You should only ever need the router node if you have a Druid cluster well into the terabyte range. The router node can be used to route queries to different broker nodes. By default, the broker routes queries based on how [Rules](../operations/rule-configuration.html) are set up. For example, if 1 month of recent data is loaded into a `hot` cluster, queries that fall within the recent month can be routed to a dedicated set of brokers. Queries outside this range are routed to another set of brokers. This set up provides query isolation such that queries for more important data are not impacted by queries for less important data.
In this example, we have two tiers in our production cluster: `hot` and `_default_tier`. Queries for the `hot` tier are routed through the `broker-hot` set of brokers, and queries for the `_default_tier` are routed through the `broker-cold` set of brokers. If any exceptions or network problems occur, queries are routed to the `broker-cold` set of brokers. In our example, we are running with a c3.2xlarge EC2 node. We assume a `common.runtime.properties` already exists.
The router module uses several of the default modules in [Configuration](../configuration/index.html) and has the following set of configurations as well:
|`druid.router.tierToBrokerMap`|An ordered JSON map of tiers to broker names. The priority of brokers is based on the ordering.|Queries for a certain tier of data are routed to their appropriate broker.|{"_default_tier": "<defaultBrokerServiceName>"}|
|`druid.router.avatica.balancer.type`|String representing an AvaticaConnectionBalancer name|Class to use for balancing Avatica queries across brokers|rendezvousHash|
|`druid.router.http.maxRequestBufferSize`|Maximum size of the buffer used to write requests when forwarding them to the broker. This should be set to atleast the maxHeaderSize allowed on the broker|8 * 1024|
The router has a configurable list of strategies for how it selects which brokers to route queries to. The order of the strategies matter because as soon as a strategy condition is matched, a broker is selected.
### timeBoundary
```json
{
"type":"timeBoundary"
}
```
Including this strategy means all timeBoundary queries are always routed to the highest priority broker.
### priority
```json
{
"type":"priority",
"minPriority":0,
"maxPriority":1
}
```
Queries with a priority set to less than minPriority are routed to the lowest priority broker. Queries with priority set to greater than maxPriority are routed to the highest priority broker. By default, minPriority is 0 and maxPriority is 1. Using these default values, if a query with priority 0 (the default query priority is 0) is sent, the query skips the priority selection logic.
### JavaScript
Allows defining arbitrary routing rules using a JavaScript function. The function is passed the configuration and the query to be executed, and returns the tier it should be routed to, or null for the default tier.
*Example*: a function that sends queries containing more than three aggregators to the lowest priority broker.
JavaScript-based functionality is disabled by default. Please refer to the Druid <ahref="../development/javascript.html">JavaScript programming guide</a> for guidelines about using Druid's JavaScript functionality, including instructions on how to enable it.
All Avatica JDBC requests with a given connection ID must be routed to the same broker, since Druid brokers do not share connection state with each other.
To accomplish this, Druid provides two built-in balancers that use rendezvous hashing and consistent hashing of a request's connection ID respectively to assign requests to brokers.
Note that when multiple routers are used, all routers should have identical balancer configuration to ensure that they make the same routing decisions.
### Rendezvous Hash Balancer
This balancer uses [Rendezvous Hashing](https://en.wikipedia.org/wiki/Rendezvous_hashing) on an Avatica request's connection ID to assign the request to a broker.
To use this balancer, specify the following property:
```
druid.router.avatica.balancer.type=rendezvousHash
```
If no `druid.router.avatica.balancer` property is set, the Router will also default to using the Rendezvous Hash Balancer.
### Consistent Hash Balancer
This balancer uses [Consistent Hashing](https://en.wikipedia.org/wiki/Consistent_hashing) on an Avatica request's connection ID to assign the request to a broker.
To use this balancer, specify the following property:
```
druid.router.avatica.balancer.type=consistentHash
```
This is a non-default implementation that is provided for experimentation purposes. The consistent hasher has longer setup times on initialization and when the set of brokers changes, but has a faster broker assignment time than the rendezous hasher when tested with 5 brokers. Benchmarks for both implementations have been provided in `ConsistentHasherBenchmark` and `RendezvousHasherBenchmark`. The consistent hasher also requires locking, while the rendezvous hasher does not.
