Feature: Pooling

The Pooling feature provides the ability to load-balance work across a “pool” of active-active Drools-PDP hosts. This particular implementation uses a kafka topic for communication between the hosts within the pool.

The pool is adjusted automatically, with no manual intervention when:

• a new host is brought online

• a host goes offline, whether gracefully or due to a failure in the host or in the network

Assumptions and Limitations

• Session persistence is not required

• Data may be lost when processing is moved from one host to another

• The entire pool may shut down if the kafka topic becomes inaccessible

Key Points

• Requests are received on a common kafka topic

• Uses a single, kafka topic for all inter-host communication

• Allocates buckets to each host

  • Requests are assigned to buckets based on their respective “request IDs”

• No session persistence

• No objects copied between hosts

• Requires feature(s): distributed-locking

Example Scenario

1. Incoming message is received on a topic — all hosts are listening, but only one random host receives the message

2. Decode message to determine “request ID” key (message-specific operation)

3. Hash request ID to determine the bucket number

4. Look up host associated with hash bucket (most likely remote)

5. Publish “forward” message to internal topic, including remote host, bucket number, topic information, and message body

6. Remote host verifies ownership of bucket, and routes the message to its own rule engine for processing

The figure below shows several different hosts in a pool. Each host has a copy of the bucket assignments, which specifies which buckets are assigned to which hosts. Incoming requests are mapped to a bucket, and a bucket is mapped to a host, to which the request is routed. The host table includes an entry for each active host in the pool, to which one or more buckets are mapped.

Bucket Reassignment

• When a host goes up or down, buckets are rebalanced

• Attempts to maintain an even distribution

• Leaves buckets with their current owner, where possible

• Takes a few buckets from each host to assign to new hosts

For example, in the diagram below, the left side shows how 32 buckets might be assigned among four different hosts. When the first host fails, the buckets from host 1 would be reassigned among the remaining hosts, similar to what is shown on the right side of the diagram. Any requests that were being processed by host 1 will be lost and must be restarted. However, the buckets that had already been assigned to the remaining hosts are unchanged, thus requests associated with those buckets are not impacted by the loss of host 1.

Usage

For pooling to be enabled, the distributed-locking feature must be also be enabled.

Enable Feature Pooling

 policy stop

 features enable distributed-locking
 features enable pooling-messages

The configuration is located at:

• $POLICY_HOME/config/feature-pooling-messages.properties

Start the PDP-D using pooling

 policy start

Disable the pooling feature

 policy stop
 features disable pooling-messages
 policy start

End of Document