From ControlTier

Intent

Define an object that encapsulates the lifecycle of a software service.
Abstract the startup and shutdown methods to control the runtime state of a service.

Problem

We want to control the runtime state of application services, but each service must be controlled through proprietary or operating specific interfaces, and often must be invoked differently depending on application environment context.

Discussion

While the Deployment type provides a standard interface and properties to manage the life cycle of a software configuration, it does not assume the software has runtime state. For example, imagine that following the installation and configuration of a Deployment, a file must be executed to start a long running process to handle requests. One would need to know how to check if the process was already running to avoid starting multiple processes which may cause undesirable conflicts. Likewise, if one had to shut down the service, one might need to consider what to do if the normal stop operation fails.

When one considers the common details and requirements for services they can be organized into different categories:

parameters: these are key attributes that describe the service
state management: these are the sequence of procedures needed to start,

stop and check status of the service.

Because service implementations vary widely and do not present a consistent management interface, the administrator must often possess either intimate knowledge of the application code or operating system features to control a service's runtime state.

Compounding the need to have a consistent approach to starting and stopping a wide variety of services, the administrator must also adjust for environmental differences; varying command line options or running commands in different directories.

A preferred option would be to define an abstraction that builds on the Deployment type which already includes support for coordinating the installation and configuration of a software component but to add to that, standard strategies for managing the runtime state of a service.

Using this model, a generic logical skeleton for starting up and shutting down a service can be embodied yet the details for interfacing with the underlying service executables and or operating system can be provided by classes derived from the generic abstraction.

This approach is particularly useful because it separates the variant and invariant behavior, minimizing the amount of code to be written.

Structure

The Service type inherits the capabilities and relationships from Deployment incorporating startup and shutdown logic to the deployment update sequence.

The essential service properties are:

Properties
Name	Description
startup-rank	A value specifying a level relative to other services.

Example

The Service type defines an object that provides methods to manage the runtime state and deployment of a software service.

The diagram below describes the startup of an Apache web server managed via the Service abstraction. One can begin integrating the Service's procedural interfaces using the provided apachectl command. For example, startService can be defined as apachectl start and stopService as apachectl stop. But one might find themselves in the situation where they are unable to use the standard apachectl command due to the way Apache was installed or must be invoked. In these cases, the Service runtime state management commands can be defined to meet the specific needs yet the administrator can still rely on the standard management interfaces: Stop, Start and Status

Check List

Identify the procedures for starting and stopping the application service
Define the assertServiceIsUp, assertServiceIsDown, startService and stopService commands using the identified procedures.