Dependencies

Control the order of guarantee execution using requires, after, and before clauses.

Why Dependencies Matter

Sometimes guarantees must happen in a specific order:

# Wrong - might try to encrypt before file exists!
ensure encrypted on file "secrets.db"
ensure exists on file "secrets.db"

EnsuraScript's implication system handles many dependencies automatically (encrypted implies exists), but sometimes you need explicit control.

Dependency Clauses

requires - Logical Prerequisite

ensure <condition-A> requires <condition-B>

Condition A requires condition B to be satisfied first.

ensure backed_up requires encrypted

Before backing up, ensure encryption is complete.

after - Temporal Ordering

ensure <condition> after <resource>

This guarantee runs after all guarantees on the specified resource.

ensure startup on service "app" after file "config.yaml" exists

Don't start the service until the config file is ready.

before - Reverse Temporal Ordering

ensure <condition> before <resource>

This guarantee runs before all guarantees on the specified resource.

ensure exists on file "config.yaml" before service "app" startup

Ensure config exists before the service starts.

Examples

Requires - Logical Dependencies

on file "database.db" {
  ensure exists
  ensure encrypted with AES:256 key "env:DB_KEY"
  ensure backed_up requires encrypted
}

The backup won't start until encryption is verified.

After - Service Startup

resource file "/etc/app/config.yaml" as config
resource service "myapp" as app

ensure exists on config
ensure readable on config

ensure running on app after config

(Note: service resource type is parsed but not yet implemented)

Before - Preparation Steps

ensure exists on file "/var/log/app.log" before file "/usr/bin/app" startup

Create the log file before starting the application.

Combining Dependencies

You can use multiple dependency clauses:

ensure backed_up requires encrypted after database "mydb" ready

This reads as: "Ensure backed_up, but only after:

1. encrypted is satisfied (requires)
2. All guarantees on database mydb are satisfied (after)

Implication vs. Explicit Dependencies

Automatic (Implication)

ensure encrypted on file "secrets.db"
# Automatically ensures: exists, readable, writable FIRST

Explicit (Dependencies)

ensure backed_up requires encrypted
# YOU specify that backed_up depends on encrypted

Use implication when possible (less code), use explicit dependencies for business logic ordering.

Dependency Graph

EnsuraScript builds a dependency graph from:

1. Implication rules (automatic)
2. requires clauses (explicit)
3. after/before clauses (explicit)

Then it performs topological sorting to determine execution order.

View the execution order:

ensura plan config.ens

View the dependency graph as DOT format:

ensura compile config.ens --graph

Cycle Detection

If dependencies create a cycle, compilation fails:

ensure A requires B
ensure B requires C
ensure C requires A  # CYCLE!

Error:

Compilation error: Dependency cycle detected: A → B → C → A

Next Steps

Continue to Collections & Invariants to learn how to enforce guarantees across multiple resources.