EventBridge

Event Router / Event Bus

What it’s for (mental model)

• A central event router with content-based routing to many targets.
• Strong fit for: domain events, microservice choreography, agent workflow events, auditability + replay.

Key capabilities

• Rules: match on event patterns (JSON) → route to targets.
• Event buses: default + custom; supports cross-account patterns (with IAM role requirements). ([AWS Documentation][11])
• Archive + replay: production debugging + reprocessing. ([AWS Documentation][12])
• Pipes: “managed glue” from source to target with optional filter/transform/enrich. ([AWS Documentation][13])
• Scheduler: at-scale cron/one-time schedules (serverless). ([Amazon Web Services, Inc.][14])

Cost mental model (these are the big ones)

• Custom events: $1 / 1M events ingested (64KB chunks). ([Amazon Web Services, Inc.][14])
• Cross-account delivery can add cost (e.g., $1 / 1M for some cross-account delivery scenarios shown). ([Amazon Web Services, Inc.][14])
• Pipes: $0.40 / 1M requests after filtering (and batching helps). ([Amazon Web Services, Inc.][14])
• Scheduler: 14M invocations/month free, then $1 / 1M. ([Amazon Web Services, Inc.][14])
• Payload chunking: 256KB event counts as 4 events. ([Amazon Web Services, Inc.][14])

Design heuristics (when to choose it vs SNS/SQS)

• Choose EventBridge when you need:

  • many event types, many consumers, routing by content, cross-account, replay

• Choose SNS when you need:

  • simple fanout + per-subscriber filtering, minimal event governance

• Choose SQS when you need:

  • durable buffering and worker pull model, not broadcast routing

Terraform: Custom bus → rule → SQS target (classic routing)

resource "aws_cloudwatch_event_bus" "bus" {
  name = "app-bus"
}

resource "aws_sqs_queue" "target_q" {
  name                    = "eb-target"
  sqs_managed_sse_enabled = true
}

resource "aws_cloudwatch_event_rule" "rule" {
  name           = "route-job-events"
  event_bus_name = aws_cloudwatch_event_bus.bus.name

  event_pattern = jsonencode({
    source      = ["app.jobs"]
    "detail-type" = ["JobStateChanged"]
    detail = {
      status = ["FAILED", "SUCCEEDED"]
    }
  })
}

resource "aws_cloudwatch_event_target" "to_sqs" {
  rule           = aws_cloudwatch_event_rule.rule.name
  event_bus_name = aws_cloudwatch_event_bus.bus.name
  arn            = aws_sqs_queue.target_q.arn
}

# Allow EventBridge to send to SQS
data "aws_iam_policy_document" "sqs_allow_eventbridge" {
  statement {
    effect = "Allow"
    principals {
      type        = "Service"
      identifiers = ["events.amazonaws.com"]
    }
    actions   = ["sqs:SendMessage"]
    resources = [aws_sqs_queue.target_q.arn]

    condition {
      test     = "ArnEquals"
      variable = "aws:SourceArn"
      values   = [aws_cloudwatch_event_rule.rule.arn]
    }
  }
}

resource "aws_sqs_queue_policy" "target_q_policy" {
  queue_url = aws_sqs_queue.target_q.id
  policy    = data.aws_iam_policy_document.sqs_allow_eventbridge.json
}

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