SearchStax Cloud Help Center

The SearchStax Help Center Frequently Asked Questions page includes FAQs about SearchStax Cloud, our hosted Apache Solr Cloud service.

What is a Collection/ Core/ Shard/ Replica? What is HA/ FT/ DR?

SearchStax Cloud is a fully-managed hosted Solr SaaS solution that automates, manages and scales Solr infrastructure.

Here is a short glossary to help you understand the components of a SearchStax Solr Cloud deployment and how they provide High Availability, Fault Tolerance, and Disaster Recovery.

Note: In Solr terminology, there is a sharp distinction between the logical parts of an index (collections, shards) and the physical manifestations of those parts (cores, replicas). In this diagram, the “logical” concepts are dashed/transparent, while the “physical” items are solid.

SearchStax Solr Glossary
Element Description
Node • In SearchStax Cloud deployments, one Solr node corresponds to one physical server.
• Single-node systems are for testing and development. They do not provide the high-availability/fault-tolerant (HA/FT) behavior needed by production systems.
• A single-node deployment uses an Nginx load-balancer. The log files are not available to the user.
• The node hosts a single Zookeeper instance on the same server as Solr.
• If Solr stops, query service stops.
• A single-node system can be upgraded to increase memory, CPUs, and disk space. It cannot be expanded to create a cluster.
Cluster • A SearchStax “Nextgen” cluster has at least two Solr nodes to provide high-availability/fault-tolerant behavior. If a Solr issue degrades or stops one node, query traffic automatically continues on the other node(s).
• A cluster has a dedicated Load Balancer. It stores a week of log files.
• The cluster includes a three-node Zookeeper ensemble. The Zookeeper and Solr instances are not co-resident on the same servers.
• There is no upper limit on the number of Solr nodes in a cluster.
Zookeeper Ensemble • Zookeeper ensures that changes to config files and updates to index segments are automatically distributed across the nodes of the cluster.
• It also determines which Solr server is the “leader” node for each of the collections.
Collection • A “collection” is a single logical index in its entirety, regardless of how many replicas or shards it has.
• One Solr node can host multiple collections. (A single Sitecore site typically generates over a dozen Solr collections.)
Shard • A “shard” is a logical subset of the documents in a collection. Shards let us split a massive index across multiple servers.  
• SearchStax clients don’t usually subdivide their collections, so for our purposes a “shard” and a “collection” are the same thing. We rarely speak of shards.
• Sharding multiplies the number of servers required to achieve high-availability/fault-tolerant behavior.
• Sharding greatly complicates backup and restore operations.
If your index can fit comfortably on one server, then use one shard. This is Solr’s default behavior.
Core and Replica • A “core” is a complete physical index on a Solr node. In a typical SearchStax Cloud deployment, a “core” is replicated to multiple nodes. Therefore, “replicas” are cores.
• Due to the details of index segment processing, replicas of an index are not all the same physical size.
• To achieve high-availability/fault-tolerant (HA/FT) behavior, every node of the cluster must have a replica of every collection.
• When you create a collection, set replicationFactor equal to the number of nodes in the cluster. Replication is automatic after that step.
• It is a SearchStax convention to speak of replicas rather than cores, hopefully avoiding confusion.
High-Availability • “High-availability (HA)” refers to a cluster of redundant Solr nodes, all of which can respond to queries. If one node goes down, the other nodes can temporarily take up the query load.
Fault-Tolerant • “Fault-tolerance (FT)” enables Solr to continue to serve queries, possibly at a reduced level, rather than failing completely when a component of Solr gets into difficulty.
• Solr/Zookeeper has fault-tolerant strategies such as automatically rebuilding a replica that has become unresponsive. These features handle mild issues if given enough time, but can become overwhelmed.
Rolling Restart • When a Solr system has been severely stressed, its fault-tolerant features can get stuck in a degraded state. There are many possible ways for this to happen, frustrating a direct link from cause to result.
• The “root cause” of this situation is usually that the client sent too many /update requests in too short a time, or that the updates were inefficient. CPU and memory became overloaded, and unneeded subsystems were shut down (but were not automatically restarted).
• A “rolling restart” shuts down each Solr node in the cluster and restarts it. This returns Solr to a known state. Due to the cluster’s High Availability, this does not interrupt query service.
• SearchStax provides a button on the Deployment Details screen to let you do a rolling restart at any time.
Disaster Recovery Disaster Recovery refers to:
◦ A full duplicate production deployment (“Hot” DR) that can take over the query load at a moment’s notice.
◦ A reduced-size production deployment than can be rapidly expanded (“Warm” DR).
◦ Or a remotely-stored recent backup (“Cold” DR).

• DR failover occurs automatically when High-Availability fails (all nodes have been unresponsive for five minutes ).


Do not hesitate to contact the SearchStax Support Desk.

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