains available in the event of any outage circumstance, planned or unplanned, but what of the business applications themselves? Clearly, as part of the business continuity implementation, mechanisms must be in place to ensure continued availability of both the data and the applications which operate on that data if business services are to be maintained – this is where HP Pathway domains comes in.

Pathway is a mainstay of today’s mission-critical systems. It implements application scalability by distributing workloads across dynamic pools of application server processes that in turn are spread across multiple HP NonStop server CPUs. It offers high levels of application availability by monitoring and automatically restarting server processes in the event of failure.

The Pathway model delivers excellent availability for a single HP NonStop server, providing local fault tolerance. But this model still necessitates application outages for certain planned system maintenance activities, and it offers no protection against unplanned system outages.

These limitations are addressed by Pathway domains, which allow applications to be distributed across HP NonStop server nodes, with user requests being load-balanced between them. If one of the Pathway environments (PATHMONs) or nodes in the domain takes a planned or unplanned outage, the user requests are automatically routed to the remaining active Pathway environments in the domain, without need for operator intervention or special applications programming.

By configuring applications within a Pathway domain across multiple HP NonStop nodes, application scalability is increased by automatic workload distribution across those systems. In the event of planned or unplanned outages, user requests are automatically routed to the remaining systems, thereby preserving application availability, even when catastrophic failures take out a system, an entire data center, or a geographic region.

While there are some benefits in other modes, it is in active/active system architectures where the combination of Shadowbase software and Pathway domains really complement each other to provide significant benefits, with Pathway domains facilitating active/active application processing and Shadowbase replication facilitating active/active database processing.

Each PATHMON in an active/active configuration is included in a Pathway domain configuration on each node, with a subset/superset of the same applications running in each of them. Transactions can land on any active node, and Pathway domains handles the distribution of workload between the PATHMONs configured in the domain. Shadowbase technology is providing bi-directional replication to keep all active database copies synchronized, and enabling read-write database access to applications even while replication is in process.

When a node is taken down for maintenance or suffers an unplanned outage, Application Clustering Services (ACS) detects that the PATHMONs running on that node are no longer available and stops sending requests to them. The workload is thus automatically routed by ACS to the remaining node(s). Since each node also has a current copy of the online database (due to bi-directional replication), business transactions can proceed with no noticeable downtime.

It is worth making a point about the use of Pathway domains and Shadowbase data replication in the context of eliminating planned downtime for system upgrades and migrations – the use of the two products to avoid planned outages is as applicable as it is to avoiding unplanned outages. In the case of system maintenance or upgrade, the online workload is routed by Pathway domains to another system in the domain to maintain business services while the maintenance is carried out. Once the maintenance is completed, the system is brought back online and begins processing transactions. Then if required, another system can be taken down for maintenance while the original system takes care of the online processing, and so on. All the while business services remain available to end users as Pathway domains redistributes the workload to other available systems in the domain and Shadowbase maintains database synchronization. This technique can also be used to migrate to a new system without any downtime, a so-called zero downtime migration (ZDM).

Another common requirement while performing system maintenance is that disaster recovery capabilities are not compromised in the meantime. In a two node architecture, while one system is offline for maintenance a failure of the other system brings business services to a halt, which is unacceptable for many applications. Companies are increasingly deploying multi-node configurations to meet this need. In such configurations, multiple systems are capable of taking over the business services, located in different data centers. In a Pathway domain, there can be up to four such data centers, with Shadowbase data replication keeping the databases synchronized between them. While one system is down for maintenance, another local system can take over active processing. If either data center should then experience a complete outage, there will remain at least one other system available to continue active processing.

In summary, it takes two to tango! It is of no use having data available on backup systems if the applications required to provide the business services are not also available when needed. Shadowbase data replication solutions provide all the capabilities necessary to keep databases replicated and synchronized across multiple systems, whether it be a uni-directional (active/passive), bi-directional (sizzling-hot-takeover, active/active), heterogeneous, homogeneous, or ZDM environment. The introduction of Pathway domains helps to ensure the applications are available too. Thus, the combination of Shadowbase software and Pathway domains provides a firm basis to achieve increased levels of availability for your business services. In short, Pathway domains and Shadowbase data replication solutions are perfect together!

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