Stream Manager has a built-in mechanism for selecting & providing the best (optimal) node for an appropriate streaming operation. This mechanism allows the stream manager to select an optimal node based on the state of available resources on it.

Best node selection logic uses a metric-weight evaluation mechanism to determine the optimal candidate. When a publisher/subscriber request arrives on the Stream Manager requesting an origin/edge for publishing/subscribing, it creates a pool of nodes suitable for the request. Stream Manager then evaluates the node-score for each candidate node in the pool. The node having the best node-score is then returned to the client as per request.

The InstanceMetricsRuleManager component in Stream Manager uses predefined metric rules, where specified metrics are configured with predefined metric weights which project their importance relative to other metrics.

Each metric in the metric evaluation system can be put in one of the two categorizes – LOWERBETTER or HIGHERBETTER. For metric of type LOWERBETTER, a increase in value leads to decrease in weight and for a metric of type HIGHERBETTER increase in value causes increase in weight.

Currently all of defined metrics are of LOWERBETTER.

Stream Manager, internally deals with two types of metrics :

• STATIC : Metrics that are directly available via node reports.
• DYNAMIC : Metrics that are not directly available, but are derived using STATIC metrics.

Metrics that affect the best node selection algorithm are defined in the configuration file applicationContext.xml.

A current net weight or node score is evaluated for all candate nodes for a node type using all the metrics defined that influence the algorithm, the current value of each metric and assigned weight for the respective metric.The node score refers to the sum of all metric weights for a node. The node with the highest score is preferred over other candidate nodes.