A java utility for managing the deterministic execution of start and stop tasks for a directed acyclic graph of interdependent services.
Tasks are triggered on a service when all its dependencies have successfully executed their tasks. Either an individual service or all services can be started or stopped interactively. The topological order of managed services is calculated by the service starter to determine the correct execution order of tasks.
In many systems services execute independently but need to co-ordinate their lifecycle with each other. A service may require all downstream services to be started before starting and becoming available to accept upstream requests. If this order is not respected then unpredictable behaviour can occur, possibly resulting in application errors.
Similarly, if a downstream service becomes unavailable all upstream services will need to be notified and take appropriate actions, gracefully shutting down services starting with external facing services and working back towards the failed service.
As systems grow a complex graph of interdependent services quickly arises. The difficulty in correctly managing lifecycle overwhelms a handwritten manual solution and can result in brittle non-deterministic behaviour that services may rely upon.
Service starter is a utility that manages the lifecycle of independent services, executing start and stop tasks associated with a particular service at the correct time.
An application is an event driven system with independent processes A,B,C and D providing application functionality. When the application is running events flow from A to B and C in parallel, then events are pushed to D from both B and C. For A to function correctly B,C and D must be running.
For the example above all services are in a stopped state and a request is made to start A.
To start A ServiceManager produces the following outputs, and processes state change inputs from the application:
| Step | Action |
|---|---|
| 1 | ServiceManager produces task list with start task for D |
| 2 | D completes task and sends a notification D has started successfully to the ServiceManager |
| 3 | ServiceManager produces task list with B and C start tasks |
| 4 | B completes task and sends a notification B has started successfully to the ServiceManager |
| 5 | Service manager does nothing, as C has not started |
| 6 | C completes task and sends a notification C has started successfully to the ServiceManager |
| 7 | ServiceManager produces task list with start task for A |
| 8 | A completes task and sends a notification A has started successfully to the ServiceManager |
| 9 | ServiceManager produces no task list as there are no dependents on A to start |