Cache storage

Copy Markdown View Source

ImagePipe has two caches: cache stores complete processed responses, and input_cache stores originals fetched from remote sources. Both take {ImagePipe.Cache.FileSystem, options}. The adapter's options are listed in ImagePipe.Cache.FileSystem, and the server's [cache] keys in the server configuration reference. Setting up the caches is covered in Caching processed images, and how long cached images stay valid in Caching and freshness.

Requests that use the cache

The cache is read after the request is parsed, its signature checked, and its source resolved. A request that fails any of those steps returns its error without reading the cache or fetching the source.

ImagePipe.run/4 uses the same caches when its input is {:source, identifier} and it gets the same ImagePipe.config/1 as the Plug. It selects the same cached copy as a Plug request only when given the same accept and request_inputs (see sharing cache entries). {:file, path} and {:binary, bytes} inputs never use either cache.

Source cache settings

The source_cache_policy option sets the cache policy for every source. A source's own cache_policy option replaces it field by field, and the S3 adapter also accepts one per bucket:

source_cache_policy: [
  storage: :origin,                 # :origin | :allow | :deny
  freshness: {:fallback, 300},      # :origin | {:fallback, seconds} | {:force, seconds}
  stale_while_revalidate: :origin   # :origin | :disabled | {:force, seconds}
]

ImagePipe.Source.CachePolicy describes each field. The other per-source settings, stable, internal_cache, and http_cache, are listed in ImagePipe.Source.CacheSettings:

  • stable: :immutable marks a source whose identifiers always name the same bytes.
  • internal_cache: :disabled turns off both caches for the source. The default, :auto, caches whatever the source's storage policy allows.
  • http_cache is described in HTTP cache headers.

What is stored

The processed-image cache stores successful responses only. A failed request, and a crop that fell back to a default because content detection failed, are never stored. A response larger than the cache's max_body_bytes is delivered but not stored.

A watermarked response is stored or reused only when the original and every watermark source permit storage and have internal caching enabled.

A stored response keeps its body, content type, and two response headers: vary and cache-control. Header names are stored in lower case.

Each original's version and its origin freshness headers are also stored in the processed-image cache. They let processed images outlive their original, and count against the processed-image cache's size.

Cache key inputs

Each stored response is selected by:

  • The original's version (see originals and processed images).
  • The processing options. Two spellings of the same options select the same entry.
  • The output format. For a URL without a format option this is the format chosen from Accept, not the Accept header itself.
  • The content detector and model, when the crop uses detection.
  • The URL's cachebuster.
  • The values of the request headers and cookies named in storage_inputs.
  • The credentials and headers the source sends to its origin, as a hash.
    • ImagePipe.Source.S3 uses the configured credentials: static keys, or the provider and its options. Temporary credentials from a provider don't change the key when they rotate.
    • ImagePipe.Source.HTTP uses the resolved auth: value in its req_options. A function or MFA tuple is called once per request, so a new token selects new entries. With auth: :netrc or {:netrc, path}, the source reads the file again about a second after it changes, and its new credentials select new entries too.

The URL's expiry, signature, filename, attachment option, and debug option don't select a different entry.

The originals cache uses the original's identifier, the cachebuster, the storage_inputs values, and the origin credentials. Processing options and output format don't, so every size and format of one original shares one stored copy.

A source's name isn't part of the key, so renaming a source keeps its entries. For a custom source, every option apart from the cache settings stable, cache_policy, internal_cache, and http_cache is part of the key. Within one source, the identity a custom adapter returns from resolve/3 must name everything that selects different origin bytes, or two different originals can share cache entries (see Writing a custom source).

A function in the options of any source is hashed by its module, its place in that module, and the values it captures. The auth: function of ImagePipe.Source.HTTP is the exception, described above. A redeploy keeps the key unless the function or the code around it changes, and functions that capture different values select different entries. An option that holds a pid or a reference, or a function that captures one, can change the key after a restart or redeploy.

Originals cache

input_cache accepts only ImagePipe.Cache.FileSystem, with a root separate from the processed-image cache's. Neither root may be inside the other. Each cache has its own size limit and eviction. Serving a processed image from the cache doesn't count as a request for its original.

  • Originals are stored byte for byte, including their EXIF and ICC data.
  • Local files are copied in only when their source sets copy: :keep (see copy files from network storage).
  • A download is stored only once it completes.
  • When a request fails to decode a stored original, the cache hashes it again and removes it only if it no longer matches the SHA-256 hash recorded when it was stored. An intact original stays cached for the other sizes and formats made from it.
  • Making a new processed image from a stored original applies the current max_body_bytes and pixel limits. Processed images already in the cache are still served after you lower those limits.
  • Downloads in progress, and originals being read by a request, can take the cache over its max_size_bytes for as long as those requests run.

Some originals, such as those the originals cache keeps, are written to a temporary file while they download, in the image_pipe directory under the system temporary directory (see System.tmp_dir/0). Only the user running ImagePipe can open that directory. When that path is a symlink, a file, or another user's directory, those originals are held in memory instead, up to max_body_bytes each. On a host shared with other users, set TMPDIR to a directory of your own.

Writing entries

A response is processed and streamed to the client at the same time, and written to the cache as it streams. The entry is stored only when:

  • the encoder finished,
  • every chunk reached the client,
  • and the body stayed within max_body_bytes.

A client disconnect, a failure after the first chunk, or a body over max_body_bytes leaves nothing in the cache. The client still receives the full response in the last case.

Cache errors never fail a request. A failed write is logged and reported in telemetry, and the response is delivered without being stored. A failed or invalid read is treated as a miss, and the image is processed again under the same key.

Coordination limits

Each node coordinates fetching originals, making processed images, and background checks (see request coalescing):

  • Up to 64 originals can be fetched or checked at once, with up to 1,024 requests waiting for them. Past either limit, a request fetches its original itself, without waiting. A request waiting for an HTTP or S3 original gives up after waiting that source's fetch_timeout (see ImagePipe.Source.HTTP and ImagePipe.Source.S3) plus one second, and answers 504.
  • Up to 16 background checks run at once. Each has a 60-second deadline. After a check of an original finishes, that original isn't checked in the background again for one second.
  • Up to 64 processed images can be made at once while other requests wait for them, with up to 1,024 requests waiting. A request waits at most 60 seconds. Past any of these limits, a request processes the image itself, still subject to the processing pool.

Files on disk

ImagePipe.Cache.FileSystem names files by hash. Paths never contain request paths, source identifiers, header values, or cookie values. Each entry is a body file and a metadata file holding the body's size and SHA-256 hash. Each body is flushed to disk before it appears in the cache, so a crash or power loss can't leave a cached body with missing data.

Before reading a cached body, the cache checks that its size matches its metadata. An entry that fails the check, has invalid metadata, or can't be read is logged and treated as a miss. Only the size is checked when a body is read, so never change cache files in place: a changed body of the same size is served as it is. A body removed after the check, for example by eviction in bounded mode, is still sent in full from the file already opened, as a chunked response without content-length.

A path that leads outside root through a symlink fails as a cache error. Each node checks each entry's directory under root the first time it uses it and remembers each directory that passes. A directory later replaced by a symlink leading outside root isn't noticed until the node restarts, so restart nodes after changing symlinks under root. Anyone who can create files in root can change cached images anyway, so only the user running ImagePipe must be able to write there.

ImagePipe.Cache.FileSystem.get/2 returns the whole body as a binary, for callers outside the Plug.

Leftover files

A node that stops abruptly, for example after a crash or kill -9, can leave temporary entry files behind, links that requests used to hold originals open, and bodies that no metadata file names. A running node can leave them too, for example when a request is killed while its image is being written. ImagePipe deletes them in the background once they are more than an hour old, so nodes that share a root can clean it while the others keep running:

  • In an unbounded cache's root, when an ImagePipe instance starts (see ImagePipe.child_spec/1) and every 24 hours while it runs. image_pipe_server always runs an instance. An unbounded cache without an instance isn't cleaned.
  • In a bounded cache's root, after its startup scan and after each re-scan.
  • In the image_pipe directory under the system temporary directory (TMPDIR, or /tmp), where originals are staged while they download, when the :image_pipe application starts and every 24 hours after.

The [:cache, :sweep] event reports what each cleanup removed (see cache events).

Bounded mode

max_size_bytes turns on bounded mode. Without it the cache grows without limit. The other bounded-mode options, listed in ImagePipe.Cache.FileSystem, require max_size_bytes, and all except node_id have defaults.

Node IDs and supervision

Bounded mode needs node_id, the name of this node's state file <node_id>.state. It must stay the same across restarts and differ between nodes that share a root (see run several replicas).

Each root and node_id pair runs one process that tracks the cache's size and chooses which entries to keep. A bounded cache must be configured on an instance, which starts that process (see ImagePipe.child_spec/1 and Bound the cache size).

Size cap and directory scans

max_size_bytes is a soft cap on the total size of stored bodies in the root, including bodies other nodes that share it wrote:

  • Each write either stores the new entry, evicting less valuable entries to make room, or rejects it. Rejected and replaced bodies are deleted from disk.
  • An entry larger than max_size_bytes is always rejected.
  • A write that needs to evict more than eviction_victim_limit entries (64 by default) evicts that many and starts background eviction for the rest. Until background eviction finishes, the cache can exceed the cap. If the entries it needs to evict include ones read recently, the write is rejected instead, because background eviction would remove the new entry before them.
  • On startup the process scans the entries already on disk in the background, evicts until the cache is at or under the cap, and then deletes leftover files.
  • Every reconcile_interval (60 seconds by default) it evicts again until the cache is at or under the cap.
  • Every rescan_interval (300 seconds by default, moved earlier or later by up to a fifth at random) it lists the root again. It starts counting entries other nodes wrote, stops counting entries they deleted, evicts until the cache is at or under the cap, and deletes leftover files.

A node counts what other nodes write only when it re-scans, so a shared root can exceed the cap by what all its nodes write between two re-scans.

Background eviction removes entries in batches so requests can run between batches. Each batch removes at most eviction_victim_limit entries. Background eviction reports its evictions with trigger: :reconcile.

A write that waits more than 5 seconds for the cache process is reported as a cache write error with error: :admission_timeout. The cache process still handles the queued write and stores or rejects the entry as usual.

Warm start from peers

Each node writes its request counts to <node_id>.state in state_dir every flush_interval. On startup a node merges the counts from every peer state file younger than state_ttl, so a new node keeps entries that are popular across the cluster. Older peer files are deleted every cleanup_interval. On startup a node deletes state files it left half written. Peers' half-written files are deleted with their old state files. Counts of responses requested only once are not saved.

Bounded-mode limitations

  • All writes to one root and node_id go through one process.
  • A crash between writing a body and writing its metadata can leave a body file the cache doesn't track. It doesn't count against max_size_bytes until a startup at least an hour after the crash deletes it.
  • Two writes to the same key at once both store their body. The last one wins, and the other body is deleted.
  • Nodes that share a root count entry sizes separately. When one node deletes or replaces an entry, the other nodes keep counting its old size until their next re-scan, or until they read, evict, delete, or rewrite that entry. Until then a node's count is off by the difference, so it evicts too early or too late.
  • Nodes that share a root and re-scan at about the same time can each evict entries to remove the same excess, so the cache briefly drops below the cap.

The bounded-mode telemetry events are listed in cache events.