There are three types of layers that can be contributed to an image
build layers – the directory will be accessible by subsequent buildpacks,cache layers – the directory will be included in the cache,launch layers – the directory will be included in the run image as a single layer,A fourth type of layer
ignored layersare available to buildpack authors for use as temporary layers.
In this section we look at caching each layer type.
buildpacks ensure byte-for-byte reproducibility of layers. File creation time is normalized to January 1, 1980 to ensure reproducibility. Byte-for-byte reproducibility means previous layers can be reused. However, we want to invalidate previously cached layers if
A layer built using a buildpack at API version 0.7 should be considered invalid if the API version for that buildpack has been updated to 0.8. Similarly, if a layer is changed from being a cache-only layer to being a cache and launch layer, then the cache should be considered invalid.
In addition to general cache invalidation conditions a buildpack should invalidate a previous layer if an important property changes, such as:
requests moduleLaunch layers are exported to an OCI registry and layer metadata is stored with the launch layer. The layer metadata is commonly used when deciding if a launch layer should be re-used from cache.
Caching during the production of an application image is necessarily very flexible. Most buildpacks that wish to contribute a layer to the application image need only to
launch = true.This will guarantee that the previously published application image layer in the registry is re-used if the layer metadata matches the requested metadata. In this most straightforward use-case launch is true and both build and cache are set to false. That is to say, the most common case is where cache = false, build = false and launch = true. It is important to note that the layer is re-used on the OCI registry. This avoids expensive rebuilds of the layer and expensive pulls of the layer to the host running the build.
Setting build = true makes a layer available to subsequent buildpacks. Therefore binaries installed to the bin directory on a build = true layer are available to subsequent buildpacks during the build phase. It is also the case that lib directories on a build = true later are added to the LD_LIBRARY_PATH during the build phase of subsequent buildpacks. Environment variables defined in a build = true layer are similarly available. For any layer where launch = true and build = true, a launch layer from the OCI registry can no longer be reused. Instead, the layer must be made available locally so that subsequent buildpacks can use it.
Setting cache = true allows additional fine-grained control over caching. The cache = true flag caches a layer and allows a buildpack to make content level decisions about the validity of the cache (as opposed to using the less granular metadata). As an example, suppose a layer where launch = true installs a jq binary with version 1.5 and sets version=1.5 in the layer metadata. By default, this layer will not be re-used from the registry when a buildpack requests jq with version=1.6 to be installed. However, setting cache = true makes a previously built layer available during the build. A buildpack could then prefer to implement logic to restore jq with version=1.5 instead of performing a potentially expensive download of jq with version=1.6. The cache = true setting allows for cache validation decisions to be made at a level of granularity that is much finer grained than layer metadata.
Setting cache = false, build = false, and launch = true is the most common configuration. If cache = false, build = false, and launch = true is not appropriate for your layer, then cache = true, build = true, and launch = true should be the next combination to evaluate:
cache = true, build = true, launch = true, explicitly setting build = true makes the layer available, to subsequent buildpacks, during the build phase. As cache = true the layer is restored from local cache before proceeding to the build phase. For example, a Python distribution could be provided in a cached, build and launch layer. The build phase could verify that the restored cached version of the Python distribution contains Python 3.10 but disregard the patch number of the Python interpreter.cache = true, build = true, launch = true is an appropriate setting for a layer providing a distribution or runtime such as a Python interpreter or NodeJS runtime.Other common configurations include
cache = true, build = false, launch = true Allows the same caching behavior as cache = true, build = true, launch = true, but the layer is not available to subsequent buildpacks. For example, the build phase can restore a Python distribution disregarding the patch number of the major.minor.patch number stored in the metadata. As build = false the python interpreter is unavailable to subsequent buildpacks.cache = true, build = true, launch = false This configuration is useful where a build time dependency is provided. For example, a JDK could be provided as a cached build layer that is not added as a launch layer. Instead, a JRE could be provided as a launch layer in the application image.cache = true, build = false, launch = false This configuration is particularly useful in layers that download resources. Using a cache-only layer supports allows a buildpack to re-use cached downloads during installation. For example, pip wheels could be downloaded as a cache-only layer and the same buildpack could install the wheels in to a launch layer.There are other boolean combinations of cache, build and launch. These provide significant flexibility in the caching system. Users of less common caching strategies need a good understanding of the buildpacks specification on Layer Types.