Caching

Caching dramatically reduces repeated processing costs and shrinks latency for popular images. imgforge integrates with the Foyer cache engine to offer three backends: in-memory, disk, and hybrid. This document explains how to configure each mode and how the cache interacts with the request lifecycle.

Cache architecture diagram

┌─────────────────────────────────────────────────────────────────────────┐
│                         Cache Architecture                              │
└─────────────────────────────────────────────────────────────────────────┘

    Request arrives
         │
         ▼
    ┌────────────────────────┐
    │   Generate cache key   │  → Full path hash (signature + options + source)
    │   from request path    │
    └───────────┬────────────┘
                │
                ▼
    ┌───────────────────────────────────────────────────────────────┐
    │              Check IMGFORGE_CACHE_TYPE                        │
    └───────────────────────────────────────────────────────────────┘
                │
                │
        ┌───────┴───────┬──────────────┬────────────────┐
        │               │              │                │
        ▼               ▼              ▼                ▼
    ┌────────┐    ┌─────────┐   ┌──────────┐     ┌──────────┐
    │  None  │    │ Memory  │   │   Disk   │     │  Hybrid  │
    └────┬───┘    └────┬────┘   └─────┬────┘     └─────┬────┘
         │             │              │                │
         │             ▼              ▼                ▼
         │      ┌────────────┐  ┌──────────────┐  ┌──────────────────┐
         │      │Foyer       │  │Foyer Block   │  │Memory (hot) +    │
         │      │In-Memory   │  │Engine        │  │Disk (spillover)  │
         │      │LRU Cache   │  │Persistent    │  │                  │
         │      └─────┬──────┘  └──────┬───────┘  └─────────┬────────┘
         │            │                │                    │
         │         HIT│    MISS        │HIT      MISS       │HIT    MISS
         │            │                │                    │
         │            └────────────────┴────────────────────┘
         │                            │
         │                           MISS
         └────────────────────────────┘
                                      │
                                      ▼
                          ┌──────────────────────┐
                          │  Process Image       │
                          │  (full pipeline)     │
                          └──────────┬───────────┘
                                     │
                                     ▼
                          ┌──────────────────────┐
                          │  Populate Cache      │
                          │  (if enabled)        │
                          └──────────┬───────────┘
                                     │
                                     ▼
                          ┌──────────────────────┐
                          │  Return Response     │
                          └──────────────────────┘


    Cache Backend Comparison:
    ┌─────────────┬──────────────┬───────────────┬─────────────────────┐
    │   Type      │   Speed      │  Persistence  │     Best For        │
    ├─────────────┼──────────────┼───────────────┼─────────────────────┤
    │  Memory     │  Fastest     │  Volatile     │  Edge nodes, dev    │
    │  Disk       │  Medium      │  Persistent   │  Long-lived cache   │
    │  Hybrid     │  Fast + Med  │  Persistent   │  High-traffic prod  │
    │  None       │  N/A         │  N/A          │  Testing only       │
    └─────────────┴──────────────┴───────────────┴─────────────────────┘

How caching works

Key derivation: The cache key is the full request path (including processing options, cache_buster, and output format). Different signatures or parameters yield different cache entries.
Population: After successfully processing an image, imgforge inserts the rendered bytes into the configured cache backend.
Invalidation: Caches are size-limited, so least-recently-used entries are evicted automatically. Use the cache_buster option to force a miss when you update upstream assets.

Metrics:

Metric	Description
`cache_hits_total{cache_type="memory"}`	Number of successful lookups.
`cache_misses_total{cache_type="memory"}`	Number of misses (including disabled caches).

Enabling a cache backend

Set IMGFORGE_CACHE_TYPE to one of memory, disk, or hybrid. If unset, the cache is disabled and every request hits the processing pipeline.

Common environment variables

Variable	Description
`IMGFORGE_CACHE_TYPE`	Backend selector: `memory`, `disk`, or `hybrid`.
`IMGFORGE_CACHE_MEMORY_CAPACITY`	Maximum number of items retained in memory (default `1000`). Applies to memory and hybrid caches.
`IMGFORGE_CACHE_DISK_PATH`	Directory for on-disk storage. Required for disk and hybrid caches. Ensure it exists and is writable before starting the server.
`IMGFORGE_CACHE_DISK_CAPACITY`	Maximum number of entries stored on disk (default `10000`).

Memory cache

Configure for short-lived services or low-latency responses:

export IMGFORGE_CACHE_TYPE=memory
export IMGFORGE_CACHE_MEMORY_CAPACITY=5000

Backed entirely by RAM.
Fastest option but volatile—entries are lost on restart.
Ideal for front-line edge nodes where disk is unavailable.

Disk cache

Persist cache entries across restarts using local or network storage:

export IMGFORGE_CACHE_TYPE=disk
export IMGFORGE_CACHE_DISK_PATH=/var/cache/imgforge
export IMGFORGE_CACHE_DISK_CAPACITY=20000

Uses Foyer’s block engine to store bytes on disk.
Suitable when CPU-intensive renders must be reused across deploys.
Place /var/cache/imgforge on SSD-backed storage to minimize latency.

Hybrid cache

Combine a memory hot set with disk-backed persistence:

export IMGFORGE_CACHE_TYPE=hybrid
export IMGFORGE_CACHE_MEMORY_CAPACITY=5000
export IMGFORGE_CACHE_DISK_PATH=/var/cache/imgforge
export IMGFORGE_CACHE_DISK_CAPACITY=50000

Frequently accessed objects stay in memory; less popular ones spill to disk.
Balances latency and durability for high-traffic services.

Operational tips

Provision storage – Ensure the disk path exists and ownership matches the user running imgforge. For containers, mount a persistent volume at the desired location.
Monitor hit ratios – Scrape Prometheus metrics and alert on low hit rates; adjust memory capacity or investigate signature churn.
Warm caches – Precompute popular assets by hitting imgforge ahead of peak traffic. Automate via a job triggered after deploys.
Eviction strategy – Cache capacity is entry-based. If stored objects vary significantly in size, monitor disk usage separately and prune old entries if necessary.
Security – When storing on shared disks, restrict permissions (0700) to the imgforge user to prevent other processes from reading cached content.
Replication – imgforge does not provide distributed caching. For multi-node deployments, rely on CDN layers or object storage if cross-node sharing is required.

Troubleshooting caching issues

Misses despite configuration: Confirm IMGFORGE_CACHE_TYPE is set and no typos exist. Check logs for Failed to initialize cache messages.
Permission denied: Ensure the disk directory is writable. In containers, mount with the correct UID/GID or use chown during image build.
Unexpected eviction: Increase IMGFORGE_CACHE_*_CAPACITY values and monitor resource usage. Also verify that cache_buster values are not changing unnecessarily.

For broader operational strategies, see Performance and Deployment.

Caching

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