Difference between revisions of "Qonos-scheduling-service"

Revision as of 20:40, 3 May 2013

• Launchpad Entry: QonoS scheduling service
• Created: 3 May 2013
• Contributors: Alex Meade, Eddie Sheffield, Andrew Melton, Iccha Sethi, Nikhil Komawar, Brian Rosmaita

Summary

This document describes the design and API of QonoS, a distributed high-availability scheduling service that has been implemented for the cloud^[1]. QonoS is currently used as the scheduling component of a scheduled images service that is invoked by a Nova extension, so many of the examples in this document discuss that use case.

Service responsibilities include:

• Create scheduled tasks
• Perform scheduled tasks
• Handle rescheduling failed jobs
• Maintain persistent schedules

QonoS was designed to work with OpenStack and uses OpenStack common components.

Conceptual Overview

The system consists of:

• a REST API
• a database
• one or more schedulers, and
• one or more workers.

The API handles communication, both external requests and internal communication. It creates the schedule for a request and stores it in the database.

The scheduler examines schedules and creates jobs.

A job describes a task that must be performed.

A worker performs a task. It obtains a task by polling the API and picking up the first task it is capable of handling.

Job Lifecycle

Jobs have the following statuses:

• queued : the job is ready to be processed by a worker
• processing : the job has been picked up by a worker
• done : the worker processing this job has decided that the job has been successfully completed
• timeout : the worker processing this job has decided the job is taking too long and has stopped processing it. A job in this state can be picked up by another worker.
• error : the worker notes that something went wrong, but the job could be retried
• canceled : the worker decides that the job can't be done and should not be retried

Job Timeouts

There are two kinds of timeouts:

• hard timeout: once reached, the job is no longer available for retries
• soft timeout: is renewed by the worker, indicates that the worker is still doing the task (similar to a heartbeat)

Job Failures

Job failures are reported as job faults and stored in the database.

Scalability

Address or remove?

Reliability

Address or remove?

Overall System Diagram

Design

API

Schedules

Create Schedule

POST <version>/schedules
    {"schedule":
        {
            "tenant": "tenant_username",
            "action": "snapshot",
            "minute": 30,
            "hour": 2,
            "day": 3,
            "day_of_week": 5,
            "day_of_month": 23,
            "metadata":
            {
                "instance_id": "some_uuid",
                "retention": "3"
            }
        }
    }

List schedules

GET <version>/schedules
{
    "schedules":
    [
        {
            # schedule as above
        },
        {
            # schedule as above
        },
        ...
    ]
}

Query filters

• next_run_after - only list schedules with next_run value >= this value
• next_run_before - only list schedules with next_run value <= this value

Example

List schedules which start in the next five minutes

GET <version>/schedules?next_run_after={Current_DateTime}&next_run_before={Current_DateTime+5_Minutes}
GET <version>/schedules?next_run_after=2012-05-16T15:27:36Z&next_run_before=2012-05-16T15:32:36Z

Get a specific schedule

GET /v1/schedules/{id}

Update a schedule

PUT <version>/schedules/{id}
    {"schedule":
        {
            "minute": 45,
            "hour": 3
        }
    }

Delete a schedule

DELETE <version>/schedules/{id}

Jobs

Create job from schedule

POST <version>/jobs
    {"job": {"schedule_id": "some_uuid"}}

The action, tenant_id, and metadata gets copied from the schedule to the job.

Get a specific job

GET <version>/jobs/{id}
{
    "job":{
    {
        "id": "{some_uuid}",
        "created_at": "{DateTime}",
        "updated_at": "{DateTime}",
        "schedule_id": "{some_uuid}",
        "worker_id": "{some_uuid}",
        "tenant": "tenant_username",
        "action": "snapshot",
        "status": "queued",
        "retry_count": 0,
        "hard_timeout": "{DateTime}",
        "timeout": "{DateTime}",

        "metadata":
        {
            "key1": "value1",
            "key2", "value2"
        }
    }
}

List current jobs

GET <version>/jobs
{
    "jobs":
    [
        {
            # job as above
        },
        {
            # job as above
        },
        ...
    ]
}

Update status of a job

PUT <version>/jobs/{id}/status
{
    "status":
    {
        "status": "some_status",
        "timeout": "{datetime of next timeout}" (optional)
        "error_message":"some message" (optional)
    }
}

NOTE: The error_message field is only looked for if the status is ERROR. In the event of an ERROR status, an entry is created in the job_faults table capturing as much info as possible from the job. If an error_message is provided, it is included in the job fault entry.

Delete(finish) a specific job

DELETE <version>/jobs/{id}

Metadata

Set schedule/job metadata

PUT <version>/schedules/{id}/metadata
or
PUT <version>/jobs/{id}/metadata

Note: The resulting metadata for a schedule/job will exactly match what is provided.

{
    "metadata":
    {
        "each": "someval",
        "meta": "someval",
        "key": "someval",
    }
}

List all metadata for a schedule/job

GET <version>/schedules/{id}/metadata
or
GET <version>/jobs/{id}/metadata
{
    "metadata":
    {
        "instance_id": "some_uuid",
        "retention": "3"
    }
}

Workers

Register worker with API

POST <version>/workers
    {"worker":
        { "host": "a.host.name"}
    }

List workers registered with API

GET <version>/workers

# Not shown - id, created_at, and updated_at fields for each worker

{
    "workers":
    [
        { 
            "host": "a.host.name",
        },
        {
            "host": "a.host.name2"",
        },
        ...
    ]
}

Get a specific worker registered with API

GET <version>/workers/{id}
# Not shown - id, created_at, and updated_at fields for each worker
{
    "worker":
    {
        "host": "a.host.name"
    }
}

Unregister worker with API

DELETE <version>/workers/{id}

Grab next job for worker

This can also be interpreted as "Assign a new job to the worker". Note: this call doesn't map cleanly to normal RESTful practices since it is a POST but it has a return

POST <version>/workers/{id}/jobs

Request Body

 {"job":{"action":"snapshot"}}

Response Body

If an appropriate job is found:

{
    "job":
    {
        # job as returned below
    }
}

If no job is found:

{
    "job": None
}

Example Usage

As an example, consider a Nova API extension that allows users to request that daily snapshots automatically be taken of a server.

1. User makes request to Nova extension
2. Nova extension picks a random time for the snapshot of this server to be taken (or it could use some more sophisticated algorithm, key thing is we want the requests to be uniformly distributed) and makes a create schedule request to the QonoS API.
3. The QonoS API adds a schedule entry to the database
4. Worker polls DB for schedules needing action
5. Worker creates job entry in DB
6. Worker initiates image snapshot
7. Worker waits for completion while updating 'last_touched' field in the job table (to indicate the Worker has not died)
8. Worker updates DB to show the job has been completed
9. 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)

Code Repository

• https://github.com/rackspace-titan/qonos

References

1. ↑ QonoS was first described in https://wiki.openstack.org/wiki/Scheduled-images-service (29 October 2012).