Proposal: add optional cache for feature flags

[drjayvee]

I'm trialing PostHog for our SaaS app. For us, the lack of caching for feature flags are a total no-go. (We're currently canary-releasing a feature that requires a decide on every single page view.)

So I've added caching by extending the PostHog\Client and overriding fetchFeatureVariants.

<?php

use PostHog\Client;
use PostHog\HttpClient;
use Psr\Cache\CacheItemPoolInterface;

class CachingClient extends Client {
	
	private const DEFAULT_EXPIRY_DURATION = 'PT5M';    // five minutes
	
	private ?CacheItemPoolInterface $cache;
	private \DateInterval $cacheExpiryInterval;
	
	public function __construct(
		string $apiKey,
		array $options = [],
		?HttpClient $httpClient = null,
		string $personalAPIKey = null,
		CacheItemPoolInterface $cache = null
	) {
		parent::__construct($apiKey, $options, $httpClient, $personalAPIKey);
		
		$this->cache = $cache;
		$this->cacheExpiryInterval = new \DateInterval($options['cacheDuration'] ?? self::DEFAULT_EXPIRY_DURATION);
	}
	
	public function fetchFeatureVariants(string $distinctId, array $groups = [], array $personProperties = [], array $groupProperties = []): array {
		$callParent = fn() => parent::fetchFeatureVariants($distinctId, $groups, $personProperties, $groupProperties);
		
		if (!$this->cache) {
			return $callParent();
		}
		
		$query = substr(md5(json_encode([$groups, $personProperties, $groupProperties])), 0, 8);	// yes, md5 is totally suitable here: it's fast and provides a good distribution
		$cacheItem = $this->cache->getItem("FeatureFlags.dId=$distinctId.q=$query");
		
		if ($cacheItem->isHit()) {
			return $cacheItem->get();
		}
		
		$result = $callParent();
		
		$this->cache->save(
			$cacheItem
				->expiresAfter($this->cacheExpiryInterval)
				->set($result)
		);
		
		return $result;
	}
	
}

I'd be happy to create a PR to add this to the base class.