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DynamoDB Stream filters comparing values in NewImage and OldImage

1

I have a table MyTable with count (numeric) among its fields.

I want my CountListener function to be triggered each time the value of count gets updated, for any item in the table.

I've enabled the stream (New and old images) for the table and now I need to write a filter for the stream events in order to invoke CountListener all and only each time count changes.

From the docs I see how to write a filter that compares the value of a field with a constant value: I wonder if I can compare the value of count in NewImage against the one in OldImage and trigger the lambda if and only if they're different.

Topics
AnalyticsServerlessComputeDatabase
Tags
ServerlessAmazon Kinesis Data StreamsComputeAWS LambdaAmazon DynamoDB
Language
English

asked 4 years ago2.8K views

1 Answer
1
Accepted Answer

It is not possible to achieve your goal with Lambda event filtering at the time of writing this for two reasons:

  1. There is no "numeric not equal" operator
  2. It is not possible to compare two fields of the stream data with each other.
AWS

answered 4 years ago

EXPERT

reviewed 2 years ago

  • cionzo
    4 years ago

    Thank you for your answer.

    So, since the code knows whether the "count" field is going to be written, I could add a new (perhaps boolean) field like "updatedCount" set to true when a value for "count" is being set.

    Then, the trigger could be activated by a filter like '{"dynamodb": {"NewImage": {"updatedCount": [true]}}}'. Does it make any sense to you? If not, what would you suggest?

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