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    Replication

    Replication in RavenDB is the process of transferring data from one database instance to another.
    Your data will be highly available as modified data is automatically replicated.

    • Internal replication:
      Data is replicated among the different database instances within the same database-group.
      The number of database instances is defined by the database replication factor.

    • External replication:
      Data is replicated between two different databases, typically located on different clusters.
      This is handled by the External-Replication ongoing task.

    The transferred entities are:

    • Documents
    • Revisions
    • Attachments
    • Conflicts

    How does Replication works

    • The replication process sends data over a TCP connection by the modification order, from the oldest to the newest.
    • Every database has an ETag, which is incremented on every modification in the database's storage.
    • Each replication process has a source , destination and a last confirmed ETag which is the cursor to where the replication process is.
    • The data is sent in batches. When the destination confirms getting the data, the last accepted ETag is then advanced and the next batch of data is sent.

    Replication Failure

    In case of failure it will re-start with the Initial Handshake Procedure, which will make sure we will start replicating from the last accepted ETag.

    Replication Transaction Boundary

    The boundary of a transaction is extended across multiple nodes.
    If there are several documents in the same transaction they will be sent in the same replication batch to keep the data consistent.

    However this doesn't always ensure the data consistency, since the same document can be modified in a different transaction and be sent in a different batch.

    Replication consistency can be achieved by -

    • Using Write Assurance.
    • Enabling Revisions.
      When documents that own revisions are replicated, their revisions will be replicated with them.

    Let's see how the replication of revisions helps data consistency.

    Consider a scenario in which two documents, Users/1 and Users/2, are created in the same transaction, and then Users/2 is modified in a different transaction.

    * **How will `Users/1` and `Users/2` be replicated?**  
  When RavenDB creates replication batches, it keeps the
  [transaction boundary](../../../server/clustering/replication/replication#replication-transaction-boundary)
  by always sending documents that were modified in the same transaction,
  **in the same batch**.  
  In our scenario, however, `Users/2` was modified after its creation, it
  is now recognized by its Etag as a part of a different transaction than
  that of `Users/1`, and the two documents may be replicated in two different
  batches, `Users/1` first and `Users/2` later.  
  If this happens, `Users/1` will be replicated to the destination without `Users/2`
  though they were created in the same transaction, causing a data inconsistency that
  will persist until the arrival of `Users/2`.

* **The scenario will be different if revisions are enabled.**  
  In this case the creation of `Users/1` and `Users/2` will also create revisions
  for them both. These revisions will continue to carry the Etag given to them
  at their creation, and will be replicated in the same batch.  
  When the batch arrives at the destination, data consistency will be kept:  
  `Users/1` will be stored, and so will the `Users/2` revision, that will become
  a live `Users/2` document.

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