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