open-consul/consul/tombstone_gc.go

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package consul
import (
"fmt"
"sync"
"time"
)
// TombstoneGC is used to track creation of tombstones
// so that they can be garbage collected after their TTL
// expires. The tombstones allow queries to provide monotonic
// index values within the TTL window. The GC is used to
// prevent monotonic growth in storage usage. This is a trade off
// between the length of the TTL and the storage overhead.
//
// In practice, this is required to fix the issue of delete
// visibility. When data is deleted from the KV store, the
// "latest" row can go backwards if the newest row is removed.
// The tombstones provide a way to ensure time doesn't move
// backwards within some interval.
//
type TombstoneGC struct {
ttl time.Duration
granularity time.Duration
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// enabled controls if we actually setup any timers.
enabled bool
// expires maps the time of expiration to the highest
// tombstone value that should be expired.
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expires map[time.Time]*expireInterval
// expireCh is used to stream expiration
expireCh chan uint64
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// lock is used to ensure safe access to all the fields
lock sync.Mutex
}
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// expireInterval is used to track the maximum index
// to expire in a given interval with a timer
type expireInterval struct {
maxIndex uint64
timer *time.Timer
}
// NewTombstoneGC is used to construct a new TombstoneGC given
// a TTL for tombstones and a tracking granularity. Longer TTLs
// ensure correct behavior for more time, but use more storage.
// A shorter granularity increases the number of Raft transactions
// and reduce how far past the TTL we perform GC.
func NewTombstoneGC(ttl, granularity time.Duration) (*TombstoneGC, error) {
// Sanity check the inputs
if ttl <= 0 || granularity <= 0 {
return nil, fmt.Errorf("Tombstone TTL and granularity must be positive")
}
t := &TombstoneGC{
ttl: ttl,
granularity: granularity,
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enabled: false,
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expires: make(map[time.Time]*expireInterval),
expireCh: make(chan uint64, 1),
}
return t, nil
}
// ExpireCh is used to return a channel that streams the next index
// that should be expired
func (t *TombstoneGC) ExpireCh() <-chan uint64 {
return t.expireCh
}
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// SetEnabled is used to control if the tombstone GC is
// enabled. Should only be enabled by the leader node.
func (t *TombstoneGC) SetEnabled(enabled bool) {
t.lock.Lock()
defer t.lock.Unlock()
if enabled == t.enabled {
return
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}
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// Stop all the timers and clear
if !enabled {
for _, exp := range t.expires {
exp.timer.Stop()
}
t.expires = make(map[time.Time]*expireInterval)
}
// Update the status
t.enabled = enabled
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}
// Hint is used to indicate that keys at the given index have been
// deleted, and that their GC should be scheduled.
func (t *TombstoneGC) Hint(index uint64) {
expires := t.nextExpires()
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t.lock.Lock()
defer t.lock.Unlock()
if !t.enabled {
return
}
// Check for an existing expiration timer
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exp, ok := t.expires[expires]
if ok {
// Increment the highest index to be expired at that time
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if index > exp.maxIndex {
exp.maxIndex = index
}
return
}
// Create new expiration time
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t.expires[expires] = &expireInterval{
maxIndex: index,
timer: time.AfterFunc(expires.Sub(time.Now()), func() {
t.expireTime(expires)
}),
}
}
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// PendingExpiration is used to check if any expirations are pending
func (t *TombstoneGC) PendingExpiration() bool {
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t.lock.Lock()
defer t.lock.Unlock()
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return len(t.expires) > 0
}
// nextExpires is used to calculate the next experation time
func (t *TombstoneGC) nextExpires() time.Time {
expires := time.Now().Add(t.ttl)
remain := expires.UnixNano() % int64(t.granularity)
adj := expires.Add(t.granularity - time.Duration(remain))
return adj
}
// expireTime is used to expire the entries at the given time
func (t *TombstoneGC) expireTime(expires time.Time) {
// Get the maximum index and clear the entry
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t.lock.Lock()
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exp := t.expires[expires]
delete(t.expires, expires)
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t.lock.Unlock()
// Notify the expires channel
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t.expireCh <- exp.maxIndex
}