The problem with ACS….

…is that it currently has no published SLAs and there’s no out-of-the-box solution for Disaster Recovery in an active scenario (as in – web services, REST services)

Let me expand on these two points –

I could not find any SLA information about ACS, so I asked Microsoft who responded that, as things stand ACS is a free service and they do not offer SLAs for free. This might change as and when ACS is properly rolled into Microsoft Azure Active Directory with its paid, Premium option, but that’s not the case at the moment.

Pragmatically – I have not seen any ACS issues in the past and there are several Azure services that do carry SLA that take a dependency on ACS, namely the Service Bus and BizTalk Services. Make of that what you will – you have to either accept or reject this risk.

To make matters worse – there’s no easy solution for DR for ACS in an active scenario.

Federated identity follows two approaches – when dealing with users and browsers the passive approach is used and that relies on browser redirects. This means that the web app is responsible for redirecting users to the Identity Provider (IdP) and holds the configuration of where the IdP is. That means, for example, a web role deployed across data centres and fronted by Traffic Manager, can have separate configurations to redirect users to different ACS namespaces, depending on the environment.

When dealing with services (SOAP with WS-Federation or REST with oAuth) the service client holds the configuration about where the IdP is (in the SOAP case this is part of the service contract in oAuth it is simply exchanged out of band) and it is the service client’s responsibility to go and get a token from the IdP before calling the service.  This means that if the ACS namespace changes (for example due to outage and switch to DR), the client will need to know to go to the alternative the service won’t tell it automatically.

With this in mind, what are the options?

I can think of 3 or 4. would love to hear additional views –

1. Accept the risk and the need to reconfigure clients when switching to DR.

2. If you have control over the client – implement an operation to automatically (on failure) and/or regularly request the metadata from your web role, which would allow it to auto-recover

3. If either of the above are not acceptable – roll your own oAuth solution on your web role which would mean that client access the same url (managed by traffic manager) to access the service and the IdP.

A better alternative is to use the bit of ACS that is rolled into Active Directory through the configuration of Applications for both the Web API and the client. I’ll post an entry on this shortly. the upside is that if you then make sure your directory is a Premium one you are covered by SLAs, although I’m still not sure what the DR story for AD is.

The main downside of this approach at this point in time is that, currently at least, there’s not much configuration that can be done – it only supports a client secret and JWT tokens with no control over token TTL etc. I also the the process is quite fiddly, requiring tempering with the app manifest manually etc, but I’m sure all of this will improve over time.

[cross posted on the Solidsoft Blog]

About Yossi Dahan
I work as a cloud solutions architect in the Azure team at Microsoft UK. I spend my days working with customers helping be successful in the cloud with Microsoft Azure.

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