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CUE (Configure, Unify, Execute) is another well-regarded schema language, useful for defining valid fields, types, and values for a config. CUE was developed at Google by Marcel van Lohuizen, one of the original creators of Go. The original motivation stemmed from Google’s internal struggles in managing complex, large-scale configurations for infrastructure and applications. JSON and YAML were too weak for expressing constraints, logic, or composition, so CUE set out to solve that. Instead of building and managing external tools to validate, patch, or generate configs, CUE itself is a fully programmable config engine. This means CUE wholly encapsulates the configuration data, the schema, and the logic (how to derive those config values).

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Example

Below is an example CUE schema:
CUE
@miru(config_type="communication")

control_loop_rate_hz: number & >=1 & <=1000 | *50
watchdog_timeout_ms: number & >=100 & <=5000 | *500

network: {
	max_latency_ms: number & >=10 & <=1000 | *100
	connection_timeout_ms: number & >=100 & <=10000 | *2000
	reconnect_attempts: number & >=1 & <=10 | *3
	reconnect_interval_ms: number & >=100 & <=5000 | *1000
	heartbeat_interval_ms: number & >=50 & <=1000 | *250
}

logging: {
	enable_packet_logging: bool | *true
	log_level: "debug" | "info" | "warn" | "error" | *"info"
	max_log_size_mb: number & >=1 & <=1024 | *100
	retain_logs_days: number & >=1 & <=90 | *30
}

error_handling: {
	max_consecutive_failures: number & >=1 & <=20 | *5
	failure_timeout_ms: number & >=100 & <=30000 | *5000
	enable_auto_recovery: bool | *true
}
Although many of CUE’s capabilities are omitted in this example, you’ll find some of the most commonly used features, including:
  • type specification - number, string, bool, etc.
  • numeric constraints- >=, <=, ==, !=, =~, !~, etc.
  • default values - | *value
  • enumerations - | "value1" | "value2" | "value3"
  • intersection of constraints - &
As you can see, CUE inlines constraints directly with type definitions, making schemas significantly more concise than JSON Schema. For more information on the supported features of CUE, check out the CUE tour on the official CUE website.

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CUE version

Miru fully supports CUE and stays up to date with the latest CUE features and syntax. Currently, Miru uses CUE API v0.14.2.

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File formats

Unlike JSON Schema, CUE can only be written in the CUE file format and must have the .cue extension:
  • CUE (.cue)
Similar to JSON Schema, CUE schemas can validate config instances in formats other than CUE. The following formats are supported:
  • JSON (.json)