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rfc-0030-add-support-for-file-based-service-binding.md

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  • Name: Add Support for File based Service Binding Information
  • Start Date: 2024-03-11
  • Author(s): @beyhan
  • Contributing Authors: @stephanme, @loewenstein
  • Status: Accepted
  • RFC Pull Request: community#804

Summary

The current contract between the CF platform and applications for service binding information is based on an environment variable. The Linux Kernel defines size limit per environment variable and there are also other limitations with this approach. That is why, CF should add support for an alternative option to provide service binding information which can address the limitations of the current approach.

Problem

The CF platform provides service binding Information to hosted applications via the VCAP_SERVICES environment variable. There are following challenges with this approach:

  • The environment variable length has a hard limit of 131072 bytes per variable which is controlled by the underlying Linux kernel. The environment variable size is defined by MAX_ARG_STRLEN, which is a constant with the value PAGE_SIZE*32 where page size is 4096 bytes. This means that to change the size limit for the CF platform a recompiled kernel with updated value for MAX_ARG_STRLEN is required. This limit could be an issue for applications using many services. If the limit is reached by an application, it will fail to stage as discussed in this issue.
  • Updates of the service binding Information require restage. This is not optimal for an eventual support of Binding rotation specification from the OSBI API spec.

Proposal

The CFF community should implement an alternative approach for service binding information based on tmpfs file(s). Using a file or files to provide service binding information to applications will address the challenges listed in the problem section because:

  • The file size limit can be controlled by the CF platform
  • Already today CF uses tmpfs for Instance Identity Credentials which are rotated without restarting the application every 24h by default

The two approaches should be supported in parallel. Users should be able to select which approach Cloud Controller should use to deliver the binding information. Applications with binding information >130KB have to go with the file option and adopt it. There are two alternatives regarding service binding file organization:

  1. The VCAP_SERVICES content is stored in a file which location is specified via the VCAP_SERVICES_FILE_PATH env var in the same format as the VCAP_SERVICES environment variable
    • Advantages:
      • Less disruptive for applications consuming the VCAP_SERVICES env var
      • Less implementation effort for the Cloud Controller
    • Disadvantages:
      • Can’t make use of tools and libraries from the Cloud Native community because K8s specifies a different file structure and format for the service binding information
  2. Implement the K8s service binding specification. The environment variable SERVICE_BINDING_ROOT defines the location for the service bindings. The name of the file and the format follow the K8s specification:
    • Advantages:
      • CF community could re-use service binding libraries from the Cloud Native community
      • Moving application between CF or K8s deployments will be easier
    • Disadvantages
      • Higher implementation efforts for the Cloud Controller

The 2) alternatives offers more than just addressing the issue of this RFC. It suggests an option to evolve the CF platform towards a different service binding specification defined by the Cloud Native community. This means higher implementation efforts for the CF platform and application developers, but possible benefits from the Cloud Native community. This RFC has a light preference for the 2) alternative because of the listed advantages but the feedback of the CF community is wanted here.

Additionally, the application environment is stored in the CCDB and BBS DB that is why we should define a limit for the size of it, which makes it possible to be stored in the according DBs and doesn’t impact the performance of the communication between Cloud Controller and the Diego API. That is why this RFC suggests a limit of 1MB, which is roughly ten times higher than the current one of 130KB. This is subject for evaluation during the implementation of this RFC.

Note

The voting in the CF community selected the option 2). That is why, this RFC focuses on option 2) in the next sections. The voting results are available here.

Implementation Overview

Cloud Controller should introduce a new app feature for activation of the file-based approach. This means that the App Features API could be used here and a new feature flag called “file-based-service-bindings" should be introduced. The contract between Cloud Controller and Diego should be extended so that file name and file content for the application container can be specified. E.g. the run action could look like this when file approach is selected:

action := &models.RunAction{
  Path: "/path/to/executable",
  Args: []string{"some", "args to", "pass in"},
  Dir: "/path/to/working/directory",
  User: "username",
  EnvironmentVariables: []*models.EnvironmentVariable{
    {
      Name: "ENVNAME",
      Value: "ENVVALUE",
    },
  },
 VolumeMountedFiles: []*models.File{
    {
      Path: "/etc/cf-instance-binding",
      Content: "VALUE",
    },
  },
  ResourceLimits: &models.ResourceLimits{
    Nofile: 1000,
  },
  LogSource: "some-log-source",
  SuppressLogOutput: false,
}

Workstreams

App Runtime Interfaces WG

Cloud Controller should add a new App Feature for activation of the new file-based service binding option. If the file-based service binding feature is active for an application the Cloud Controller should generate a Run action, which configures the service bindings to be stored as tmpfs file(s) in the application container instead of VCAP_SERVICES environment variable. Additionally, Cloud Controller should set the SERVICE_BINDING_ROOT environment variable accordingly. The translation from VCAP_SERVICES to file based bindings should follow the CNB translation in libcnb. The implementation in libcnb could be found here and it does following:

  • The credentials object is translated into key/value files where key is the file name and value is the file content. If a key in credentials has a nested JSON value, then the JSON is written as the content of the file.
  • The label from VCAP_SERVICES is translated to type.
  • The top-level key from the VCAP_SERVICES is translated to provider.

The RFC doesn't mention all attributes documented for VCAP_SERVICES but the same approach should be followed for them also. E.g. plan should be the file name and the service plan the file content. Attributes with underscore in their names like binding_guid should be translated to dashes binding-guid to be conform with the supported characters in the K8s file name specification. Translating the VCAP_SERVICES attributes in this way should be fine with the K8s binding specification because it allows any entry as documented in the Provisioned Service section.

Cloud Controller should ensure that each binding has a unique name. In case of name collisions Cloud Controller should provide a meaningful error message. E.g. with the current implementation for VCAP_SERVICES it is not possible to create two service instances with the same name or two service bindings with the same name, but it is possible to create a service binding with the name foobar from an arbitrary service instance and another service binding without a binding name from a service instance called foobar. This would mean that both binding entries end up with name=foobar. Switching to file based service bindings in this situation will result into an error from Cloud Controller which is not a backward compatible behaviour but this should be acceptable for an opt-in feature.

Additionally, the suggested limit of 1MB for the size should be implemented.

App Runtime Platform WG

Diego should add support for the new argument of the Run action to create files with the desired content. Like the Instance Identity credentials implementation, the Diego Executor should be extended to prepare the tmpfs mount and create the required files for an application container. For reference there is a CredManager , InstanceIdentityHandler and the tmpfs mount is configured in the Diego release for the current implementation of the Instance Identity Credentials. The files should have security permissions like the Instance Identity Credentilas 644 or even higher 600 owned by the uid running the app if possible.

Possible Future Work

The App Features API aren’t supported currently in the CF CLI and app manifest. To make the use of this proposal for CF operators easier this should be addressed.

App Manifest Attributes Proposal

The CF app manifest is additive not declarative. That is why if we want to disable app feature flags via the app manifest a status input like enabled or disabled will be required. Like:

---
applications:
- name: test-app
  features:
  - file-based-service-bindings: true

or as alternative proposal:

---
applications:
- name: test-app
  features:
  - name: file-based-service-bindings
    enabled: true

CF CLI new Commands Proposal

  • app-feature-flags Retrieve list of available app feature flags with status
  • app-feature-flag Retrieve an individual app feature flag with status
  • enable-app-feature-flag Allow use of an app feature
  • disable-app-feature-flag Disable use of an app feature