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