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HA - Availability - Common “Nines”

Availability is generally expressed as “Nines”, common ‘Nines’ are listed below.

Availability % Downtime per year Downtime per month Downtime per week Downtime per day
99%(Two Nines) 3.65 days 7.31 hours 1.68 hours 14.40 minutes
99.5%(Two and a half Nines) 1.83 days 3.65 hours 50.40 minutes 7.20 minutes
99.9%(Three Nines) 8.77 hours 43.83 minutes 10.08 minutes 1.44 minutes
99.95%(Three and a half Nines) 4.38 hours 21.92 minutes 5.04 minutes 43.20 seconds
99.99%(Four Nines) 52.60 minutes 4.38 minutes 1.01 minutes 8.64 seconds
99.995%(Four and a half Nines) 26.30 minutes 2.19 minutes 30.24 seconds 4.32 seconds
99.999%(Five Nines) 5.26 minutes 26.30 seconds 6.05 seconds 864.0 ms



HA - Availability Serial Components

A System with components is operating in the series If the failure of a part leads to the combination becoming inoperable.

For example, if LB in our architecture fails, all access to app tiers will fail. LB and app tiers are connected serially.

The combined availability of the system is the product of individual components availability

A = Ax x Ay x …..



HA - Availability Parallel Components

A System with components is operating in parallel If the failure of a part leads to the other part taking over the operations of the failed part.

If we have more than one LB and if the rest of the LBs can take over the traffic during one LB failure then LBs are operating in parallel

The combined availability of the system is

A = 1 - ( (1-Ax) x (1-Ax) x ….. )



HA - Core Principles

Elimination of single points of failure (SPOF) This means adding redundancy to the system so that the failure of a component does not mean failure of the entire system.

Reliable crossover In redundant systems, the crossover point itself tends to become a single point of failure. Reliable systems must provide for reliable crossover.

Detection of failures as they occur If the two principles above are observed, then a user may never see a failure




WHAT: Never implement and always eliminate single points of failure.

WHEN TO USE: During architecture reviews and new designs.

HOW TO USE: Identify single instances on architectural diagrams. Strive for active/active configurations. At the very least we should have a standby to take control when active instances fail.

WHY: Maximize availability through multiple instances.

KEY TAKEAWAYS: Strive for active/active rather than active/passive solutions. Use load balancers to balance traffic across instances of a service. Use control services with active/passive instances for patterns that require singletons.

HA - Reliable Crossover

WHAT: Ensure when system components failover they do so reliably.

WHEN TO USE: During architecture reviews, failure modeling, and designs.

HOW TO USE: Identify how available a system is during the crossover and ensure it is within acceptable limits.

WHY: Maximize availability and ensure data handling semantics are preserved.

KEY TAKEAWAYS: Strive for active/active rather than active/passive solutions, they have a lesser risk of cross over being unreliable. Use LB and the right load balancing methods to ensure reliable failover. Model and build your data systems to ensure data is correctly handled when crossover happens. Generally, DB systems follow active/passive semantics for writes. Masters accept writes and when the master goes down, the follower is promoted to master(active from being passive) to accept writes. We have to be careful here that the cutover never introduces more than one master. This problem is called a split brain.

Applications in SRE role

  1. SRE works on deciding an acceptable SLA and make sure the system is available to achieve the SLA
  2. SRE is involved in architecture design right from building the data center to make sure the site is not affected by a network switch, hardware, power, or software failures
  3. SRE also run mock drills of failures to see how the system behaves in uncharted territory and comes up with a plan to improve availability if there are misses.

Post our understanding about HA, our architecture diagram looks something like this below HA Block Diagram