There are no leaders nor followers, the underlying architecture is leaderless.

In fact, with Scylla, we have even removed the concept of seed nodes found in other gossip implementations.

This gossip mechanism is also used in case of topology changes, such as adding or removing a node, or in case of an unexpected node outage, providing strong resiliency to a Scylla cluster.

In this way, even if a node is lost, the data still resides somewhere in the cluster.

In such cases, even if two out of the three copies of the data are unavailable, the data will reside somewhere live in the cluster.

With a properly set replication factor, zero downtime is achievable.

Users can determine their own replication factor, based on their use case.

There are times when a replication factor of two may be sufficient, and times when a replication factor of five may be called for.

Scylla automatically takes care of replicating the data in the background.

You just set the replication factor, and the cluster handles the rest.

Scylla focuses on high availability (A) and partition tolerance (P), so is referred to as an AP-mode system.

Any distributed database requires partition tolerance the ability to continue operating even if part of the system becomes off-line due to network or server failure.

Thats the goal for high availability database systems.

The way Scylla achieves zero downtime is through a few mechanisms, including rack and datacenter awareness, as well as multi-datacenter replication.

Data in Scylla is automatically synchronized across datacenters in an eventually consistent manner without requiring users to create any sort of streaming or batch processing to ensure the clusters communicate changes.

It uses snitches to know which rack and which datacenter a node belongs to.

That way, if a rack goes out, or even a whole datacenter, your data is still available.

For instance, the primary datacenter may have a RF of 3, and a separate satellite datacenter may be set to an RF of 2.

This allows you to determine the resiliency of your data per site.

But when those situations occur, the system has to battle against entropy and bring the cluster back to full operation.

The following processes and features are designed to mitigate that.

When the node returns to service, the Hinted Handoffs allow the node to catch up on what transpired while it was offline.

This process can be managed with a command-line interface, called nodetool repair, or from within Scylla Manager, which can also restore data from backups.

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Scylla Seed Node