Why Are We Still Writing Callback Hell for Event Processing in 2026?

Complex Event Processing has a dirty secret: detecting a simple sequence like "transaction opens, processes through multiple steps, then closes" requires absurd amounts of boilerplate in most engines. Let's compare. The problem: match A → B+ → C and compute aggregates over the run A common pattern in transaction monitoring: detect a transaction that opens, goes through one or more processing steps, and closes — then compute statistics across those steps. Simple enough, right? Apama (MonitorScript) The classic approach uses nested on listeners in a monitor: monitor TransactionMonitor { action onload() { on all TransactionOpen() as open { sequence<ProcessingStep> steps := new sequence<ProcessingStep>; on all ProcessingStep(tx_id=open.tx_id) as step and not TransactionClose(tx_id=open.tx_id) { steps.append(step); } on TransactionClose(tx_id=open.tx_id) as close { float totalDuration := 0.0; integer maxErrors := 0; float totalThroughput := 0.0; ProcessingStep s; for s in steps { totalDurat