Stack Association

This topic describes the stack association feature for Cloud Foundry (CF) buildpacks.

Note: This functionality is supported as of CAPI v1.58.0 and cf CLI v6.39.0.

WARNING: To avoid security exposure, ensure that you migrate your apps and custom buildpacks to use the cflinuxfs3 stack based on Ubuntu 18.04 (Bionic Beaver). cflinuxfs2 is based on Ubuntu 14.04 (Trusty Tahr), which reaches end of general support (EOGS) in April 2019.

Overview

Each buildpack in your CF deployment is associated with a stack. You can see this when you run cf buildpacks: there is a stack column in the output that shows a corresponding stack for each buildpack. See the following example:

$ cf buildpacks
Getting buildpacks...

buildpack                position   enabled   locked   filename                                             stack
staticfile_buildpack     1          true      false    staticfile_buildpack-cached-cflinuxfs2-v1.4.29.zip   cflinuxfs2
java_buildpack_offline   2          true      false    java-buildpack-offline-cflinuxfs2-v4.12.1.zip        cflinuxfs2
ruby_buildpack           3          true      false    ruby_buildpack-cached-cflinuxfs2-v1.7.21.zip         cflinuxfs2
. . . 

Because of this stack association, buildpacks do not have to be uniquely named. This helps in managing similar buildpacks that are compatible with different stacks.

The buildpack packager includes a -stack option. If you use this option and upload a buildpack to CF, the Cloud Controller detects the stack association and creates a stack record for the buildpack.

Buildpacks without a Stack Record

Some buildpacks may have a missing stack record. For example, if you uploaded a custom buildpack before CF introduced stack assocation. The output of cf buildpacks shows a blank stack column if the buildpack does not have a stack record.

In this case, you must manually assign a stack to the buildpack. To do this, run cf update-buildpack BUILDPACK-NAME --assign-stack stack.

Buildpacks with a missing stack record will continue to work, but are more manageable when the stack record is present.

Consider the following behavior when pushing apps to a deployment that has buildpacks with a missing stack record:

• If you push an app and specify a stack with cf push app-name -s stack, CF uses that stack. Otherwise, it uses the system default, cflinuxfs2.
• You may see additional logging in the buildpack detection output of the cf push command when CF detects buildpacks without a stack record.

Managing Stack Association with the cf CLI

The cf CLI commands for managing buildpacks include functionality to support assocation between buildpacks and stacks. The update-buildpack, rename-buildpack, and delete-buildpack commands all include a -s flag for specifying a stack.

When operating on buildpacks with the cf CLI, consider the following:

• You cannot upload a buildpack with cf create-buildpack if a buildpack of the same name already exists and it has a missing stack record.
• When using cf create-buildpack, you may inadvertently create a duplicate buildpack with a nil stack. cf create-buildpack does not disallow creation of buildpacks with no stack association.
• The -s flag is required when there are two buildpacks with the same name. If you are operating on a uniquely named buildpack, you do not need to specify its stack.
• If you have two buildpacks of the same name, one with a stack record and one without, running cf CLI commands without -s operates on the buildpack with the missing stack record.

Example Scenarios

See the following examples of managing buildpacks with the cf CLI, which are applicable when running cf update-buildpack, cf rename-buildpack, or cf delete-buildpack:

• Updating, renaming, or deleting a uniquely-named buildpack:
  • You have a single buildpack named my-buildpack, and it is associated with stack_a. If you want to delete the buildpack, you can run cf delete-buildpack my-buildpack.
    
    You can also provide -s stack_a, but the option is not required if you have a uniquely-named buildpack.
• Updating, renaming, or deleting a uniquely-named buildpack that has a nil stack:
  • You have a single buildpack named my-buildpack, and it is not associated with a stack. If you want to delete the buildpack, you can run cf delete-buildpack my-buildpack.
• Updating, renaming, or deleting a buildpack when another buildpack exists with the same name, and both buildpacks have stack associations:
  • You have two buildpacks named my-buildpack, one that is associated with stack_a and the other associated with stack_b. If you want to delete the buildpack that uses stack_a, you can run cf delete-buildpack my-buildpack -s stack_a.
• Updating, renaming, or deleting a buildpack when another buildpack exists with the same name. One buildpack has a stack association, and the other buildpack has a nil stack:
  • You have two buildpacks named my-buildpack, one associated with stack_a and the other associated with no (nil) stack association:
    • If you want to delete the buildpack that uses stack_a, you can run cf delete-buildpack my-buildpack -s stack_a.
    • If you want to delete the buildpack that is associated with the nil stack, run cf delete-buildpack my-buildpack.