foundation-singleton

Singleton pattern for single-row configuration tables

What it is for

# foundation-singleton

Singleton pattern for single-row configuration tables.

Singletons ensure exactly one row exists per type, with:

• Safe concurrent access via transaction handling
• Corruption detection (multiple rows)
• Deletion prevention
• Optional environment variable fallback

Example

use foundation_singleton::{Singleton, SingletonError};

struct SiteSettings {
id: i32,
site_name: String,
maintenance_mode: bool,
}

impl Singleton for SiteSettings {
fn id(&self) -> i32 {
self.id
}
}

let settings = SiteSettings {
id: 1,
site_name: "My Site".to_string(),
maintenance_mode: false,
};

// Singleton ID is always 1
assert_eq!(settings.id(), 1);
assert!(settings.is_valid_singleton());

// Deletion is not allowed
assert!(settings.delete().is_err());

Capabilities

No public items.

How to use it

From this crate's own rustdoc:

use foundation_singleton::{Singleton, SingletonError};

struct SiteSettings {
    id: i32,
    site_name: String,
    maintenance_mode: bool,
}

impl Singleton for SiteSettings {
    fn id(&self) -> i32 {
        self.id
    }
}

let settings = SiteSettings {
    id: 1,
    site_name: "My Site".to_string(),
    maintenance_mode: false,
};

// Singleton ID is always 1
assert_eq!(settings.id(), 1);
assert!(settings.is_valid_singleton());

// Deletion is not allowed
assert!(settings.delete().is_err());

Module structure

No public modules: the crate root is its whole surface.

Public surface

No public items.

Re-exports. Exported here, defined elsewhere.

Boundary

Depends on no other workspace tier.

Shares tier foundation with 27 other crates: foundation-audit-log, foundation-basemodels, foundation-bounded-io, foundation-conversation-closure, foundation-crypto-sign, foundation-decisioning, foundation-encounter-vocabulary, foundation-fs-metadata, … (27 total).

_What this crate deliberately does NOT own is a judgment. No committed registry records one for it, so none is stated here._

Where it sits

Dependencies

Runtime, in this workspace.

Runtime, from outside the workspace.

Development, from outside the workspace.

Build. None.

Depended on by. 1 workspace crate.

Signal flow — what reaches this crate, and what it reaches.

flowchart LR
  n_application_singleton["application-singleton"] -->|uses| SELF
  SELF["foundation-singleton"]
  SELF -->|runtime| n_foundation_basemodels["foundation-basemodels"]
  classDef self fill:#1f883d,stroke:#1f883d,color:#fff;
  class SELF self;

Feature flags

No Cargo features are defined: every capability is unconditional, so no consumer can receive a half-wired crate.

Targets

Error model

No public error type was detected: no public item declares a type named *Error, and no public signature returns one.

Operational characteristics

No unsafe block, unsafe fn, unsafe impl or unsafe trait was found by the parser anywhere in this crate's source.

Configuration

No environment variable is read with a literal name anywhere in this crate. A variable whose key is computed at run time cannot be listed here, and is not claimed to be absent.

Related capabilities

1 workspace crate depends on this one: application-singleton.

Verification

What the tests establish, by name:

• test_delete_error_is_deletion_not_allowed — src/lib.rs
• test_delete_error_message — src/lib.rs
• test_delete_returns_error — src/lib.rs
• test_env_fallback_db_takes_precedence_over_env — src/lib.rs
• test_env_fallback_returns_db_value_when_present — src/lib.rs
• test_env_fallback_returns_default_when_empty — src/lib.rs
• test_env_fallback_returns_env_var_when_db_empty — src/lib.rs
• test_fields_persist_correctly — src/lib.rs
• test_get_env_var_name — src/lib.rs
• test_get_value_source_database — src/lib.rs
• test_get_value_source_default — src/lib.rs
• test_get_value_source_environment — src/lib.rs
• test_has_value_false_when_empty — src/lib.rs
• test_has_value_true_for_db — src/lib.rs
• test_has_value_true_for_env — src/lib.rs
• test_is_valid_singleton_false_for_negative — src/lib.rs
• test_is_valid_singleton_false_for_wrong_id — src/lib.rs
• test_is_valid_singleton_false_for_zero — src/lib.rs
• test_is_valid_singleton_true_for_id_1 — src/lib.rs
• test_multiple_instances_have_same_singleton_id — src/lib.rs
• test_singleton_error_display_corrupted — src/lib.rs
• test_singleton_error_display_deletion — src/lib.rs
• test_singleton_id_is_one — src/lib.rs
• test_singleton_id_returns_correct_value — src/lib.rs
• test_singleton_id_with_custom_id — src/lib.rs
• test_value_source_clone — src/lib.rs
• test_value_source_debug — src/lib.rs
• test_value_source_equality — src/lib.rs

Documentation coverage

Metrics

pie showData
    title Rust source composition
    "Code" : 315
    "Blank or comment" : 266

Generation

Rendered by tools-corpus corpus readme from repository evidence alone, renderer schema 2, lexicon current. No model, network service or database was consulted. Regenerate with tools-corpus corpus readme --write; verify with --check.