CloudPatterns.org

Problem

Certain IT resources can only be accessed using an assigned path (hyperlink) that leads to the location of the IT resources. The path can be inadvertently lost or incorrectly defined by the cloud consumer or changed by the cloud provider. When a cloud consumer no longer possesses the correct and exclusive path to an IT resource, this IT resource becomes inaccessible and unavailable. When this unavailability occurs without warning at runtime, exception conditions can result that compromise the stability of larger cloud solutions that depend on the IT resource's availability.

Figure 1 - Physical Server A is connected to LUN A via a single fiber channel, and uses LUN A to store different types of data (1). The fiber channel connection becomes unavailable due to a HBA card failure and invalidates the path used by Physical Server A (2), which has consequently lost access to LUN A and all of the data it is currently storing.

Solution

A multipathing system is established to provide alternative paths to IT resources providing cloud consumers with a means of programmatically or manually overcoming path failures.

Figure 2 - A multipathing system providing alternative paths to a cloud storage device.

Application

The application of this pattern requires the use of a multipathing system and the creation of alternative paths (or hyperlinks) that are assigned to specific IT resources. The alternative paths may be physical or virtual. The multipathing system resides on the server or hypervisor, and ensures that each IT resource can be seen via each alternative path identically.

Figure 3 - An example of a multipathing system.

1. Physical Server A is connected to the LUN A storage device via two different paths.
2. The LUN A is seen as different LUNs from each of the two paths.
3. The multipathing system is put in place and configured.
4. LUN A is seen as one identical LUN from both paths.
5. Physical Server A has access to LUN A from two different paths.
6. A link failure occurs and one of the paths becomes unavailable.
7. Physical Server A can still use LUN A because the other link remains active.

Figure 4 - In some cases, a specific driver is required by the operating system to ensure that it understands the redundant paths and does view two paths leading to the same IT resource as two separate IT resources.

NIST Reference Architecture Mapping

This pattern relates to the highlighted parts of the NIST reference architecture, as follows: