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Editorial Team
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Asked: 2026-05-12T21:54:04+00:00

In practice we are finding the default NHibernate (v2.0 & 2.1) FlushMode=Auto to be

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In practice we are finding the default NHibernate (v2.0 & 2.1) FlushMode=Auto to be extremely expensive. Reviewing the NHibernate source suggests that the algorithms for determining what needs to be flushed rely on brute-force of looping through all entities in session, and this occurs for every query run in a transaction.

In some production scenario with updates on many items, with multiple queries we have seen the process 100 times longer with FlushMode=Auto compared to FlushMode=Commit.

Any thoughts/advice/best practices for usage of FlushMode when performing ‘complex’ session logic involving multiple updates, multiple queries etc.

Any ideas on optimizing the AutoFlush algorithms in nHibernate?

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1 Answer

  1. Editorial Team

    This slowness is a known issue and is tracked in NH as NH-1365/GitHib Issue 857

    There are three flush modes in NH:

    • FlushMode.Auto = Flush when needed (on commit and before queries, if needed). This is the default.
    • FlushMode.Commit = flush on commit of NH transaction only
    • FlushMode.Never = never flush (until Flush is called). This will still go to DB on insert of entities that use native (identity) PK generator.
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