Elasticity

Elasticity refers to the ability for you adjust your database to changing workload demands by adding or removing nodes. When your database experiences high demand, you can add new nodes or start stopped nodes to increase the amount of compute available. When your database experiences lower demands (such as during holidays or weekends) you can stop or terminate nodes to save money. You can also gradually add nodes over time as your database demands grow.

All nodes in an Eon Mode database belong to a subcluster. By choosing which subclusters get new nodes, you can affect how the new nodes impact your database. There are two goals you can achieve when adding nodes to your database:

• Improve query throughput: higher throughput means your database processes more queries simultaneously. You often want to improve throughput when you have a workload of "dashboard queries": many relatively short-running queries. In this case, speeding up the processing of individual queries is not as important as having more queries run in parallel.
• Improve query performance: higher query performance means that your complex in-depth analytic queries complete faster.

Scaling for Query Throughput

To scale for query throughput, add additional nodes to your database in one or more new subclusters. Subclusters independently process queries: a query only runs on the nodes in the subcluster containing the initiator node. By adding one or more subclusters, your database can process more queries at the same time. For the best performance, add the same number of nodes to each new subcluster as there are shards in the database. For example, if you have 6 shards in your database, add 6 nodes to each new subcluster you create.

To take advantage of the improved throughput offered by the new subclusters, clients must connect to them. The best way to ensure your users take advantage of the subclusters you have added for throughput scaling is to create connection load balancing policies that spread client connections across the all nodes in all of these subclusters. See Connection Load Balancing Policies for more information.

Subclusters also organize nodes into groups that can easily be stopped or started together. This feature makes expanding and shrinking your database easier. See Starting and Stopping Subclusters for details.

Scaling for Query Performance

To improve the performance of individual queries in a subcluster, add more nodes to it. Queries perform faster when there is more computing power available to process them.

Adding nodes is especially effective if your subcluster has less nodes than there are shards in the database. In this case, nodes are responsible for processing data in multiple shards. When you add more nodes, the newly-added nodes take over responsibility for some of the shards. With less data to process, each node finishes their part of the query faster, resulting in better overall performance. For the best performance, make the number of nodes in the subcluster an even divisor of (or equal to) the number of shards in the database. For example, in a 12-shard database, make the number of nodes in the subcluster 3, 6, or 12.

You can further improve query performance by adding more nodes than there are shards in the database. When nodes outnumber shards, multiple nodes in the subcluster subscribe to the same shard. In this case, when processing a query, Vertica uses a feature called elastic crunch scaling (ECS) to have all of the nodes in the subcluster take part in the query. ECS assigns a subset of the data in each shard to the shard's subscribers. For example, in six-node subcluster in a a three-shard database, each shard has two subscribers. ECS assigns each of the subscribers half of the data in the shard to process during queries. In most cases, with less data to process, the nodes finish executing the query faster. When adding more nodes than shards to a subcluster, make the number of nodes a multiple of the number of shards to ensure an even distribution. For example, in a three-shard database, make the number of nodes in the subcluster 6, 9, 12, and so on.

Using Different Subclusters for Different Query Types

You do not have to choose one form of elasticity over the other in your database. You can create a group of subclusters to improve query throughput and one or more subclusters that improve query performance. The difference between the two subcluster types is mainly the number of subclusters you create and the number of nodes they contain. To improve throughput, add a multiple subclusters that contain a number of nodes that is equal to or less than the number of shards in the database. The more subclusters you add, the greater the throughput you achieve. To improve query performance, add one or more subclusters where the number of nodes is a multiple of the number of shards in the database.

Once you have created your set of subclusters, you must have clients connect to the correct subcluster for the types of queries they will run. For clients executing frequent, simple dashboard queries, create a connection load balancing policy that connects them to nodes in the throughput scaling subclusters. For clients running more complex analytic queries, create another load balancing policy that connects them to nodes in the performance scaling subcluster.

For details on scaling your Eon Mode database, see Scaling Your Eon Mode Database.