harmony/adr/015-higher-order-topologies/example.rs

//! Example of Higher-Order Topologies in Harmony.
//! Demonstrates how `FailoverTopology<T>` automatically provides failover for *any* capability
//! supported by a sub-topology `T` via blanket trait impls.
//! 
//! Key insight: No manual impls per T or capability -- scales effortlessly.
//! Users can:
//! - Write new `Topology` (impl capabilities on a struct).
//! - Compose with `FailoverTopology` (gets capabilities if T has them).
//! - Compile fails if capability missing (safety).

use async_trait::async_trait;
use tokio;

/// Capability trait: Deploy and manage PostgreSQL.
#[async_trait]
pub trait PostgreSQL {
    async fn deploy(&self, config: &PostgreSQLConfig) -> Result<String, String>;
    async fn get_replication_certs(&self, cluster_name: &str) -> Result<ReplicationCerts, String>;
}

/// Capability trait: Deploy Docker.
#[async_trait]
pub trait Docker {
    async fn deploy_docker(&self) -> Result<String, String>;
}

/// Configuration for PostgreSQL deployments.
#[derive(Clone)]
pub struct PostgreSQLConfig;

/// Replication certificates.
#[derive(Clone)]
pub struct ReplicationCerts;

/// Concrete topology: Kubernetes Anywhere (supports PostgreSQL).
#[derive(Clone)]
pub struct K8sAnywhereTopology;

#[async_trait]
impl PostgreSQL for K8sAnywhereTopology {
    async fn deploy(&self, _config: &PostgreSQLConfig) -> Result<String, String> {
        // Real impl: Use k8s helm chart, operator, etc.
        Ok("K8sAnywhere PostgreSQL deployed".to_string())
    }

    async fn get_replication_certs(&self, _cluster_name: &str) -> Result<ReplicationCerts, String> {
        Ok(ReplicationCerts)
    }
}

/// Concrete topology: Linux Host (supports Docker).
#[derive(Clone)]
pub struct LinuxHostTopology;

#[async_trait]
impl Docker for LinuxHostTopology {
    async fn deploy_docker(&self) -> Result<String, String> {
        // Real impl: Install/configure Docker on host.
        Ok("LinuxHost Docker deployed".to_string())
    }
}

/// Higher-Order Topology: Composes multiple sub-topologies (primary + replica).
/// Automatically derives *all* capabilities of `T` with failover orchestration.
/// 
/// - If `T: PostgreSQL`, then `FailoverTopology<T>: PostgreSQL` (blanket impl).
/// - Same for `Docker`, etc. No boilerplate!
/// - Compile-time safe: Missing `T: Capability` → error.
#[derive(Clone)]
pub struct FailoverTopology<T> {
    /// Primary sub-topology.
    pub primary: T,
    /// Replica sub-topology.
    pub replica: T,
}

/// Blanket impl: Failover PostgreSQL if T provides PostgreSQL.
/// Delegates reads to primary; deploys to both.
#[async_trait]
impl<T: PostgreSQL + Send + Sync + Clone> PostgreSQL for FailoverTopology<T> {
    async fn deploy(&self, config: &PostgreSQLConfig) -> Result<String, String> {
        // Orchestrate: Deploy primary first, then replica (e.g., via pg_basebackup).
        let primary_result = self.primary.deploy(config).await?;
        let replica_result = self.replica.deploy(config).await?;
        Ok(format!("Failover PG deployed: {} | {}", primary_result, replica_result))
    }

    async fn get_replication_certs(&self, cluster_name: &str) -> Result<ReplicationCerts, String> {
        // Delegate to primary (replica follows).
        self.primary.get_replication_certs(cluster_name).await
    }
}

/// Blanket impl: Failover Docker if T provides Docker.
#[async_trait]
impl<T: Docker + Send + Sync + Clone> Docker for FailoverTopology<T> {
    async fn deploy_docker(&self) -> Result<String, String> {
        // Orchestrate across primary + replica.
        let primary_result = self.primary.deploy_docker().await?;
        let replica_result = self.replica.deploy_docker().await?;
        Ok(format!("Failover Docker deployed: {} | {}", primary_result, replica_result))
    }
}

#[tokio::main]
async fn main() {
    let config = PostgreSQLConfig;

    println!("=== ✅ PostgreSQL Failover (K8sAnywhere supports PG) ===");
    let pg_failover = FailoverTopology {
        primary: K8sAnywhereTopology,
        replica: K8sAnywhereTopology,
    };
    let result = pg_failover.deploy(&config).await.unwrap();
    println!("Result: {}", result);

    println!("\n=== ✅ Docker Failover (LinuxHost supports Docker) ===");
    let docker_failover = FailoverTopology {
        primary: LinuxHostTopology,
        replica: LinuxHostTopology,
    };
    let result = docker_failover.deploy_docker().await.unwrap();
    println!("Result: {}", result);

    println!("\n=== ❌ Would fail to compile (K8sAnywhere !: Docker) ===");
    // let invalid = FailoverTopology {
    //     primary: K8sAnywhereTopology,
    //     replica: K8sAnywhereTopology,
    // };
    // invalid.deploy_docker().await.unwrap(); // Error: `K8sAnywhereTopology: Docker` not satisfied!
    // Very clear error message :
    // error[E0599]: the method `deploy_docker` exists for struct `FailoverTopology<K8sAnywhereTopology>`, but its trait bounds were not satisfied
    //   --> src/main.rs:90:9
    //    |
    //  4 | pub struct FailoverTopology<T> {
    //    | ------------------------------ method `deploy_docker` not found for this struct because it doesn't satisfy `FailoverTopology<K8sAnywhereTopology>: Docker`
    // ...
    // 37 | struct K8sAnywhereTopology;
    //    | -------------------------- doesn't satisfy `K8sAnywhereTopology: Docker`
    // ...
    // 90 | invalid.deploy_docker(); // `T: Docker` bound unsatisfied
    //    |         ^^^^^^^^^^^^^ method cannot be called on `FailoverTopology<K8sAnywhereTopology>` due to unsatisfied trait bounds
    //    |
    // note: trait bound `K8sAnywhereTopology: Docker` was not satisfied
    //   --> src/main.rs:61:9
    //    |
    // 61 | impl<T: Docker + Send + Sync> Docker for FailoverTopology<T> {
    //    |         ^^^^^^                ------     -------------------
    //    |         |
    //    |         unsatisfied trait bound introduced here
    // note: the trait `Docker` must be implemented
}