Load balancing for distributed search queries

Load balancing for distributed search queries.

DSE Search uses algorithms to balance the load for distributed search queries by minimizing the number of shards that are queried and reducing the amount of data that is transferred from non-local nodes. Strategies are per search index (per core) and can be changed with dsetool set_core_property. Changes are recognized with and do not require restarting the node. Different search indexes can have different values.

Core properties

The core properties for load balancing distributed search queries are:

shard.set.cover.finder

The shard set cover finder calculates how to set cover for a query and specify how one node is selected over others for reading the search data.

The value can be one of:

STATIC


Use	Results
Faster `shard.set.cover.finder=STATIC`	For a given index, a particular coordinator accesses the same token ranges from the respective shards. Creates fewer token filters. Load balancing on the client side is required to achieve uniform utilization of shards by the coordinator nodes.

DYNAMIC


Use	Results
`DYNAMIC` is the default in DSE 6.0 `shard.set.cover.finder=DYNAMIC`	There is no fixed distribution of shard requests for a given coordinator. For two queries, there may be two different sets of shard requests. Creates a large number of unique token filters because different queries may yield shard requests accessing different sets of token ranges. This scenario is often times a problem especially with vnodes because there is a much greater number of possible combinations. In your development environment, compare the load balancing performance when you test using the `STATIC` or `DYNAMIC` setting. The DSE 6.0 default of `DYNAMIC` may not be optimal for your search queries.

shard.shuffling.strategy

When shard.set.cover.finder=DYNAMIC, you can change the shard shuffling strategy to one of these values:

HOST - Shards are selected based on the host that received the query.
QUERY - Shards are selected based on the query string.
HOST_QUERY - Shards are selected by host x query.
RANDOM - Different random set of shards are selected with each request (default).
SEED - Selects the same shard from one query to another.

shard.set.cover.finder.inertia

When shard.set.cover.finder=STATIC, you can change the shard cover finder inertia value. Increasing the inertia value from the default of 1 may improve performance for clusters with more than 1 vnode and more than 20 nodes. The default is appropriate for most workloads.

Changing core properties

Note: Changing core properties is an advanced operation that sets properties in the dse-search.properties resource for the search index.

These example commands show how to change core properties for the demo keyspace and the health_data table.

To change the shard set cover finder:

dsetool set_core_property demo.health_data shard.set.cover.finder=STATIC

Only when shard.set.cover.finder=DYNAMIC, you can change the shard shuffling strategy:
```
dsetool set_core_property demo.health_data shard.shuffling.strategy=query
```
To recognize the changes on the node, reload the search index:
```
RELOAD SEARCH INDEX ON demo.health_data
```
To view the state of the properties in the dse-search.properties resource:
```
dsetool list_core_properties demo.health_data
```
Result:
```
shard.set.cover.finder=STATIC
```

Tip: Log files show the loaded DSE search properties. The dsetool list_core_properties command shows only the state of the properties in the dse-search.properties resource.