High Availability Overview and Design for Business Systems

High Availability (HA) describes a system designed to minimize downtime and maintain a high level of service availability.

For non‑repairable systems, Mean Time To Failure (MTTF) measures the average operational time before a failure; for repairable systems, Mean Time Between Failures (MTBF) measures the interval between successive failures, while Mean Time To Repair (MTTR) quantifies the average repair duration. Availability (A) is calculated as A = MTBF / (MTBF + MTTR).

Typical business‑system HA targets four‑nine (99.99%) availability, translating to roughly 53 minutes of annual downtime and requiring automatic fault‑recovery capabilities.

HA is a system‑wide concern involving hardware infrastructure, software architecture, governance, and control mechanisms, and must be driven by business goals from the earliest planning stages.

The HA planning process consists of five phases that form a continuous improvement loop:

Assessment : Define HA objectives aligned with business goals and identify improvement opportunities.

Planning : Develop HA strategies and a roadmap.

Design : Create detailed IT infrastructure and software designs that satisfy non‑functional requirements.

Implementation : Deploy solutions efficiently while controlling costs.

Operation : Conduct drills, monitor risks, and refine the HA posture.

During the assessment phase, business goals are clarified, current hardware/software capabilities are evaluated, and failure impact analyses (e.g., FMEA) are performed. Metrics such as TPMC and business‑level models are used to gauge capacity needs.

The planning phase translates assessment findings into concrete HA strategies covering IT infrastructure, middleware, application architecture, governance, and security.

The design phase details the future IT environment, including server, storage, network, middleware selection, and software architecture that meets performance and reliability requirements.

Implementation focuses on cost‑effective, rapid enhancements, whether for greenfield systems or existing deployments, emphasizing monitoring, performance tuning, and incremental optimization.

Operational HA relies on ITSM/ITIL processes, shifting from reactive incident handling to proactive risk management, with Service Level Agreements (SLAs) guiding alerting and escalation.

Performance modeling assesses required compute, storage, and network resources based on peak user load, transaction volume, data growth, and future expansion, using formulas such as TPM = TASK × 80% × S × F / (T × C) where TASK is daily peak transactions, S reflects transaction complexity, C is CPU utilization target, and F accounts for growth reserve.

Key HA objectives encompass high reliability, high performance, and high scalability, which are interdependent: reliability often relies on redundant HA designs, performance demands efficient architecture, and scalability provides the foundation for both.

Evaluating current HA status involves identifying non‑functional gaps, analyzing single points of failure, and proposing technical solutions across infrastructure, middleware, and database layers, including active‑active clustering, real‑time replication, and load‑balancing.

Overall, achieving robust high availability requires coordinated effort across architecture, implementation, and operations, guided by systematic assessment, planning, design, deployment, and continuous improvement.