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Background

Web-hooks would be used to notify external services of events in auth gear.

Proposed Designs

Events

Web-hook events are triggered when some mutating operation is performed by auth gear.

Each operation will trigger two events: BEFORE and AFTER:

  • BEFORE events would be triggered before the operation is performed; the operation can be aborted by web-hook handler.
  • AFTER events would be trigger after the operation is performed.

Additionally, a user_sync event is provided for synchronizing user state to self-managed database.

Both events has the same event payload.

Delivery

The events would be delivered to web-hook handlers as an HTTP POST request to the handler endpoint.

The handler endpoint can be specified as full URL or an absolute path. Absolute paths will be resolved to full URL using an inferred URL scheme and authority:

  • from HTTP request URL if the event is generated from it; otherwise,
  • from the tenant configuration.

Each event type can have multiple handlers; the order of deliveries is unspecified for AFTER events, and same order as in skygear.yaml for BEFORE events.

BEFORE events will always be delivered before AFTER events. BEFORE events will be delivered in well-defined order during a request. AFTER events will be delivered in unspecified order.

Web-hook handler should be idempotent, since AFTER events may be delivered multiple times due to retries.

Web-hook handler must return a status code within the 2XX range. Responding with status code outside the range, including 3XX & 5XX, would be considered a failed delivery.

BEFORE Events

BEFORE events would be delivered to web-hook handlers synchronously, right before committing the result of the operation into database.

Web-hook handler should respond with a JSON-formatted body to indicated whether the operation should be failed, for example:

// Allowed
{
    "is_allowed": true
}

// Disallowed
{
    "is_allowed": false,
    "reason": "the metadata does not match the required format.",
    "data": {
        "email": "invalid email format"
    }
}

If the operation is disallowed, a non-empty reason must be provided. Optional additional information can be included. If any of the delievery responses disallowed the operation, the operation is considered as failed, with the disallowing reasons and additional information as part of the error. For example:

{
    "error": {
        "name": "WebHookError",
        "code": 10000,
        "message": "Operation is disallowed by web-hook",
        "info": {
            "errors": [
                {
                    "reason": "the metadata does not match the required format.",
                    "data": {
                        "email": "invalid email format"
                    }
                }
            ]
        }
    }
}

Web-hook handler can request mutations, see mutation section for details.

The time spent in a BEFORE event delivery must not exceed 5 seconds, otherwise would be considered a failed delivery. Also, the total time spent in all deliveries of the event must not exceed 10 seconds, otherwise the operation would be considered failed. Both timeout are configurable in app config.

BEFORE events would not be persisted and their failed deliveries would not be retried.

The operation is considered as failed if any of the deliveries failed. A failed operation would not trigger AFTER events.

AFTER Events

AFTER events would be delivered to web-hook handlers asynchronously after the operation is performed (i.e. commited into database).

The time spent in an AFTER event delivery must not exceed 60 seconds, otherwise would be considered a failed delivery.

All AFTER events with web-hook handler are presisted into database, with minimum retention period of 30 days.

The response body of web-hook handler is ignored.

If any delivery failed, all deliveries will be retried after some time, regardless whether some deliveries may succeed. The retry will be performed with a variant of exponential back-off algorithm. If Retry-After HTTP header is present in the response, the delivery will not be retried before the specified time.

If the delivery continue to fail after 3 days from the time of first attempted delivery, the event is marked as permanently failed and will not be retried automatically.

Mutations

BEFORE events allow web-hook handler to request mutation on some fields before committing to database.

Web-hook handler cannot request mutation if the operation is disallowed. Web-hook handler should indicate the mutations to perform through its response, for example:

{
    "is_allowed": true,
    "mutations": {
        "metadata": {
            "username": "test"
        },
        "is_verified": false
    }
}
  • If a field is absent: no mutation would be performed for that field.
  • If a field is present: the field would be set to the provided value.

If failed to perform the mutations, the operation would be failed. If the operation failed, the mutations are rollbacked and not visible.

Mutations would not generate additional events.

The mutated value is be propagated along the next events.

Developer is responsible for ensuring correct order of event delivery. For example, in most case, developer would like mutating event-handlers (e.g. populating default value for some fields) before non-mutating event-handlers (e.g. validating field values).

user_sync

user_sync is a special event. It is delivered like an AFTER event.

When an API that would potentially update the user state (including user object, identity, or session) is used, this event would be generated, regardless whether the user state is actually updated.

This event can be used to synchronize user state to user-managed database.

Event Management

Alerts

If an event delivery is permanently failed, an ERROR level log entry is generated to notify developers.

Past Events

An API is provided to list past events. This API can be used to reconcile self-managed database with the failed events.

NOTE: BEFORE events are not persisted, regardless of success/failure.

Manual Re-delivery

Developer can manually trigger a re-delivery of failed event, bypassing the retry interval limit.

NOTE: BEFORE events cannot be re-delivered.

Security

Web-hook delivery process can be modeled as following:

 Int. Net. ┊    Internet    ┊      Internal Network
           ┊                ┊
 Auth Gear─┊─────┬──────────┊>Gateway────────────>Internal Handler
           ┊     │          └┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄
           ┊     └───────────>External Handler

Considering the authenticity, integrity and confidentiality of the process:

We assume Gateway -> Internal Handler data-path is secure with respect to the mentioned 3 properties; the admins of internal network (e.g. cluster admin) is responsible for its security.

To protect the data-paths Auth Gear -> Gateway and Auth Gear -> External Handler, we provide some security measures:

  • Require HTTPS: ensure integrity and confidentiality of the delivery
  • Signed Request: ensure authenticity of the delivery

HTTPS

We require all external web-hook handler HTTP endpoint to use HTTPS.

For internal handlers, network admin is responsible to ensure the request URL passed to auth gear is secure.

Signature

Each web-hook event request is signed with a secret key shared between auth gear and web-hook handler. Developer must validate the signature and reject requests with invalid signature to ensure the request is originated from auth gear.

For the detail on signature generation and validation, refer to #300.

For advanced end-to-end security scenario, some network admin may wish to use mTLS for authentication. We do not support this at the moment.

Considerations

Recursive Web-hooks

A ill-designed web-hook handler may be called recursively. For example, updating user metadata when handling after_user_update event.

Developer is responsible to ensure:

  • web-hook handlers would not be called recursively; or
  • recursive web-hook handlers have well-defined termination condition.

Delivery Reliablity

The main purpose of web-hook in auth gear is to allow external services to observe state changes in auth gear.

Therefore, AFTER events should be persistent, immutable, and delivered reliably; otherwise, external services may observe inconsistent changes.

It is not recommended to perform side-effect in BEFORE event handlers; otherwise, developer should consider how to compensate for the side-effect for possibility of operation failure. In general, use cases that require BEFORE events may instead consider:

  • use authorization policy to allow/deny request (TBD)
  • wrap auth gear functions to validate requests

Eventual Consistency

Fundamentally, auth gear with web-hooks is a distributed system. When web-hook handlers have side-effects, we need to choose between guaranteeing consistency or availability of the system (see appendix: CAP theorem).

We decided to ensure the availability of the system. To maintain consistency, developer should take eventual consistency into account when designing system.

Developer should regular check the past event list for unprocessed events to ensure system consistency.

We considered ensuring the consistency of the system through distributed transaction protocols (e.g. two-phase commit). However, its implementation requires much effort from both auth gear and web-hook handlers, and introduce a single point of failure (coordinator). We decided on availability instead.

Event Timing

There are four possible delivery timing of events: sync BEFORE, async BEFORE, sync AFTER, async AFTER.

Async BEFORE is mostly useless: the request may not be successful, and handler cannot affect the operation. Therefore, we do not offer async BEFORE events.

Sync AFTER cannot be used safely:

  • If it is not within the operation transaction, async AFTER can be used instead.
  • If it is within the operation transaction, then the transaction should rollback on web-hook delivery failure, otherwise async AFTER can be used instead. So:
    • If the handler has no side-effect, sync BEFORE can be used instead (e.g. custom validation)
    • If the handler has side-effect, it is subject to distributed transaction problem. We use eventual consistency so async AFTER should be used.

Therefore, we do not offer sync AFTER events.

Appendix

Overview of Request Lifecycle (including web-hooks)

  1. Receive request
  2. Decode & validate Request
  3. Begin transaction
  4. Perform operations
  5. Deliver BEFORE events to web-hook handlers
  6. If disallowed: rollback transaction & abort request
  7. If mutation requested: perform mutation
  8. Commit transaction
  9. Return response
  10. Deliver AFTER events to web-hook handlers

Web-hook Event Details

Web-hook Configuration

Web-hook Management API

Web-hook Use Cases

CAP Theorem

To simplify, the CAP theorem state that, a distributed data store can satify only two of the three properties simultaneously:

  • Consistency
  • Availability
  • Network Partition Tolerance

Since network partition cannot be avoided practically, distributed system would need to choose between consistency and availabilty. Most microservice architecture prefer availabilty over strong consistency, and instead application state is eventually consistent.

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