Constants¶

A constants block declares a value once and lets the rest of the configuration refer to it by name.

constants {
  skus_to_skip = ["SKU-1", "SKU-2", "SKU-3"]
  page_size    = 500
  region       = env("REGION", "us")
}

Every part of a configuration reads them the same way — constants.<name>:

constants {
  skus_to_skip = ["SKU-1", "SKU-2", "SKU-3"]
  page_size    = 500
  region       = env("REGION", "us")
}

flow "list_items" {
  from {
    connector = "api"
    operation = "GET /items"
  }
  to {
    connector = "db"
    query     = "SELECT * FROM items LIMIT ${constants.page_size}"
  }
}

flow "process_item" {
  from {
    connector = "rabbit"
    operation = "items.updated"
  }

  accept {
    when      = "!(input.body.sku in constants.skus_to_skip)"
    on_reject = "ack"
  }

  transform {
    sku    = "input.body.sku"
    region = "constants.region"
  }

  to {
    connector = "db"
    target    = "items"
  }
}

One name, two machines¶

The two references above are evaluated by different things. ${...} is HCL's, folded in when the configuration is read; constants.region inside a transform is CEL's, evaluated for every message. A constant is put in front of both, so which one you are writing for is not something you have to know.

That is the whole point of the block. A constant that resolved in a query and not in a filter would be worse than having none, because the failure is per-expression and nothing announces it.

What a constant holds¶

A literal: a string, a number, a boolean, a list, a map, or anything built out of those — including an env() call, which is a literal by the time anything else runs.

constants {
  retries      = 3
  debug        = false
  regions      = ["us", "uk", "fr"]
  thresholds   = { warn = 100, alert = 500 }
  database_url = env("DATABASE_URL")
}

What a constant is not is a value worked out from a message. There is no constants.total = "input.price * input.quantity": that is what a transform is for. Constants are read once, when the configuration is, and do not change while the service runs.

Where they can be declared¶

In any .mycel file, as many blocks as you like. They are collected before anything else is read, so a flow may use a constant a file later in the directory declares — nothing about the order files are walked in matters.

# constants.mycel
constants {
  page_size = 500
}

Declaring the same name twice is refused, naming both files:

constant "page_size" is declared twice: in constants.mycel and in limits.mycel
— a constant holds one value, so which file wins cannot be left to the order
they are read in

Writing one¶

mycel add constants --value page_size=500 --value region=us

Checking them¶

mycel validate lists what it found:

  Constants: 3
    - page_size = 500
    - region = us
    - skus_to_skip = [SKU-1 SKU-2 SKU-3]