Hook0 Kotlin SDK

Send and verify webhooks, blocking or suspending, inside whichever runtime you brought

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What is this?

The Kotlin SDK for Hook0, the open source Webhooks-as-a-Service platform for SaaS applications. It sends events, declares the event types your application uses, verifies the signature of a webhook you receive, and calls every operation the API declares through generated, documented types.

Every call comes twice: one that blocks, and one that suspends. The suspending half is written against kotlin.coroutines, the language's own, so it suspends inside whichever coroutine runtime you brought: kotlinx.coroutines, Ktor's, Spring's, or none. It brings nothing with it, and kotlin-stdlib is the only thing it declares.

Features

• Send events - under an ID the client mints, so a retry cannot duplicate one
• Declare event types - upsert the ones your application emits, in one call
• Verify signatures - HMAC-SHA256 over a bilateral clock window
• The whole API, typed - data classes for schemas, enum classes for closed lists, one exception per problem
• Bounded everywhere - data class options, so copy names the one bound you meant
• Nothing read through !! - a member the API does not declare required is a nullable type, everything else is not

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Quick Start

1. Install

Not on Maven Central yet. The com.hook0 namespace has not been claimed on the Central Portal, which is not something a pipeline can do; ci/release-no-publish-job.toml records what is missing. The pom already carries everything else a Central release needs, under a release profile. Until the namespace is claimed, build the jar from a checkout and install it into your local repository.

git clone https://gitlab.com/hook0/hook0.git
mvn -f hook0/clients/kotlin/pom.xml install

That puts com.hook0:hook0-client-kotlin:2.0.3 in ~/.m2, where your own build resolves it:

<dependency>
  <groupId>com.hook0</groupId>
  <artifactId>hook0-client-kotlin</artifactId>
  <version>2.0.3</version>
</dependency>

Kotlin 2.4 and Java 21 or later.

2. Send an event

Hook0Client("https://app.hook0.com/api/v1", applicationId, token).use { client ->
  val sent: UUID = client.sendEvent(
    Event(
      eventType = "billing.invoice.paid",
      payload = """{"invoice":"in_123"}""",
      payloadContentType = "application/json",
      labels = mapOf("environment" to "production")
    )
  )
}

3. Verify a webhook you receive

Webhooks.verify(
  request.getHeader("X-Hook0-Signature"),
  rawBody,
  headersAsTheyArrived,
  subscriptionSecret,
  Duration.ofMinutes(5)
)

The clock window is bilateral, so a delivery dated too far ahead is refused exactly like one dated too far behind, because a window that only looked backwards is one a sender widens by dating its own delivery in the future. A header the signature covers but the request did not carry is refused before any code is computed.

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Configuration

Every bound one send is held to is yours to set, and every one has a default.

Bound	Default	What it holds back
maxAttempts	4	requests one send issues, capped at 16 whatever a policy says
initialBackoff	100 ms	the ceiling of the wait before the first retry
maxBackoff	2 s	the ceiling no single wait between attempts crosses
maxTotalDelay	5 s	the budget every wait of one send shares
requestTimeout	10 s	how long one attempt is given
maxPayloadBytes	1 MiB	the payload, refused before a socket is opened
maxResponseBytes	8 MiB	the body read off a socket
maxHeadBytes	16 KiB	the head of an answer, every line taken together
maxResponseHeaders	64	header lines one answer may carry
maxHeaderBytes	64 KiB	one header line

Every default comes from clients/conformance/bounds.json, the corpus every Hook0 SDK reads. A number changed there fails every SDK still carrying the old one, so no two of them can bound different things.

The last three bound what the other end may cost you. A server that is broken or hostile can otherwise stream a head, a header or a body of any length into your process.

val options = Options.defaults().copy(
  retryPolicy = RetryPolicy(
    maxAttempts = 4,
    initialBackoff = Duration.ofMillis(100),
    maxBackoff = Duration.ofMillis(2000),
    maxTotalDelay = Duration.ofMillis(5000)
  ),
  requestTimeout = Duration.ofSeconds(10),
  maxPayloadBytes = 1024 * 1024,
  maxResponseBytes = 8L * 1024 * 1024
)

val client = Hook0Client("https://app.hook0.com/api/v1", applicationId, token, options)

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Usage

Sending is idempotent, and retried

sendEvent sends every event under an ID it knows, either the one set on the event or a UUIDv7 it mints when the event carries none. Passing no ID does not mean the ID comes from Hook0. The value comes from the client, travels with the request, and is what sendEvent answers.

That is what makes a retry safe. Hook0 keys events on their ID, so a request repeated after a network failure or a server error ingests the event once rather than twice. Without a client-chosen ID, the repeated request would create a second event and deliver it to every subscriber.

Retrying is limited to what could end differently. A request that got no answer, a server error and an instance saying it is being reached faster than it accepts are all retried. A 429 naming a spent quota is not, because a quota clears when a plan changes or a day turns, and no send can wait for that. A Retry-After the answer carries is honoured, clamped to what is left of the delay budget. A retried request Hook0 answers with EventAlreadyIngested reports success, since an earlier attempt of that same send reached the API. The same answer to a first attempt is a genuine conflict, and is reported as an error.

Declaring the event types you use

val created: List<String> = client.upsertEventTypes(
  listOf("billing.invoice.paid", "billing.invoice.voided")
)

Only the ones your application does not declare yet are created, and those are what comes back.

Calling the rest of the API

Every operation the API declares is a method of a generated group, one group per entity.

val application = ApplicationsApi(client.transport).get(applicationId)

val suspended = ApplicationsSuspendingApi(client.transport).get(applicationId)

The compiler tells you when the API grows a problem

Each problem the API can report is an exception of its own under a sealed class ProblemException, so a catch may name one problem, or the base, or a when may match over the closed set and be told by the compiler the day the API grows another.

Headers, however they arrived

Verification takes them as a Map<String, String> or as a List<Pair<String, String>>, since a request may carry the same header twice.

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Development

clients/kotlin/src/main/kotlin/com/hook0/kotlin/generated/ is written by hook0-sdkgen from the OpenAPI snapshot the API commits, and is rewritten whole on every regeneration. A hand edit there is reverted the next time anyone regenerates, and the drift guard says so before that. Change the generator, then run:

UPDATE_SDK=kotlin cargo test -p hook0-sdkgen sdk_targets

Everything beside it, the transport, the retry loop, the signature verification, the JSON reader and the suspending machinery, is hand-written and never regenerated, and so is test/kotlin, which sits outside src on purpose, since a test file under a generated tree would be deleted without a word at the next regeneration.

What a send retries, the bounds it is held to and how a signature is verified are dictated by the shared corpus at clients/conformance, which every SDK's suite reads, so a verdict changed there fails this client until it agrees again.

Every case runs against a real Hook0 over a loopback socket. Nothing here stands in for a part of the client.

mvn ktlint:check verify

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License

The Hook0 Kotlin SDK is free and open source, released under the MIT License. Use it, change it, ship it, in open source and in commercial work alike, as long as the copyright notice travels with it.

Hook0 itself is open source too. Read what Hook0 is, visit hook0.com, join the community, or write to support@hook0.com.

Maintained by David Sferruzza and François-Guillaume Ribreau.