Symfony 7.4’s Share Directory Solves the Cache Problem for Kubernetes Apps

The release of Symfony 7.4 marks a pivotal moment for PHP developers, particularly those of us architecting high-performance, cloud-native applications. While the changelog is packed with the usual array of developer experience (DX) improvements and performance tweaks, one feature stands out as a game-changer for infrastructure teams and backend leads alike: the Share Directory. Share Directory For years, we’ve wrestled with the “Cache Dilemma” in multi-server environments. We’ve written hacky scripts, managed complex NFS mounts and accepted performance penalties — all to keep our application state synchronized. With Symfony 7.4, the core team has finally addressed this architectural friction point head-on. In this artice, we will explore the new var/share directory, understand the critical problem it solves and implement a robust, production-ready configuration using Symfony 7.4, Doctrine and Flysystem. var/share The “Cache Dilemma”: A History of Performance Bottlenecks To appreciate the elegance of the Share Directory, we must first revisit the pain of the past. In a standard Symfony application, the var/cache directory has historically been the dumping ground for two very different types of data: var/cache System Cache: This includes the compiled service container, URL matching routes, optimized Twig templates and class maps. This data is immutable (or should be) after the cache:warmup phase. It is specific to the version of the code currently running. Application Cache: This includes your cache.app pool, API responses, serialized objects and potentially temporary file storage. This data is volatile and shared. If Server A caches a heavy database query, Server B should be able to read it. System Cache: This includes the compiled service container, URL matching routes, optimized Twig templates and class maps. This data is immutable (or should be) after the cache:warmup phase. It is specific to the version of the code currently running. System Cache: immutable cache:warmup Application Cache: This includes your cache.app pool, API responses, serialized objects and potentially temporary file storage. This data is volatile and shared. If Server A caches a heavy database query, Server B should be able to read it. Application Cache: cache.app volatile shared The Problem in a Cluster Imagine you are deploying a Symfony app to a Kubernetes cluster with 10 pods. Scenario A (Local Cache):You keep var/cache local to each pod.Result: The System Cache is fast (local SSD). However, the Application Cache is fragmented. If a user hits Pod 1, their data is cached there. If their next request hits Pod 2, the cache is missed and the database is hit again. Data consistency is lost. Scenario B (Shared Storage):You mount var/cache to a shared volume (NFS, EFS).Result: The Application Cache is consistent across all pods. Great! But now, every time Symfony needs to load a class or check a route (System Cache), it has to traverse the network to read from the shared storage. This introduces significant I/O latency, often slowing down response times by 10–20% simply due to filesystem overhead. Scenario A (Local Cache):You keep var/cache local to each pod.Result: The System Cache is fast (local SSD). However, the Application Cache is fragmented. If a user hits Pod 1, their data is cached there. If their next request hits Pod 2, the cache is missed and the database is hit again. Data consistency is lost. Scenario A (Local Cache): Result: Scenario B (Shared Storage):You mount var/cache to a shared volume (NFS, EFS).Result: The Application Cache is consistent across all pods. Great! But now, every time Symfony needs to load a class or check a route (System Cache), it has to traverse the network to read from the shared storage. This introduces significant I/O latency, often slowing down response times by 10–20% simply due to filesystem overhead. Scenario B (Shared Storage): Result: We were stuck choosing between consistency and performance. consistency performance Enter the Share Directory (var/share) Symfony 7.4 introduces a dedicated architectural concept to solve this: the Share Directory. Share Directory This feature formally decouples the location of system-specific files from application-shared files. var/cache: Remains the home for System Cache. It should stay local to the server (ephemeral storage in K8s terms). It requires no synchronization. var/share: The new home for Application Data. This directory is designed to be mounted on shared storage (like NFS, AWS EFS, or a persistent volume). var/cache: Remains the home for System Cache. It should stay local to the server (ephemeral storage in K8s terms). It requires no synchronization. var/cache: System Cache var/share: The new home for Application Data. This directory is designed to be mounted on shared storage (like NFS, AWS EFS, or a persistent volume). var/share: Application Data This separation allows us to have the best of both worlds: blazing-fast, local system caches and a consistent, shared application state. Implementation Guide Let’s get our hands dirty. We will build a Symfony 7.4 application that leverages the Share Directory for three common use cases: Shared Application Cache Shared File Storage (Uploads) Shared SQLite Database (for simple state) Shared Application Cache Shared File Storage (Uploads) Shared SQLite Database (for simple state) Prerequisites and Installation Ensure you are running PHP 8.2 or higher and have the Symfony CLI installed. symfony new my_app --webapp cd my_app composer require symfony/framework-bundle:^7.4 symfony new my_app --webapp cd my_app composer require symfony/framework-bundle:^7.4 Basic Configuration In a fresh Symfony 7.4 installation, the framework is already aware of this concept. Open your .env file. You will see a new environment variable: .env # .env APP_SHARE_DIR=$APP_PROJECT_DIR/var/share # .env APP_SHARE_DIR=$APP_PROJECT_DIR/var/share Under the hood, the Kernel class now implements a getShareDir() method. If you are upgrading an existing application, you might need to add this manually or rely on the default fallback (which points to the cache directory for backward compatibility). Kernel getShareDir() To enforce this structure in an upgraded app, update your Kernel.php if necessary, or simply define the environment variable. Kernel.php Configuring Shared Cache Pools The most immediate win is moving your application cache to the shared directory. By default, Symfony’s file system cache adapter saves to var/cache/pools. We want to move this to var/share/pools. var/cache/pools var/share/pools Open config/packages/cache.yaml. We will configure the default app cache to use the new %kernel.share_dir% parameter. config/packages/cache.yaml %kernel.share_dir% # config/packages/cache.yaml framework: cache: # distinct_id helps avoid collisions if multiple apps share the same storage prefix_seed: 'my_app_%env(APP_ENV)%' # Default app cache app: cache.adapter.filesystem # System cache (stays in var/cache, FAST!) system: cache.adapter.system # Configure filesystem adapter to use the share directory default_redis_provider: 'redis://localhost' pools: # We explicitly tell the 'cache.app' pool to use the share directory cache.app: adapter: cache.adapter.filesystem provider: ~ # The 'directory' option is available for the filesystem adapter # We use the new container parameter default_lifetime: 3600 provider: 'cache.default_marshaller' services: # We need to configure the specific directory for the filesystem adapter # Since we can't easily pass the directory in the simplified YAML above for the main adapter, # we can define a custom adapter or rely on the global default if Symfony 7.4 automates it. # However, for explicit control, let's define a shared pool: cache.adapter.shared_filesystem: parent: 'cache.adapter.filesystem' tags: ['cache.pool'] arguments: $directory: '%kernel.share_dir%/pools' framework: cache: app: cache.adapter.shared_filesystem # config/packages/cache.yaml framework: cache: # distinct_id helps avoid collisions if multiple apps share the same storage prefix_seed: 'my_app_%env(APP_ENV)%' # Default app cache app: cache.adapter.filesystem # System cache (stays in var/cache, FAST!) system: cache.adapter.system # Configure filesystem adapter to use the share directory default_redis_provider: 'redis://localhost' pools: # We explicitly tell the 'cache.app' pool to use the share directory cache.app: adapter: cache.adapter.filesystem provider: ~ # The 'directory' option is available for the filesystem adapter # We use the new container parameter default_lifetime: 3600 provider: 'cache.default_marshaller' services: # We need to configure the specific directory for the filesystem adapter # Since we can't easily pass the directory in the simplified YAML above for the main adapter, # we can define a custom adapter or rely on the global default if Symfony 7.4 automates it. # However, for explicit control, let's define a shared pool: cache.adapter.shared_filesystem: parent: 'cache.adapter.filesystem' tags: ['cache.pool'] arguments: $directory: '%kernel.share_dir%/pools' framework: cache: app: cache.adapter.shared_filesystem In standard Symfony 7.4, the default filesystem adapter might still default to cache_dir. By explicitly creating a service cache.adapter.shared_filesystem pointing to %kernel.share_dir%/pools, we ensure that our application cache is stored in the correct location. cache_dir cache.adapter.shared_filesystem %kernel.share_dir%/pools To verify this, clear your cache and warm it up. php bin/console cache:clear php bin/console cache:pool:clear cache.app php bin/console cache:clear php bin/console cache:pool:clear cache.app Then, generate some cache entries (e.g., by visiting a page). Check the directory structure: ls -la var/share/pools ls -la var/share/pools You should see a folder structure created by the cache adapter. Shared File Storage with Flysystem Handling user uploads in a cluster is a classic use case for shared storage. We will use league/flysystem-bundle to map a storage adapter to var/share/storage. league/flysystem-bundle var/share/storage Installation: Installation: composer require league/flysystem-bundle composer require league/flysystem-bundle Configuration: Open config/packages/flysystem.yaml. Configuration: config/packages/flysystem.yaml # config/packages/flysystem.yaml flysystem: storages: default.storage: adapter: 'local' options: # Use the new kernel parameter directory: '%kernel.share_dir%/storage' # config/packages/flysystem.yaml flysystem: storages: default.storage: adapter: 'local' options: # Use the new kernel parameter directory: '%kernel.share_dir%/storage' Now, let’s create a service to handle file uploads using this storage. We will use PHP 8 Attributes for dependency injection. PHP 8 Attributes namespace App\Service; use League\Flysystem\FilesystemOperator; use Symfony\Component\DependencyInjection\Attribute\Target; use Symfony\Component\HttpFoundation\File\UploadedFile; class FileManager { public function __construct( #[Target('default.storage')] private readonly FilesystemOperator $storage ) {} public function uploadUserFile(UploadedFile $file, string $userId): string { $filename = sprintf('%s/%s.%s', $userId, uniqid(), $file->guessExtension()); $stream = fopen($file->getPathname(), 'r'); // Write to var/share/storage/... $this->storage->writeStream($filename, $stream); if (is_resource($stream)) { fclose($stream); } return $filename; } public function getFileContent(string $filename): string { return $this->storage->read($filename); } } namespace App\Service; use League\Flysystem\FilesystemOperator; use Symfony\Component\DependencyInjection\Attribute\Target; use Symfony\Component\HttpFoundation\File\UploadedFile; class FileManager { public function __construct( #[Target('default.storage')] private readonly FilesystemOperator $storage ) {} public function uploadUserFile(UploadedFile $file, string $userId): string { $filename = sprintf('%s/%s.%s', $userId, uniqid(), $file->guessExtension()); $stream = fopen($file->getPathname(), 'r'); // Write to var/share/storage/... $this->storage->writeStream($filename, $stream); if (is_resource($stream)) { fclose($stream); } return $filename; } public function getFileContent(string $filename): string { return $this->storage->read($filename); } } When you deploy this to Kubernetes, you will mount a Persistent Volume Claim (PVC) to /app/var/share. If Pod A uploads a file, it is written to the PVC. Pod B can immediately read that file via the same path, because they share the underlying storage volume. /app/var/share Shared SQLite Database (The “Lite” Cluster) While usually not recommended for heavy concurrent writes, using SQLite in var/share is a fantastic pattern for read-heavy, low-write data, or simple internal tooling dashboards that need to persist across deployments without a full MySQL/PostgreSQL setup. var/share Configuration: Open config/packages/doctrine.yaml. Configuration: config/packages/doctrine.yaml doctrine: dbal: # Use the standard SQLite driver driver: 'pdo_sqlite' # Point the path to the share directory path: '%kernel.share_dir%/db/app.sqlite' charset: UTF8 doctrine: dbal: # Use the standard SQLite driver driver: 'pdo_sqlite' # Point the path to the share directory path: '%kernel.share_dir%/db/app.sqlite' charset: UTF8 Ensure the directory exists: mkdir -p var/share/db mkdir -p var/share/db Now, when you run migrations: php bin/console doctrine:migrations:migrate php bin/console doctrine:migrations:migrate The .sqlite file is created in the shared directory. All servers in your cluster will read from this single file. .sqlite Warning: SQLite over NFS (Network File System) can have locking issues. Ensure your shared storage solution supports file locking correctly (e.g., AWS EFS with appropriate mounting options), or reserve this for scenarios with very low write frequency. Warning: DevOps: The Deployment Strategy To fully utilize this feature, your deployment manifest (e.g., docker-compose.yml or Kubernetes deployment.yaml) needs to be updated. docker-compose.yml deployment.yaml Docker Compose Example Here is how you simulate the production environment locally. services: app_1: build: . environment: APP_SHARE_DIR: /app/var/share volumes: - shared_data:/app/var/share app_2: build: . environment: APP_SHARE_DIR: /app/var/share volumes: - shared_data:/app/var/share volumes: shared_data: services: app_1: build: . environment: APP_SHARE_DIR: /app/var/share volumes: - shared_data:/app/var/share app_2: build: . environment: APP_SHARE_DIR: /app/var/share volumes: - shared_data:/app/var/share volumes: shared_data: In this setup: System Cache (var/cache) is inside the container (fast, isolated). Shared Data (var/share) is a named volume shared between app_1 and app_2. System Cache (var/cache) is inside the container (fast, isolated). System Cache (var/cache) Shared Data (var/share) is a named volume shared between app_1 and app_2. Shared Data (var/share) Kubernetes (K8s) Strategy In Kubernetes, you would use a PersistentVolumeClaim. apiVersion: apps/v1 kind: Deployment metadata: name: symfony-app spec: replicas: 3 template: spec: containers: - name: php image: my-symfony-app:7.4 env: - name: APP_SHARE_DIR value: "/var/www/html/var/share" volumeMounts: - mountPath: "/var/www/html/var/share" name: shared-storage volumes: - name: shared-storage persistentVolumeClaim: claimName: efs-pvc apiVersion: apps/v1 kind: Deployment metadata: name: symfony-app spec: replicas: 3 template: spec: containers: - name: php image: my-symfony-app:7.4 env: - name: APP_SHARE_DIR value: "/var/www/html/var/share" volumeMounts: - mountPath: "/var/www/html/var/share" name: shared-storage volumes: - name: shared-storage persistentVolumeClaim: claimName: efs-pvc Verification: Verification: Exec into Pod 1: kubectl exec -it symfony-app-pod-1 — bash Create a file: touch var/share/test.txt Exec into Pod 2: kubectl exec -it symfony-app-pod-2 — bash Check file: ls var/share/ You should see test.txt. Exec into Pod 1: kubectl exec -it symfony-app-pod-1 — bash kubectl exec -it symfony-app-pod-1 — bash Create a file: touch var/share/test.txt touch var/share/test.txt Exec into Pod 2: kubectl exec -it symfony-app-pod-2 — bash kubectl exec -it symfony-app-pod-2 — bash Check file: ls var/share/ ls var/share/ You should see test.txt. You should see test.txt Migration Guide: Upgrading to 7.4 If you are maintaining a legacy application, adopting the Share Directory requires a few careful steps. Update Composer: Update composer.json to allow symfony/*: 7.4.*. Run composer update “symfony/*”. Update .env: Add APP_SHARE_DIR to your .env and .env.test. Update .gitignore: You generally do not want to commit the contents of the share directory, but you might want to keep the gitkeep. Refactor Paths: Search your codebase for hardcoded references to kernel.cache_dir or var/cache. If the code was using the cache directory to store data that should be persisted or shared (like generated PDF reports, temporary exports), change it to use %kernel.share_dir%. Update Composer: Update composer.json to allow symfony/*: 7.4.*. Run composer update “symfony/*”. Update .env: Add APP_SHARE_DIR to your .env and .env.test. .env: Add APP_SHARE_DIR .env .env.test Update .gitignore: You generally do not want to commit the contents of the share directory, but you might want to keep the gitkeep. .gitignore Refactor Paths: Search your codebase for hardcoded references to kernel.cache_dir or var/cache. If the code was using the cache directory to store data that should be persisted or shared (like generated PDF reports, temporary exports), change it to use %kernel.share_dir%. Refactor Paths: kernel.cache_dir var/cache %kernel.share_dir% Conclusion Symfony 7.4’s Share Directory is more than just a new folder structure; it is a maturity signal. It acknowledges that modern PHP applications live in the cloud, across clusters and need to scale horizontally without fighting the framework. Share Directory By adopting var/share, you simplify your infrastructure configuration, improve the performance of your system caches and provide a clear, standardized location for your application’s state. var/share Ready to scale? Start by upgrading a non-critical microservice to Symfony 7.4 today. Implement the var/share directory, remove those complex NFS mounts for your system cache and watch your average response times drop. Ready to scale? var/share Keep in touch! Did this article help you clarify your upgrade path? Follow me on [LinkedIn:https://www.linkedin.com/in/matthew-mochalkin/]. Let’s build better software, together. Keep in touch! https://www.linkedin.com/in/matthew-mochalkin/ https://www.linkedin.com/in/matthew-mochalkin/