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Error Handling: Wait and Retry Example

This section demonstrates the implementation of the OnError.action='wait' property in a wait and retry error handling strategy at the sink level.

Wait and Retry Strategy​

In a scenario where connections to external systems are intermittent, the OnError.action='wait' property enables a sink to temporarily hold event publication until the connection is re-established. This behavior introduces back pressure, preventing new events from being consumed until the issue is resolved.

Here is an example demonstrating how to configure a sink to handle connection errors by using a wait and retry strategy:

-- Defines the stream for handling glucose readings
CREATE STREAM GlucoseReadingStream (locationRoom string, locationBed string, timeStamp string, sensorID long, patientFirstName string, patientLastName string, sensorValue double);

-- Sink configuration with the OnError.action='wait' property for wait and retry strategy
CREATE SINK AbnormalGlucoseReadingStream WITH (type = 'http', OnError.action='wait', publisher.url = "http://<customer's host>:8080/logger", method = "POST", map.type = 'json') (timeStampInLong long, locationRoom string, locationBed string, sensorID long, patientFullName string, sensorReadingValue double);

@info(name='abnormal-reading-identifier')
insert into AbnormalGlucoseReadingStream
select math:parseLong(timeStamp) as timeStampInLong, locationRoom, locationBed, sensorID, str:concat(patientFirstName, " ", patientLastName) as patientFullName, sensorValue as sensorReadingValue
from GlucoseReadingStream[sensorValue > 220];

Prerequisites​

Download the mock logger service.

Input and Output​

The example below demostrates wait and retry.

  • In the case of a service unavailability, the sink will retry sending the event as per the configured wait and retry logic.
  • Upon service restoration, events that were held will be published to the HTTP endpoint and logged accordingly.

Example:

  1. The event sent to GlucoseReadingStream should be formatted as follows:

    ['Get-1024', 'Level2', '1576829362', 10348, 'Alex', 'John', 250]

  1. execute the command to start the mock logger service.

    java -jar logservice-1.0.0.jar

  2. Now, you could see the event sent in step #1 is get logged in the logger service console as given below.

    LoggerService:42 - {event={timeStampInLong=1.576829362E9, locationRoom=Get-1024, locationBed=Level2, sensorID=10348.0, patientFullName=Alex John, sensorReadingValue=250.0}}