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# Data Gaps and WAN Capacity

A data gap consists of persistent message data that has not yet been replicated to persistence services at the disaster recovery site. This data is at risk, and could be lost in a disaster.

Replication of persistence data to a disaster recovery site requires sufficient WAN capacity to transfer data in a timely manner. You must ensure sufficient WAN capacity for expected peak data volume.

A small data gap will always exist because replication to the disaster recovery site is asynchronous. That is, publishers of a persistent message stream do not wait for replication to the disaster recovery site (even if they wait for confirmation from the main site).

The data gap can grow in two situations:

- Peak message activity exceeds the WAN capacity. The data gap grows until the message volume decreases. When message volume decreases, replication at the disaster recovery site can catch up to the data state at the main site, and the data gap shrinks.
  Tip: It is good practice for administrators to cross-check the active and standby persistence services during peak periods. If the history values are not identical, raise the capacity of the WAN links as needed.
- The WAN link malfunctions. Replication completely stops, unless you have redundant WAN lines for fault tolerance, and the data gap grows rapidly. Even with redundant lines, a data gap can grow if the backup capacity is less than the message volume. The data gap can also grow if failover introduces a significant delay.

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