Cache storage

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The cache option stores complete processed responses as files on disk. It takes a keyword list, such as [root: "/var/cache/image_pipe/processed", max_size_bytes: 5_000_000_000]. In Elixir, the cache needs an ImagePipe instance (see ImagePipe.child_spec/1). image_pipe_server runs one itself. The options are listed in ImagePipe.Cache.FileSystem, and the server's [cache] keys in the server configuration reference. Setting up the cache 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 vary_values (see sharing cache entries). {:file, path} and {:binary, bytes} inputs never use the 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 caching for the source: its processed images aren't stored, and its originals aren't remembered or shared between requests. The default, :enabled, 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.

An instance keeps the bytes of up to 64 recently used watermark images, each up to 1 MiB, in memory, so a watermark from an origin isn't downloaded for every processed image. A stale watermark is checked with its origin, as an original is.

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

An instance keeps each original's version and its origin freshness headers in memory, so processed images outlive their original.

Cache key inputs

Each stored response is selected by:

  • The original's version, and each watermark image'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 configured output settings that apply to the image, such as the default qualities, encoder options, and metadata stripping.
  • Where each watermark comes from and its configured opacity. Renaming a watermark keeps its entries.
  • The content detector and model, when the crop uses detection.
  • The URL's cachebuster.
  • The values of the request headers and cookies named in vary_on.
  • 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.

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.

Downloaded originals

Originals from HTTP and S3 sources are written to a temporary file while they download and while requests use them, in the image_pipe directory under the system temporary directory (see System.tmp_dir/0). Local files are read where they are. 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. The first request that can't use the directory logs a warning naming it. With the default Logger attached, each request whose original is held in memory has staging=memory on its line. 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 response never waits for the cache's disk: each chunk is written as it arrives, and the entry is synced and stored after the response is sent. 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.

An original downloaded for a request is stored in the background too, while the request processes it.

A cache drops a write when:

  • it has 64 writes in progress,
  • its unfinished writes hold more than 256 MiB, or a twentieth of its max_size_bytes when that is smaller,
  • the client is more than 8 MiB ahead of a response's write,
  • a response's write is still unfinished 30 seconds after the response started.

The response is still delivered, and the [:cache, :stage] event reports the dropped write.

A write counts toward the unfinished writes as it goes. A write that is the only one in progress passes that limit at any size, so a cache of 100 MB, for example, stores an image over 5 MB only when no other write is in progress. The cache's files can take up to max_size_bytes, plus the limit on unfinished writes or the one write in progress, whichever is larger, plus evicted entries that requests are still reading.

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 4,096 originals can be shared between requests at once, with up to 1,024 requests waiting for them. Past either limit, a request fetches its original itself, without waiting. A request that waits while another request downloads or checks an HTTP or S3 original gives up after that source's fetch_timeout (see ImagePipe.Source.HTTP and ImagePipe.Source.S3) plus one second, and answers 504. When a download finishes, every request waiting for it uses the downloaded original.
  • Up to 16 background checks run at once. Each has a 60-second deadline. After a background check for a processed image finishes, no other check for that image starts for one second. Checks for other sizes or formats of the same original can still start.
  • Up to 4,096 processed images can be shared between requests at once, with up to 1,024 requests waiting for them. Each wait lasts at most 60 seconds, and at most 2 seconds after the image was sent if it isn't stored by then. 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, 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.

Cache root owner file

Each root serves one node. When an ImagePipe instance (see ImagePipe.child_spec/1) starts, it writes a .image-pipe-owner file naming its hostname into each cache's root. The instance rewrites the file every 5 seconds and deletes it when it stops. A node that restarts with the same hostname takes the root over at once.

When the file names another hostname, the instance logs a warning, starts without that cache, and checks the file every 25 seconds:

  • While the other node keeps rewriting the file, the instance logs an error once.
  • Once the file is gone, or hasn't changed since the last check, the instance takes the root over, reads the file back 5 seconds later, and starts using the cache. If another node took the root at the same time, the instance keeps checking.

A waiting instance starts using the cache:

  • about 30 seconds after it starts, when the other node was already gone, for example killed with kill -9
  • within 30 seconds of the other node stopping and deleting the file
  • within about a minute of the other node stopping without deleting the file

A running node that finds its file rewritten by another node logs an error and runs without that cache, watching the file as a starting instance does. This happens when two nodes with the same hostname share a root (the node that started later keeps it), or when a node stops rewriting its file for 25 to 50 seconds, depending on when the waiting node checks, while another node waits for the root, as on a hung network mount.

If the instance can't write the file, for example because root isn't writable, it logs an error and uses the cache anyway. Each image it can't store then counts as a write error.

Leftover files

A node that stops abruptly, for example after a crash or kill -9, can leave temporary entry files behind, 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. After a version change that changes the format of metadata files, the ones the cache can't parse count as left behind too, and so do their bodies. ImagePipe deletes leftover files in the background once they are more than an hour old:

  • In the cache's root, during its startup scan and every 24 hours while the instance runs.
  • 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 records the metadata format it last checked in a .metadata-version file in root. It reads every metadata file only when that file is missing or names another format, as after an upgrade or a downgrade. The startup scan then takes longer.

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

Size limit

The cache evicts entries to stay under max_size_bytes. It keeps the entries that are requested often and took the most work to make for their size, so an AVIF made with automatic quality outlasts a JPEG thumbnail requested as often.

max_size_bytes counts each entry as the disk it takes: its body rounded up to whole 4 KiB blocks, plus one more block. A 30-byte BlurHash counts 8 KiB, so a 1 GB cache holds at most about 120,000 of them.

Cache process and restarts

Each root runs one process that tracks the cache's size and chooses which entries to keep. Only an instance starts that process (see ImagePipe.child_spec/1 and Add a processed-image cache).

Request counts live only in memory. After a restart they start from zero, and entries already on disk are next in line for eviction, roughly those that took the least work for their size first, until they are requested again.

The process also keeps the metadata of recently served entries in memory, so a repeated hit reads only the body file from disk. The process holds one entry's metadata for each 50 KB of max_size_bytes, from 1,000 to 50,000 entries, at about 1.1 KB each: about 22 MB for a 1 GB cache, and at most about 55 MB. Set max_remembered_entries to change the number of entries. This memory isn't counted against max_size_bytes. A restart empties it. A request with debug headers reads its metadata from disk.

Work to make an entry

The work to make a processed image is estimated from what its request did:

  • the original's format and size, and whether it has transparency
  • the output's format, size, and encoder effort
  • operations with a cost of their own, such as trim, blur, sharpen, and rotate by an angle other than a quarter turn, each by the size of the image it runs on
  • each quality that automatic quality tries
  • object detection

The estimate isn't timed, so a busy server doesn't make entries look more expensive.

Eviction and startup scan

max_size_bytes caps the total size of stored bodies in the root:

  • To make room for a write, the cache picks 16 entries at random, chooses the one least worth keeping, and repeats until the write fits. If the new entry is worth more than the chosen entries on average, the cache evicts them and stores it. Otherwise it rejects the write and evicts nothing. A rejected entry requested at least six times is stored anyway one time in
    1. The stored bodies never exceed the cap, except briefly while the startup scan runs: until the scan has counted the entries on disk, writes are measured against only the entries counted so far.
  • A rejected write's body is deleted without being synced to disk. A kept write is synced, then stored, evicting the entries it was compared with. If other writes filled the cache during the sync so that those entries no longer make room, the kept write is compared with more entries and can still be rejected.
  • An entry larger than the limit on unfinished writes is stored only when no other write is in progress (see writing entries).
  • On startup the process lists the entries on disk in the background, reading only file names except after a metadata format change, and deletes leftover files as it goes. If the cache is over the cap, for example after you lower max_size_bytes, it then evicts in batches until it's under, so requests can use the cache between batches.

A write that waits more than 5 seconds for the cache process is reported as a cache write error with error: :admission_timeout:

  • A write that timed out before it was kept or rejected is dropped.
  • A kept write that timed out before it was stored is still stored or rejected by the cache process.

Eviction limitations

  • A crash between writing a body and writing its metadata can leave a body file the cache doesn't track. Its size isn't counted against max_size_bytes. The first cleanup of leftover files after it is an hour old deletes it, which can be up to a day later.
  • Two writes to the same key at once both store their body. The last one wins, and the other body is deleted.
  • A metadata file damaged after the cache last checked every metadata file, for example by a disk fault, makes its entry a miss, but the cleanup of leftover files doesn't delete it. Its body counts against max_size_bytes until eviction deletes the entry.
  • Entries an earlier ImagePipe version stored in another format aren't counted against max_size_bytes. A cleanup of leftover files deletes them, which can be up to a day after the upgrade for files written in its last hour.

The admission and eviction telemetry events are listed in cache events.

Usage and hit rate

ImagePipe.cache_stats/1 returns an :output entry for cache, with the cache's :bytes, :entries, and :max_bytes, and its :hits, :misses, :read_errors, and :write_errors since the VM started:

  • Each request counts once: a hit when the processed image is served from the cache, a miss when it's made.
  • A read error is a stored entry the cache couldn't read. The request treats it as a miss, so it counts as a read error and as a miss.
  • A write error is an image the cache failed to store, for example because root isn't writable or its disk is full. The image is still served.
  • :counting? is true until the startup scan has counted the entries on disk, deleted leftover files, and evicted down to max_size_bytes. Until then, :bytes and :entries leave out the ones it hasn't reached.

The hit rate is hits / (hits + misses). Each node counts only its own requests. A cache that isn't configured isn't listed. A cache whose root another node owns isn't used by this node, so its entry has only refused: true and :max_bytes. A cache whose process is restarting, or busy for more than a second, can't report its size. Its entry has available: false, refused: false, :max_bytes, and the lookup counts, but no :bytes, :entries, or :counting?. Every other entry has available: true and refused: false.