Information Technology Reference
In-Depth Information
Client
Save session
[“age”] = 16
Read session
[“age”]
Load Balancer
Session
Saved
[age] = ?
Instance 1
Instance 2
FIGURE 7.7
Session state problem with multiple web roles.
different web role for each request it makes, this will cause the application
to malfunction. There are various ways to solve this problem but the quick-
est and easiest is to use the Azure AppFabric cache to hold the session state
outside the web roles. Also, this cache can be used for in-memory buffering
of static contents of the application, hence improving the performance.
As far as the hosted service is concerned, it is up to the certain subscriber
how many web role instances the subscriber wants to create. Microsoft
requires at least two web roles to ensure 99.9% uptime. In this case, three
web role instances are running the application. In case of failure of a web
role, other web roles take over and keep serving requests. As soon as Azure
detects the failure, it replaces the faulty web role.
As Azure keeps by default three copies of storage and databases in three
different domains of Microsoft data centers, there is no need to keep a backup
database server or storage infrastructure. Once a fault is detected within
a database, Azure automatically disconnects the faulty copy, acquires a new
database instance from the Cloud, and synchronizes it with the remaining
copies. As a result, when a failure occurs, it can have some effect on perfor-
mance until the new database instance is ready. The same is true for storage.
7.5 Evaluation
For evaluation purposes, the applicative database has been populated with
150,000 demonstration road accident records for Scotland. To functionally
test the system, a set of tasks has been devised to search the records in the
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