• Core
  • Rewrite Network Stack (cluster, REST API) based on Boost Asio, Beast, Coroutines
    • Technical concept: #7041
    • Requires package updates: Boost >1.66 (either from packages.icinga.com, EPEL or backports). SLES11 & Ubuntu 14 are EOL.
    • Require TLS 1.2 and harden default cipher list
  • Improved Reload Handling (umbrella process, now 3 processes at runtime)
    • Support running Icinga 2 in (Docker) containers natively in foreground
  • Quality: Use Modern JSON for C++ library instead of YAJL (dead project)
  • Quality: Improve handling of invalid UTF8 strings
• API
  • Fix crashes on Linux, Unix and Windows from Nessus scans #7431
  • Locks and stalled waits are fixed with the core rewrite in #7071
  • schedule-downtime action supports all_services for host downtimes
  • Improve storage handling for runtime created objects in the _api package
• Cluster
  • HA aware features & improvements for failover handling #2941 #7062
  • Improve cluster config sync with staging #6716
  • Fixed that same downtime/comment objects would be synced again in a cluster loop #7198
• Checks & Notifications
  • Ensure that notifications during a restart are sent
  • Immediately notify about a problem after leaving a downtime and still NOT-OK
  • Improve reload handling and wait for features/metrics
  • Store notification command results and sync them in HA enabled zones #6722
• DSL/Configuration
  • Add getenv() function
  • Fix TimePeriod range support over midnight
  • concurrent_checks in the Checker feature has no effect, use the global MaxConcurrentChecks constant instead
• CLI
  • Permissions: node wizard/setup, feature, api setup now run in the Icinga user context, not root
  • ca list shows pending CSRs by default, ca remove/restore allow to delete signing requests
• ITL
  • Add new commands and missing attributes
• Windows
  • Update bundled NSClient++ to 0.5.2.39
  • Refine agent setup wizard & update requirements to .NET 4.6
• Documentation
  • Service Monitoring: How to create plugins by example, check commands and a modern version of the supported plugin API with best practices
  • Features: Better structure on metrics, and supported features
  • Technical Concepts: TLS Network IO, Cluster Feature HA, Cluster Config Sync
  • Development: Rewritten for better debugging and development experience for contributors including a style guide. Add nightly build setup instructions.
  • Packaging: INSTALL.md was integrated into the Development chapter, being available at https://icinga.com/docs too.

Upgrading docs: https://icinga.com/docs/icinga2/snapshot/doc/16-upgrading-icinga-2/