Craig Kitterman

Enhancements to multi-region databases in Astra DB provide additional consistency options (EACH_QUORUM and SERIAL) to ensure data is immediately written to a majority of replicas on all configured regions. SERIAL consistency also enables the use of Lightweight Transactions (LWTs) in multi-region deployments.
Last year we <a href="https://www.datastax.com/blog/introducing-only-serverless-database-multiple-regions-and-multiple-clouds">announced</a> support for multi-region serverless databases in Astra DB. Astra DB multi-region databases enable a powerful yet simple option for business continuity, and provide ultra-low latency experiences to end users globally from a single database&mdash;all with lower total cost of ownership (TCO) than traditional multi-region deployment architectures. Enabling this capability is as easy as a few clicks in the Astra DB portal, or a simple API call through the Astra DevOps API.&nbsp;
In Astra DB, all regions in a multi-region database are &ldquo;active&rdquo; and can accept reads and writes.&nbsp; The replication between regions is handled automatically. In the case of a typical write,&nbsp; LOCAL_QUORUM consistency would be observed, ensuring that the data was successfully written to a majority of replicas within the region accepting the initial write, before returning a response to the requesting client. Replication to the other region(s) would then be a &ldquo;fire and forget&rdquo; operation, passing the data along, but not blocking on a response from the other regions.&nbsp; Any cross-region write operation that failed would be handled by a periodic asynchronous repair operation, ensuring eventual consistency between the regions. Unfortunately, given the lag introduced by the asynchronous repair, it was often difficult to know with certainty whether a write had reached all regions and data in all the regions was in sync. For some application types, the potential for this uncertainty is untenable.&nbsp;
 <img style="display: block; margin-left: auto; margin-right: auto;" src="https://cdn.sanity.io/images/bbnkhnhl/production/770e99d1dc4d238c2a5cf5520939f22067261d1f-1440x1234.png" />
<h2>Ensure data replication to configured regions</h2>
Today, DataStax is introducing support for an &lsquo;EACH_QUORUM&rsquo; consistency model for multi-region Astra DB databases. With EACH_QUORUM, Astra DB will ensure the data has been successfully written to a majority of replicas on all configured regions prior to returning a response to the client. This provides instant assurance that a write has reached all configured regions, and the data in all the regions is in sync, and no additional repair operation is needed.&nbsp;
DataStax is also introducing support for a SERIAL consistency model using <a href="https://www.datastax.com/blog/lightweight-transactions-cassandra-20">Lightweight Transactions (LWTs)</a> for multi-region deployments. The SERIAL consistency model ensures that data is consistent and correct across regions using a distributed "compare-and-set" operation implemented via the Paxos algorithm. Compare-and-set manages consistency without the need for error-prone locking mechanisms that can lead to concurrency issues.&nbsp;&nbsp;
Additionally, this update introduces better support for cross-region hinted handoff, to regain consistency quicker in case of failures without having to wait for repair operations.
For more information on configuring multi-region databases in Astra DB, see the <a href="https://docs.datastax.com/en/astra-serverless/docs/manage/db/manage-multi-region.html">Astra DB serverless documentation</a>. We look forward to your continued feedback!

Enhanced Multi-Region Consistency in Astra DB

Craig Kitterman

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