Dovecot Pro Palomar Architecture

The Dovecot Pro Palomar Architecture in v3.x replaces the director component in older Dovecot versions with a new cluster service.

The cluster service provides:

• Load balancer between Dovecot Proxies and Backends
• User-stickiness to backends
• High availability for backends and other sites
• Load redistribution / user moving
• Multi-site support

Users are always attempted to be accessed only via a single backend at a time. This allows caching to work efficiently. When using object storage and multiple sites, it's possible that the user is accessed simultaneously by multiple sites when the sites' networks don't see each others (split brain). The obox format handles this by eventually merging the changes and moving the user handling back to a single site soon after the split brain is over.

Overview

User Groups

Users are assigned to groups. Only groups can be moved between backends, not individual users. The number of groups should be sized approximately by the number of backends. For example counting 100 groups per backend can allow changing the backend load by 1% increments when moving groups.

Ideally all the groups have equal "load", i.e. moving any of the groups in a backend elsewhere would reduce the backend's load the same amount. A later Palomar version will support automatically moving users between groups to try to make them more balanced.

Features

Group Moving

See Palomar Group Moving.

Automatic Load Rebalancing

See Palomar Load Rebalancing.

Automatic Group Size Rebalancing

See Palomar Group Size Rebalancing.

Logging

See Palomar Logging.