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Disaster Recovery Models & Gauging Nine(s) | Maintaining Server Redundancy in a Highly Available Environment

As a 24/7/365 radiology company, vRad’s technology is the backbone of everything we do. In order to deliver patient care, our client facilities must be able to communicate with our servers – and for us, that’s every day, every night, every weekend, every holiday… you get the picture.

In short, vRad systems must be highly available.

Definition Highly Available System: A system that can be accessed consistently with very little to no down-time.

Today I’m going to discuss how we prepare for a disaster in order to minimize down-time and ensure the continuous delivery of services to our clients and the 5.9 million patients we serve each year.


Up-time: Goals

In order to measure success and failure when it comes to server up-time, it’s important to have an up-time goal to measure against. For the uninitiated, calculating up-time is a simple equation:

Up-time = (total time – down-time) / total time

vRad does a large amount of emergency medicine. We handle cases where seconds and minutes matter. With regard to up-time, those seconds and minutes are precious.

Seconds and minutes with 99.95% up-time means:

  • Being down less than 2 seconds per Hour,
  • Being down less than 1 minute per Day, and
  • Being down less than 25 minutes per Month.

We don’t have room for failure at vRad. If a car crash trauma case with a suspected intracranial hemorrhage comes to an ER we serve, we have to be ready to serve that patient, all day, every day. Those seconds matter to our patients and they matter to us.


Up-time: Gauging Nine(s)

At vRad, we target 100% up-time.

We monitor the stability of our platform using a metric we call nine(s) – the number of 9s of uptime we have (e.g., 99.99%). We use this information to decide how much change we allow in our platform.

As a leading radiology company, continually improving our systems and processes is part of our company’s DNA, and that means consistently enhancing our platform.  Balancing the amount of change by gauging our nine(s) is part of that process.  While we have many processes in place for ensuring high quality changes, making changes inherently introduces risk.  When we approach an up-time of 99.95%, we become very selective about making changes to our platform and only allow the implementation of critical fixes. This reduces potential risk and allows our platform up-time numbers to stabilize.

Diligence and consistent tracking help us to maintain our up-time goal across all systems, which we monitor in our Technology Service Availability Report:



We also investigate any down-time events to garner a deep understanding of the root cause(s) and make corrections/adjustments based on our analysis.

Stringent development and maintenance practices go a long way toward making our up-time goal of 100% achievable – but at a certain point the biggest threat to up-time is an unexpected disaster taking out our primary data center.

A strong disaster response model is critical to gauging nine(s), as any data center related issues can cause major outages with little that can be done to protect your nine(s) if you haven’t prepared ahead of time.


Cold, Warm and Hot Site Disaster Recovery Models

We take outages very seriously. There are many different disaster recovery strategies including Cold Sites, Warm Sites, and Hot Sites – each of which provides differing costs and time to recovery.

At vRad, we use a Hot Site model to maximize our up-time. Let’s explore why that’s important.




Cold Sites

Cold Site strategies rely on a secondary datacenter location that is ready for equipment in the case of a disaster. The location is not connected to the internet beforehand and the equipment must be relocated and set up from scratch.

The Cold Site disaster recovery strategy is easily the cheapest to maintain but requires the longest potential outage duration, typically lasting weeks.


Warm Sites

A Warm Site strategy consists of a datacenter pre-populated with equipment and connectivity.  Yet, the backup equipment and software may be out of date and the datacenter would only ever become active in the case of a failover. Because a Warm Site datacenter is not in use until needed, you simply don’t know the status of the site – extra outage time is typically incurred as unforeseen issues arise during failover.

Failover to this type of data center still involves a significant outage duration, hours at best and most likely days, although shorter than a Cold Site Strategy.


Hot Sites

The Hot Site strategy consists of having a duplicate datacenter configured to handle full production load and near real time synchronization of data.  In the event of a failover, the production load will be re-directed to the Hot Site and disruption of service is minimal: hours, possibly minutes.

Even with Hot Sites, where outages are measured in hours, we architect the system differently as we cannot tolerate hours of outages.  We further break down the Hot Site strategy into Active/Passive (outages measured hour(s)) or Active/Active (outages measured in minute(s)).


Hot Site in an Active/Passive Configuration

In an Active/Passive configuration, the systems in the disaster recovery datacenter are passive and will only receive load in the event of a failover.  Because the systems are passive, functionality is not assured. Over time, software and hardware is updated and maintained, but typically are not tested under a production load. When the passive configuration is needed due to an outage event, unpleasant surprises can arise and increase your downtime.


Hot Site in an Active/Active Configuration

vRad uses an Active/Active hot site configuration to maximize up-time. The systems in the disaster recovery datacenter are included as part of the day-to-day production load.  The Network, Storage systems, Servers, Software … take a percentage of the production load at all times.  This strategy ensures that that the disaster recovery datacenter is functional as these systems are part of production. We know our recovery center will work in a disaster simply because it already is!

I hope you enjoyed this high-level view of vRad’s disaster recovery model – and I hope you join me throughout the year as I dive deeper into how we maintain our servers here at vRad.

Author Keith Gardner

Keith leads the DBAs and Technical Debt teams at vRad and collaborates with the Principal Engineers to set the vRad’s technical direction. He has over 30 years of experience building highly scalable/available systems and is passionate about logging, anomaly alerting and building self-correcting systems. In his free time, he enjoys reef marine aquariums, traveling, playing tennis, taking walks, watching football (both kinds) and spending time with his family.

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