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TensorFlow Version Semantics

Semantic Versioning 2.0

Once we reach version 1.0, TensorFlow will follow Semantic Versioning 2.0 (semver). For details, see http://semver.org.  Each release version of TensorFlow has the form MAJOR.MINOR.PATCH.  Changes to the each number have the following meaning:

  • MAJOR:  Backwards incompatible changes.  Code and data that worked with a previous major release will not necessarily work with a new release. However, in some cases existing TensorFlow data (graphs, checkpoints, and other protobufs) may be migratable to the newer release; see below for details on data compatibility.

  • MINOR: Backwards compatible features, speed improvements, etc.  Code and data that worked with a previous minor release and which depends only the public API will continue to work unchanged.  For details on what is and is not the public API, see below.

  • PATCH: Backwards compatible bug fixes.

Before 1.0, semver allows backwards incompatible changes at any time.  However, to support users now, we will use the format 0.MAJOR.MINOR (shifted one step to the right).  Thus 0.5.0 to 0.6.0 may be backwards incompatible, but 0.6.0 to 0.6.1 will include only backwards compatible features and bug fixes.

At some point (especially as we approach 1.0) we will likely use prerelease versions such as X.Y.Z-alpha.1, but we do not yet have specific plans (beyond the restrictions of semver).

Public API

Only the public API of TensorFlow is backwards compatible across minor and patch versions.  The public API consists of

The public C++ API is exposed through the header files in tensorflow/core/public. The public Python API is unfortunately not everything available through the tensorflow python module and its submodules, since we do not yet use __all__ everywhere (#421).  Please refer to the documentation to determine whether a given Python feature is part of the public API. For now, the protocol buffers are defined in tensorflow/core/framework/*.proto (#484).

Details That Are Not Public

The following are specifically not part of the public API: they are allowed to change without notice across minor releases and even patch releases if bug fixes require it:

  • Details of composite ops:  Many public functions in Python expand to several primitive ops in the graph, and these details will be part of any graphs saved to disk as GraphDefs.  These details are allowed to change for minor releases. In particular, regressions tests that check for exact matching between graphs are likely to break across minor releases, even though the behavior of the graph should be unchanged and existing checkpoints will still work.

  • Floating point numerical details: The specific floating point values computed by ops may change at any time: users should rely only on approximate accuracy and numerical stability, not on the specific bits computed.  Changes to numerical formulas in minor and patch releases should result in comparable or improved accuracy, with the caveat that in machine learning improved accuracy of specific formulas may result in worse accuracy for the overall system.

  • Random numbers: The specific random numbers computed by the random ops may change at any time: users should rely only on approximately correct distributions and statistical strength, not the specific bits computed.  However, we will make changes to random bits rarely and ideally never for patch releases, and all such intended changes will be documented.

Compatibility for Graphs and Checkpoints {#graphs}

Many users of TensorFlow will be saving graphs and trained models to disk for later evaluation or more training, often changing versions of TensorFlow in the process.  First, following semver, any graph or checkpoint written out with one version of TensorFlow can be loaded and evaluated with a later version of TensorFlow with the same major release.  However, we will endeavour to preserve backwards compatibility even across major releases when possible, so that the serialized files are usable over long periods of time.

There are two main classes of saved TensorFlow data: graphs and checkpoints. Graphs describe the data flow graphs of ops to be run during training and inference, and checkpoints contain the saved tensor values of variables in a graph.

Graphs are serialized via the GraphDef protocol buffer.  To facilitate (rare) backwards incompatible changes to graphs, each GraphDef has an integer version separate from the TensorFlow version.  The semantics are:

  • Each version of TensorFlow supports an interval of GraphDef versions.  This interval with be constant across patch releases, and will only grow across minor releases.  Dropping support for a GraphDef version will only occur for a major release of TensorFlow.

  • Newly created graphs use the newest GraphDef version.

  • If a given version of TensorFlow supports the GraphDef version of a graph, it will load and evaluate with the same behavior as when it was written out (except for floating point numerical details and random numbers), regardless of the major version of TensorFlow.  In particular, all checkpoint files will be compatible.

  • If the GraphDef upper bound is increased to X in a (minor) release, there will be at least six months before the lower bound is increased to X.

For example (numbers and versions hypothetical), TensorFlow 1.2 might support GraphDef versions 4 to 7.  TensorFlow 1.3 could add GraphDef version 8 and support versions 4 to 8.  At least six months later, TensorFlow 2.0.0 could drop support for versions 4 to 7, leaving version 8 only.

Finally, when support for a GraphDef version is dropped, we will attempt to provide tools for automatically converting graphs to a newer supported GraphDef version.

C++ API Compatibility

Only patch releases will be binary compatible at the C++ level.  That is, minor releases are backwards compatible in terms of behavior but may require a recompile for downstream C++ code.  As always, backwards compatibility is only provided for the public C++ API.