Strand

Postgres-native durable workflow engine for Swift 6.3.

No separate coordination service. No Redis. No Cassandra. Just Swift workers and Postgres.

struct OrderWorkflow: Workflow {
    typealias Input  = OrderInput
    typealias Output = ShipResult

    mutating func run(
        context: WorkflowContext<Self>,
        input: OrderInput
    ) async throws -> ShipResult {
        let charge = try await context.runActivity(
            ChargeCardActivity.self,
            input: .init(amount: input.amount)
        )
        return try await context.runActivity(
            ShipOrderActivity.self,
            input: .init(paymentID: charge.paymentID)
        )
    }
}

If a worker crashes mid-workflow the next worker that picks it up resumes from the last checkpoint. No work is duplicated, no state is lost.

Documentation

• Getting started — installation, first workflow, first worker
• Core concepts — execution model, checkpointing, determinism
• Activities — I/O, retries, heartbeats, timeouts
• Workflows — orchestration, fan-out, sleep, signals
• Scheduling — cron, intervals, catch-up
• Signals and events — external wakeup
• Retry strategies — backoff, non-retryable errors, deadlines
• Testing — integration test helpers
• Data pipelines — fan-out, multi-queue, crash recovery

Quick start

1. Start Postgres

# docker-compose.yml
services:
  db:
    image: postgres:18-alpine
    environment:
      POSTGRES_USER: strand
      POSTGRES_PASSWORD: strand
      POSTGRES_DB: strand_dev
    ports:
      - "5499:5432"

docker compose up -d
psql "postgresql://strand:strand@localhost:5499/strand_dev" -f strand.sql

2. Add to Package.swift

.package(url: "https://github.com/thoven87/strand", from: "0.1.0"),

3. Run

import Strand
import ServiceLifecycle

let postgres = PostgresClient(configuration: .init(
    host: "localhost", port: 5499,
    username: "strand", password: "strand",
    database: "strand_dev", tls: .disable
))
let client = StrandClient(postgres: postgres, queue: "default")
let worker = StrandWorker(
    postgres: postgres,
    options: WorkerOptions(queue: "default"),
    workflows: [OrderWorkflow.self],
    activities: [ChargeCardActivity(), ShipOrderActivity()]
)
let group = ServiceGroup(configuration: .init(
    services: [.init(service: postgres), .init(service: worker)],
    gracefulShutdownSignals: [.sigterm, .sigint]
))

// Start a workflow — returns a handle you can poll for the result:
let handle = try await client.startWorkflow(
    OrderWorkflow.self,
    input: OrderInput(amount: 99_00, orderID: "ord-1")
)

try await group.run()

Scheduling

Declare recurring schedules directly on StrandScheduler — they are upserted to the database when the service starts:

let scheduler = StrandScheduler(
    client: client,
    schedules: [
        .workflow(
            "daily-report",
            pattern: .daily(offset: "PT9H"),          // 09:00 UTC every day
            workflowType: DailyReportWorkflow.self,
            input: ReportInput()
        ),
        .workflow(
            "market-open",
            pattern: .cron("30 8 * * 1-5",
                           timezone: TimeZone(identifier: "America/New_York")!),
            workflowType: MarketOpenWorkflow.self,
            input: StrandVoid()
        ),
    ]
)

let group = ServiceGroup(configuration: .init(
    services: [
        .init(service: postgres),
        .init(service: worker),
        .init(service: scheduler),
    ],
    gracefulShutdownSignals: [.sigterm, .sigint]
))
try await group.run()

For schedules created at runtime (e.g. from an HTTP API), call client.schedule(name:pattern:workflowType:input:) directly — it is always a live database write.

See Scheduling for patterns, catch-up behaviour, time zones, and runtime management.

Examples

See Examples/ for complete runnable examples:

Example	What it shows
Greeting	Minimal activity + workflow
MultipleActivities	Sequential and parallel activities
Schedule	Cron and interval scheduling
ChildWorkflows	Fan-out orchestration

Requirements

• Swift 6.3+
• PostgreSQL 15+

Dependencies

Package	Version
PostgresNIO	1.32.2
Hummingbird	2.22.0
swift-service-lifecycle	2.11.0
swift-log	1.12.0
swift-metrics	2.10.1
swift-distributed-tracing	1.4.1
swift-collections	1.0.0+
swift-nio	2.77+

License

Apache 2.0