What is an AWS Service Level Agreement?
An AWS service Level Agreement (SLA) is an agreement between an AWS user and AWS regarding the level of availability that AWS guarantees. So what defines an AWS service level agreement?
The level of availability that AWS guarantees is specific to each service it offers, and it applies separately to each account using those services.
For example, if you visit the latest AWS Service Level Agreements at https://aws.amazon.com/compute/sla/?did=sla_card&trk=sla_card for Compute (EC2 etc)
you will see varying percentages of availability guarantees. These commitments that AWS makes are also broken two into two different groups:
The first group is a Region-Level Service Level Agreement that will cover services deployed across more than one Availability Zone (AZ) or AWS Region.
The Second group is an Instance-Level Service Level Agreement that describes AWS commitments on the individual Compute (EC2) instance.
The reason for specifying two different groups, with two different levels, is due to the fact that AWS can generally offer greater commitments
when services are used across multiple Availability Zones. Availability Zones are geographically separate “data centers” so to speak. So if one AZ goes
down due to failures or a natural disaster, the other AZ’s would likely be unaffected. For more information about Availability Zones and Regions see
What Are AWS Availability Zones?. At the time of writing this article, a single EC2 instance in one
Availability Zone has a guaranteed uptime percentage of 99.5%. A multi Availability Zone EC2
concurrent deployment has a monthly uptime percentage guarantee of 99.99%. This is a simple calculation of .995 x .995 = .99 or 99%. For more information
about calculating availability for your AWS systems see How Do I Calculate Availability in AWS?
What happens if AWS fails to meet its Service Level Agreements?
AWS is a well managed cloud service with high levels of availability, but failures can happen. So what happens when AWS fails to meet the percentage
uptime commitments that it guarantees it’s end users? If a Service Level Agreement is not met AWS offers Service Credit Percentages. A Service Credit
Percentage is a percentage credit towards your bill for that service during the time AWS did not meet the monthly uptime percentage.
As an example, if a
regional level EC2 deployment is under 99.99% but equal to or greater than 99.0% AWS will give you a service credit percentage or 10%. The service credit
percentages increase as the monthly uptime percentage drops further below what was promised. Credit percentages are typically not applied automatically.To
request a service credit you should open a case with the AWS Support Center.
