On a high level, Revapi has a simple architecture consisting of a linear pipeline that processes a tree of individual API elements.

The API elements come from archives. The implementations of the archive interface are responsible for providing the bytes the archive consists of and optionally its version.

A set of archives containing the old version of API is passed to an archive analyzer that produces a (multi) tree of API elements (internally called ElementForest). The archives of the new version are analyzed in the same way.

The two API forests are then consistently sorted and traversed in parallel. New or missing elements are detected and equivalent elements are compared using an element analyzer, producing a set of reports that contain the differences found while comparing the elements. The differences are then simply reported.

The following diagram depicts the high level workflow graphically.


Each of the above steps is configurable by the API author. For example, it is usually the difference transforms that the library author uses to add justifications and assign criticality to individual API changes found during the analysis.

Of course, a closer look at the architecture reveals more complex details, but the high level look is still valid.

For more in-depth look at the architecture, see Extending Revapi.


From an API author point of view, you will most usually handle the differences that Revapi finds in the APIs.

A difference consists of the following data:


This is a unique textual ID of the difference.


A human readable name of the difference.


Detailed description of what the difference represents. This is meant for human consumption.


A mapping between a compatibility type and severity of the problem expressing how severe the difference is in given compatibility "modes" (e.g. a binary incompatible change might be source compatible)


How critical the difference is as judged by the authors. This is distinct from the severity because it is meant to be assigned manually by the API authors during the evolution of the API. It should express how the API change is dealt with (e.g. documented) instead of how it affects the users of the API (which is what the severity expresses).


These are programmatically assigned additional "qualities" of the difference that can be used (or sometimes are necessary) to reference the difference in the configuration (e.g. when writing down the justification for the API change, etc.).

Revapi recognizes 4 types of compatibility:

  • SOURCE - old and new API is source compatible if the code using the old API can be compiled against the new API without modification.

  • BINARY - old and new API is binary compatible if the code compiled against the old API can run using the new API with modification and error.

  • SEMANTIC - old and new API is semantically compatible if they behave the same

  • OTHER - other type of compatibility not captured by the previous three.

A difference can have 1 of the 4 severities (per compatibility type):

  • BREAKING - the differences breaks the API compatibility (of given type)

  • POTENTIALLY_BREAKING - the difference may break the API compatibility (of given type) under some specific circumstances

  • NON_BREAKING - the difference doesn’t break the API

  • EQUIVALENT - "there is no change" - this is provided so that transforms and other tools can declare that certain changes are not even non-breaking - they are effectively non-existent.

A criticality is identified by its name and has a level associated with it. The higher the level, the more critical the criticality is. There are 4 predefined criticalities:

  • allowed - for API changes that are allowed to happen and might not even be tracked in some kind of generated report. The level of this is set to 1000.

  • documented - for API changes that are justified and documented in some generated report. The level of this is set to 2000.

  • highlight - essentially the same as documented but more "severe". Such API changes should be somehow highlighted in the generated reports because they are very important to take note of by the users. The level of this is set to 3000.

  • error - These changes should not be allowed in a release. The level of this is set to the maximum integer value. There can be no more severe criticality than this.

The criticality is not generally assigned directly by the difference analyzer. It is meant to be assigned by the API author that configures the analysis through configuring difference transform extensions to assign the criticality based on some criteria.

The recognized criticalities can be configured in the pipeline configuration where one can define a completely new set of criticalities known to the analysis or just augment the levels of the default ones.