SLA: Controller

	At a Glance
	Catalog	Dynamic
	Category	Application Controller
	User volumes	yes
	Min. memory	256 MB
	OS	Linux
	Constraints	no
	Questions/Comments	Ask Forum

Functional Overview

The policy is configurable through a simple web interface (GUI) exposed by SLA. The GUI is accessed using the application IP and the configured SLA port. A policy comprises:

• counter entity (i.e., CPU Summary)
• counter name (i.e., CPU Total)
• start value (i.e., 30)
• stop value (i.e., 10)
• trailing average period (i.e., 15 minutes)

SLA periodically calculates a trailing average for the counter by averaging the counter values of the running appliances in the appliance group over the trailing average period. When the trailing average passes the start value, SLA starts an appliance in the appliance group. When the trailing average passes the stop value, SLA stops an appliance in the appliance group.

SLA is typically used to start and stop individual web servers within the scalable web server appliances WEBx4/WEBx8 in response to HTTP request load.

SLA works only with MON appliances version 1.1.3 and above. To verify the version of any MON appliance in your application, right-click on the appliance and select view class.

Boundary

Resources

Terminals

User Volumes

This volume is used to store:

• a private key file which provides authentication as a normal user on the grid controller
• the policy
• temporary files
• a log of SLA operations (provided the log terminal is unconnected)

This volume should be 10MB in size. See Preparing for Use for information on setting up a private key on the config volume.

Properties

Performance

Additional resources do not affect performance.

Error Messages

The following messages may appear in either the appliance log file or the system log of the grid controller when the appliance fails to start:

• httpd did not start
• sla setup failed
• /var/www/html/.rc.local script is not executable
• /var/www/html/.rc.local script failed, ret=ret

Operational Constraints and Behavior

• SLA tracks the values of only a single counter (i.e. Total CPU Usage) for all the running appliances in the appliance_group (i.e. web servers within a scalable web server appliance such as WEBx4 or WEBx8).
• When the trailing average of the counter falls outside of the configured range, SLA starts or stops an appliance in the appliance_group.
• SLA never stops all appliances with the appliance_group.
• If all appliances within the appliance_group are running and the trailing average passes beyond the start value, SLA logs a message on the AppLogic Dashboard.
• Counter data is collected from the mtr terminal every 5 seconds.
• Counter data is retained to an historical depth of 24 hours.
• Policy enforcement occurs once per minute.
• Once an appliance is started or stopped, policy enforcement does not resume until 6 minutes later.

Web Interface (GUI)

SLA exposes a web interface which is used to define the policy. This GUI is accessed using the application IP on the configured port port_no.

Above is an example of the GUI when it is first accessed. The Commit button commits the policy defined in the GUI for enforcement.

Above is an example of the GUI when a policy is being enforced. The graph shows the last 24 hours of policy enforcement and includes:

• a graph of the counter values averaged over the running appliances
• a graph of the trailing average
• a graph of the number of running appliances in appliance_group
• a horizontal line marking the start value
• a horizontal line marking the stop value
• a dashed vertical line marking the trailing average period

The buttons at the top of the GUI are used to:

button	action
	Stop enforcement of the policy. While enforcement is stopped, SLA will not start or stop appliances in accordance with the policy.
	Start enforcement of the policy. SLA resumes enforcement and will start and stop appliances in accordance with the policy.
	Edit the policy.
	Contract the graph width.
	Expand the graph width.
	Manually stop an appliance in appliance_group.
	Manually start an appliance in appliance_group.

Preparing to Use SLA in an Application

These steps must be taken before SLA can be used to dynamically start and stop appliances within an application. A step-by-step example is provided here.

• SLA must be able to authenticate on the grid controller as a normal user. To ensure this, create a public/private key pair in openssh format without a passphrase. Create a user on the grid whose public key corresponds to the generated public key. Put the generated private key file, with the name grid.private.key, on the root of the config volume of the appliance.

Typical Usage

LampX4

The diagram below shows how SLA can be incorporated in the LampX4 reference application. The INSSL gateway properties are set to forward tcp traffic on port 8080 to its aux terminal. Pointing a browser at port 8080 of the LampX4 application IP or resolvable domain name brings up the web interface for SLA.

Note: the SLA appliance should not be connected directly between an IN gateway and the rest of the application, but rather to the aux terminal of an INSSL gateway.

A step-by-step example for setting up the LampX4 use case is provided here.

Notes and Links

Notes

• It is good practice to set the username and password properties of SLA to prevent unauthorized users from accessing the SLA GUI.
• If SLA fails to start or stop an appliance in appliance_group, a message is logged on the AppLogic Dashboard. SLA will not try to start or stop the failed appliance again until SLA has been re-started.
• On start SLA accesses the grid controller through its net terminal. SLA must start after any appliance connected to its net terminal.
• SLA fails to start if any of these conditions is true:
  • the property grid_ctl_ip is incorrect
  • the grid.private.key file does not exist
  • the grid.private.key file does not authenticate on the grid controller
  • the appliance_group property does not resolve to two or more appliances within the application

Related Documents

• Implementation Design
• Test Plan
• Release Notes

• Step-by-step Example

• Live Demo Application?

Questions and Comments

To post a question about this appliance, visit our forum.

-- StephenQ - 17 Nov 2007