Lesson 3: Availability & Reliability#

Reliability vs. Availability#

• Reliability: The probability that a system will function correctly without failure for a specified period. It’s about correctness.
• Availability: The percentage of time a system is operational and accessible. It’s about uptime.

A system can be available but not reliable (e.g., it returns 500 errors but is “up”).

Measuring Availability#

Availability is often measured in “nines”:

Achieving High Availability#

Redundancy#

The key to availability is eliminating Single Points of Failure (SPOF). This is done via redundancy.

• Active-Passive: One server handles traffic; the other is on standby.
• Active-Active: Both servers handle traffic. If one fails, the other takes over the full load.

Failover#

The process of switching to a redundant system upon failure. This can be manual or automatic.

🛠️ Sruja Perspective: Modeling Redundancy#

You can explicitly model redundant components in Sruja to visualize your high-availability strategy.

```sruja
system Payments "Payment System" {
    container PaymentService "Payment Service" {
        technology "Java"
    }

    // Modeling a primary and standby database
    container PrimaryDB "Primary Database" {
        technology "MySQL"
        tags ["primary"]
    }

    container StandbyDB "Standby Database" {
        technology "MySQL"
        tags ["standby"]
        description "Replicates from PrimaryDB. Promoted to primary if PrimaryDB fails."
    }

    PaymentService -> PrimaryDB "Reads/Writes"
    PrimaryDB -> StandbyDB "Replicates data"
}