github.com/bigcommerce/nomad@v0.9.3-bc/scheduler/feasible.go

package scheduler

import (
	"fmt"
	"reflect"
	"regexp"
	"strconv"
	"strings"

	version "github.com/hashicorp/go-version"
	"github.com/hashicorp/nomad/nomad/structs"
	psstructs "github.com/hashicorp/nomad/plugins/shared/structs"
)

// FeasibleIterator is used to iteratively yield nodes that
// match feasibility constraints. The iterators may manage
// some state for performance optimizations.
type FeasibleIterator interface {
	// Next yields a feasible node or nil if exhausted
	Next() *structs.Node

	// Reset is invoked when an allocation has been placed
	// to reset any stale state.
	Reset()
}

// JobContextualIterator is an iterator that can have the job and task group set
// on it.
type ContextualIterator interface {
	SetJob(*structs.Job)
	SetTaskGroup(*structs.TaskGroup)
}

// FeasibilityChecker is used to check if a single node meets feasibility
// constraints.
type FeasibilityChecker interface {
	Feasible(*structs.Node) bool
}

// StaticIterator is a FeasibleIterator which returns nodes
// in a static order. This is used at the base of the iterator
// chain only for testing due to deterministic behavior.
type StaticIterator struct {
	ctx    Context
	nodes  []*structs.Node
	offset int
	seen   int
}

// NewStaticIterator constructs a random iterator from a list of nodes
func NewStaticIterator(ctx Context, nodes []*structs.Node) *StaticIterator {
	iter := &StaticIterator{
		ctx:   ctx,
		nodes: nodes,
	}
	return iter
}

func (iter *StaticIterator) Next() *structs.Node {
	// Check if exhausted
	n := len(iter.nodes)
	if iter.offset == n || iter.seen == n {
		if iter.seen != n {
			iter.offset = 0
		} else {
			return nil
		}
	}

	// Return the next offset
	offset := iter.offset
	iter.offset += 1
	iter.seen += 1
	iter.ctx.Metrics().EvaluateNode()
	return iter.nodes[offset]
}

func (iter *StaticIterator) Reset() {
	iter.seen = 0
}

func (iter *StaticIterator) SetNodes(nodes []*structs.Node) {
	iter.nodes = nodes
	iter.offset = 0
	iter.seen = 0
}

// NewRandomIterator constructs a static iterator from a list of nodes
// after applying the Fisher-Yates algorithm for a random shuffle. This
// is applied in-place
func NewRandomIterator(ctx Context, nodes []*structs.Node) *StaticIterator {
	// shuffle with the Fisher-Yates algorithm
	shuffleNodes(nodes)

	// Create a static iterator
	return NewStaticIterator(ctx, nodes)
}

// DriverChecker is a FeasibilityChecker which returns whether a node has the
// drivers necessary to scheduler a task group.
type DriverChecker struct {
	ctx     Context
	drivers map[string]struct{}
}

// NewDriverChecker creates a DriverChecker from a set of drivers
func NewDriverChecker(ctx Context, drivers map[string]struct{}) *DriverChecker {
	return &DriverChecker{
		ctx:     ctx,
		drivers: drivers,
	}
}

func (c *DriverChecker) SetDrivers(d map[string]struct{}) {
	c.drivers = d
}

func (c *DriverChecker) Feasible(option *structs.Node) bool {
	// Use this node if possible
	if c.hasDrivers(option) {
		return true
	}
	c.ctx.Metrics().FilterNode(option, "missing drivers")
	return false
}

// hasDrivers is used to check if the node has all the appropriate
// drivers for this task group. Drivers are registered as node attribute
// like "driver.docker=1" with their corresponding version.
func (c *DriverChecker) hasDrivers(option *structs.Node) bool {
	for driver := range c.drivers {
		driverStr := fmt.Sprintf("driver.%s", driver)

		// COMPAT: Remove in 0.10: As of Nomad 0.8, nodes have a DriverInfo that
		// corresponds with every driver. As a Nomad server might be on a later
		// version than a Nomad client, we need to check for compatibility here
		// to verify the client supports this.
		if driverInfo, ok := option.Drivers[driver]; ok {
			if driverInfo == nil {
				c.ctx.Logger().Named("driver_checker").Warn("node has no driver info set", "node_id", option.ID, "driver", driver)
				return false
			}

			return driverInfo.Detected && driverInfo.Healthy
		}

		value, ok := option.Attributes[driverStr]
		if !ok {
			return false
		}

		enabled, err := strconv.ParseBool(value)
		if err != nil {
			c.ctx.Logger().Named("driver_checker").Warn("node has invalid driver setting", "node_id", option.ID, "driver", driver, "val", value)
			return false
		}

		if !enabled {
			return false
		}
	}
	return true
}

// DistinctHostsIterator is a FeasibleIterator which returns nodes that pass the
// distinct_hosts constraint. The constraint ensures that multiple allocations
// do not exist on the same node.
type DistinctHostsIterator struct {
	ctx    Context
	source FeasibleIterator
	tg     *structs.TaskGroup
	job    *structs.Job

	// Store whether the Job or TaskGroup has a distinct_hosts constraints so
	// they don't have to be calculated every time Next() is called.
	tgDistinctHosts  bool
	jobDistinctHosts bool
}

// NewDistinctHostsIterator creates a DistinctHostsIterator from a source.
func NewDistinctHostsIterator(ctx Context, source FeasibleIterator) *DistinctHostsIterator {
	return &DistinctHostsIterator{
		ctx:    ctx,
		source: source,
	}
}

func (iter *DistinctHostsIterator) SetTaskGroup(tg *structs.TaskGroup) {
	iter.tg = tg
	iter.tgDistinctHosts = iter.hasDistinctHostsConstraint(tg.Constraints)
}

func (iter *DistinctHostsIterator) SetJob(job *structs.Job) {
	iter.job = job
	iter.jobDistinctHosts = iter.hasDistinctHostsConstraint(job.Constraints)
}

func (iter *DistinctHostsIterator) hasDistinctHostsConstraint(constraints []*structs.Constraint) bool {
	for _, con := range constraints {
		if con.Operand == structs.ConstraintDistinctHosts {
			return true
		}
	}

	return false
}

func (iter *DistinctHostsIterator) Next() *structs.Node {
	for {
		// Get the next option from the source
		option := iter.source.Next()

		// Hot-path if the option is nil or there are no distinct_hosts or
		// distinct_property constraints.
		hosts := iter.jobDistinctHosts || iter.tgDistinctHosts
		if option == nil || !hosts {
			return option
		}

		// Check if the host constraints are satisfied
		if !iter.satisfiesDistinctHosts(option) {
			iter.ctx.Metrics().FilterNode(option, structs.ConstraintDistinctHosts)
			continue
		}

		return option
	}
}

// satisfiesDistinctHosts checks if the node satisfies a distinct_hosts
// constraint either specified at the job level or the TaskGroup level.
func (iter *DistinctHostsIterator) satisfiesDistinctHosts(option *structs.Node) bool {
	// Check if there is no constraint set.
	if !(iter.jobDistinctHosts || iter.tgDistinctHosts) {
		return true
	}

	// Get the proposed allocations
	proposed, err := iter.ctx.ProposedAllocs(option.ID)
	if err != nil {
		iter.ctx.Logger().Named("distinct_hosts").Error("failed to get proposed allocations", "error", err)
		return false
	}

	// Skip the node if the task group has already been allocated on it.
	for _, alloc := range proposed {
		// If the job has a distinct_hosts constraint we only need an alloc
		// collision on the JobID but if the constraint is on the TaskGroup then
		// we need both a job and TaskGroup collision.
		jobCollision := alloc.JobID == iter.job.ID
		taskCollision := alloc.TaskGroup == iter.tg.Name
		if iter.jobDistinctHosts && jobCollision || jobCollision && taskCollision {
			return false
		}
	}

	return true
}

func (iter *DistinctHostsIterator) Reset() {
	iter.source.Reset()
}

// DistinctPropertyIterator is a FeasibleIterator which returns nodes that pass the
// distinct_property constraint. The constraint ensures that multiple allocations
// do not use the same value of the given property.
type DistinctPropertyIterator struct {
	ctx    Context
	source FeasibleIterator
	tg     *structs.TaskGroup
	job    *structs.Job

	hasDistinctPropertyConstraints bool
	jobPropertySets                []*propertySet
	groupPropertySets              map[string][]*propertySet
}

// NewDistinctPropertyIterator creates a DistinctPropertyIterator from a source.
func NewDistinctPropertyIterator(ctx Context, source FeasibleIterator) *DistinctPropertyIterator {
	return &DistinctPropertyIterator{
		ctx:               ctx,
		source:            source,
		groupPropertySets: make(map[string][]*propertySet),
	}
}

func (iter *DistinctPropertyIterator) SetTaskGroup(tg *structs.TaskGroup) {
	iter.tg = tg

	// Build the property set at the taskgroup level
	if _, ok := iter.groupPropertySets[tg.Name]; !ok {
		for _, c := range tg.Constraints {
			if c.Operand != structs.ConstraintDistinctProperty {
				continue
			}

			pset := NewPropertySet(iter.ctx, iter.job)
			pset.SetTGConstraint(c, tg.Name)
			iter.groupPropertySets[tg.Name] = append(iter.groupPropertySets[tg.Name], pset)
		}
	}

	// Check if there is a distinct property
	iter.hasDistinctPropertyConstraints = len(iter.jobPropertySets) != 0 || len(iter.groupPropertySets[tg.Name]) != 0
}

func (iter *DistinctPropertyIterator) SetJob(job *structs.Job) {
	iter.job = job

	// Build the property set at the job level
	for _, c := range job.Constraints {
		if c.Operand != structs.ConstraintDistinctProperty {
			continue
		}

		pset := NewPropertySet(iter.ctx, job)
		pset.SetJobConstraint(c)
		iter.jobPropertySets = append(iter.jobPropertySets, pset)
	}
}

func (iter *DistinctPropertyIterator) Next() *structs.Node {
	for {
		// Get the next option from the source
		option := iter.source.Next()

		// Hot path if there is nothing to check
		if option == nil || !iter.hasDistinctPropertyConstraints {
			return option
		}

		// Check if the constraints are met
		if !iter.satisfiesProperties(option, iter.jobPropertySets) ||
			!iter.satisfiesProperties(option, iter.groupPropertySets[iter.tg.Name]) {
			continue
		}

		return option
	}
}

// satisfiesProperties returns whether the option satisfies the set of
// properties. If not it will be filtered.
func (iter *DistinctPropertyIterator) satisfiesProperties(option *structs.Node, set []*propertySet) bool {
	for _, ps := range set {
		if satisfies, reason := ps.SatisfiesDistinctProperties(option, iter.tg.Name); !satisfies {
			iter.ctx.Metrics().FilterNode(option, reason)
			return false
		}
	}

	return true
}

func (iter *DistinctPropertyIterator) Reset() {
	iter.source.Reset()

	for _, ps := range iter.jobPropertySets {
		ps.PopulateProposed()
	}

	for _, sets := range iter.groupPropertySets {
		for _, ps := range sets {
			ps.PopulateProposed()
		}
	}
}

// ConstraintChecker is a FeasibilityChecker which returns nodes that match a
// given set of constraints. This is used to filter on job, task group, and task
// constraints.
type ConstraintChecker struct {
	ctx         Context
	constraints []*structs.Constraint
}

// NewConstraintChecker creates a ConstraintChecker for a set of constraints
func NewConstraintChecker(ctx Context, constraints []*structs.Constraint) *ConstraintChecker {
	return &ConstraintChecker{
		ctx:         ctx,
		constraints: constraints,
	}
}

func (c *ConstraintChecker) SetConstraints(constraints []*structs.Constraint) {
	c.constraints = constraints
}

func (c *ConstraintChecker) Feasible(option *structs.Node) bool {
	// Use this node if possible
	for _, constraint := range c.constraints {
		if !c.meetsConstraint(constraint, option) {
			c.ctx.Metrics().FilterNode(option, constraint.String())
			return false
		}
	}
	return true
}

func (c *ConstraintChecker) meetsConstraint(constraint *structs.Constraint, option *structs.Node) bool {
	// Resolve the targets. Targets that are not present are treated as `nil`.
	// This is to allow for matching constraints where a target is not present.
	lVal, lOk := resolveTarget(constraint.LTarget, option)
	rVal, rOk := resolveTarget(constraint.RTarget, option)

	// Check if satisfied
	return checkConstraint(c.ctx, constraint.Operand, lVal, rVal, lOk, rOk)
}

// resolveTarget is used to resolve the LTarget and RTarget of a Constraint.
func resolveTarget(target string, node *structs.Node) (interface{}, bool) {
	// If no prefix, this must be a literal value
	if !strings.HasPrefix(target, "${") {
		return target, true
	}

	// Handle the interpolations
	switch {
	case "${node.unique.id}" == target:
		return node.ID, true

	case "${node.datacenter}" == target:
		return node.Datacenter, true

	case "${node.unique.name}" == target:
		return node.Name, true

	case "${node.class}" == target:
		return node.NodeClass, true

	case strings.HasPrefix(target, "${attr."):
		attr := strings.TrimSuffix(strings.TrimPrefix(target, "${attr."), "}")
		val, ok := node.Attributes[attr]
		return val, ok

	case strings.HasPrefix(target, "${meta."):
		meta := strings.TrimSuffix(strings.TrimPrefix(target, "${meta."), "}")
		val, ok := node.Meta[meta]
		return val, ok

	default:
		return nil, false
	}
}

// checkConstraint checks if a constraint is satisfied. The lVal and rVal
// interfaces may be nil.
func checkConstraint(ctx Context, operand string, lVal, rVal interface{}, lFound, rFound bool) bool {
	// Check for constraints not handled by this checker.
	switch operand {
	case structs.ConstraintDistinctHosts, structs.ConstraintDistinctProperty:
		return true
	default:
		break
	}

	switch operand {
	case "=", "==", "is":
		return lFound && rFound && reflect.DeepEqual(lVal, rVal)
	case "!=", "not":
		return !reflect.DeepEqual(lVal, rVal)
	case "<", "<=", ">", ">=":
		return lFound && rFound && checkLexicalOrder(operand, lVal, rVal)
	case structs.ConstraintAttributeIsSet:
		return lFound
	case structs.ConstraintAttributeIsNotSet:
		return !lFound
	case structs.ConstraintVersion:
		return lFound && rFound && checkVersionMatch(ctx, lVal, rVal)
	case structs.ConstraintRegex:
		return lFound && rFound && checkRegexpMatch(ctx, lVal, rVal)
	case structs.ConstraintSetContains, structs.ConstraintSetContainsAll:
		return lFound && rFound && checkSetContainsAll(ctx, lVal, rVal)
	case structs.ConstraintSetContainsAny:
		return lFound && rFound && checkSetContainsAny(lVal, rVal)
	default:
		return false
	}
}

// checkAffinity checks if a specific affinity is satisfied
func checkAffinity(ctx Context, operand string, lVal, rVal interface{}, lFound, rFound bool) bool {
	return checkConstraint(ctx, operand, lVal, rVal, lFound, rFound)
}

// checkAttributeAffinity checks if an affinity is satisfied
func checkAttributeAffinity(ctx Context, operand string, lVal, rVal *psstructs.Attribute, lFound, rFound bool) bool {
	return checkAttributeConstraint(ctx, operand, lVal, rVal, lFound, rFound)
}

// checkLexicalOrder is used to check for lexical ordering
func checkLexicalOrder(op string, lVal, rVal interface{}) bool {
	// Ensure the values are strings
	lStr, ok := lVal.(string)
	if !ok {
		return false
	}
	rStr, ok := rVal.(string)
	if !ok {
		return false
	}

	switch op {
	case "<":
		return lStr < rStr
	case "<=":
		return lStr <= rStr
	case ">":
		return lStr > rStr
	case ">=":
		return lStr >= rStr
	default:
		return false
	}
}

// checkVersionMatch is used to compare a version on the
// left hand side with a set of constraints on the right hand side
func checkVersionMatch(ctx Context, lVal, rVal interface{}) bool {
	// Parse the version
	var versionStr string
	switch v := lVal.(type) {
	case string:
		versionStr = v
	case int:
		versionStr = fmt.Sprintf("%d", v)
	default:
		return false
	}

	// Parse the version
	vers, err := version.NewVersion(versionStr)
	if err != nil {
		return false
	}

	// Constraint must be a string
	constraintStr, ok := rVal.(string)
	if !ok {
		return false
	}

	// Check the cache for a match
	cache := ctx.VersionConstraintCache()
	constraints := cache[constraintStr]

	// Parse the constraints
	if constraints == nil {
		constraints, err = version.NewConstraint(constraintStr)
		if err != nil {
			return false
		}
		cache[constraintStr] = constraints
	}

	// Check the constraints against the version
	return constraints.Check(vers)
}

// checkAttributeVersionMatch is used to compare a version on the
// left hand side with a set of constraints on the right hand side
func checkAttributeVersionMatch(ctx Context, lVal, rVal *psstructs.Attribute) bool {
	// Parse the version
	var versionStr string
	if s, ok := lVal.GetString(); ok {
		versionStr = s
	} else if i, ok := lVal.GetInt(); ok {
		versionStr = fmt.Sprintf("%d", i)
	} else {
		return false
	}

	// Parse the version
	vers, err := version.NewVersion(versionStr)
	if err != nil {
		return false
	}

	// Constraint must be a string
	constraintStr, ok := rVal.GetString()
	if !ok {
		return false
	}

	// Check the cache for a match
	cache := ctx.VersionConstraintCache()
	constraints := cache[constraintStr]

	// Parse the constraints
	if constraints == nil {
		constraints, err = version.NewConstraint(constraintStr)
		if err != nil {
			return false
		}
		cache[constraintStr] = constraints
	}

	// Check the constraints against the version
	return constraints.Check(vers)
}

// checkRegexpMatch is used to compare a value on the
// left hand side with a regexp on the right hand side
func checkRegexpMatch(ctx Context, lVal, rVal interface{}) bool {
	// Ensure left-hand is string
	lStr, ok := lVal.(string)
	if !ok {
		return false
	}

	// Regexp must be a string
	regexpStr, ok := rVal.(string)
	if !ok {
		return false
	}

	// Check the cache
	cache := ctx.RegexpCache()
	re := cache[regexpStr]

	// Parse the regexp
	if re == nil {
		var err error
		re, err = regexp.Compile(regexpStr)
		if err != nil {
			return false
		}
		cache[regexpStr] = re
	}

	// Look for a match
	return re.MatchString(lStr)
}

// checkSetContainsAll is used to see if the left hand side contains the
// string on the right hand side
func checkSetContainsAll(ctx Context, lVal, rVal interface{}) bool {
	// Ensure left-hand is string
	lStr, ok := lVal.(string)
	if !ok {
		return false
	}

	// Regexp must be a string
	rStr, ok := rVal.(string)
	if !ok {
		return false
	}

	input := strings.Split(lStr, ",")
	lookup := make(map[string]struct{}, len(input))
	for _, in := range input {
		cleaned := strings.TrimSpace(in)
		lookup[cleaned] = struct{}{}
	}

	for _, r := range strings.Split(rStr, ",") {
		cleaned := strings.TrimSpace(r)
		if _, ok := lookup[cleaned]; !ok {
			return false
		}
	}

	return true
}

// checkSetContainsAny is used to see if the left hand side contains any
// values on the right hand side
func checkSetContainsAny(lVal, rVal interface{}) bool {
	// Ensure left-hand is string
	lStr, ok := lVal.(string)
	if !ok {
		return false
	}

	// RHS must be a string
	rStr, ok := rVal.(string)
	if !ok {
		return false
	}

	input := strings.Split(lStr, ",")
	lookup := make(map[string]struct{}, len(input))
	for _, in := range input {
		cleaned := strings.TrimSpace(in)
		lookup[cleaned] = struct{}{}
	}

	for _, r := range strings.Split(rStr, ",") {
		cleaned := strings.TrimSpace(r)
		if _, ok := lookup[cleaned]; ok {
			return true
		}
	}

	return false
}

// FeasibilityWrapper is a FeasibleIterator which wraps both job and task group
// FeasibilityCheckers in which feasibility checking can be skipped if the
// computed node class has previously been marked as eligible or ineligible.
type FeasibilityWrapper struct {
	ctx         Context
	source      FeasibleIterator
	jobCheckers []FeasibilityChecker
	tgCheckers  []FeasibilityChecker
	tg          string
}

// NewFeasibilityWrapper returns a FeasibleIterator based on the passed source
// and FeasibilityCheckers.
func NewFeasibilityWrapper(ctx Context, source FeasibleIterator,
	jobCheckers, tgCheckers []FeasibilityChecker) *FeasibilityWrapper {
	return &FeasibilityWrapper{
		ctx:         ctx,
		source:      source,
		jobCheckers: jobCheckers,
		tgCheckers:  tgCheckers,
	}
}

func (w *FeasibilityWrapper) SetTaskGroup(tg string) {
	w.tg = tg
}

func (w *FeasibilityWrapper) Reset() {
	w.source.Reset()
}

// Next returns an eligible node, only running the FeasibilityCheckers as needed
// based on the sources computed node class.
func (w *FeasibilityWrapper) Next() *structs.Node {
	evalElig := w.ctx.Eligibility()
	metrics := w.ctx.Metrics()

OUTER:
	for {
		// Get the next option from the source
		option := w.source.Next()
		if option == nil {
			return nil
		}

		// Check if the job has been marked as eligible or ineligible.
		jobEscaped, jobUnknown := false, false
		switch evalElig.JobStatus(option.ComputedClass) {
		case EvalComputedClassIneligible:
			// Fast path the ineligible case
			metrics.FilterNode(option, "computed class ineligible")
			continue
		case EvalComputedClassEscaped:
			jobEscaped = true
		case EvalComputedClassUnknown:
			jobUnknown = true
		}

		// Run the job feasibility checks.
		for _, check := range w.jobCheckers {
			feasible := check.Feasible(option)
			if !feasible {
				// If the job hasn't escaped, set it to be ineligible since it
				// failed a job check.
				if !jobEscaped {
					evalElig.SetJobEligibility(false, option.ComputedClass)
				}
				continue OUTER
			}
		}

		// Set the job eligibility if the constraints weren't escaped and it
		// hasn't been set before.
		if !jobEscaped && jobUnknown {
			evalElig.SetJobEligibility(true, option.ComputedClass)
		}

		// Check if the task group has been marked as eligible or ineligible.
		tgEscaped, tgUnknown := false, false
		switch evalElig.TaskGroupStatus(w.tg, option.ComputedClass) {
		case EvalComputedClassIneligible:
			// Fast path the ineligible case
			metrics.FilterNode(option, "computed class ineligible")
			continue
		case EvalComputedClassEligible:
			// Fast path the eligible case
			return option
		case EvalComputedClassEscaped:
			tgEscaped = true
		case EvalComputedClassUnknown:
			tgUnknown = true
		}

		// Run the task group feasibility checks.
		for _, check := range w.tgCheckers {
			feasible := check.Feasible(option)
			if !feasible {
				// If the task group hasn't escaped, set it to be ineligible
				// since it failed a check.
				if !tgEscaped {
					evalElig.SetTaskGroupEligibility(false, w.tg, option.ComputedClass)
				}
				continue OUTER
			}
		}

		// Set the task group eligibility if the constraints weren't escaped and
		// it hasn't been set before.
		if !tgEscaped && tgUnknown {
			evalElig.SetTaskGroupEligibility(true, w.tg, option.ComputedClass)
		}

		return option
	}
}

// DeviceChecker is a FeasibilityChecker which returns whether a node has the
// devices necessary to scheduler a task group.
type DeviceChecker struct {
	ctx Context

	// required is the set of requested devices that must exist on the node
	required []*structs.RequestedDevice

	// requiresDevices indicates if the task group requires devices
	requiresDevices bool
}

// NewDeviceChecker creates a DeviceChecker
func NewDeviceChecker(ctx Context) *DeviceChecker {
	return &DeviceChecker{
		ctx: ctx,
	}
}

func (c *DeviceChecker) SetTaskGroup(tg *structs.TaskGroup) {
	c.required = nil
	for _, task := range tg.Tasks {
		c.required = append(c.required, task.Resources.Devices...)
	}
	c.requiresDevices = len(c.required) != 0
}

func (c *DeviceChecker) Feasible(option *structs.Node) bool {
	if c.hasDevices(option) {
		return true
	}

	c.ctx.Metrics().FilterNode(option, "missing devices")
	return false
}

func (c *DeviceChecker) hasDevices(option *structs.Node) bool {
	if !c.requiresDevices {
		return true
	}

	// COMPAT(0.11): Remove in 0.11
	// The node does not have the new resources object so it can not have any
	// devices
	if option.NodeResources == nil {
		return false
	}

	// Check if the node has any devices
	nodeDevs := option.NodeResources.Devices
	if len(nodeDevs) == 0 {
		return false
	}

	// Create a mapping of node devices to the remaining count
	available := make(map[*structs.NodeDeviceResource]uint64, len(nodeDevs))
	for _, d := range nodeDevs {
		var healthy uint64 = 0
		for _, instance := range d.Instances {
			if instance.Healthy {
				healthy++
			}
		}
		if healthy != 0 {
			available[d] = healthy
		}
	}

	// Go through the required devices trying to find matches
OUTER:
	for _, req := range c.required {
		// Determine how many there are to place
		desiredCount := req.Count

		// Go through the device resources and see if we have a match
		for d, unused := range available {
			if unused == 0 {
				// Depleted
				continue
			}

			// First check we have enough instances of the device since this is
			// cheaper than checking the constraints
			if unused < desiredCount {
				continue
			}

			// Check the constraints
			if nodeDeviceMatches(c.ctx, d, req) {
				// Consume the instances
				available[d] -= desiredCount

				// Move on to the next request
				continue OUTER
			}
		}

		// We couldn't match the request for the device
		return false
	}

	// Only satisfied if there are no more devices to place
	return true
}

// nodeDeviceMatches checks if the device matches the request and its
// constraints. It doesn't check the count.
func nodeDeviceMatches(ctx Context, d *structs.NodeDeviceResource, req *structs.RequestedDevice) bool {
	if !d.ID().Matches(req.ID()) {
		return false
	}

	// There are no constraints to consider
	if len(req.Constraints) == 0 {
		return true
	}

	for _, c := range req.Constraints {
		// Resolve the targets
		lVal, lOk := resolveDeviceTarget(c.LTarget, d)
		rVal, rOk := resolveDeviceTarget(c.RTarget, d)

		// Check if satisfied
		if !checkAttributeConstraint(ctx, c.Operand, lVal, rVal, lOk, rOk) {
			return false
		}
	}

	return true
}

// resolveDeviceTarget is used to resolve the LTarget and RTarget of a Constraint
// when used on a device
func resolveDeviceTarget(target string, d *structs.NodeDeviceResource) (*psstructs.Attribute, bool) {
	// If no prefix, this must be a literal value
	if !strings.HasPrefix(target, "${") {
		return psstructs.ParseAttribute(target), true
	}

	// Handle the interpolations
	switch {
	case "${device.model}" == target:
		return psstructs.NewStringAttribute(d.Name), true

	case "${device.vendor}" == target:
		return psstructs.NewStringAttribute(d.Vendor), true

	case "${device.type}" == target:
		return psstructs.NewStringAttribute(d.Type), true

	case strings.HasPrefix(target, "${device.attr."):
		attr := strings.TrimPrefix(target, "${device.attr.")
		attr = strings.TrimSuffix(attr, "}")
		val, ok := d.Attributes[attr]
		return val, ok

	default:
		return nil, false
	}
}

// checkAttributeConstraint checks if a constraint is satisfied. nil equality
// comparisons are considered to be false.
func checkAttributeConstraint(ctx Context, operand string, lVal, rVal *psstructs.Attribute, lFound, rFound bool) bool {
	// Check for constraints not handled by this checker.
	switch operand {
	case structs.ConstraintDistinctHosts, structs.ConstraintDistinctProperty:
		return true
	default:
		break
	}

	switch operand {
	case "!=", "not":
		// Neither value was provided, nil != nil == false
		if !(lFound || rFound) {
			return false
		}

		// Only 1 value was provided, therefore nil != some == true
		if lFound != rFound {
			return true
		}

		// Both values were provided, so actually compare them
		v, ok := lVal.Compare(rVal)
		if !ok {
			return false
		}

		return v != 0

	case "<", "<=", ">", ">=", "=", "==", "is":
		if !(lFound && rFound) {
			return false
		}

		v, ok := lVal.Compare(rVal)
		if !ok {
			return false
		}

		switch operand {
		case "is", "==", "=":
			return v == 0
		case "<":
			return v == -1
		case "<=":
			return v != 1
		case ">":
			return v == 1
		case ">=":
			return v != -1
		default:
			return false
		}

	case structs.ConstraintVersion:
		if !(lFound && rFound) {
			return false
		}

		return checkAttributeVersionMatch(ctx, lVal, rVal)
	case structs.ConstraintRegex:
		if !(lFound && rFound) {
			return false
		}

		ls, ok := lVal.GetString()
		rs, ok2 := rVal.GetString()
		if !ok || !ok2 {
			return false
		}
		return checkRegexpMatch(ctx, ls, rs)
	case structs.ConstraintSetContains, structs.ConstraintSetContainsAll:
		if !(lFound && rFound) {
			return false
		}

		ls, ok := lVal.GetString()
		rs, ok2 := rVal.GetString()
		if !ok || !ok2 {
			return false
		}

		return checkSetContainsAll(ctx, ls, rs)
	case structs.ConstraintSetContainsAny:
		if !(lFound && rFound) {
			return false
		}

		ls, ok := lVal.GetString()
		rs, ok2 := rVal.GetString()
		if !ok || !ok2 {
			return false
		}

		return checkSetContainsAny(ls, rs)
	case structs.ConstraintAttributeIsSet:
		return lFound
	case structs.ConstraintAttributeIsNotSet:
		return !lFound
	default:
		return false
	}

}