<h1>RabbitMQ 4.0.x Release Notes</h1>
<h2>Introduction</h2>
<p>RabbitMQ has rolled out significant updates in versions 4.0.0 and 4.0.1. These major releases introduce several key improvements and breaking changes that users need to be aware of before upgrading from the 3.x series.</p>
<h2>Key Improvements</h2>
<h3>Khepri Support</h3>
<p>One of the notable changes is the full support for Khepri as an alternative to the Mnesia database. This can potentially offer improved performance and scalability for certain workloads.</p>
<h3>AMQP 1.0 as Core Protocol</h3>
<p>AMQP 1.0 has been included as a core protocol, enhancing the versatility and compatibility of RabbitMQ with different systems.</p>
<h3>Quorum Queue Enhancements</h3>
<p>Efficiency in quorum queue recovery has been improved. Additionally, these updates now support priorities within quorum queues, enabling more granular control over message processing.</p>
<h2>Breaking Changes</h2>
<h3>Removal of Classic Queue Mirroring</h3>
<p>Mirroring for classic queues has been removed. Users are urged to transition to quorum queues or other suitable alternatives.</p>
<h3>Deprecated Configurations</h3>
<p>Several configurations have been deprecated, requiring users to review and update their setups accordingly.</p>
<h3>Default Maximum Message Size</h3>
<p>The default maximum message size has been reduced. Users need to verify their existing workloads to ensure compatibility.</p>
<h2>Known Issues</h2>
<p>Despite the numerous enhancements, one known issue includes incorrect version reporting in generic binary builds. Users are advised to verify versions separately if needed.</p>
<h2>Upgrade Instructions</h2>
<p>Before upgrading to RabbitMQ 4.0.x, it is crucial to enable all feature flags from version 3.13.x to ensure a smooth transition. Carefully follow the provided upgrade path and review the breaking changes to avoid disruptions.</p>
<h2>Conclusion</h2>
<p>The RabbitMQ 4.0.x series brings substantial improvements along with several breaking changes. Users should thoroughly review the new features and changes to fully leverage the benefits of these updates while ensuring compatibility and continuity in their operations.</p>


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RabbitMQ 4.0.x introduces major updates, including full Khepri support, AMQP 1.0 as a core protocol, and improvements in quorum queue efficiency. Key breaking changes involve the removal of classic queue mirroring, deprecated configurations, and a reduced default max message size. Before upgrading from the 3.x series, users must enable all feature flags from version 3.13.x and review the breaking changes carefully.

Release RabbitMQ 4.0.x: Major Improvements and Breaking Changes