Release Process

The Zephyr project releases on a time-based cycle, rather than a feature-driven one. Zephyr releases represent an aggregation of the work of many contributors, companies, and individuals from the community.

A time-based release process enables the Zephyr project to provide users with a balance of the latest technologies and features and excellent overall quality. A roughly 4-month release cycle allows the project to coordinate development of the features that have actually been implemented, allowing the project to maintain the quality of the overall release without delays because of one or two features that are not ready yet.

The Zephyr release model is loosely based on the Linux kernel model:

  • Release tagging procedure:

    • linear mode on main branch,

    • release branches for maintenance after release tagging.

  • Each release period will consist of a merge window period followed by one or more release candidates on which only stabilization changes, bug fixes, and documentation can be merged in.

    • Merge window mode: all changes are accepted (subject to approval from the respective maintainers.)

    • When the merge window is closed, the release owner lays a vN-rc1 tag and the tree enters the release candidate phase

    • CI sees the tag, builds and runs tests; QA analyses the report from the build and test run and gives an ACK/NAK to the build

    • The release owner, with QA and any other needed input, determines if the release candidate is a go for release

    • If it is a go for a release, the release owner lays a tag release vN at the same point

  • Development on new features continues in topic branches. Once features are ready, they are submitted to mainline during the merge window period and after the release is tagged.

Release Cycle

Release Cycle

Merge Window

A relatively straightforward discipline is followed with regard to the merging of patches for each release. At the beginning of each development cycle, the “merge window” is said to be open. At that time, code which is deemed to be sufficiently stable (and which is accepted by the development community) is merged into the mainline tree. The bulk of changes for a new development cycle (and all of the major changes) will be merged during this time.

The merge window lasts for approximately two months. At the end of this time, the release owner will declare that the window is closed and release the first of the release candidates. For the codebase release which is destined to be 0.4.0, for example, the release which happens at the end of the merge window will be called 0.4.0-rc1. The -rc1 release is the signal that the time to merge new features has passed, and that the time to stabilize the next release of the code base has begun.

Over the next weeks, only patches which fix problems should be submitted to the mainline. On occasion, a more significant change will be allowed, but such occasions are rare and require a TSC approval (Change Control Board). As a general rule, if you miss the merge window for a given feature, the best thing to do is to wait for the next development cycle. (An occasional exception is made for drivers for previously unsupported hardware; if they do not touch any other in-tree code, they cannot cause regressions and should be safe to add at any time).

As fixes make their way into the mainline, the patch rate will slow over time. The mainline release owner releases new -rc drops once or twice a week; a normal series will get up to somewhere between -rc4 and -rc6 before the code base is considered to be sufficiently stable and the quality metrics have been achieved at which point the final 0.4.x release is made.

At that point, the whole process starts over again.

Here is the description of the various moderation levels:

  • Low:

    • Major New Features

    • Bug Fixes

    • Refactoring

    • Structure/Directory Changes

  • Medium:

    • Bug Fixes, all priorities

    • Enhancements

    • Minor “self-contained” New Features

  • High:

    • Bug Fixes: P1 and P2

    • Documentation + Test Coverage

Release Quality Criteria

The current backlog of prioritized bugs shall be used as a quality metric to gate the final release. The following counts shall be used:

Bug Count Release Thresholds








The “low” bug count target of <50 will be a phased appoach starting with 150 for release 2.4.0, 100 for release 2.5.0, and 50 for release 2.6.0


The following syntax should be used for releases and tags in Git:

  • Release [Major].[Minor].[Patch Level]

  • Release Candidate [Major].[Minor].[Patch Level]-rc[RC Number]

  • Tagging:

    • v[Major].[Minor].[Patch Level]-rc[RC Number]

    • v[Major].[Minor].[Patch Level]

    • v[Major].[Minor].99 - A tag applied to main branch to signify that work on v[Major].[Minor+1] has started. For example, v1.7.99 will be tagged at the start of v1.8 process. The tag corresponds to VERSION_MAJOR/VERSION_MINOR/PATCHLEVEL macros as defined for a work-in-progress main branch version. Presence of this tag allows generation of sensible output for “git describe” on main branch, as typically used for automated builds and CI tools.


Zephyr Code and Releases

Long Term Support (LTS)

Long-term support releases are designed to be supported and maintained for an extended period and is the recommended release for products and the auditable branch used for certification.

An LTS release is defined as:

  • Product focused

  • Extended Stabilisation period: Allow for more testing and bug fixing

  • Stable APIs

  • Quality Driven Process

  • Long Term: Maintained for an extended period of time (at least 2.5 years) overlapping previous LTS release for at least half a year.

Product Focused

Zephyr LTS is the recommended release for product makers with an extended support and maintenance which includes general stability and bug fixes, security fixes.

An LTS includes both mature and new features. API and feature maturity is documented and tracked. The footprint and scope of mature and stable APIs expands as we move from one LTS to the next giving users access to bleading edge features and new hardware while keeping a stable foundation that evolves over time.

Extended Stabilisation Period

Zephyr LTS development cycle differs from regular releases and has an extended stabilization period. Feature freeze of regular releases happens 3-4 weeks before the scheduled release date. The stabilisation period for LTS is extended by 3 weeks with the feature freeze occurring 6-7 weeks before the anticipated release date. The time between code freeze and release date is extended in this case.

Stable APIs

Zephyr LTS provides a stable and long-lived foundation for developing products. To guarantee stability of the APIs and the implementation of such APIs it is required that any release software that makes the core of the OS went through the Zephyr API lifecycle and stabilised over at least 2 releases. This guarantees that we release many of the highlighted and core features with mature and well-established implementations with stable APIs that are supported during the lifetime of the release LTS.

  • API Freeze (LTS - 2)

    • All stable APIs need to be frozen 2 releases before an LTS. APIs can be extended with additional features, but the core implementation is not modified. This is valid for the following subsystems for example:

      • Device Drivers (i2c.h, spi.h)…

      • Kernel (k_*):

      • OS services (logging,debugging, ..)

      • DTS: API and bindings stability

      • Kconfig

    • New APIs for experimental features can be added at any time as long as they are standalone and documented as experimental or unstable features/APIs.

  • Feature Freeze (LTS - 1) - No new features or overhaul/restructuring of code covering major LTS features.

    • Kernel + Base OS

    • Additional advertised LTS features

    • Auxiliary features on top of and/or extending the base OS and advertised LTS features can be added at any time and should be marked as experimental if applicable

Quality Driven Process

The Zephyr project follows industry standards and processes with the goal of providing a quality oriented releases. This is achieved by providing the following products to track progress, integrity and quality of the software components provided by the project:

  • Compliance with pubished coding guidelines, style guides and naming conventions and documentation of deviations.

  • Regular static analysis on the complete tree using available commercial and open-source tools and documentation of deviations and false positives.

  • Documented components and APIS

  • Requirements Catalog

  • Verification Plans

  • Verification Reports

  • Coverage Reports

  • Requirements Traceability Matrix (RTM)

  • SPDX License Reports

Each release is created with the above products to document the quality and the state of the software when it was released.

Long Term Support and Maintenance

A Zephyr LTS release is published every 2 years and is branched and maintained independently from the main tree for at least 2.5 years after it was released. Support and maintenance for an LTS release stops at least half a year after the following LTS release is published.

Long Term Support Release

Long Term Support Release

Changes and fixes flow in both directions. However, changes from main branch to an LTS branch will be limited to fixes that apply to both branches and for existing features only.

All fixes for an LTS branch that apply to the mainline tree shall be submitted to mainline tree as well.

Auditable Code Base

An auditable code base is to be established from a defined subset of Zephyr OS features and will be limited in scope. The LTS, development tree, and the auditable code bases shall be kept in sync after the audit branch is created, but with a more rigorous process in place for adding new features into the audit branch used for certification.

This process will be applied before new features move into the auditable code base.

The initial and subsequent certification targets will be decided by the Zephyr project governing board.

Processes to achieve selected certification will be determined by the Security and Safety Working Groups and coordinated with the TSC.

Release Procedure

This section documents the Release manager responsibilities so that it serves as a knowledge repository for Release managers.


The following graphic shows the timeline of phases and milestones associated with each release:

Release Milestones

Release milestones

This shows how the phases and milestones of one release overlap with those of the next release:

Release Milestones

Release milestones with planning

Milestone Description





Planning Kickoff

Start Entering Requirements


TSC Agrees on Major Features and Schedule


Merge Window Open

All features, Sized, and AssignedMerge Window Is Opened


M1 Checkpoint

Major Features Ready for Code Reviews
Test Plans Reviewed and Approved


Feature Merge Window Close

Feature Freeze
Feature Development Complete (including Code Reviews and Unit Tests Passing)
P1 Stories Implemented
Feature Merge Window Is Closed
Test Development Complete
Technical Documentation Created/Updated and Ready for Review
CCB Control Starts


Code Freeze

Code Freeze
RC3 Tagged and Built



TSC Reviews the Release Criteria Report and Approves Release
Final RC Tagged
Make the Release

Release Checklist

Each release has a GitHub issue associated with it that contains the full checklist. After a release is complete, a checklist for the next release is created.


The final release and each release candidate shall be tagged using the following steps:


Tagging needs to be done via explicit git commands and not via GitHub’s release interface. The GitHub release interface does not generate annotated tags (it generates ‘lightweight’ tags regardless of release or pre-release). You should also upload your gpg public key to your GitHub account, since the instructions below involve creating signed tags. However, if you do not have a gpg public key you can opt to remove the -s option from the commands below.


This section uses tagging 1.11.0-rc1 as an example, replace with the appropriate release candidate version.

  1. Update the version variables in the VERSION file located in the root of the Git repository to match the version for this release candidate. The EXTRAVERSION variable is used to identify the rc[RC Number] value for this candidate:

  2. Post a PR with the updated VERSION file using release: Zephyr 1.11.0-rc1 as the commit subject. Merge the PR after successful CI.

  3. Tag and push the version, using an annotated tag:

    $ git pull
    $ git tag -s -m "Zephyr 1.11.0-rc1" v1.11.0-rc1
    $ git push v1.11.0-rc1
  4. Once the tag is pushed, a github action will create a draft release in Github with a shortlog since the last tag. The action will also create a SPDX manifest of the Zephyr tree and will add the file as an asset in the release.

    Go to the draft release that was created and edit as needed. If this step fails for a reason, it can be done manually following the steps below:

    1. Create a shortlog of changes between the previous release (use rc1..rc2 between release candidates):

      $ git shortlog v1.10.0..v1.11.0-rc1
    2. Find the new tag at the top of the releases page and edit the release with the Edit tag button with the following:

      • Name it Zephyr 1.11.0-rc1

      • Copy the shortlog into the release notes textbox (don’t forget to quote it properly so it shows as unformatted text in Markdown)

      • Check the “This is a pre-release” checkbox

  5. Send an email to the mailing lists (announce and devel) with a link to the release

Listing all closed GitHub issues

The release notes for a final release contain the list of GitHub issues that have been closed during the development process of that release.

In order to obtain the list of issues closed during the release development cycle you can do the following:

  1. Look for the last release before the current one and find the day it was tagged:

    $ git show -s --format=%ci zephyr-v1.10.0
    tag zephyr-v1.10.0
    Tagger: Kumar Gala <>
    Zephyr 1.10.0
    2017-12-08 13:32:22 -0600
  2. Use available release tools to list all the issues that have been closed between that date and the day of the release.