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On cloud services web roles you must create at least 2 instances to get Azure's 99.95% SLA because when they do updates to the servers' OS etc, Azure will need to restart the machines (one at a time). This is well documented and you will be shouted at by the portal when running a one instance web role for doing this.

With Azure websites you don't get such warnings and I cannot find any documentation suggesting 2 instances for the 99.9% SLA. However, I also can't find any documentation saying that you don't need 2 instances for this SLA.

So which is it? Do I need 2 instances for Azure websites' SLA like web roles or is that a thing of the past?

BritishDeveloper
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3 Answers3

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A developer on the Azure Websites team has replied here that

the SLA for any Azure Website on the Basic or Standard tier is 99.95%

Just having one instance of the site is enough

Community
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mvark
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    Thanks for finally answering this important question. FYI I have now been running one instance websites without any signs of downtime since January 2015 – BritishDeveloper May 03 '15 at 09:14
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You can download the Azure Web Sites SLA document here: http://azure.microsoft.com/en-us/support/legal/sla/

It does not state that you need 2 instances to get the 99.9% SLA but clearly state that the Free and Shared Web Site Tiers are not covered by this SLA.

CSharpRocks
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    I read that when I was researching. The thing that bothers me is that it although it doesn't state that you do need two instances for the SLA it doesn't state you *don't* either. Given how much emphasis azure put on needing two web role instances I would have thought they would explicitly state that it doesn't apply to websites. Hence the confusion :( – BritishDeveloper Oct 31 '14 at 14:27
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We guarantee that Web Apps, Mobile Apps, Logic Apps, and API Apps running in a customer subscription will be available 99.9% of the time, with a 99.95% SLA provided when multiple instances and traffic manager are used. No SLA is provided for Apps or sites under either the Free or Shared tiers.

Daniele Grandini
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    Where is the 99.9% for 1 instance and 99.95% for multiple instances, statement is mentioned on the Azure documentation? It just says here: https://azure.microsoft.com/en-us/support/legal/sla/app-service/v1_4/ We guarantee that Apps running in a customer subscription will be available 99.95% of the time. No SLA is provided for Apps under either the Free or Shared tiers. – user1763470 May 04 '17 at 10:36