Haizea

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# Copyright 2006-2008, University of Chicago                                 #

# Copyright 2008, Distributed Systems Architecture Group, Universidad        #

# Complutense de Madrid (dsa-research.org)                                   #

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"""Haizea uses a policy manager that allows certain scheduling decisions to

be delegated to pluggable policies. This is done so scheduling policies

can be (1) modified without having to modify core components of Haizea, and

(2) implemented by writing a single Python class that implements a given

interface for pluggable policies.

Three policies are currently pluggable: lease preemptability ("Can lease X

preempt lease Y?"), host selection ("I want to deploy a VM, what host should

I select for this?") and lease admission ("Should I accept/reject this lease

request?"). Haizea provides several simple policy modules in the

haizea.policies package. The policy to use is selected in the configuration

file. See the Haizea Documentation for more details on how this is done.

This module provides Haizea's policy manager and the base classes for

pluggable policies.  

"""

from haizea.common.utils import abstract

from haizea.core.leases import Lease

from mx.DateTime import DateTimeDelta

import operator

class PolicyManager(object):

    """The Policy Manager

    This class manages the policy modules and provides methods to

    access these modules.

    """    

    def __init__(self, admission, preemption, host_selection):

        """Constructor

        Expects fully-constructed policies (these are currently

        loaded in the Manager class, based on the config file).

        Arguments:

        admission -- A child of LeaseAdmissionPolicy

        preemption -- A child of PreemptabilityPolicy

        host_selection -- A child of HostSelectionPolicy

        """

        self.admission = admission

        self.preemption = preemption

        self.host_selection = host_selection

    def sort_leases(self, preemptor, preemptees, time):

        """Sorts a list of leases by their preemptability

        Takes a list of leases (the "preemptees"), determines their preemptability

        by another lease (the "preemptor"), and returns a list with the

        leases sorted by decreasing preemptability score (most preemptable

        leases first)

        See documentation of PreemptabilityPolicy.get_lease_preemptability_score

        for more details on the preemptability score.

        Argument

        preemptor -- Preemptor lease

        preemptees -- List of preemptee leases

        time -- Time at which preemption would take place        

        """              

        leases_score = [(preemptee, self.get_lease_preemptability_score(preemptor,preemptee, time)) for preemptee in preemptees]

        leases_score = [(preemptee,score) for preemptee,score in leases_score if score != -1]

        leases_score.sort(key=operator.itemgetter(1), reverse=True)

        return [preemptee for preemptee,score in leases_score]

    def sort_hosts(self, nodes, time, lease):

        """Sorts a list of hosts by their score

        Takes a list of hosts, determines their score, and sorts them in

        order of decreasing score (most desireable hosts first)

        See documentation of HostSelectionPolicy.get_host_score for more details.

        Arguments:

        nodes -- List of physical node (the integer identifier used in the slot table)

        time -- Time at which the lease might be scheduled

        lease -- Lease that is being scheduled.

        """        

        nodes_score = [(node, self.get_host_score(node, time, lease)) for node in nodes]

        nodes_score.sort(key=operator.itemgetter(1), reverse=True)

        return [node for node,score in nodes_score]

    def accept_lease(self, lease):

        """Lease admission function

        Returns True if the lease can be accepted, False if it should be rejected.

        Argument

        lease -- Lease request

        """        

        return self.admission.accept_lease(lease)

    def get_lease_preemptability_score(self, preemptor, preemptee, time):

        """Computes the lease preemptability score

        See documentation of PreemptabilityPolicy.get_lease_preemptability_score

        for more details.

        Arguments:

        preemptor -- Preemptor lease

        preemptee -- Preemptee lease

        time -- Time at which preemption would take place

        """                

        return self.preemption.get_lease_preemptability_score(preemptor, preemptee, time)

    def get_host_score(self, node, time, lease):

        """Computes the score of a host

        See documentation of HostSelectionPolicy.get_host_score for more details.

        Arguments:

        node -- Physical node (the integer identifier used in the slot table)

        time -- Time at which the lease might be scheduled

        lease -- Lease that is being scheduled.

        """               

        return self.host_selection.get_host_score(node, time, lease)

class LeaseAdmissionPolicy(object):

    """Lease Admission policy

    This is the parent class of lease admission policies. A lease admission

    policy determines whether a given lease request should be accepted or not

    by Haizea. Note that this is distinct from whether the lease can be

    scheduled or not (although this could certainly be a part of the

    policy); the policy simply decides whether the lease can be considered for

    scheduling or not. For example, a user could submit an AR lease that must

    start in 5 hours, but the policy could dictate that all ARs must be notified

    at least 24 hours in advance (and the lease would be rejected, regardless of

    whether there was resources available for it in 5 hours). Similarly, an

    AR lease could be requested 48 hours in advance, be accepted by the lease

    admission policy, but then be rejected by the scheduler if there are no

    resources available.

    """       

    def __init__(self, slottable):

        """Constructor

        Argument

        slottable -- A fully constructed SlotTable

        """

        self.slottable = slottable

    def accept_lease(self, lease):

        """Lease admission function

        Returns True if the lease can be accepted, False if it should be rejected.

        Argument

        lease -- Lease request

        """        

        abstract()

class PreemptabilityPolicy(object):

    """Lease Preemptability policy

    This is the parent class of lease preemptability policies. This type of

    policy is used to determine whether a lease can be preempted by another

    lease at a given time. However, the policy doesn't return True or False but,

    rather, a "preemptability score" (see get_lease_preemptability_score for

    more details)

    """           

    def __init__(self, slottable):

        """Constructor

        Argument

        slottable -- A fully constructed SlotTable

        """        

        self.slottable = slottable

    def get_lease_preemptability_score(self, preemptor, preemptee, time):

        """Computes the lease preemptability score

        Given a lease that needs to preempt resources (the "preemptor"),

        another lease (the "preemptee") that may be preempted by it, and a time,

        this method determines the preemptability score of the preemptee or

        "how preemptable is the preemptee by the preemptor at the given time".

        The score can be the following:

        -1 : Cannot be preempted under any circumstances

        0.0 <= x <= 1.0: Lease can be preempted. The higher the score,

        the "more preemptable" it is (this is a relative measure; the score

        should be used to determine which of several leases is a better

        candidate for preemption)

        Arguments:

        preemptor -- Preemptor lease

        preemptee -- Preemptee lease

        time -- Time at which preemption would take place

        """             

        abstract()    

    def _get_aging_factor(self, lease, time):

        """Returns an aging factor for the preemptability score

        This is a convenience function that can be used to "age" a

        preemptability score (allowing leases that have been submitted

        long ago to avoid preemption). The method returns a factor

        between 0 and 1 that can be multiplied by the score, reducing

        the score based on the lease's "age".

        Currently, this method uses a hard-coded horizon of 31 days

        (any lease older than 7 days cannot be preempted, and leases

        less than 7 days are assigned a factor proportional to their age)

        Arguments:

        lease -- Lease that is going to be preempted

        time -- Time at which preemption would take place        

        """            

        # TODO: Make horizon configurable

        horizon = time - DateTimeDelta(7)

        if lease.submit_time <= horizon:

            return -1

        else:

            seconds = (time - lease.submit_time).seconds

            horizon_seconds = DateTimeDelta(31).seconds

            return float(horizon_seconds - seconds) / horizon_seconds        

class HostSelectionPolicy(object):

    """Host Selection policy

    This is the parent class of host selection policies. When mapping VMs

    to physical hosts, this policy determines what hosts are more desireable.

    For example, an energy-saving policy might value hosts that already have

    VMs running (to leave as many empty machines as possible, which could then

    be turned off), whereas another policy might prefer empty hosts to make

    sure that VMs are spread out across nodes.

    To do this, the policy will assign a score to each host. See the documentation

    for get_host_score for more details.

    """             

    def __init__(self, slottable):

        """Constructor

        Argument

        slottable -- A fully constructed SlotTable

        """        

        self.slottable = slottable

    def get_host_score(self, node, time, lease):

        """Computes the score of a host

        Given a physical host, a time, and a lease we would like to

        schedule at that time, this method returns a score indicating

        how desireable that host is for that lease at that time.

        The score can be between 0.0 and 1.0. The higher the score,

        the "more desireable" the physical host is (this is a relative measure; 

        the score should be used to determine which of several physical hosts

        is more desireable for this lease).

        Arguments:

        node -- Physical node (the integer identifier used in the slot table)

        time -- Time at which the lease might be scheduled

        lease -- Lease that is being scheduled.

        """               

        abstract()