This is all about details on High Availability of Applications running in Azure. Would cover on fundamentals of High Availability in Azure and discuss in depth on PaaS (High Availability of Web Role and Worker Role).

1. High Availability of Azure
Applications(PaaS)
Himanshu Sahu
Mindfire Solutions
himanshus@mindfiresolutions.com

2. Agenda
Introduction
Windows Azure Role Architecture
Fault Domains in Windows Azure
Update Domains in Windows Azure
Windows Azure Host OS Updates
Windows Azure Guest OS Updates
Techniques for High Availability

3. High Availability in Azure
Introduction
ALWAYS ON
Reliability and Scalability
Design for failure
Implement separation of function
Use a service-oriented architecture

4. Windows Azure Role Architecture

5. Fault Domains in Windows Azure
Fault Domains
Fault Domain is a physical unit of failure, and is closely related to
the physical infrastructure in the data centers. In Windows Azure
the rack can be considered a fault domain. However there is no 1:1
mapping between fault domain and rack.
Windows Azure Fabric is responsible to deploy the instances of
your application in different fault domains. Right now Fabric makes
sure that your application uses at least 2 (two) fault domains.
As a developer have no direct control over how many fault
domains your application will use.

6. Update Domains in Windows Azure
Update Domains
Upgrade Domain is a logical unit, which determines how particular
service will be upgraded.
The default number of upgrade domains that are configured for
your application is 5 (five). You can control how many upgrade
domains your application will use through the upgradeDomain
configuration setting in your service definition file (CSDEF).

7. Windows Azure Host Updates
When and Why
Windows Azure deploys updates to the host OS approximately
once per month. This ensures that Windows Azure provides a
reliable, efficient and secure platform for hosting your applications.
The HA consists of multiple subcomponents, such as the Network
Agent (NA) that manages virtual machine VLANs and the Virtual
Machine virtual disk driver that connects Virtual Machine disks to
the blobs containing their data in Windows Azure Storage. Azure
therefore update the HA and its subcomponents at different
intervals, depending on when a fix or new functionality is ready.

8. Windows Azure Host Updates

9. Windows Azure Host Updates
How
The host OS reboots instances and the fabric controller ensures that only
instances from one upgrade domain at a time will be rebooted.
Virtual machines running on the server that have an Input Endpoint in
their role’s service model are removed from the load balancer rotation so
that no new requests will come to the virtual machine and instead new
requests are sent to other instances of that role as per the Azure load-
balancing policies.
Each virtual machine hosting a Web or Worker Role receives a Stopping
event, whereas VM Roles receive a standard Windows shutdown event.
Worker, Web, and Virtual machine roles are allowed five minutes to
respond to the stopping and shutdown event before they are forcibly
stopped.

10. Windows Azure Host Updates
How
After all guest virtual machines are stopped, the root partition OS
shuts down and the server reboots.
The updated root partition OS starts.
The virtual machines hosted on the server boot and start their
application code.
Virtual machines hosting service roles with Input Endpoints
reconnect to the load balancer, enabling them to receive client
request

11. Windows Azure Guest Updates
Once the Host OS has finished upgrading across the datacenter
then the Guest OS will be upgraded for services which are
configured to use automatic Guest OS versions and this upgrade
will proceed using standard upgrade domain rules for your service.
Your VM will be rebooted and the Windows Partition (the D drive)
will be reimaged with the upgraded OS.
The Guest OS update process is much faster than the Host OS
update since the fabric only has to coordinate the update within
your hosted service and your upgrade domains.

12. Availability
An available application considers the availability of its underlying
infrastructure and dependent services. Available applications
remove single points of failure through redundancy and resilient
design
Azure SLA
More Instances in Azure
Make Guest OS Update Manual

13. Availability
Scalability directly affects availability—an application that fails
under increased load is no longer available. Scalable applications
are able to meet increased demand with consistent results in
acceptable time windows.
Auto Scaling in Azure

14. Availability
Protection against hardware failures
Because every application is made up of multiple instances of each
role, hardware failures—a disk crash, a network fault, or the death
of a server machine—won’t take down the application. To help
with this, the fabric controller doesn’t choose machines for an
application’s instances at random. Instead, different instances of
the same role are placed in different fault domains. A fault domain
is a set of hardware—computers, switches, and more—that share a
single point of failure. (For example, all of the computers in a single
fault domain might rely on the same switch to connect to the
network.) Because of this, a single hardware failure can’t take
down an entire application. The application might temporarily lose
some instances, but it will continue to behave correctly.

15. Availability
Protection against software failures
The fabric controller can also detect failures caused by software. If
the code in an instance crashes or the VM in which it’s running
goes down, the fabric controller will start either just the code or, if
necessary, a new VM for that role. While any work the instance
was doing when it failed will be lost, the new instance will become
part of the application as soon as it starts running.

16. Availability
The ability to update applications with no application
downtime
When a new version of the application needs to be deployed, the
fabric controller can shut down the instances in just one update
domain, update the code for these, then create new instances
from that new code. Once those instances are running, it can do
the same thing to instances in the next update domain, and so on.
While users might see different versions of the application during
this process, depending on which instance they happen to interact
with, the application as a whole remains continuously available.

17. Availability
The ability to update Windows and other supporting
software with no application downtime.
Answer is Update Domain. :)

18. Resources
https://msdn.microsoft.com/enus/library/azure/dn251004.aspx
http://blogs.msdn.com/b/kwill/archive/2011/05/05/windows-azure-r
http://blog.toddysm.com/2010/04/upgrade-domains-and-fault-doma
http://blogs.msdn.com/b/kwill/archive/2012/09/19/role-
instance-restarts-due-to-os-upgrades.aspx

19. Questions?

20. Thank you!