peter siemen, dipl.-inf.

Let's say you ask a friend, Monica, to water your plants while you are away on holidays (your country is not in corona lockdown anymore). You might not be sure if having put the well-being of your plants into Monica's botanical hands was a good idea and you would like to express this new uncertainty in your life with numbers.

A technically inclined friend told you that you can easily use measurements like availability and durability to express your feelings about a potentially unsafe situation like this.

This is how it works.

For the sake of simplicity let's say you are going away for 365 days of a non-leap year and you have a neighbor, Carlos (Monica does not know Carlos). He lives across the street from you and is constantly watching your flowers in the window from his flat.

The percentage of time during those 365 days Carlos is seeing your flowers in an up & alive and happy state is expressed as availability. If for example Monica is keeping your flowers properly watered (not too much and not too little) so that for only exactly 8.77 hours (~ 8 hours and 45 minutes) the flowers look deadlyish she has sucessfully ensured a flower availability of 99.9%.

Availability

Availability is usually expressed as percentage of uptime in a given year. Common availabilities (sidenote: in the AWS world) are "three nines", "three and a half nines" and "four nines".

In case you are not only interested in the looks of your flowers's but rather want a number to express how they really are - e.g. alive/dead - you can use durability.

Let's say Monica's job is to take care of 100 flowers in your flat and she signs a contract with you to ensure a durability of 99%. By signing this contract you unromantically just said goodbye to 1 of your 100 flowers.

Maybe you rather want to agree on 99.9% durability and make life a little rosier for your flowers. With a durability of 99.9% for your 100 flowers you would only have to expect the loss of one single flower during this critical year in which Monica is taking care of them with a likelihood of 10%. Much better!

Durability

In case you want to know how durable AWS S3 is....

Amazon S3 Standard, S3 Standard–IA, S3 One Zone-IA, and S3 Glacier are all designed to provide 99.999999999% durability of objects over a given year. This durability level corresponds to an average annual expected loss of 0.000000001% of objects. For example, if you store 10,000,000 objects with Amazon S3, you can on average expect to incur a loss of a single object once every 10,000 years. In addition, Amazon S3 Standard, S3 Standard-IA, and S3 Glacier are all designed to sustain data in the event of an entire S3 Availability Zone loss.

^{https://aws.amazon.com/s3/faqs/#How_durable_is_Amazon_S3}

There are two other, related measures that you can use to express your unsureness about the flower situation. However bending them into the flower example might get weird but let's try.

Recovery Time Objective

RTO represents how many hours it takes you to return to a working state after a disaster.

How long does it take Monica to get a flower that looks like the one that was lost? How far away is the next flower shop? How willing is she to get a lookalike replacement as soon as possible? Will she go into all this trouble at all only to give Carlos a good view on your flowers?

Recovery Point Objective

RPO, which is also expressed in hours, represents how much data you could lose when a disaster happens.

Alright, let's assume that Carlos likes to watch the flowers grow, further that his eyesight allows him to distinguish sizes up from the range of 1cm and that none of your flowers grow more than 1cm per day.

In order to keep the neighbors (Carlos) from talking about your potentially insufficient care for your plants you might want to agree on a RPO of 24 hours with Monica. In order to ensure the RPO of 24 hours Monica would go to the flower shop in your neighborhood every night at 1:00am and tell them the exact size of every flower of yours. The flower shop will take this information to "provision" 100 flowers as exact replicas of yours (at 1:00am in the night :)). Should a flower leave you, Monica can walk over to the flower shop, get the flower that "was" the exact replica of the now dead flower yesterday at 1:00am, put it back in your window and hence give Carlos no reason whatsoever to badmouth about your insufficient botanical care for your plants while you are on holidays because he is too shortsighted to see a difference anyways. Everybody wins.

Thank you for reading and please make sure to give your flowers someone you trust.

If you found someone for your plants but still need professional help with AWS, get in touch...