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Cache storage backends

Cache storage backends

To ensure fast builds, BuildKit automatically caches the build result in its own internal cache. Additionally, BuildKit also supports exporting build cache to an external location, making it possible to import in future builds.

An external cache becomes almost essential in CI/CD build environments. Such environments usually have little-to-no persistence between runs, but it’s still important to keep the runtime of image builds as low as possible.

Warning

If you use secrets or credentials inside your build process, ensure you manipulate them using the dedicated --secret option. Manually managing secrets using COPY or ARG could result in leaked credentials.

Backends

Buildx supports the following cache storage backends:

  • inline : embeds the build cache into the image.

    The inline cache gets pushed to the same location as the main output result. Note that this only works for the image exporter.

  • registry : embeds the build cache into a separate image, and pushes to a dedicated location separate from the main output.

  • local : writes the build cache to a local directory on the filesystem.

  • gha : uploads the build cache to GitHub Actions cache (beta).

  • s3 : uploads the build cache to an AWS S3 bucket (unreleased).

  • azblob : uploads the build cache to Azure Blob Storage (unreleased).

Command syntax

To use any of the cache backends, you first need to specify it on build with the --cache-to option to export the cache to your storage backend of choice. Then, use the --cache-from option to import the cache from the storage backend into the current build. Unlike the local BuildKit cache (which is always enabled), all of the cache storage backends must be explicitly exported to, and explicitly imported from. All cache exporters except for the inline cache requires that you select an alternative Buildx driver.

Example buildx command using the registry backend, using import and export cache:

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=registry,ref=<registry>/<cache-image>[,parameters...] \
  --cache-from type=registry,ref=<registry>/<cache-image>[,parameters...] .

Warning

As a general rule, each cache writes to some location. No location can be written to twice, without overwriting the previously cached data. If you want to maintain multiple scoped caches (for example, a cache per Git branch), then ensure that you use different locations for exported cache.

Multiple caches

BuildKit currently only supports a single cache exporter. But you can import from as many remote caches as you like. For example, a common pattern is to use the cache of both the current branch and the main branch. The following example shows importing cache from multiple locations using the registry cache backend:

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=registry,ref=<registry>/<cache-image>:<branch> \
  --cache-from type=registry,ref=<registry>/<cache-image>:<branch> \
  --cache-from type=registry,ref=<registry>/<cache-image>:main .

Configuration options

This section describes some configuration options available when generating cache exports. The options described here are common for at least two or more backend types. Additionally, the different backend types support specific parameters as well. See the detailed page about each backend type for more information about which configuration parameters apply.

The common parameters described here are:

  • Cache mode
  • Cache compression
  • OCI media type

Cache mode

When generating a cache output, the --cache-to argument accepts a mode option for defining which layers to include in the exported cache. This is supported by all cache backends except for the inline cache.

Mode can be set to either of two options: mode=min or mode=max . For example, to build the cache with mode=max with the registry backend:

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=registry,ref=<registry>/<cache-image>,mode=max \
  --cache-from type=registry,ref=<registry>/<cache-image> .

This option is only set when exporting a cache, using --cache-to . When importing a cache ( --cache-from ) the relevant parameters are automatically detected.

In min cache mode (the default), only layers that are exported into the resulting image are cached, while in max cache mode, all layers are cached, even those of intermediate steps.

While min cache is typically smaller (which speeds up import/export times, and reduces storage costs), max cache is more likely to get more cache hits. Depending on the complexity and location of your build, you should experiment with both parameters to find the results that work best for you.

Cache compression

The cache compression options are the same as the exporter compression options. This is supported by the local and registry cache backends.

For example, to compress the registry cache with zstd compression:

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=registry,ref=<registry>/<cache-image>,compression=zstd \
  --cache-from type=registry,ref=<registry>/<cache-image> .

OCI media types

The cache OCI options are the same as the exporter OCI options. These are supported by the local and registry cache backends.

For example, to export OCI media type cache, use the oci-mediatypes property:

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=registry,ref=<registry>/<cache-image>,oci-mediatypes=true \
  --cache-from type=registry,ref=<registry>/<cache-image> .

This property is only meaningful with the --cache-to flag. When fetching cache, BuildKit will auto-detect the correct media types to use.

Inline cache

Inline cache

The inline cache storage backend is the simplest way to get an external cache and is easy to get started using if you’re already building and pushing an image. However, it doesn’t scale as well to multi-stage builds as well as the other drivers do. It also doesn’t offer separation between your output artifacts and your cache output. This means that if you’re using a particularly complex build flow, or not exporting your images directly to a registry, then you may want to consider the registry cache.

Synopsis

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=inline \
  --cache-from type=registry,ref=<registry>/image .

No additional parameters are supported for the inline cache.

To export cache using inline storage, pass type=inline to the --cache-to option:

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=inline .

Alternatively, you can also export inline cache by setting the build argument BUILDKIT_INLINE_CACHE=1 , instead of using the --cache-to flag:

$ docker buildx build --push -t <registry>/<image> \
  --arg BUILDKIT_INLINE_CACHE=1 .

To import the resulting cache on a future build, pass type=registry to --cache-from which lets you extract the cache from inside a Docker image in the specified registry:

$ docker buildx build --push -t <registry>/<image> \
  --cache-from type=registry,ref=<registry>/<image> .

Further reading

For an introduction to caching see Optimizing builds with cache.

For more information on the inline cache backend, see the BuildKit README.

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Local cache

Local cache

The local cache store is a simple cache option that stores your cache as files in a directory on your filesystem, using an OCI image layout for the underlying directory structure. Local cache is a good choice if you’re just testing, or if you want the flexibility to self-manage a shared storage solution.

Note

This cache storage backend requires using a different driver than the default docker driver - see more information on selecting a driver here. To create a new driver (which can act as a simple drop-in replacement):

$ docker buildx create --use --driver=docker-container

Synopsis

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=local,dest=path/to/local/dir[,parameters...] \
  --cache-from type=local,src=path/to/local/dir .

The following table describes the available CSV parameters that you can pass to --cache-to and --cache-from .

Name Option Type Default Description
src cache-from String  Path of the local directory where cache gets imported from.
digest cache-from String  Digest of manifest to import, see cache versioning.
dest cache-to String  Path of the local directory where cache gets exported to.
mode cache-to min , max min Cache layers to export, see cache mode.
oci-mediatypes cache-to true , false true Use OCI media types in exported manifests, see OCI media types.
compression cache-to gzip , estargz , zstd gzip Compression type, see cache compression.
compression-level cache-to 0..22 Â Compression level, see cache compression.
force-compression cache-to true , false false Forcibly apply compression, see cache compression.

If the src cache doesn’t exist, then the cache import step will fail, but the build will continue.

Cache versioning

This section describes how versioning works for caches on a local filesystem, and how you can use the digest parameter to use older versions of cache.

If you inspect the cache directory manually, you can see the resulting OCI image layout:

$ ls cache
blobs  index.json  ingest
$ cat cache/index.json | jq
{
  "schemaVersion": 2,
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.index.v1+json",
      "digest": "sha256:6982c70595cb91769f61cd1e064cf5f41d5357387bab6b18c0164c5f98c1f707",
      "size": 1560,
      "annotations": {
        "org.opencontainers.image.ref.name": "latest"
      }
    }
  ]
}

Like other cache types, local cache gets replaced on export, by replacing the contents of the index.json file. However, previous caches will still be available in the blobs directory. These old caches are addressable by digest, and kept indefinitely. Therefore, the size of the local cache will continue to grow (see moby/buildkit#1896 for more information).

When importing cache using --cache-to , you can specify the digest parameter to force loading an older version of the cache, for example:

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=local,dest=path/to/local/dir \
  --cache-from type=local,ref=path/to/local/dir,digest=sha256:6982c70595cb91769f61cd1e064cf5f41d5357387bab6b18c0164c5f98c1f707 .

Further reading

For an introduction to caching see Optimizing builds with cache.

For more information on the local cache backend, see the BuildKit README.

Read article
Registry cache

Registry cache

The registry cache storage can be thought of as an extension to the inline cache. Unlike the inline cache, the registry cache is entirely separate from the image, which allows for more flexible usage - registry -backed cache can do everything that the inline cache can do, and more:

  • Allows for separating the cache and resulting image artifacts so that you can distribute your final image without the cache inside.
  • It can efficiently cache multi-stage builds in max mode, instead of only the final stage.
  • It works with other exporters for more flexibility, instead of only the image exporter.

Note

This cache storage backend requires using a different driver than the default docker driver - see more information on selecting a driver here. To create a new driver (which can act as a simple drop-in replacement):

$ docker buildx create --use --driver=docker-container

Synopsis

Unlike the simpler inline cache, the registry cache supports several configuration parameters:

$ docker buildx build --push -t <registry>/<image> \
  --cache-to type=registry,ref=<registry>/<cache-image>[,parameters...] \
  --cache-from type=registry,ref=<registry>/<cache-image> .

The following table describes the available CSV parameters that you can pass to --cache-to and --cache-from .

Name Option Type Default Description
ref cache-to , cache-from String  Full name of the cache image to import.
dest cache-to String  Path of the local directory where cache gets exported to.
mode cache-to min , max min Cache layers to export, see cache mode.
oci-mediatypes cache-to true , false true Use OCI media types in exported manifests, see OCI media types.
compression cache-to gzip , estargz , zstd gzip Compression type, see cache compression.
compression-level cache-to 0..22 Â Compression level, see cache compression.
force-compression cache-to true , false false Forcibly apply compression, see cache compression.

You can choose any valid value for ref , as long as it’s not the same as the target location that you push your image to. You might choose different tags (e.g. foo/bar:latest and foo/bar:build-cache ), separate image names (e.g. foo/bar and foo/bar-cache ), or even different repositories (e.g. docker.io/foo/bar and ghcr.io/foo/bar ). It’s up to you to decide the strategy that you want to use for separating your image from your cache images.

If the --cache-from target doesn’t exist, then the cache import step will fail, but the build will continue.

Further reading

For an introduction to caching see Optimizing builds with cache.

For more information on the registry cache backend, see the BuildKit README.

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Amazon S3 cache

Amazon S3 cache

Warning

This cache backend is unreleased. You can use it today, by using the moby/buildkit:master image in your Buildx driver.

The s3 cache storage uploads your resulting build cache to Amazon S3 file storage service, into a specified bucket.

Note

This cache storage backend requires using a different driver than the default docker driver - see more information on selecting a driver here. To create a new driver (which can act as a simple drop-in replacement):

$ docker buildx create --use --driver=docker-container

Synopsis

$ docker buildx build --push -t <user>/<image> \
  --cache-to type=s3,region=<region>,bucket=<bucket>,name=<cache-image>[,parameters...] \
  --cache-from type=s3,region=<region>,bucket=<bucket>,name=<cache-image> .

The following table describes the available CSV parameters that you can pass to --cache-to and --cache-from .

Name Option Type Default Description
region cache-to , cache-from String  Geographic location.
bucket cache-to , cache-from String  Name of the S3 bucket used for caching
name cache-to , cache-from String  Name of the cache image
access_key_id cache-to , cache-from String  See authentication
secret_access_key cache-to , cache-from String  See authentication
session_token cache-to , cache-from String  See authentication
mode cache-to min , max min Cache layers to export, see cache mode.

Authentication

access_key_id , secret_access_key , and session_token , if left unspecified, are read from environment variables on the BuildKit server following the scheme for the AWS Go SDK. The environment variables are read from the server, not the Buildx client.

Further reading

For an introduction to caching see Optimizing builds with cache.

For more information on the s3 cache backend, see the BuildKit README.

Read article
Garbage collection

Garbage collection

While docker builder prune or docker buildx prune commands run at once, garbage collection runs periodically and follows an ordered list of prune policies.

Garbage collection runs in the BuildKit daemon. The daemon clears the build cache when the cache size becomes too big, or when the cache age expires. The following sections describe how you can configure both the size and age parameters by defining garbage collection policies.

Configuration

Depending on the driver used by your builder instance, the garbage collection will use a different configuration file.

If you’re using the docker driver, garbage collection can be configured in the Docker Daemon configuration. file:

{
  "builder": {
    "gc": {
      "enabled": true,
      "defaultKeepStorage": "10GB",
      "policy": [
          {"keepStorage": "10GB", "filter": ["unused-for=2200h"]},
          {"keepStorage": "50GB", "filter": ["unused-for=3300h"]},
          {"keepStorage": "100GB", "all": true}
      ]
    }
  }
}

For other drivers, garbage collection can be configured using the BuildKit configuration file:

[worker.oci]
  gc = true
  gckeepstorage = 10000
  [[worker.oci.gcpolicy]]
    keepBytes = 512000000
    keepDuration = 172800
    filters = [ "type==source.local", "type==exec.cachemount", "type==source.git.checkout"]
  [[worker.oci.gcpolicy]]
    all = true
    keepBytes = 1024000000

Default policies

Default garbage collection policies are applied to all builders if not already set:

GC Policy rule#0:
        All:            false
        Filters:        type==source.local,type==exec.cachemount,type==source.git.checkout
        Keep Duration:  48h0m0s
        Keep Bytes:     512MB
GC Policy rule#1:
        All:            false
        Keep Duration:  1440h0m0s
        Keep Bytes:     26GB
GC Policy rule#2:
        All:            false
        Keep Bytes:     26GB
GC Policy rule#3:
        All:            true
        Keep Bytes:     26GB
  • rule#0 : if build cache uses more than 512MB delete the most easily reproducible data after it has not been used for 2 days.
  • rule#1 : remove any data not used for 60 days.
  • rule#2 : keep the unshared build cache under cap.
  • rule#3 : if previous policies were insufficient start deleting internal data to keep build cache under cap.

Note

“Keep bytes” defaults to 10% of the size of the disk. If the disk size cannot be determined, it defaults to 2GB.

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