62 lines
2.4 KiB
Go
62 lines
2.4 KiB
Go
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package api
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import (
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"context"
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"sync"
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"time"
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"github.com/ponzischeme89/memby/server/internal/store"
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)
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// featurePolicyTTL is how stale the cached feature policy may be.
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//
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// The document is read on the request path from sixteen places and by /v1/status, which
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// every open television polls every ten seconds — so on a four-set household the policy
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// was fetched from Postgres somewhere over twenty times a minute to answer a question
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// whose answer changes when an operator presses a switch. Each read is a millisecond and
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// none of them was ever the reason a screen was slow; what they were is a standing draw
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// on a connection pool that the requests which *are* slow have to queue behind.
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//
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// Five seconds because the operator is the only writer and their own write clears this
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// instance's copy outright: the window is not "how long until my change takes effect" but
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// "how long until an instance that did not make the change notices", which is the same
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// question WatchMaintenance answers with thirty.
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const featurePolicyTTL = 5 * time.Second
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// featurePolicyCache is a read-through cache with a deliberately simple concurrency
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// story: a stale read takes the lock, refreshes, and every other caller waits for that
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// one refresh rather than starting its own. That is worth stating because the opposite —
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// releasing the lock to query — is what turns one expired entry into a thundering herd of
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// identical queries, which is the failure this exists to prevent.
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type featurePolicyCache struct {
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mu sync.Mutex
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value store.FeaturePolicy
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valid bool
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fetched time.Time
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}
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// read returns the cached value, refreshing through load when it has expired. load is
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// only ever called with the lock held, so it must not itself read the cache.
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func (c *featurePolicyCache) read(
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ctx context.Context, load func(context.Context) store.FeaturePolicy,
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) store.FeaturePolicy {
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c.mu.Lock()
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defer c.mu.Unlock()
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if c.valid && time.Since(c.fetched) < featurePolicyTTL {
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return c.value
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}
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c.value = load(ctx)
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c.valid = true
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c.fetched = time.Now()
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return c.value
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}
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// invalidate drops the copy so the next read goes to Postgres. Called by the operator's
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// own write: a console that showed the old answer back to the person who had just
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// changed it would read as a save that failed.
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func (c *featurePolicyCache) invalidate() {
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c.mu.Lock()
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defer c.mu.Unlock()
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c.valid = false
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}
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