Force Refresh a Symfony Cache Item with beta: INF

Here's some Symfony Cache Contracts code you've probably written a hundred times:

$value = $cache->get('weather', function (ItemInterface $item) {
    $item->expiresAfter(3600);

    return $this->weatherApi->getForecast();
});

Return the cached value if it exists, otherwise run the callback and save its result. But get() accepts more than a key and a callback - here's the full signature:

public function get(
    string $key,
    callable $callback,
    ?float $beta = null,
    ?array &$metadata = null,
): mixed;

That third argument - $beta - is one I ignored for years. It turns out to be a knob on the algorithm Symfony uses to prevent cache stampedes. And if we turn that knob all the way up, we get a genuinely useful application-level feature for free.

Wait... What's beta?

Despite the name, this has nothing to do with beta software. The name comes from the math behind the cache stampede prevention algorithm Symfony uses.

Symfony's implementation is based on an algorithm called XFetch, from the paper Optimal Probabilistic Cache Stampede Prevention. In it, the Greek letter beta - β - is the parameter that controls an exponential probability distribution used to decide how early a cached value might be recomputed.

So $beta isn't just some funky Symfony parameter name. It's basically a little math knob.

And notice that the default value is null:

beta: null

That doesn't mean "disable beta". It means: let the cache adapter choose its default beta value. For Symfony's standard cache adapters, that normally means a beta of 1.0, giving you the usual probabilistic early-expiration behavior - which is exactly what you want most of the time, so you never need to think about this argument at all.

Avoiding a Cache Stampede

Why does any of this exist?

Imagine a popular cache item expires at exactly 2:00 PM. At 1:59:59, everything is great. Then 2:00 arrives and a bunch of requests hit the site at once.

Without any protection, all of them could notice:

Hey! This cache item expired!

And suddenly they're all trying to perform the same expensive work. That's a cache stampede, also known as the "thundering herd" problem, and Symfony's Cache Contracts have two built-in ways to help prevent it.

Both are on by default - there's nothing to enable, and if you're using the Cache Contracts you're already getting them.

First, there's locking. If multiple requests need to compute the same missing value, Symfony makes sure only one does the work at a time. The others wait for it to finish.

That's already much better than having every request hammer our database or API. But waiting isn't ideal either, and this is where Symfony's second trick - probabilistic early expiration - gets interesting.

Instead of waiting until the cached value is completely gone, Symfony can decide a little early:

I'm going to refresh this now.

That one request recomputes the value while everyone else can keep happily using the existing cached value. Once the recomputation finishes, the fresh value replaces the old one. No herd - and, even better, no requests waiting around for the cache to warm back up.

The $beta argument controls how aggressive that early expiration is: a larger beta makes Symfony more eager to recompute early.

Turn the knob the other way:

beta: 0.0

and early expiration is disabled entirely. The value is then only recomputed once it has genuinely expired. Locking still applies, so we're not completely unprotected - but that's the "everybody waits" behavior we just talked about.

Turning the Knob All the Way Up

So what happens if we take that math knob to infinity?

$value = $cache->get(
    'weather',
    function (ItemInterface $item) {
        $item->expiresAfter(3600);

        return $this->weatherApi->getForecast();
    },
    beta: INF,
);

The early-expiration test always wins. In other words:

Treat this cache item as expired and recompute it now.

The callback runs, its result is stored, and we get the fresh value back. Which makes beta: INF a lovely way to say "force refresh" - though your first instinct for that is probably to delete the key:

$cache->delete('weather');

$value = $cache->get('weather', $callback);

That works, but it creates an actual cache miss, and the cost lands on everyone else. Once we've deleted the key, the old value is gone, so every other request that needs it while we're rebuilding is stuck waiting on that same expensive callback. Our own request waits for the callback either way - that part is unavoidable, and usually fine, since the request doing the refreshing is a deliberate one: an admin action, a console command, a Messenger message.

With beta: INF, we're saying something more precise:

Get this value, but force this request to refresh it.

Nobody else pays for our refresh: normal traffic keeps reading the existing cached value until the new one is ready to replace it.

A Nice "Refresh" Operation

Imagine an admin action that can optionally force some cached API data to refresh. Instead of one code path for the normal cache lookup and another for deleting and rebuilding, we can keep everything together:

$products = $cache->get(
    'products',
    function (ItemInterface $item) {
        $item->expiresAfter(3600);

        return $this->productApi->fetchProducts();
    },
    beta: $forceRefresh ? INF : null,
);

When $forceRefresh is false, we pass beta: null and the cache adapter uses its normal probabilistic behavior. When it's true, we pass beta: INF and Symfony recomputes the value immediately.

$forceRefresh can come from wherever fits the app - a ?forceRefresh=1 query parameter, a --refresh-cache console option, a button in an admin panel - just restrict who's allowed to trigger it.

Normal request? Use the cache normally.

Explicit refresh? Recompute it now.

One argument covers both cases, and at no point does the cached value disappear out from under everyone else. Not bad for a parameter most of us have scrolled past a hundred times without ever wondering what it was for.

Happy cache-herd wrangling!