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Osmos provides on-disk ordered key-value tables for Erlang, based on a sort-merge machine with user-defined merging semantics. This allows a very high volume of updates to be handled efficiently while still supporting a variety of useful operations with transactional safety, e.g., adding to a counter, taking the union of sets, or simply replacing a record. Osmos is ideal for situations where updates are much more frequent than queries, for example, collecting statistics for reporting, and periodically generating reports.