Defaults

Repeating the same pace, limit, seed, or locale in every generator makes a scenario harder to change. Put shared settings under defaults, then override only the generator that should behave differently.

The result we want

Our marketplace needs two customer events but only one daily promotion:

customers       CUST-00
dailyPromotion  free-shipping
customers       CUST-01

Both generators inherit a limit of two. The promotion replaces its inherited limit with one.

Shared defaults with one override

YAML defaults-inheritance.yaml

defaults:
  seed: 42
  faker:
    locale: en_GB
  maxEvents: 2

generators:
  - name: customers
    value:
      customerId: =seq(format=CUST-%02d)
      name: =faker(person.fullName)

  - name: dailyPromotion
    config:
      maxEvents: 1
    value:
      promotion: free-shipping
      market: UK

Press Try it. You should see two customers events and one dailyPromotion event.

defaults.seed: 42 repeats calculated random values.
defaults.faker.locale: en_GB applies to every Faker expression.
defaults.maxEvents: 2 applies to both generators.
dailyPromotion.config.maxEvents: 1 replaces only that generator’s inherited limit.

The override does not change customers, and it does not replace the seed or Faker locale.

Everything you can set

The entire defaults block is optional. This table shows every supported setting and what Synthtraffic uses when you leave it out.

Setting	Accepted values	Value when omitted
`seed`	An integer used for repeatable calculated values	Generates a seed and prints it so you can reuse it
`clock.start`	An RFC3339 timestamp such as `2026-03-01T09:00:00Z`	The time the command starts
`faker.locale`	`en_US`, `en_IN`, `en_AU`, or `en_GB`	`en_US`
`rate`	`unlimited`, a value such as `10/s`, or an advanced rate-window object	`unlimited`
`interval`	A duration such as `500ms` or an expression such as `=uniformDuration(100ms, 500ms)`	Not set
`maxEvents`	A positive event count	`10000` per generator when `maxDuration` is also omitted
`maxDuration`	A positive duration such as `30s` or `5m`	Not set
`history.maxEvents`	A positive retained-event count used by relationships	`100000` per referenced source generator
`delay`	Delivery-delay settings	Disabled
`discard`	Probabilistic delivery-drop settings	Disabled
`repeat`	Probabilistic duplicate-delivery settings	Disabled

The behavior of rate, interval, and limits is covered in Time and pace. Delay, discard, and repeat are covered in Delivery behavior.

Generator config can replace rate, interval, maxEvents, maxDuration, history, delay, discard, or repeat for that generator. seed, clock.start, and faker.locale apply to the whole scenario.

Make a run repeatable

seed and clock.start repeat different parts of a run:

seed repeats calculated random values from expressions such as uuid(), faker(), and uniform().
clock.start repeats the first scenario-clock timestamp used by now().

Set both when random values and timestamps must match:

defaults:
  seed: 42
  clock:
    start: 2026-03-01T09:00:00Z

With the same scenario, seed, and clock start, another run produces the same random choices and the same scenario timestamps. A seed alone does not freeze timestamps because the scenario clock otherwise begins when the command starts.

Choose a Faker locale

faker.locale selects the regional data used by every faker() expression:

en_US — United States
en_IN — India
en_AU — Australia
en_GB — United Kingdom

For example:

defaults:
  faker:
    locale: en_GB

Next: relate orders to customers that already exist.