Queue

The Queue interface is another simple API for pushing and pulling messages to and from a queue.

public interface Queue {
  String pull();
  void push(String task);
  String getName();
  void setName(String name);
  void startPolling();
  void stopPolling();
}

Currently this interface is implemented by the AWSQueue class which relies on the AWS Simple Queue Service, and by LocalQueue which is based on a ConcurrentLinkedQueue. The implementation to load is configurable through:

para-application.conf

para.q = "sqs"
# or
para.q = "local"

AWSQueue

The AWS SQS plugin uses Amazon’s SQS as a river by long polling for messages from a given queue.

The code continuously pulls messages from an SQS queue. After a message is processed, it gets deleted from the queue. To enable this functionality, set para.queue_link_enabled = true in the Para configuration file.

Sample code:

AWSQueue q = new AWSQueue("myQueue");
AWSQueueUtils.pushMessages(q.getUrl(), listOfMessages);
List<String> result = AWSQueueUtils.pullMessages(q.getUrl(), 10);

To configure, put this in your para-application.conf:

para-application.conf

para.queue.polling_sleep_seconds: 60
para.queue.polling_interval_seconds: 20

Messages are in the same format as all other Para objects (see Sysprop):

{
  "id": "id_OR_null",
  "appid": "para_app_id",
  "type": "para_data_type",
  "properties": { "key1": "value1" ...}
}

Note

• If a '_delete': true field exists, the object in the message will be deleted.
• If '_create': true field exists, the object will be recreated, overwriting anything with the same id.

When the queue is empty the river will sleep for sleep seconds before sending a new request for messages to the queue. Long polling is done by the Amazon SQS client using the waitTimeSeconds attribute which is set to longpolling_interval (must be between 0 and 20).

LocalQueue

This queue implementation is good for local development, testing and single-machine Para servers. It uses an in-memory queue which is not persisted in any way.