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Glossary : Etags

What are Etags?

RavenDB uses etags to track changes of data. Etags are 64-bit numbers which always increase with each change. If comparing etags for two versions of the same document on a specific RavenDB instance, the version with a higher etag is more up-to-date.

Note

Etags are local to each node and are meaningless in the context of the cluster.

Etags vs. Change Vectors

Both etags and change vectors are used to track document changes. Etags have meaning local to cluster nodes, and change vectors have meaning cluster-wide.

  • An etag is only incremented when a document is modified locally
  • Change Vector may be changed when a document is modified locally or replicated

Etags In-Depth

In RavenDB, etags track changes on many different types of server entities.

Etags are used to track changes to the following:

  • Documents
  • Indexes
  • Cluster Topology
  • ETL

For developing with the RavenDB client API, etags are not needed. Etags are used as a "building block" of a Change Vector.

Etags and Clustering

When looking at etags in the context of a RavenDB cluster, etags are only meaningful within a single database on a specific node. This means etags are not universally unique per different databases on a single node and are not unique across cluster nodes.