3. ABI Policy

3.1. Description

This document details the management policy that ensures the long-term stability of the DPDK ABI and API.

3.2. General Guidelines

  1. Major ABI versions are declared no more frequently than yearly. Compatibility with the major ABI version is mandatory in subsequent releases until a new major ABI version is declared.
  2. Major ABI versions are usually but not always declared aligned with a LTS release.
  3. The ABI version is managed at a project level in DPDK, and is reflected in all non-experimental library’s soname.
  4. The ABI should be preserved and not changed lightly. ABI changes must follow the outlined deprecation process.
  5. The addition of symbols is generally not problematic. The modification of symbols is managed with ABI Versioning.
  6. The removal of symbols is considered an ABI breakage, once approved these will form part of the next ABI version.
  7. Libraries or APIs marked as experimental may be changed or removed without prior notice, as they are not considered part of an ABI version. The experimental status of an API is not an indefinite state.
  8. Updates to the minimum hardware requirements, which drop support for hardware which was previously supported, should be treated as an ABI change.

Note

Please note that this policy does not currently apply to the Windows build.

3.2.1. What is an ABI?

An ABI (Application Binary Interface) is the set of runtime interfaces exposed by a library. It is similar to an API (Application Programming Interface) but is the result of compilation. It is also effectively cloned when applications link to dynamic libraries. That is to say when an application is compiled to link against dynamic libraries, it is assumed that the ABI remains constant between the time the application is compiled/linked, and the time that it runs. Therefore, in the case of dynamic linking, it is critical that an ABI is preserved, or (when modified), done in such a way that the application is unable to behave improperly or in an unexpected fashion.

../_images/what_is_an_abi.svg

Fig. 3.2 Illustration of DPDK API and ABI.

3.2.2. What is an ABI version?

An ABI version is an instance of a library’s ABI at a specific release. Certain releases are considered to be milestone releases, the yearly LTS release for example. The ABI of a milestone release may be declared as a ‘major ABI version’, where this ABI version is then supported for some number of subsequent releases and is annotated in the library’s soname.

ABI version support in subsequent releases facilitates application upgrades, by enabling applications built against the milestone release to upgrade to subsequent releases of a library without a rebuild.

More details on major ABI version can be found in the ABI versioning guide.

3.3. The DPDK ABI policy

A new major ABI version is declared no more frequently than yearly, with declarations usually aligning with a LTS release, e.g. ABI 21 for DPDK 20.11. Compatibility with the major ABI version is then mandatory in subsequent releases until the next major ABI version is declared, e.g. ABI 22 for DPDK 21.11.

At the declaration of a major ABI version, major version numbers encoded in libraries’ sonames are bumped to indicate the new version, with the minor version reset to 0. An example would be librte_eal.so.21.3 would become librte_eal.so.22.0.

The ABI may then change multiple times, without warning, between the last major ABI version increment and the HEAD label of the git tree, with the condition that ABI compatibility with the major ABI version is preserved and therefore sonames do not change.

Minor versions are incremented to indicate the release of a new ABI compatible DPDK release, typically the DPDK quarterly releases. An example of this, might be that librte_eal.so.21.1 would indicate the first ABI compatible DPDK release, following the declaration of the new major ABI version 21.

An ABI version is supported in all new releases until the next major ABI version is declared. When changing the major ABI version, the release notes will detail all ABI changes.

../_images/abi_stability_policy.svg

Fig. 3.3 Mapping of new ABI versions and ABI version compatibility to DPDK releases.

3.3.1. ABI Changes

The ABI may still change after the declaration of a major ABI version, that is new APIs may be still added or existing APIs may be modified.

Warning

Note that, this policy details the method by which the ABI may be changed, with due regard to preserving compatibility and observing deprecation notices. This process however should not be undertaken lightly, as a general rule ABI stability is extremely important for downstream consumers of DPDK. The API should only be changed for significant reasons, such as performance enhancements. API breakages due to changes such as reorganizing public structure fields for aesthetic or readability purposes should be avoided.

The requirements for changing the ABI are:

  1. At least 3 acknowledgments of the need to do so must be made on the dpdk.org mailing list.

    • The acknowledgment of the maintainer of the component is mandatory, or if no maintainer is available for the component, the tree/sub-tree maintainer for that component must acknowledge the ABI change instead.
    • The acknowledgment of three members of the technical board, as delegates of the technical board acknowledging the need for the ABI change, is also mandatory.
    • It is also recommended that acknowledgments from different “areas of interest” be sought for each deprecation, for example: from NIC vendors, CPU vendors, end-users, etc.

  2. Backward compatibility with the major ABI version must be maintained through ABI Versioning, with forward-only compatibility offered for any ABI changes that are indicated to be part of the next ABI version.

    • In situations where backward compatibility is not possible, read the section on ABI Breakages.
    • No backward or forward compatibility is offered for API changes marked as experimental, as described in the section on Experimental APIs and Libraries.
    • In situations in which an experimental symbol has been stable for some time. When promoting the symbol to become part of the next ABI version, the maintainer may choose to provide an alias to the experimental tag, so as not to break consuming applications.

  3. If a newly proposed API functionally replaces an existing one, when the new API becomes non-experimental, then the old one is marked with __rte_deprecated.

    • The deprecated API should follow the notification process to be removed, see Examples of Deprecation Notices.
    • At the declaration of the next major ABI version, those ABI changes then become a formal part of the new ABI and the requirement to preserve ABI compatibility with the last major ABI version is then dropped.
    • The responsibility for removing redundant ABI compatibility code rests with the original contributor of the ABI changes, failing that, then with the contributor’s company and then finally with the maintainer.

Note

Note that forward-only compatibility is offered for those changes made between major ABI versions. As a library’s soname can only describe compatibility with the last major ABI version, until the next major ABI version is declared, these changes therefore cannot be resolved as a runtime dependency through the soname. Therefore any application wishing to make use of these ABI changes can only ensure that its runtime dependencies are met through Operating System package versioning.

Note

Updates to the minimum hardware requirements, which drop support for hardware which was previously supported, should be treated as an ABI change, and follow the relevant deprecation policy procedures as above: 3 acks, technical board approval and announcement at least one release in advance.

3.3.2. ABI Breakages

For those ABI changes that are too significant to reasonably maintain multiple symbol versions, there is an amended process. In these cases, ABIs may be updated without the requirement of backward compatibility being provided. These changes must follow the same process described above as non-breaking changes, however with the following additional requirements:

  1. ABI breaking changes (including an alternative map file) can be included with deprecation notice, in wrapped way by the RTE_NEXT_ABI option, to provide more details about oncoming changes. RTE_NEXT_ABI wrapper will be removed at the declaration of the next major ABI version.
  2. Once approved, and after the deprecation notice has been observed these changes will form part of the next declared major ABI version.

3.3.3. Examples of ABI Changes

The following are examples of allowable ABI changes occurring between declarations of major ABI versions.

  • DPDK 20.11 release defines the function rte_foo() ; rte_foo() is part of the major ABI version 21.
  • DPDK 21.02 release defines a new function rte_foo(uint8_t bar). This is not a problem as long as the symbol rte_foo@DPDK_21 is preserved through ABI Versioning.
    • The new function may be marked with the __rte_experimental tag for a number of releases, as described in the section Experimental.
    • Once rte_foo(uint8_t bar) becomes non-experimental, rte_foo() is declared as __rte_deprecated and an deprecation notice is provided.
  • DPDK 20.11 is not re-released to include rte_foo(uint8_t bar), the new version of rte_foo only exists from DPDK 21.02 onwards as described in the note on forward-only compatibility.
  • DPDK 21.02 release defines the experimental function __rte_experimental rte_baz(). This function may or may not exist in the DPDK 21.05 release.
  • An application dPacket wishes to use rte_foo(uint8_t bar), before the declaration of the DPDK 22 major ABI version. The application can only ensure its runtime dependencies are met by specifying DPDK (>= 21.2) as an explicit package dependency, as the soname can only indicate the supported major ABI version.
  • At the release of DPDK 21.11, the function rte_foo(uint8_t bar) becomes formally part of then new major ABI version DPDK 22 and rte_foo() may be removed.

3.3.4. Examples of Deprecation Notices

The following are some examples of ABI deprecation notices which would be added to the Release Notes:

  • The Macro #RTE_FOO is deprecated and will be removed with ABI version 22, to be replaced with the inline function rte_foo().
  • The function rte_mbuf_grok() has been updated to include a new parameter in version 21.2. Backwards compatibility will be maintained for this function until the release of the new DPDK major ABI version 22, in DPDK version 21.11.
  • The members of struct rte_foo have been reorganized in DPDK 21.02 for performance reasons. Existing binary applications will have backwards compatibility in release 21.02, while newly built binaries will need to reference the new structure variant struct rte_foo2. Compatibility will be removed in release 21.11, and all applications will require updating and rebuilding to the new structure at that time, which will be renamed to the original struct rte_foo.
  • Significant ABI changes are planned for the librte_dostuff library. The upcoming release 21.02 will not contain these changes, but release 21.11 will, and no backwards compatibility is planned due to the extensive nature of these changes. Binaries using this library built prior to ABI version 22 will require updating and recompilation.

3.4. New ABI versions

A new ABI version may be declared aligned with a given release. The requirement to preserve compatibility with the previous major ABI version is then dropped for the duration of this release cycle. This is commonly known as the ABI breakage window, and some amended rules apply during this cycle:

  • The requirement to preserve compatibility with the previous major ABI version, as described in the section ABI Changes does not apply.
  • Contributors of compatibility preserving code in previous releases, are now required to remove this compatibility code, as described in the section ABI Changes.
  • Symbol versioning references to the old ABI version are updated to reference the new ABI version, as described in the section Deprecating an entire ABI version.
  • Contributors of aliases to experimental in previous releases, as described in section Aliasing experimental symbols, are now required to remove these aliases.
  • Finally, the ABI breakage window is not permission to circumvent the other aspects of the procedures to make ABI changes described in ABI Changes, that is, 3 ACKs of the requirement to break the ABI and the observance of a deprecation notice are still considered mandatory.

3.5. Experimental

3.5.1. APIs

APIs marked as experimental are not considered part of an ABI version and may be changed or removed without prior notice. Since changes to APIs are most likely immediately after their introduction, as users begin to take advantage of those new APIs and start finding issues with them, new DPDK APIs will be automatically marked as experimental to allow for a period of stabilization before they become part of a tracked ABI version.

Note that marking an API as experimental is a multi step process. To mark an API as experimental, the symbols which are desired to be exported must be placed in an EXPERIMENTAL version block in the corresponding libraries’ version map script. Experimental symbols must be commented so that it is clear in which DPDK version they were introduced.

EXPERIMENTAL {
        global:

        # added in 20.11
        rte_foo_init;
        rte_foo_configure;

        # added in 21.02
        rte_foo_cleanup;
...

Secondly, the corresponding prototypes of those exported functions (in the development header files), must be marked with the __rte_experimental tag (see rte_compat.h). The DPDK build makefiles perform a check to ensure that the map file and the C code reflect the same list of symbols. This check can be circumvented by defining ALLOW_EXPERIMENTAL_API during compilation in the corresponding library Makefile.

In addition to tagging the code with __rte_experimental, the doxygen markup must also contain the EXPERIMENTAL string, and the MAINTAINERS file should note the EXPERIMENTAL libraries.

For removing the experimental tag associated with an API, deprecation notice is not required. Though, an API should remain in experimental state for at least one release. Thereafter, the normal process of posting patch for review to mailing list can be followed.

After the experimental tag has been formally removed, a tree/sub-tree maintainer may choose to offer an alias to the experimental tag so as not to break applications using the symbol. The alias is then dropped at the declaration of next major ABI version.

3.5.2. Libraries

Libraries marked as experimental are entirely not considered part of an ABI version. All functions in such libraries may be changed or removed without prior notice.

3.5.3. Promotion to stable

An API’s experimental status should be reviewed annually, by both the maintainer and/or the original contributor. Ordinarily APIs marked as experimental will be promoted to the stable ABI once a maintainer has become satisfied that the API is mature and is unlikely to change.

In exceptional circumstances, should an API still be classified as experimental after two years and is without any prospect of becoming part of the stable API. The API will then become a candidate for removal, to avoid the accumulation of abandoned symbols.

Should an ABI change, usually due to a direct change to the API’s signature, it is reasonable for the review and expiry clocks to reset. The promotion or removal of symbols will typically form part of a conversation between the maintainer and the original contributor.