Actually, the title was a mistake.
In this post, we walk you through the new message filtering feature, and how to use it to clean up unnecessary logic in your components, and reduce the number of topics in your architecture. In the above scenario, every subscriber receives the same message published to the topic, allowing them to process the message independently.
For many use cases, this is sufficient. However, in more complex scenarios, the subscriber may only be interested in a subset of the messages being published.
The onus, in that case, is on each subscriber to ensure that they are filtering and only processing those messages in which they are actually interested. To avoid this additional filtering logic on each subscriber, many organizations have adopted a practice in which the publisher is now responsible for routing different types of messages to different topics.
When the publisher posts a new message to the topic, SNS attempts to match the incoming message attributes to the filter policy set on each subscription, to determine whether a particular subscriber is interested in that incoming event.
If there is a match, SNS then pushes the message to the subscriber in question. The new attribute-based message filtering approach is depicted in the following diagram.
Message filtering in action Look at how message filtering works. The following example is based on a sports merchandise ecommerce website, which publishes a variety of events to an SNS topic. First, create the single SNS topic to which all shopping events are published.
A subscription filter policy is set as a subscription attribute, by the subscription owner, as a simple JSON object, containing a set of key-value pairs. This object defines the kind of event in which the subscriber is interested.
Message attributes allow you to provide structured metadata items such as time stamps, geospatial data, event type, signatures, and identifiers about the message. Message attributes are optional and separate from, but sent along with, the message body.
You can include up to 10 message attributes with your message. The first message published in this example is related to an order that has been placed on the ecommerce website. Therefore, only the Lambda function subscribed to the SNS topic is notified about this navigation event.
As described earlier, checkout events are pushed only to the SQS queue, whereas navigation events are pushed to the Lambda function only. Message filtering criteria It is important to remember the following things about subscription filter policy matching: For a filter policy to match a message, the message must contain all the attribute keys listed in the policy.
Attributes of the message not mentioned in the filtering policy are ignored. The value of each key in the filter policy is an array containing one or more values.
The policy matches if any of the values in the array match the value in the corresponding message attribute. If the value in the message attribute is an array, then the filter policy matches if the intersection of the policy array and the message array is non-empty.
The matching is exact character-by-characterwithout case-folding or any other string normalization. The values being matched follow JSON rules: Strings enclosed in quotes, numbers, and the unquoted keywords true, false, and null.
Number matching is at the string representation level. When should I use message filtering? We recommend using message filtering and grouping subscribers into a single topic only when all of the following is true: This option could result in an unnecessarily large topic, which could potentially impact your message delivery latency.
Also, you would lose the ability to implement fine-grained access control on your topics. Finally, if you already use SNS, but had to add filtering logic in your subscribers or routing logic in your publishers topic-based filteringyou can now immediately benefit from message filtering.
This new approach lets you clean up any unnecessary logic in your components, and reduce the number of topics in your architecture.
Add filter policies to your subscriptions on the SNS console .This website is the only official on-line source of information regarding John Edward, his schedule, tour dates or other appearances.
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