image_pipe

# Cache storage

ImagePipe has two caches: `cache` stores complete processed responses, and
`input_cache` stores originals fetched from remote sources. Both store files
on disk and take a keyword list, such as
`[root: "/var/cache/image_pipe/processed", max_size_bytes: 5_000_000_000]`.
In Elixir, each 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](../../image_pipe_server/docs/server-configuration.md#cache).
Setting up the caches is covered in
[Caching processed images](caching-processed-images.md), and how long cached
images stay valid in [Caching and freshness](caching-and-freshness.md).

## 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](combined-usage.md#processing-in-a-job)).
`{: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:

```elixir
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, `:enabled`, caches whatever the source's storage policy allows.
- `http_cache` is described in [HTTP cache headers](cdn-http-cache.md#per-source-modes).

## 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](content-aware-gravity.md) 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](caching-and-freshness.md#originals-and-processed-images),
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](caching-and-freshness.md#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](custom-sources.md)).

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` needs 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](serving-local-files.md#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 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](streaming-failures.md). 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. The details kept to recheck an original are stored
in the processed-image cache. Without an originals cache, storing those
details takes one of the processed-image cache's 64 writes, or runs in the
request when none is free.

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]`](telemetry-events.md#cache-stage) event reports the
dropped write.

A write counts toward the unfinished writes as it goes, so an entry larger
than that limit is never stored. A cache of 100 MB, for example, never
stores an image over 5 MB. The cache's files can take up to
`max_size_bytes` plus the limit on unfinished writes, plus evicted originals
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](caching-and-freshness.md#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`. When the original isn't stored 2 seconds after it was
  downloaded, the next waiting request takes over, and downloads it again
  if it still isn't stored.
- 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,
  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](processing-controls.md).

## 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](#size-limit), 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 a warning and uses the cache anyway.

### 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.
Metadata files the cache can't read, such as ones another ImagePipe version
wrote, count as left behind too, and so do their bodies. ImagePipe deletes
them in the background once they are more than an hour old:

- In each cache's `root`, after 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, :sweep]` event reports what each cleanup removed (see
[cache events](telemetry-events.md#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.

### 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](caching-processed-images.md#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.

### 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`](processing/crop.md#trim), [`blur`](processing/effects.md#blur),
  [`sharpen`](processing/effects.md#sharpen), and
  [`rotate`](processing/crop.md#rotate) by an angle other than a quarter
  turn, each by the size of the image it runs on
- each quality that [automatic quality](processing/output.md#autoquality)
  tries
- object detection

The estimate isn't timed, so a busy server doesn't make entries look more
expensive. Originals in the originals cache are scored by how long their
download took.

### 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
  128. 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.
- An entry larger than the limit on unfinished writes is never stored (see
  [writing entries](#writing-entries)).
- On startup the process lists the entries on disk in the background,
  reading only file names. If the cache is over the cap, for example after
  you lower `max_size_bytes`, it evicts in batches until it's under, so
  requests can use the cache between batches. Then it deletes
  [leftover files](#leftover-files).

A write that waits more than 5 seconds for the cache process is reported as
a [cache write](telemetry-events.md#cache-write) error with
`error: :admission_timeout`. The cache process still handles the queued write
and stores or rejects the entry as usual.

### 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](#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.
- Entries an earlier ImagePipe version stored in another format aren't
  counted against `max_size_bytes`. A cleanup of
  [leftover files](#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](telemetry-events.md#cache-events).