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Tag: Build Operability In

Availability targets

Why is it important to measure operability? What should be the trailing indicators and leading indicators of operability?


  • Reliability means balancing the risk of unavailability with the cost of sustaining availability.
  • Availability can be understood as a level of availability, from 99.0% to 99.999%.
  • Increasing an availability level incurs up to an order of magnitude more engineering effort.
  • An availability target is selected by a product manager based upon the maximum revenue loss they can tolerate for their service.


Organisations must have reliable IT applications at the heart of their business if they are to innovate in changing markets. Reliability is defined by Patrick O’Connor and Andre Kleyner in Practical Reliability Engineering as “the probability that [a system] will perform a required function without failure under stated conditions for a stated period of time”. There must be an investment in reliability if propositions are to be rapidly delivered to customers and remain highly available.

Reliability means balancing the risk of application unavailability with the cost of sustaining application availability. Application unavailability will incur opportunity costs related to lower customer revenue, loss of confidence, and reputational damage. On the other hand, sustaining application availability also incurs opportunity costs, as engineering time must be devoted to operational work instead of new product features visible to customers. In Site Reliability Engineering, Betsey Beyer et al state “cost does not increase linearly… an incremental improvement in reliability may cost 100x more than the previous increment”.

Furthermore, the user experience of application availability will be constrained by lower levels of user device availability. For example, a smartphone with 99.0% availability will not allow a user to experience a website with 99.999% availability. 100% availability is never the answer, as the cost is too high and users will not perceive any benefits. Maximising feature delivery will harm availability, maximising availability will harm feature delivery.

Availability targets

Application availability can be understood as an availability target. An availability target represents a desired level of availability, and is usually expressed as a number of nines. Each additional nine of availability represents an order of magnitude more of engineering effort. For example, 99.0% availability means “two nines”, and if its engineering effort is N then 99.9% availability would require 10N in engineering effort.

An availability target should be coupled to product risk. This will ensure a product owner translates their business goals into operational objectives, and empowers their team to strike a balance between application availability and costs. The goal is to improve the operability of an application until its availability target is met, and can be sustained.

For example, consider a Fruits R Us organisation with 3 availability targets for its applications – 99.0% (“two nines”), 99.5% (“two and a half nines”), and 99.9% (“three nines”). The 99.9% availability target allows for a maximum of 0.1% unavailability per month, which in a 30 day month equates to a maximum of 43 mins 12 seconds unavailability. It also requires 10 times more engineering effort to sustain than the 99.0% availability target.

In Site Reliability Engineering, the maximum unavailability per month for an availability target is expressed as an error budget. Error budgets are are a method of formalising the shared ownership and prioritisation of product features versus operational features, and might be used to halt production deployments during periods of sustained unavailability.

Availability target selection

A product owner should select an availability target by comparing their projected revenue impact of application unavailability with the set of possible availability targets. They need to consider if their application is tied directly or indirectly to revenue, their application payment model, what expectations users will have, and what level of service is provided by competitors in the same marketplace.

First, an organisation needs to establish a minimum Cost Of Delay revenue loss for each availability target, on loss of availability. Then a product owner should estimate the Cost Of Delay for their application being unavailable for the duration of each target. The Value Framework by Joshua Arnold et al can be used to estimate the financial impact of the loss of an application:

  • Increase Revenue – does the application increase sales levels
  • Protect Revenue – does the application sustain current sales levels
  • Reduce Costs – does the application reduce current incurred costs
  • Avoid Costs – does the application reduce potential for future incurred costs

This will allow a product owner to balance their need for application availability with the opportunity costs associated with consistently meeting that availability level.

For example, at Fruits R Us a set of revenue bands is attached to existing availability targets, based on an analysis of existing revenue streams. The 99.0% availability target is intended for applications where the Cost Of Delay on unavailability is at least £50K in 7h 12m, whereas 99.9% is for unavailability that could cost £1M or more in only 43m 12s.

A proposed Bananas application is expected to produce a monthly revenue increase of £40K. It is intended to replace an Apples application, which has an availability target of 99.0% sustained by an average of 8 engineering hours per month. The Bananas product owner believes customers will have heightened reliability expectations due to superior competitor offerings in the marketplace, and that Bananas could lose the £40K revenue increase within 2 hours of unavailability in a month. The 99.0% availability target can fit 2 hours of unavailability into its 7h 12m ceiling, but cannot fit a £40K revenue loss. The 99.5% availability target is selected, and the Bananas product owner knows at 5N engineering effort that 40 engineering hours will be needed per month to invest in operational  features.


Thanks to Thierry de Pauw for the review

Operability measures

Why is it important to measure operability? What should the trailing indicators and leading indicators of operability?


  • The trailing indicators of operability are availability rate and time to restore availability.
  • The leading indicators of operability include the frequency of Chaos Days and the time to act upon incident review findings.


In How To Measure Anything, Douglas Hubbard states organisations have a Measurement Inversion, and waste their time measuring variables with a low information value. This is certainly true of IT reliability, which is usually badly measured if at all. By proxy, this includes operability as well.

In many organisations, reliability is measured by equating failures with recorded production incidents. Incident durations are calculated for Mean Time Between Failures (MTBF) and Mean Time To Repair (MTTR), or there is just an overall incident count. These are classic vanity measures. They are easy to implement and understand, but they have a low information value due to the following:

  • Quantitative measures such as incident count have no reflection on business drivers, such as percentage of unrecoverable user errors
  • Manual recording of incidents in a ticket system can be affected by data inaccuracies and cognitive biases, such as confirmation bias and recency bias
  • Goodhart’s Law means measuring incidents will result in fewer incident reports. People adjust their behaviours based on how they are measured, and measuring incidents will encourage people to suppress incident reports with potentially valuable information.

If operability is to be built into applications, there is a need to identify trailing and leading indicators of operability that are holistic and actionable. Measures of operability that encourage system-level collaboration rather than individual productivity will pinpoint where improvements need to be made. Without those indicators, it is difficult to establish a clear picture of operability, and where changes are needed.

Effective leading and trailing indicators of software delivery should be visualised and publicly communicated throughout an organisation, via internal websites and dashboards. Information radiators help engineers, managers, and executives understand at a glance the progress being made and alignment with organisational goals. Transparency also reduces the potential for accidents and bad behaviours. As Louis Brandeis said in Other People’s Money “sunlight is said to be the best of disinfectants; electric light the most efficient policeman”.

Availability as a trailing indicator

Failures should be measured in terms of application availability targets, not production incidents. Availability measurements are easy to implement with automated time series metrics collection, easy to understand, and have a high information value. Measurements can be designed to distinguish between full and partial degradation, and between unrecoverable and recoverable user errors.

For example, a Fruits R Us organisation has 99.0%, 99.5%, and 99.9% as its availability targets A product manager for an Oranges application selects 99.5% for at least the first 3 months.

Availability should be measured in the aggregate as Request Success Rate, as described by Betsey Beyer et al in Site Reliability Engineering. Request Success Rate can approximate degradation for customer-facing or back office applications, provided a well-defined notion of successful and unsuccessful work. It covers partial and full downtime for an application, and is more fine-grained than uptime versus downtime.

When an application has a Request Success Rate lower than its availability target, it is considered a failure. The average time to restore availability can be tracked as a Mean Time To Repair metric, and visualised in a graph alongside availability.

At Fruits R Us, the Oranges application communicates with upstream consumers via a HTTPS API. Its availability is constantly measured by Request Success Rate, which is implemented by checking the percentage of upstream requests that produce a HTTP response code lower than HTTP 500. When the Request Success Rate over 15 minutes is lower than the availability target of 99.5%, it is considered a failure and a production incident is raised. An availability graph can be used to illustrate availability, incidents, and time to repair as a trailing indicator of operability.

Leading indicators of operability

Failures cannot be predicted in a production environment as it is a complex, adaptive system. In addition, it is easy to infer a false narrative of past behaviours from quantitative data. The insights uncovered from an availability trailing indicator and the right leading indicators can identify inoperability prior to a production incident, and they can be pattern matched to select the best heuristic for the circumstances.

A leading indicator should be split into an automated check and one or more exploratory tests. This allows for continuous discovery of shallow data, and frees up people to examine contextual, richer data with a higher information value. Those exploratory tests might be part of an operational readiness assessment, or a Chaos Day dedicated to particular applications. Leading indicators of operability can include:

Learning is a vital leading indicator of operability. An organisation is more likely to produce operable, reliable applications if it fosters a culture of continuous learning and experimentation. After a production incident, nothing should be more important than everyone in the organisation having the opportunity to accumulate new knowledge, for their colleagues as well as themselves.

The initial automated check of learning should be whether a post-incident review is published within 24 hours of an incident. This is easy to automate with a timestamp comparison between a post-incident review document and the central incident system, easy to communicate across an organisation, and highly actionable. It will uncover incident reviews that do not happen, are not publicly published, or happen too late to prevent information decay.

Another learning check should be the throughput of operability tasks, comprising the lead time to complete a task and interval between completing tasks. Tasks should be created and stored in a machine readable format during operability readiness assessments, Chaos Days, exploratory testing, and other automated checks of operability. Task lead time should not be more than a week, and task interval should not exceed the fastest learning source. For example, if operability readiness assessments occur every 90 days and Chaos Days are 30 days then at least one operability task should be completed per month.


Thanks as usual to Thierry de Pauw for reviewing this series

Build Operability in

What is operability, how does it promote resilience, and how does building operability into your applications drive Continuous Delivery adoption?


  • Operability refers to the ability to safely and reliably operate a production application.
  • Increasing service resilience depends on adding sources of adaptive capacity that increase operability.
  • Continuous Delivery depends on increasing service resilience.


The origins of the 20th century, pre-Internet IT As A Cost Centre organisational model can be traced to the suzerainty of cost accounting, and the COBIT management and governance framework. COBIT has recommended sequential Plan-Build-Run phases to maximise resource efficiency since its launch in 1996. The Plan phase is business analysis and product development, Build is product engineering, and Run is product support. The justification for this is the high compute costs and high transaction costs for a release in the 1990s.

With IT As A Cost Centre, Plan happens in a Product department, and Build and Run happens in an IT department. IT will have separate Delivery and Operations groups, with competing goals:

  • Delivery will be responsible for building features
  • Operations will be responsible for running applications

Delivery and Operations will consist of functionally-oriented teams of specialists. Delivery will have multiple development teams. Operations will have Database, Network, and Server teams to administer resources, a Service Transition team to check operational readiness prior to launch, and one or more Production Support teams to respond to live incidents.

Siloisation causes Discontinuous Delivery

In High Velocity Edge, Dr. Steven Spear warns over-specialisation leads to siloisation, and causes functional areas to “operate more like sovereign states”. Delivery and Operations teams with orthogonal priorities will create multiple handoffs in a technology value stream. A handoff means waiting in a queue for a downstream team to complete a task, and that task could inadvertently produce more upstream work.

Furthermore, the fundamentally opposed incentives, nomenclature, and risk appetites within Delivery and Operations teams will cause a pathological culture to emerge over time. This is defined by Ron Westrum in A Typology of Organisational Cultures as a culture of power-oriented interactions, with low cooperation and neglected responsibilities.

Plan-Build-Run was not designed for fast customer feedback and iterative product development. The goal of Continuous Delivery is to achieve a deployment throughput that satisfies product demand. Disparate Delivery and Operations teams will inject delays and rework into a technology value stream such that lead times are disproportionately inflated. If product demand dictates a throughput target of weekly deployments or more, Discontinuous Delivery is inevitable.

Robustness breeds inoperability

Most IT cost centres try to achieve reliability by Optimising For Robustness, which means prioritising a higher Mean Time Between Failures (MTBF) over a lower Mean Time To Repair (MTTR). This is based on the idea a production environment is a complicated system, in which homogeneous application processes have predictable, repeatable interactions, and failures are preventable.

Reliability is dependent on operability, which can be defined as the ease of safely operating a production system. Optimising For Robustness produces an underinvestment in operability, due to the following:

  • A Diffusion Of Responsibility between Delivery and Operations. When Operations teams are accountable for operational readiness and incident response, Delivery teams have little reason to work on operability
  • A Normalisation of Deviation within Delivery and Operations. When failures are tolerated as rare and avoidable, Delivery and Operations teams will pursue cost savings rather than an ability to degrade on failure

That underinvestment in operability will result in Delivery and Operations teams creating brittle, inoperable production systems.

Symptoms of brittleness will include:

  • Inadequate telemetry – an inability to detect abnormal conditions
  • Fragile architecture – an inability to limit blast radius on failure
  • Operator burnout – an inability to perform heroics on demand
  • Blame games – an inability to learn from experience

This is ill-advised, as failures are entirely unavoidable. A production environment is actually a complex system, in which heterogeneous application processes have unpredictable, unrepeatable interactions, and failures are inevitable. As Richard Cook explains in How Complex Systems Fail the complexity of these systems makes it impossible for them to run without multiple flaws“. A production environment is perpetually in a state of near-failure.

A failure occurs when multiple flaws unexpectedly coalesce and impede a business function, and the costs can be steep for a brittle, inoperable application. Inadequate telemetry widens the sunk cost duration from failure start to detection. A fragile architecture expands the opportunity cost duration from detection until resolution, and the overall cost per unit time. Operator burnout increases all costs involved, and blame games allow similar failures to occur in the future.

Resilience needs operability

Optimising For Resilience is a more effective reliability strategy. This means prioritising a lower MTTR over a higher MTBF. The ability to quickly adapt to failures is more important than fewer failures, although some failure classes should never occur and some safety-critical systems should never fail.

Resilience can be thought of as graceful extensibility. In The Theory of Graceful Extensibility, David Woods defines it as “a blend of graceful degradation and software extensibility”. A complex system with high graceful extensibility will continue to function, whereas a brittle system would collapse.

Graceful extensibility is derived from the capacity for adaptation in a system. Adaptive capacity can be created when work is effectively managed to rapidly reveal new problems, problems are quickly solved and produce new knowledge, and new local knowledge is shared throughout an organisation. These can be achieved by improving the operability of a system via:

  • An adaptive architecture
  • Incremental deployments
  • Automated provisioning
  • Ubiquitous telemetry
  • Chaos Engineering
  • You Build It You Run It
  • Post-incident reviews

Investing in operability creates a production environment in which applications can gracefully extend on failure. Ubiquitous telemetry will minimise sunk cost duration, an adaptive architecture will decrease opportunity cost duration, operator health will aid all aspects of failure resolution, and post-incident reviews will produce shareable knowledge for other operators. The result will be what Ron Westrum describes as a generative culture of performance-oriented interactions, high cooperation, and shared risks.

Dr. W. Edwards Deming said in Out Of The Crisis that “you cannot inspect quality into a product”. The same is true of operability. You cannot inspect operability into a product. Building operability in from the outset will remove handoffs, queues, and coordination costs between Delivery and Operations teams in a technology value stream. This will eliminate delays and rework, and allow Continuous Delivery to be achieved.


Thanks to Thierry de Pauw for the review

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