123 lines
3.8 KiB
Go
123 lines
3.8 KiB
Go
package cache
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import (
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"context"
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"fmt"
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"time"
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)
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// UpdateEvent is a struct summarising an update to a cache entry
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type UpdateEvent struct {
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// CorrelationID is used by the Notify API to allow correlation of updates
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// with specific requests. We could return the full request object and
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// cachetype for consumers to match against the calls they made but in
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// practice it's cleaner for them to choose the minimal necessary unique
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// identifier given the set of things they are watching. They might even
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// choose to assign random IDs for example.
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CorrelationID string
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Result interface{}
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Meta ResultMeta
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Err error
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}
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// Notify registers a desire to be updated about changes to a cache result.
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//
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// It is a helper that abstracts code from perfroming their own "blocking" query
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// logic against a cache key to watch for changes and to maintain the key in
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// cache actively. It will continue to perform blocking Get requests until the
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// context is canceled.
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//
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// The passed context must be cancelled or timeout in order to free resources
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// and stop maintaining the value in cache. Typically request-scoped resources
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// do this but if a long-lived context like context.Background is used, then the
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// caller must arrange for it to be cancelled when the watch is no longer
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// needed.
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//
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// The passed chan may be buffered or unbuffered, if the caller doesn't consume
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// fast enough it will block the notification loop. When the chan is later
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// drained, watching resumes correctly. If the pause is longer than the
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// cachetype's TTL, the result might be removed from the local cache. Even in
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// this case though when the chan is drained again, the new Get will re-fetch
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// the entry from servers and resume notification behaviour transparently.
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//
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// The chan is passed in to allow multiple cached results to be watched by a
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// single consumer without juggling extra goroutines per watch. The
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// correlationID is opaque and will be returned in all UpdateEvents generated by
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// result of watching the specified request so the caller can set this to any
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// value that allows them to dissambiguate between events in the returned chan
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// when sharing a chan between multiple cache entries. If the chan is closed,
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// the notify loop will terminate.
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func (c *Cache) Notify(ctx context.Context, t string, r Request,
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correlationID string, ch chan<- UpdateEvent) error {
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// Get the type that we're fetching
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c.typesLock.RLock()
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tEntry, ok := c.types[t]
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c.typesLock.RUnlock()
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if !ok {
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return fmt.Errorf("unknown type in cache: %s", t)
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}
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if !tEntry.Type.SupportsBlocking() {
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return fmt.Errorf("watch requires the type to support blocking")
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}
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// Always start at 0 index to deliver the inital (possibly currently cached
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// value).
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index := uint64(0)
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go func() {
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var failures uint
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for {
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// Check context hasn't been cancelled
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if ctx.Err() != nil {
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return
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}
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// Blocking request
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res, meta, err := c.getWithIndex(t, r, index)
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// Check context hasn't been cancelled
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if ctx.Err() != nil {
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return
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}
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// Check the index of the value returned in the cache entry to be sure it
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// changed
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if index < meta.Index {
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u := UpdateEvent{correlationID, res, meta, err}
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select {
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case ch <- u:
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case <-ctx.Done():
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return
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}
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// Update index for next request
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index = meta.Index
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}
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// Handle errors with backoff. Badly behaved blocking calls that returned
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// a zero index are considered as failures since we need to not get stuck
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// in a busy loop.
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if err == nil && meta.Index > 0 {
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failures = 0
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} else {
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failures++
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}
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if wait := backOffWait(failures); wait > 0 {
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select {
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case <-time.After(wait):
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case <-ctx.Done():
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return
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}
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}
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// Sanity check we always request blocking on second pass
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if index < 1 {
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index = 1
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}
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}
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}()
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return nil
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}
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