open-consul/agent/cache/watch.go

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package cache
import (
"context"
"fmt"
"reflect"
"time"
"github.com/hashicorp/consul/lib"
"google.golang.org/protobuf/proto"
)
// UpdateEvent is a struct summarizing an update to a cache entry
type UpdateEvent struct {
// CorrelationID is used by the Notify API to allow correlation of updates
// with specific requests. We could return the full request object and
// cachetype for consumers to match against the calls they made but in
// practice it's cleaner for them to choose the minimal necessary unique
// identifier given the set of things they are watching. They might even
// choose to assign random IDs for example.
CorrelationID string
Result interface{}
Meta ResultMeta
Err error
}
// Callback is the function type accepted by NotifyCallback.
type Callback func(ctx context.Context, event UpdateEvent)
// Notify registers a desire to be updated about changes to a cache result.
//
// It is a helper that abstracts code from performing their own "blocking" query
// logic against a cache key to watch for changes and to maintain the key in
// cache actively. It will continue to perform blocking Get requests until the
// context is canceled.
//
// The passed context must be canceled or timeout in order to free resources
// and stop maintaining the value in cache. Typically request-scoped resources
// do this but if a long-lived context like context.Background is used, then the
// caller must arrange for it to be canceled when the watch is no longer
// needed.
//
// The passed chan may be buffered or unbuffered, if the caller doesn't consume
// fast enough it will block the notification loop. When the chan is later
// drained, watching resumes correctly. If the pause is longer than the
// cachetype's TTL, the result might be removed from the local cache. Even in
// this case though when the chan is drained again, the new Get will re-fetch
// the entry from servers and resume notification behavior transparently.
//
// The chan is passed in to allow multiple cached results to be watched by a
// single consumer without juggling extra goroutines per watch. The
// correlationID is opaque and will be returned in all UpdateEvents generated by
// result of watching the specified request so the caller can set this to any
// value that allows them to disambiguate between events in the returned chan
// when sharing a chan between multiple cache entries. If the chan is closed,
// the notify loop will terminate.
func (c *Cache) Notify(
ctx context.Context,
t string,
r Request,
correlationID string,
ch chan<- UpdateEvent,
) error {
return c.NotifyCallback(ctx, t, r, correlationID, func(ctx context.Context, event UpdateEvent) {
select {
case ch <- event:
case <-ctx.Done():
}
})
}
// NotifyCallback allows you to receive notifications about changes to a cache
// result in the same way as Notify, but accepts a callback function instead of
// a channel.
func (c *Cache) NotifyCallback(
ctx context.Context,
t string,
r Request,
correlationID string,
cb Callback,
) error {
c.typesLock.RLock()
tEntry, ok := c.types[t]
c.typesLock.RUnlock()
if !ok {
return fmt.Errorf("unknown type in cache: %s", t)
}
if tEntry.Opts.SupportsBlocking {
go c.notifyBlockingQuery(ctx, newGetOptions(tEntry, r), correlationID, cb)
return nil
}
info := r.CacheInfo()
if info.MaxAge == 0 {
return fmt.Errorf("Cannot use Notify for polling cache types without specifying the MaxAge")
}
go c.notifyPollingQuery(ctx, newGetOptions(tEntry, r), correlationID, cb)
return nil
}
func (c *Cache) notifyBlockingQuery(ctx context.Context, r getOptions, correlationID string, cb Callback) {
// Always start at 0 index to deliver the initial (possibly currently cached
// value).
index := uint64(0)
failures := uint(0)
for {
// Check context hasn't been canceled
if ctx.Err() != nil {
return
}
// Blocking request
r.Info.MinIndex = index
res, meta, err := c.getWithIndex(ctx, r)
// Check context hasn't been canceled
if ctx.Err() != nil {
return
}
// Check the index of the value returned in the cache entry to be sure it
// changed
if index == 0 || index < meta.Index {
cb(ctx, UpdateEvent{correlationID, res, meta, err})
// Update index for next request
index = meta.Index
}
var wait time.Duration
// Handle errors with backoff. Badly behaved blocking calls that returned
// a zero index are considered as failures since we need to not get stuck
// in a busy loop.
if err == nil && meta.Index > 0 {
failures = 0
} else {
failures++
wait = backOffWait(c.options, failures)
c.options.Logger.
With("error", err).
With("cache-type", r.TypeEntry.Name).
With("index", index).
Warn("handling error in Cache.Notify")
}
if wait > 0 {
select {
case <-time.After(wait):
case <-ctx.Done():
return
}
}
// Sanity check we always request blocking on second pass
if err == nil && index < 1 {
index = 1
}
}
}
func (c *Cache) notifyPollingQuery(ctx context.Context, r getOptions, correlationID string, cb Callback) {
index := uint64(0)
failures := uint(0)
var lastValue interface{} = nil
for {
// Check context hasn't been canceled
if ctx.Err() != nil {
return
}
// Make the request
r.Info.MinIndex = index
res, meta, err := c.getWithIndex(ctx, r)
// Check context hasn't been canceled
if ctx.Err() != nil {
return
}
// Check for a change in the value or an index change
if index < meta.Index || !isEqual(lastValue, res) {
cb(ctx, UpdateEvent{correlationID, res, meta, err})
// Update index and lastValue
lastValue = res
index = meta.Index
}
// Reset or increment failure counter
if err == nil {
failures = 0
} else {
failures++
c.options.Logger.
With("error", err).
With("cache-type", r.TypeEntry.Name).
With("index", index).
Warn("handling error in Cache.Notify")
}
var wait time.Duration
// Determining how long to wait before the next poll is complicated.
// First off the happy path and the error path waits are handled distinctly
//
// Once fetching the data through the cache returns an error (and until a
// non-error value is returned) the wait time between each round of the loop
// gets controlled by the backOffWait function. Because we would have waited
// at least until the age of the cached data was too old the error path should
// immediately retry the fetch and backoff on the time as needed for persistent
// failures which potentially will wait much longer than the MaxAge of the request
//
// When on the happy path we just need to fetch from the cache often enough to ensure
// that the data is not older than the MaxAge. Therefore after fetching the data from
// the cache we can sleep until the age of that data would exceed the MaxAge. Sometimes
// this will be for the MaxAge duration (like when only a single notify was executed so
// only 1 go routine is keeping the cache updated). Other times this will be some smaller
// duration than MaxAge (when multiple notify calls were executed and this go routine just
// got data back from the cache that was a cache hit after the other go routine fetched it
// without a hit). We cannot just set MustRevalidate on the request and always sleep for MaxAge
// as this would eliminate the single-flighting of these requests in the cache and
// the efficiencies gained by it.
if failures > 0 {
wait = backOffWait(c.options, failures)
} else {
// Calculate when the cached data's Age will get too stale and
// need to be re-queried. When the data's Age already exceeds the
// maxAge the pollWait value is left at 0 to immediately re-poll
if meta.Age <= r.Info.MaxAge {
wait = r.Info.MaxAge - meta.Age
}
// Add a small amount of random jitter to the polling time. One
// purpose of the jitter is to ensure that the next time
// we fetch from the cache the data will be stale (unless another
// notify go routine has updated it while this one is sleeping).
// Without this it would be possible to wake up, fetch the data
// again where the age of the data is strictly equal to the MaxAge
// and then immediately have to re-fetch again. That wouldn't
// be terrible but it would expend a bunch more cpu cycles when
// we can definitely avoid it.
wait += lib.RandomStagger(r.Info.MaxAge / 16)
}
select {
case <-time.After(wait):
case <-ctx.Done():
return
}
}
}
// isEqual compares two values for deep equality. Protobuf objects
// require us to use a special comparison because cloned structs
// may have non-exported fields that differ. For non-protobuf objects,
// we use reflect.DeepEqual().
func isEqual(a, b interface{}) bool {
// TODO move this logic into an interface so that each type can determine
// its own logic for equality, rather than a centralized type-cast like this.
if a != nil && b != nil {
a, aok := a.(proto.Message)
b, bok := b.(proto.Message)
if aok && bok {
return proto.Equal(a, b)
}
}
return reflect.DeepEqual(a, b)
}