#+TITLE: High Availability, Disaster Recovery and Business Continuity
#+OPTIONS: num:nil
#+DATE: <2026-03-11 Wed 17:18>
#+filetags: :learning:notes:
#+WIP: 
#+COMMENTS: t
#+SLUG: high-availability-disaster-recovery

* HA, DR and BC Competency Log
Understanding High Availability (HA), Disaster Recovery (DR), and
Business Continuity Planning (BCP) is important when designing reliable
systems and analysing service incidents. These concepts help Microlise
(and organisations in general) to minimise downtime, recover from
failures, and maintain service availability.

*** High Availability (HA)
High Availability focuses on preventing service disruption by using
redundancy. Systems are designed with backup components so that if one
fails, another can take over automatically.

Examples include:

- Redundant servers or network paths
- Load balancing across multiple systems
- Automatic failover modes

This ensures services continue operating even when individual components
fail. We have this at Microlise where if there is a failure on one of
the data centres, we can failover to the other one.

An analogy from the article compares this to a bicycle with two brakes.
The bike can operate with only one brake, but having two provides
redundancy in case one fails.

*** Disaster Recovery (DR)
Disaster Recovery focuses on restoring systems after a major failure.
This includes recovering infrastructure, applications, and data so
services can resume operation.

Some of the common DR tools and methods include:

- Offsite backups
- Replicated environments (e.g., multiple data centres with sync)
- Recovery procedures and restoration tools

Unlike High Availability, which tries to prevent downtime, DR assumes a
failure has already occurred. In the bicycle analogy, after the crash
the rider takes a bus home and then drives to work to complete their
important task. This represents a recovery process after a major
disruption.

*** Business Continuity Planning (BCP)
BCP ensures the business can continue operating during or after a
disruption. This may involve alternative systems, temporary processes,
or backup locations to keep services running.

In the analogy, the car at home represents BCP because it allows the
rider to continue their journey despite the broken bicycle. A Business
Impact Assessment (BIA) helps determine the priority of services and the
acceptable level of downtime.

*** RTO and RPO
Two key recovery metrics are:

- Recovery Time Objective (RTO): The maximum time allowed to restore a
  service after failure. For example, if a critical service must be
  restored within 4 hours, the disaster recovery process must ensure
  systems are operational within that timeframe.
- Recovery Point Objective (RPO): The maximum acceptable amount of data
  loss.

RPO example:

- An RPO of 0 minutes means no data loss is acceptable.
- An RPO of 1 hour means up to one hour of data could be lost.

These values help define the required level of HA and DR design.

*** Application in Real Scenarios
These concepts are useful when performing incident debugging, root cause
analysis, or explaining service issues to customer support teams. For
example, engineers may check whether failover worked correctly, whether
recovery met the RTO, or whether backups allowed data to be restored
within the RPO.

In summary, the reason why we need to understand HA, DR, and BCP is to
maintain service availability and ensure business continuity.
