Scheduled-images-service

• Launchpad Entry: GlanceSpec:scheduled-images-service
• Created: 29 Oct 2012
• Contributors: Alex Meade, Eddie Sheffield

Summary

This blueprint introduces a new service, QonoS, for scheduling periodic snapshots of an instance. Although periodic snapshots could be implemented very quickly using cron or as one of nova's current periodic tasks, it would not prove reliable.

Service responsibilities include:

• Create scheduled tasks
• Allow enabling and disabling of scheduled images for a specific instance. Enabling will include specifying a 'retention' number.
• Allow a "get" operation to determine whether an instance has scheduled_images enabled and what its retention number is.
• Allow listing of all instances with scheduled images set (and each instance's retention number) for the current tenant
• Determine specific schedules with regards to load balancing
• Handle rescheduling failed jobs
• Maintain persistant schedules
• Removal of images outside of rotation count

Scalability

Creating a new, self-standing service allows for scaling the feature independently of the rest of the system.

Reliability

Users of the API may come to rely on this feature working every time or notifying them of failures.

It is important to have a scheduling service that understands information such as instances, tenants, etc if there is any desire to recover from errors or make performance decisions based on such information. This is opposed to having a more generic 'cron' service that knows nothing of the concept of an instance or image.

For example, listing schedules of a particular tenant would be much more efficient if the tenant was in a DB column instead of a blob in the DB.

System Picked Schedules

Instead of the user being able to pick the time of snapshot, the system can make (potentially) informed decisions about how to spread out the schedules. This way there could not be cases where a majority of the users pick midnight and produce too much load for that time.

Design

Entities

• Schedule
• * the general description of what the service will do
• * looks something like

{
  "tenantId" : <tenantId>,
  "scheduleId" : <scheduleId>,
  "job_type" : <keyword>,
  "metadata" : {
    // all the information for this job_type
    "key" : "value"
  }
   "schedule" : <the schedule info, exact format TBD>
}

• Job
• * a particular instance of a scheduled job_type
• * i.e., this is the thing that will be executed by a worker
• Worker
• * a process that performs a Job

The QonoS scheduling service has the following functional components:

• API
• * handles communication, both external requests and internal communication
• * creates the schedule for a request and stores it in DB
• * the only job_type we will implement is 'scheduled_image'
• Job Maker
• * creates Jobs from schedules; the idea is that the Jobs table will

consist of Jobs that are ready to be executed for the current time period

• Job Monitor
• * keeps the Job table updated
• Worker monitor
• * looks for dead workers
• Worker
• * executes a job

API

CRUD for schedules

POST /v1/schedules
GET /v1/schedules
GET /v1/schedules/{scheduleId}
DELETE /v1/schedules/{scheduleId}
PUT /v1/schedules/{scheduleId}

Request body for POST, PUT will be roughly the Schedule entity described above. POST would return the scheduleId.

CRUD for jobs

GET /v1/jobs
GET /v1/jobs/{jobId}
PUT /v1/jobs/{jobId}
content is job status
PUT /v1/jobs/{jobId}/heartbeat
heartbeat for this job

NOTE: no POST, DELETE, the job maker handles job creation/deletion

Worker related

GET /v1/workers/{workerId}/jobs/next
(return job info, format TBD)
POST /v1/workers
(returns a workerId, is done when a worker is instantiated, allows the system to keep track of the worker)
PUT /v1/workers/{workerId}
supplies a heartbeat
}}
==== Disable scheduled images ====
{{{
DELETE /servers/<id>/scheduled_images

No response body.

Show scheduled images setting

 GET /servers/<id>/scheduled_images

(If the scheduled images feature is not enabled for this instance, the API will return a 404.)

Service

The service shall consist of a set of apis, worker nodes, and a DB.

API - Provides a RESTful interface for adding schedules to the DB

Worker - References schedules in the DB to schedule and perform jobs

DB - Tracks schedules and currently executing jobs

Implementation

Typical flow of the system is as follows.

1. User makes request to Nova extension
2. Nova extension passes request to API
3. API picks time of day to schedule
4. Adds schedule entry to DB
5. Worker polls DB for schedules needing action
6. Worker creates job entry in DB
7. Worker initiates image snapshot
8. Worker waits for completion while updating 'last_touched' field in the job table (to indicate the Worker has not died)
9. Worker updates DB to show the job has been completed
10. Worker polls until a schedule needs action

Edge cases:

Worker dies in middle of job:

• A different worker will see the job has not been updated in awhile and take over, performing any cleanup it can.
• Jobs contain information of where they left off and what image they were working on (this allows a job whose worker died in the middle of an upload to be resumed)

Image upload fails

• Retry a certain number of times, afterwards leave image in error state

Instance no longer exists

• Remove schedule for instance

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