sim-time

The sim-time feature enables time simulation capabilities in es-entity, allowing you to accelerate time for testing and development purposes. This is particularly useful for testing time-dependent logic without having to wait for real time to pass.

Enabling sim-time

Add the sim-time feature to your es-entity dependency:

[dependencies]
es-entity = { version = "0.7", features = ["sim-time"] }

Configuration

The sim-time crate is configured through the TimeConfig struct. By default, sim-time operates in real-time mode. To enable simulation, you need to initialize it with a configuration:

extern crate es_entity;
extern crate tokio;
use es_entity::prelude::{sim_time, chrono};
use std::time::Duration;

#[tokio::main]
async fn main() {
    let config = sim_time::TimeConfig {
        realtime: false,
        simulation: Some(sim_time::SimulationConfig {
            // Start the simulation at a specific time
            start_at: chrono::Utc::now(),
            
            // Real milliseconds between simulation ticks
            tick_interval_ms: 10,
            
            // Simulated duration per tick
            tick_duration_secs: Duration::from_secs(86400), // 1 day per 10ms
            
            // Whether to switch to real-time when catching up to present
            transform_to_realtime: false,
        }),
    };

    // Initialize sim-time with the configuration
    sim_time::init(config);
}

Configuration Parameters

TimeConfig

• realtime: bool - When true, sim-time is deactivated and all time operations use real time. When false, simulation is enabled.
• simulation: Option<SimulationConfig> - The simulation configuration. Required when realtime is false.

SimulationConfig

• start_at: DateTime<Utc> - The starting time for the simulation. Defaults to the current time.
• tick_interval_ms: u64 - The real-world milliseconds between simulation ticks.
• tick_duration_secs: Duration - How much simulated time passes per tick.
• transform_to_realtime: bool - If true, the simulation will automatically switch to real-time mode once it catches up to the current time.

Usage

Once configured, sim-time provides several functions that work with simulated time:

extern crate es_entity;
extern crate tokio;
use es_entity::prelude::{sim_time, chrono};
use std::time::Duration;

#[tokio::main]
async fn main() {
    // Initialize sim-time with the example configuration
    let config = sim_time::TimeConfig {
        realtime: false,
        simulation: Some(sim_time::SimulationConfig {
            start_at: chrono::Utc::now(),
            tick_interval_ms: 10,
            tick_duration_secs: Duration::from_secs(86400), // 1 day per 10ms
            transform_to_realtime: false,
        }),
    };
    sim_time::init(config);

// Get the current simulated time
let current_time = sim_time::now();

// Sleep for a simulated duration
// With the example config (1 day = 10ms), this sleeps for ~0.04 real seconds
sim_time::sleep(Duration::from_secs(3600)).await; // Sleep for 1 simulated hour

// Set a timeout on an operation
async fn async_operation() -> Result<(), std::io::Error> { Ok(()) }
sim_time::timeout(Duration::from_secs(60), async_operation()).await;

// Wait until simulation catches up to real time
// (only relevant if transform_to_realtime is true)
sim_time::wait_until_realtime().await;
}

Effect on es-entity

When sim-time is enabled, it affects how es-entity handles timestamps:

1. Database Operations: The DbOp struct automatically caches the simulated time when the feature is enabled. This cached time is used instead of database NOW() for all write operations.

2. Event Timestamps: All events created during a transaction will use the same simulated timestamp, ensuring consistency.

3. Time-based Queries: Operations that depend on the current time will use the simulated time instead of real time.

Example: Testing Time-Dependent Logic

extern crate es_entity;
extern crate tokio;
extern crate sqlx;
extern crate serde;
extern crate anyhow;
use es_entity::prelude::*;
use std::time::Duration;
use es_entity::{EsEntity, EsEvent, EsRepo, TryFromEvents, IntoEvents, EsEntityError, EntityEvents};
use chrono::Datelike;

es_entity::entity_id! { SubscriptionId }

#[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
#[derive(EsEvent)]
#[serde(tag = "type")]
#[es_event(id = "SubscriptionId")]
enum SubscriptionEvent {
    Initialized { 
        id: SubscriptionId,
        expires_at: chrono::DateTime<chrono::Utc> 
    },
}

#[derive(Clone, EsEntity)]
struct Subscription {
    pub id: SubscriptionId,
    pub expires_at: chrono::DateTime<chrono::Utc>,
    pub events: EntityEvents<SubscriptionEvent>,
}

impl Subscription {
    pub fn is_expired(&self, now: chrono::DateTime<chrono::Utc>) -> bool {
        self.expires_at <= now
    }

    pub fn created_at(&self) -> chrono::DateTime<chrono::Utc> {
        // Get the timestamp from when this entity was first persisted
        self.events.entity_first_persisted_at().unwrap_or_else(|| sim_time::now())
    }
}

impl TryFromEvents<SubscriptionEvent> for Subscription {
    fn try_from_events(events: EntityEvents<SubscriptionEvent>) -> Result<Self, es_entity::EsEntityError> {
        let mut expires_at = chrono::Utc::now();
        for event in events.iter_all() {
            match event {
                SubscriptionEvent::Initialized { expires_at: exp, .. } => {
                    expires_at = *exp;
                }
            }
        }
        Ok(Self { id: events.id().clone(), expires_at, events })
    }
}

#[derive(Debug)]
struct NewSubscription {
    id: SubscriptionId,
    duration_days: i64,
}

impl IntoEvents<SubscriptionEvent> for NewSubscription {
    fn into_events(self) -> EntityEvents<SubscriptionEvent> {
        EntityEvents::init(
            self.id,
            [SubscriptionEvent::Initialized {
                id: self.id,
                expires_at: sim_time::now() + chrono::Duration::days(self.duration_days),
            }])
    }
}

#[derive(Clone, EsRepo)]
#[es_repo(entity = "Subscription", event = "SubscriptionEvent")]
struct SubscriptionRepo {
    pool: sqlx::PgPool,
}

#[tokio::main]
async fn main() -> anyhow::Result<()> {
    // Setup database connection
    let db_url = std::env::var("DATABASE_URL").unwrap_or_else(|_| "postgres://localhost/test".to_string());
    let pool = sqlx::PgPool::connect(&db_url).await.unwrap();
    let repo = SubscriptionRepo { pool };
    
    // Configure time to run 30 days per second
    let config = sim_time::TimeConfig {
        realtime: false,
        simulation: Some(sim_time::SimulationConfig {
            // Start simulation one year in the past
            start_at: chrono::Utc::now() - chrono::Duration::days(365),
            tick_interval_ms: 33,  // ~30 ticks per second
            tick_duration_secs: Duration::from_secs(86400), // 1 day per tick
            transform_to_realtime: false,
        }),
    };

    let start_time = chrono::Utc::now() - chrono::Duration::days(365);
    sim_time::init(config);

    // Create a subscription that expires in 30 days
    let subscription = repo.create(NewSubscription {
        id: SubscriptionId::new(),
        duration_days: 30,
    }).await?;

    // Verify that sim-time is working
    let created_at = subscription.created_at();
    
    // Verify sim-time is working by checking the entity was created in the simulated year/month
    assert_eq!(created_at.year(), start_time.year(), "Entity should be created in the simulated year");
    assert_eq!(created_at.month(), start_time.month(), "Entity should be created in the simulated month");
    
    // Verify that we're actually in the past (compared to real time)
    let real_now = chrono::Utc::now();
    assert!(created_at < real_now - chrono::Duration::days(300), "Entity creation time should be in the past");
    
    // The subscription should NOT be expired yet (30 days haven't passed in sim time)
    assert!(!subscription.is_expired(sim_time::now()));

    // Sleep for 30 simulated days (which takes ~1 real second with this config)
    sim_time::sleep(Duration::from_secs(30 * 86400)).await;

    // Check that the subscription is now expired
    let subscription = repo.find_by_id(subscription.id).await?;
    assert!(subscription.is_expired(sim_time::now()));

    Ok(())
}

Best Practices

1. Initialize Early: Call sim_time::init() before any other es-entity operations to ensure consistent time handling.

2. Use in Tests: The sim-time feature is primarily designed for testing. Consider using conditional compilation to only enable it in test builds:

   [dev-dependencies]
   es-entity = { version = "0.7", features = ["sim-time"] }
   

3. Consistent Time: All operations within a single database transaction will use the same timestamp, ensuring consistency in your event store.

4. Real-time Transformation: Use transform_to_realtime: true when you want to start a simulation in the past and have it automatically switch to real-time when it catches up. This is useful for replaying historical scenarios.