open-consul/agent/consul/stream/event_snapshot.go

101 lines
3.8 KiB
Go

package stream
// eventSnapshot represents the state of memdb for a given topic and key at some
// point in time. It is modelled as a buffer of events so that snapshots can be
// streamed to possibly multiple subscribers concurrently, and can be trivially
// cached by retaining a reference to a Snapshot. Once the reference to eventSnapshot
// is dropped from memory, any subscribers still reading from it may do so by following
// their pointers. When the last subscriber unsubscribes, the snapshot is garbage
// collected automatically by Go's runtime. This simplifies snapshot and buffer
// management dramatically.
type eventSnapshot struct {
// Head is the first item in the buffer containing the snapshot. Once the
// snapshot is complete, subsequent BufferItems are appended to snapBuffer,
// so that subscribers receive all the events from the same buffer.
Head *bufferItem
// snapBuffer is the Head of the snapshot buffer the fn should write to.
snapBuffer *eventBuffer
}
type snapFunc func(req *SubscribeRequest, buf SnapshotAppender) (uint64, error)
// newEventSnapshot creates a snapshot buffer based on the subscription request.
// The current buffer head for the topic requested is passed so that once the
// snapshot is complete and has been delivered into the buffer, any events
// published during snapshotting can be immediately appended and won't be
// missed. Once the snapshot is delivered the topic buffer is spliced onto the
// snapshot buffer so that subscribers will naturally follow from the snapshot
// to wait for any subsequent updates.
func newEventSnapshot(req *SubscribeRequest, topicBufferHead *bufferItem, fn snapFunc) *eventSnapshot {
buf := newEventBuffer()
s := &eventSnapshot{
Head: buf.Head(),
snapBuffer: buf,
}
go func() {
idx, err := fn(req, s.snapBuffer)
if err != nil {
s.snapBuffer.AppendItem(&bufferItem{Err: err})
return
}
// We wrote the snapshot events to the buffer, send the "end of snapshot" event
s.snapBuffer.Append([]Event{{
Topic: req.Topic,
Key: req.Key,
Index: idx,
Payload: endOfSnapshot{},
}})
s.spliceFromTopicBuffer(topicBufferHead, idx)
}()
return s
}
func (s *eventSnapshot) spliceFromTopicBuffer(topicBufferHead *bufferItem, idx uint64) {
// Now splice on the topic buffer. We need to iterate through the buffer to
// find the first event after the current snapshot.
item := topicBufferHead
for {
next := item.NextNoBlock()
switch {
case next == nil:
// This is the head of the topic buffer (or was just now which is after
// the snapshot completed). We don't want any of the events (if any) in
// the snapshot buffer as they came before the snapshot but we do need to
// wait for the next update.
s.snapBuffer.AppendItem(item.NextLink())
return
case next.Err != nil:
// This case is not currently possible because errors can only come
// from a snapshot func, and this is consuming events from a topic
// buffer which does not contain a snapshot.
// Handle this case anyway in case errors can come from other places
// in the future.
s.snapBuffer.AppendItem(next)
return
case len(next.Events) > 0 && next.Events[0].Index > idx:
// We've found an update in the topic buffer that happened after our
// snapshot was taken, splice it into the snapshot buffer so subscribers
// can continue to read this and others after it.
s.snapBuffer.AppendItem(next)
return
}
// We don't need this item, continue to next
item = next
}
}
// err returns an error if the snapshot func has failed with an error or nil
// otherwise. Nil doesn't necessarily mean there won't be an error but there
// hasn't been one yet.
func (s *eventSnapshot) err() error {
// Fetch the head of the buffer, this is atomic. If the snapshot func errored
// then the last event will be an error.
head := s.snapBuffer.Head()
return head.Err
}