StreamAlert consists of multiple AWS components connected together and managed by Terraform.
2. Inbound logs are first ingested via the “classifier” Lambda function in one of your clusters. The classifier is the first and most substantial component of StreamAlert, responsible for parsing, classifying, and performing normalization on logs. The classifier(s) can also optionally forward the resulting logs from classification to Firehose for historical retention and searching via Athena. The same results also get sent to an SQS Queue for further downstream analysis.
3. The SQS Queue that the classifier function(s) send data to is then utilized by a “rules engine” Lambda function. Note that this function is not “clustered” like the classifier(s) and is global to the StreamAlert deployment. The data read from the queue will be ran through the defined rules, and any alerts that are triggered will be sent to a DynamoDB table. Optionally, the rules engine function can read Threat Intelligence information from DynamoDB, or perform other lookup-style operations using data stored in S3.
4. The “alert merger” Lambda function regularly scans the alerts DynamoDB table. When new alerts arrive, they are either forwarded immediately (by default) or, if merge options are specified, they are bundled together with similar alerts before proceeding to the next stage.
5. The “alert processor” Lambda function is responsible for actually delivering the alert to its configured outputs. All alerts implicitly include a Firehose output, which feeds an S3 bucket that can be queried with Athena. Alerts will be retried indefinitely until they are successfully delivered, at which point they will be removed from the DynamoDB table.
6. An “athena partition refresh” Lambda function runs periodically to onboard new StreamAlert data and alerts into their respective Athena databases for historical search.
Other StreamAlert components include DynamoDB tables and Lambda functions for optional rule promotion and regularly updating threat intelligence information.