Hazelcast-Jet drainTo语法问题：Kafka GPB消息聚合输出异常

Let's get your GPB message output working properly and clear up those syntax issues!

First, Correct the Syntax Errors #

Your code has two key issues causing the compile error:

1. Typo in AggregateOperations: You used AggregateOperation instead of the correct plural AggregateOperations (Hazelcast Jet's utility class for aggregation operations).
2. Mismatched Sink Generic Type: When switching from outputting Double to your Amount GPB type, you need to explicitly specify the generic type for KafkaSinks.kafka() to match <String, Amount>.

Here's the corrected full pipeline code:

Pipeline p = Pipeline.create();
p.drawFrom(KafkaSources.<String, Balance>kafka(<properties>, <topic>))
  .map(s -> s.getValue())
  .groupingKey(x -> x.account)
  .rollingAggregator(AggregateOperations.summingDouble(Balance::amount))
  // Map the aggregated Entry<String, Double> to your Amount GPB message
  .map(entry -> Amount.newBuilder()
                      .setAccount(entry.getKey()) // Optional: include the account key if needed
                      .setAmount(entry.getValue())
                      .build())
  // Explicitly set the sink generic type to <String, Amount>
  .drainTo(KafkaSinks.<String, Amount>kafka(<prop>, <sinktopic>));

Note: I added setAccount(entry.getKey()) as an optional step—include it if your Amount GPB includes the account identifier, otherwise you can omit it.

Common Aggregation Scenarios in Hazelcast Jet #

Here are some common aggregation patterns you might use with Hazelcast Jet:

• Rolling Aggregations: Like your current use case, computes aggregates incrementally as new data arrives (no fixed window). Use rollingAggregator() with operations like:
  • AggregateOperations.summingDouble()/summingLong(): Sum numeric fields
  • AggregateOperations.averagingDouble(): Calculate average values
  • AggregateOperations.counting(): Count number of entries per group
• Windowed Aggregations: Aggregate data within fixed time or size windows (e.g., sum amounts per account every 5 minutes). Example:

  .window(WindowDefinition.tumbling(Duration.ofMinutes(5)))
  .aggregate(AggregateOperations.summingDouble(Balance::amount))
  

• Global Aggregations: Aggregate all data without grouping (e.g., total sum of all balances):

  .aggregate(AggregateOperations.summingDouble(Balance::amount))
  

• Custom Aggregations: Build your own aggregation logic using AggregateOperations.create() if the built-in operations don't fit your needs.

内容的提问来源于stack exchange，提问作者Abhishek