Show HN: Restate – low-latency durable workflows for JavaScript/Java, in Rust

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As regular functions and services, in your existing infrastructure. On FaaS, K8s, servers, containers. Self-hosted or fully managed. Restate meets you where you are.

Easy solutions for common challenges

Workflows as code

Durable Execution ensures code runs reliably to the end, even in the presence of failures.

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Failures and errors are automatically retried (unless labeled as terminal errors)Functions can memoize the results of code blocks, and actions like RPC, in a journal. Completed steps are not re-executed during retries, but replayed from the journal.Workflows are built with regular code and control flow, no custom DSLs needed.Durable sleeps let code wait and suspend for up to months

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export default restate.service({
name: “roleUpdate”,
handlers: {
applyRoleUpdate: async (ctx, update)=> {
const { userId, role, permissions }=update;
const applied=await ctx.run(“apply new role”, ()=>
applyUserRole(userId, role)
);
if (!applied) {
return;
}
for (const permission of permissions) {
await ctx.run(“apply permission”, ()=>
applyPermission(userId, permission)
);
}
}
}
});

@Service
public class RoleUpdateService {

@Handler
public void applyRoleUpdate(Context ctx, Update update) {

boolean success=ctx.run(“apply new role”, BOOLEAN,
() -> applyUserRole(update.getUserId(), update.getRole()));

if (!success) {
return;
}

for (String permission : update.getPermissions()) {
ctx.run(“apply permission”,
() -> applyPermission(update.getUserId(), permission));
}
}
}

API calls and webhooks

Reliably join synchronous code and async events like webhooks

Webhooks/events are persisted in Restate’s log and reliably delivered to servicesPersistent Promises/Futures easily join synchronous and asynchronous code pathsDurable execution ensures reliable completion, whether webhooks come after milliseconds or months, and avoid re-execution of completed steps.

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@Service
public class PaymentService {
@Handler
public void processPayment(Context ctx, PaymentRequest request) {
var webhookFuture=ctx.awakeable(SERDE);
var payment=ctx.run(“Stripe call”, SERDE, () -> submitPayment(
request, Map.of(“restate_callback_id”, webhookFuture.id())
));
if (payment.getStatus().equals(“processing”)) {
// synchronous response inconclusive, await webhook response
var updatedPayment=webhookFuture.await();
verifyPayment(updatedPayment);
} else {
verifyPayment(payment);
}
}
@Handler
public void processWebhook(Context ctx) {
var paymentEvent=verifyAndParseEvent(ctx.request());
String callbackId=paymentEvent.getMetadata().get(“restate_callback_id”);
ctx.awakeableHandle(callbackId).resolve(SERDE, paymentEvent);
}
}

Asynchronous Tasks

All functions invoked through Restate are executed durably and asynchronous.

Deploy async functions serverless or as containers or processes.Call functions synchronously, async, or delayed. Re-attach and await from anywhere.Build async patterns like fan-out, fan-in, task chains, and subtasks simply with function calls and Futures/Promises.Use persistent timers to schedule tasks into the future.Use fine-grained virtual queues (via virtual objects) to enforce strict task order and concurrency

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// —— service (=worker) ——
const asyncTaskService=restate.service({
name: “taskWorker”,
handlers: { processPayment }
});
// —— client ——
const rs=clients.connect({ url: process.env.RESTATE_URL });
const taskWorker=rs.serviceSendClient({ name: “taskWorker” });
// submit the payment task
app.post(‘/charge/:paymentId’, async (req, res)=> {
const taskHandle=await taskWorker.processPayment(
{ request: req.params },
SendOpts.from({ idempotencyKey: req.params.paymentId })
);
res.json(taskHandle);
});
// await the payment task
app.get(‘/status’, async (req,res)=> {
const taskHandle=req.body.json();
const paymentResult=await restate.result(taskHandle);
res.join(paymentResult);
});

// — start payment task —
server.createContext(“/charge”, httpExchange -> {
PaymentRequest req=parsePaymentRequest(httpExchange);
SendResponse handle=AsyncTaskServiceClient
.fromIngress(RESTATE_URI)
.send()
.processPayment(req, idempotencyKey(req.getPaymentId()));

respondJson(httpExchange, handle);
});
// — connect to payment result —
server.createContext(“/status”, httpExchange -> {
String handle=parseToHandle(httpExchange);
String response=IngressClient.defaultClient(RESTATE_URI)
.invocationHandle(handle, STRING)
.attach();
respond(httpExchange, response);
});

Stateful Event Processing

Process events (for example from Kafka) with durable functions as event handlers and get fine-grained retries and workflow-as-code semantics.

No queue subscriptions, no manual offset management, scaling, or balancingDeploy the event processing logic as serverless functions on FaaSKeep exactly-once state, delay events, run multiple asynchronous steps or API calls.Restate’s queue-per-key semantics mean no more head-of-the-line waiting effects

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@VirtualObject
public class EventEnricher {
static final StateKey USER=StateKey.of(“user”, of(User.class));
@Handler
public void userEvent(ObjectContext ctx, User event) {
ctx.set(USER, event);
// time box 100 ms to collect features before emitting result
EventEnricherClient.fromContext(ctx, ctx.key())
.send(ofMillis(100)).emit();
}
@Handler
public void featureEvent(ObjectContext ctx, Feature event) {
User user=ctx.get(USER).orElse(new User());
user.addFeature(event);
ctx.set(USER, user);
}
@Handler
public void emit(ObjectContext ctx) {
send(ctx.key(), ctx.get(USER));
ctx.clearAll();
}
}

Durable Signals

Create workflows and event handlers that reliably handle external signals, events, human input.

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Use durable Promises/Futures to intuitively model signals and conditionsCreate signals from RPCs, webhooks, or Kafka eventsSignals and events are persisted by Restate, no need for a queue

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export default workflow({
name: “verify”,
handlers: {
run: async (ctx, { email })=> {
const secret=ctx.run(“generate secret”, ()=>
crypto.randomUUID()
);
await ctx.run(“send email”, ()=> sendEmail({ email, secret }));

const clickSecret=await ctx.promise(“email.clicked”);
return clickSecret==secret;
},
click: (ctx, { secret })=> {
ctx.promise(“email.clicked”).resolve(secret);
},
},
});

@Workflow
public class SecretVerifier {

static final DurablePromiseKey EMAIL_CLICKED=
DurablePromiseKey.of(“email_clicked”, JsonSerdes.STRING);

@Workflow
public boolean run(WorkflowContext ctx, Email email) {
String secret=ctx.random().nextUUID().toString();
ctx.run(“send email”,
() -> sendEmailWithLink(email, secret));

String clickSecret=ctx.promise(EMAIL_CLICKED).awaitable().await();
return clickSecret.equals(secret);
}

@Handler
public void click(SharedWorkflowContext ctx, String secret) {
ctx.promiseHandle(EMAIL_CLICKED).resolve(secret);
}
}

Idempotency

Add idempotency to any RPC- or event handler.

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Every RPC- and event handler call accepts an idempotency keyUse idempotency keys to re-attach to an ongoing invocationCalls from within a durable execution context are automatically idempotent

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server.createContext(“/reserve”, httpExchange -> {
ReservationRequest req=parseRequest(httpExchange.getRequestBody());
// derive an idempotency key from the parameters
var idempotencyOps=CallRequestOptions.DEFAULT
.withIdempotency(req.getReservationId());
// add idempotency opts to the request to let the service automatically
// fuse repeated requests
Reservation reservation=ProductServiceClient
.fromIngress(RESTATE_RUNTIME_ENDPOINT)
.reserve(req.getProduct(), idempotencyOps);
sendResponse(httpExchange, reservation);
});

Sagas

Implements robust sagas and compensation patterns: long-running transactions that undo previous actions when they need to abort and roll back.

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Reliably pick up after failures to trigger compensationsEnsure compensations happen even upon failures during the compensation phaseUse standard Exception/Error mechanisms and control flow rather than complex DSLs.

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async function reservation(ctx, products) {
const reservations=[];
try {
for (const product of products) {
const reservation=await ctx.run(`reserve ${product}`,
()=> reserve(product));
reservations.push(reservation);
}
} catch (error) {
if (error instanceof TerminalError) {
for (const reservation of reservations) {
await ctx.run(“undo reserve”, ()=>
cancelReservation(reservation));
}
}
throw error;
}
}

@Handler
public void reserveAllProducts(Context ctx, Product[] products) {
final List reservations=new ArrayList();
try {
for (Product product : products) {
Reservation res=ctx.run(“Reserve ” + product.getId(),
RESERVE_SERDE, () -> reserve(product)
);
reservations.add(res);
}
} catch (TerminalException e) {
reservations.forEach(res -> {
ctx.run(“Undo reservation”, () -> cancelReservation(res));
});
throw e;
}
}

State machines

Create consistent and scalable State Machines without databases or transactions
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Run millions of State Machines that maintain state directly in the context of their handlersState changes commit atomically with function execution, for rock-solid consistencySingle-writer semantics for a dead simple concurrency model. A virtual queue per state machine for efficiency and scalability.State transition can be workflows with all the features from durable execution

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@VirtualObject
public class PaymentStateMachine {
@Handler
public String makePayment(ObjectContext ctx, PaymentRequest payment) {
String paymentId=ctx.key();
switch (ctx.get(STATE_STATUS).orElse(NEW)) {
case CANCELLED: return paymentId + ” was cancelled before”;
case SUCCESS: return paymentId + ” was previously completed”;
}
wireFunds(payment);
ctx.set(STATE_STATUS, SUCCESS);
ctx.set(STATE_PAYMENT_REQUEST, payment);
return paymentId + ” was successfully processed”;
}
@Handler
public void cancelPayment(ObjectContext ctx) {
Status status=ctx.get(STATE_STATUS).orElse(NEW);
if (status==SUCCESS) {
PaymentRequest payment=ctx.get(STATE_PAYMENT_REQUEST).get();
refund(payment);
}
ctx.set(STATE_STATUS, CANCELLED);
}
}