The gap recovery is the weakest recovery class and it ensures neither input nor output preservation. Therefore, a system failure results in missing results for all incoming elements for all incoming elements before Odysseus can process them again (at-most-once). Gap recovery addresses the needs of applications that operate solely on the most recent information (e.g., sensor-based environment monitoring) where dropping old data is tolerable for reduced recovery time and run-time overhead. Because elements may be aggregated, missing elements may also result in incorrect output values. (Reference: J.H. Hwang et al., High-Availability Algorithms for Distributed Stream Processing, 2005)
Documentation under progress
In Odysseus, all relevant actions like the installation of (1) a source connection, (2) a sink connection or (3) a query are logged in a system log, if the recovery feature is installed. That system log is stored in the HOME folder of Odysseus (.odysseus). After a crash of Odysseus and a restart, all source connections, sink connections and queries will be reinstalled. Queries that were running, will be started again.
Requirements: There are no requirements. The gap recovery is always active, if the recovery feature is installed and if no other recovery technique is chosen.