| Internet-Draft | YANG Versioning Requirements | January 2022 |
| Clarke | Expires 10 July 2022 | [Page] |
This document describes the problems that can arise because of the YANG language module update rules, that require all updates to YANG module preserve strict backwards compatibility. It also defines the requirements on any solution designed to solve the stated problems. This document does not consider possible solutions, nor endorse any particular solution.¶
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This requirements document initially considers some of the existing YANG module update rules, then describes the problems that arise due to those rules embracing strict backwards compatibility, and finally defines requirements on any solution that may be designed to solve these problems by providing an alternative YANG versioning strategy.¶
The YANG data modeling language [RFC7950] specifies strict rules for updating YANG modules (see section 11 "Updating a Module"). Citing a few of the relevant rules:¶
The rules described above, along with other similar rules, causes various problems, as described in the following sections:¶
The points made above lead to the logical conclusion that the standardized YANG modules have to be perfect on day one (at least the structure and meaning), which in turn might explain why IETF YANG modules take so long to standardize. Shooting for perfection is obviously a noble goal, but if the perfect standard comes too late, it doesn't help the industry.¶
As we learn from our mistakes, we're going to face more and more non-backwards-compatible YANG modules. An example is the YANG data model for L3VPN service delivery [RFC8049] , which, based on implementation experience, has been updated in a non-backwards-compatible way by [RFC8299] .¶
While Standards Development Organization (SDO) YANG modules are obviously better for the industry, we must recognize that many YANG modules are actually generated YANG modules (for example, from internal databases), which is sometimes the case for vendor modules [RFC8199] . From time to time, the new YANG modules are not backwards-compatible.¶
Old module parts that are no longer needed, no longer supported, or are not used by consumers need to be removed from modules. It is often hard to decide which parts are no longer needed/used; still the need and practice of removing old parts exist. While it is rare in standard modules it is more common in vendor YANG modules where the usage of modules is more controlled.¶
The problems described in Section 2.7 may also result in incompatible changes.¶
In such cases, it would be better to indicate how backwards-compatible a given YANG module actually is.¶
As modules are sometimes updated in an incompatible way the current assumption that once a YANG module is defined all further revisions can be freely used as they are compatible is not valid.¶
Sometimes small errors force us to make non-backwards-compatible updates. As an example imagine that we have a string with a complex pattern (e.g., an IP address). Let's assume the initial pattern incorrectly allows IP addresses to start with 355. In the next version this is corrected to disallow addresses starting with 355. Formally this is a non-backwards-compatible change as the value space of the string is decreased. In reality an IP address and the implementation behind it was never capable of handling an address starting with 355. So practically this is a backwards-compatible change, just like a correction of the description statement. Current YANG rules are ambiguous as to whether non-backwards-compatible bug fixes are allowed without also requiring a module name change.¶
A management system, SDN controller, or any other user of a module should be capable of easily determining the compatibility between two module versions. Higher level logic for a network function, something that cannot be implemented in a purely model driven way, is always dependent on a specific version of the module. If the client finds that the module has been updated on the network node, it has to decide if it tries to handle it as it handled the previous version of the model or if it just stops, to avoid problems. To make this decision the client needs to know if the module was updated in a backwards-compatible way or not.¶
This is not possible to decide today because of the following:¶
Finding status changes or violations of update rules need a line-by-line comparison of the old and new modules is a tedious task.¶
If a module (MOD-A) is imported by another one (MOD-B) the importer may specify which revision must be imported. Even if MOD-A is updated in a backwards-compatible way not all revisions will be suitable, e.g., a new MOD-B might need the newest MOD-A. However, both specifying or omitting the revision date for import leads to problems.¶
If the import by revision-date is specified¶
If the import revision-date is not specified¶
If a schema part is considered old/bad we need to be able to give advance warning that it will be removed. As this is an advance warning the part must still be present and usable in the current revision; however, it will be removed in one of the next revisions. The deprecated statement cannot be reliably used for this purpose both because deprecated nodes may not be implemented and also there is no mandate that text be provided explaining the deprecation.¶
We need the advance warning to allow users of the module time to plan/execute migration away from the deprecated functionality. Deprecation should be accompanied by information whether the functionality will just disappear or that there is an alternative, possibly more advanced solution that should be used.¶
Vendors use such warnings often, but the NMDA related redesign of IETF modules is also an example where it would be useful for IETF. As another example, see the usage of deprecated in the Java programming language.¶
The current definition of deprecated and obsolete in [RFC7950] (as quoted below) is problematic and should be corrected.¶
YANG is considered an interface contract between the server and the client. The current definitions of deprecated and obsolete mean that a schema node that is either deprecated or obsolete may or may not be implemented. The client has no way to find out which is the case except for by trying to write or read data at the leaf in question. This probing would need to be done for each separate data-node, which is not a trivial thing to do. This "may or may not" is unacceptable in a contract. In effect, this works as if there would be an if-feature statement on each deprecated schema node where the server does not advertise whether the feature is supported or not. Why is it not advertised?¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] .¶
In addition, this document uses the following terminology:¶
Non-backwards-compatible (NBC): In the context of this document, the term 'non-backwards-compatible' refers to a change or set of changes between two YANG module revisions that do not adhere to the list of allowable changes specified in Section 11 "Updating a Module" of [RFC7950] , with the following additional clarification:¶
Considering the issues described in the background, the problem definition can be summarized as follows.¶
Development of data models for a large collection of communication protocols and system components is difficult and typically only manageable with an iterative development process. Agile development approaches advocate evolutionary development, early delivery, and continual improvement. They are designed to support rapid and flexible response to change. Agile development has been found to be very successful in a world where the objects being modeled undergo constant changes.¶
The current module versioning scheme relies on the fundamental idea that a definition, once published, never changes its semantics. As a consequence, if a new definition is needed with different non-backwards-compatible semantics, then a new definition must be created to replace the old definition. The advantage of this versioning scheme is that a definition identified by a module name and a path has fixed semantics that never change. (The details are a bit more nuanced but we simplify things here a bit in order to get the problems worked out clearly.)¶
There are two main disadvantages of the current YANG versioning scheme:¶
Other problems experienced with the current YANG versioning scheme are the following:¶
Given the need to support agile development processes and the disadvantages and problems of the current YANG versioning scheme described above, it is necessary to develop requirements and solutions for a future YANG versioning scheme that better supports agile development processes, whilst retaining the ability for servers to handle clients using older versions of YANG modules.¶
The following is a list of requirements that a solution to the problems mentioned above MUST or SHOULD have. The list is grouped by similar requirements but is not presented in a set priority order.¶
Requirements related to making non-backwards-compatible updates to modules:¶
Requirements related to identifying changes between different module revisions:¶
Requirements related to supporting existing clients in a backwards-compatible way:¶
Requirements related to managing and documenting the life cycle of data nodes:¶
Requirements related to documentation and education:¶
This document grew out of the YANG module versioning design team that started after IETF 101. The following people are members of that design team and have contributed to defining the problem and specifying the requirements:¶
The design team would like to thank Christian Hopps and Vladimir Vassilev for their feedback and perspectives in shaping and fine tuning the versioning requirements.¶
One of the inspirations for solving the YANG module versioning comes from OpenConfig. The authors would like to thank Anees Shaikh and Rob Shakir for their helpful input.¶
The document does not define any new protocol or data model. There is no security impact.¶
None¶