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DSP-API Server Design Overview

Introduction

DSP-API's responsibilites are:

  • Querying, creating, updating, and deleting data
  • Creating, updating and deleting data models (ontologies)
  • Managing projects and users
  • Authentication of clients
  • Authorisation of clients' requests

DSP-API is developed with Scala and uses the Akka framework for message-based concurrency. It is designed to work with the Apache Jena Fuseki triplestore which is compliant to the SPARQL 1.1 Protocol. For file storage, it uses Sipi.

DSP-API Versions

  • DSP-API v2: the latest DSP-API that should be used.
  • DSP-API v1: has been removed after a long period of deprecation.

There is also an Admin API for administrating DSP projects.

Error Handling

The error-handling design has these aims:

  1. Simplify the error-handling code in actors as much as possible.
  2. Produce error messages that clearly indicate the context in which the error occurred (i.e. what the application was trying to do).
  3. Ensure that clients receive an appropriate error message when an error occurs.
  4. Ensure that ask requests are properly terminated with an akka.actor.Status.Failure message in the event of an error, without which they will simply time out (see Ask: Send and Receive Future).
  5. When a actor encounters an error that isn't the client's fault (e.g. a triplestore failure), log it, but don't do this with errors caused by bad input.
  6. When logging errors, include the full JVM stack trace.

A hierarchy of exception classes is defined in Exceptions.scala, representing different sorts of errors that could occur. The hierarchy has two main branches:

  • RequestRejectedException, an abstract class for errors that are the client's fault. These errors are not logged.
  • InternalServerException, an abstract class for errors that are not the client's fault. These errors are logged.

Exception classes in this hierarchy can be defined to include a wrapped cause exception. When an exception is logged, its stack trace will be logged along with the stack trace of its cause. It is therefore recommended that low-level code should catch low-level exceptions, and wrap them in one of our higher-level exceptions, in order to clarify the context in which the error occurred.

To simplify error-handling in responders, a utility method called future2Message is provided in ActorUtils. It is intended to be used in an actor's receive method to respond to messages in the ask pattern. If the responder's computation is successful, it is sent to the requesting actor as a response to the ask. If the computation fails, the exception representing the failure is wrapped in a Status.Failure, which is sent as a response to the ask. If the error is a subclass of RequestRejectedException, only the sender is notified of the error; otherwise, the error is also logged and rethrown (so that the KnoraExceptionHandler can handle the exception).

In many cases, we transform data from the triplestore into a Map object. To simplify checking for required values in these collections, the class ErrorHandlingMap is provided. You can wrap any Map in an ErrorHandlingMap. You must provide a function that will generate an error message when a required value is missing, and optionally a function that throws a particular exception. Rows of SPARQL query results are already returned in ErrorHandlingMap objects.

If you want to add a new exception class, see the comments in Exceptions.scala for instructions.

Transformation of Exception to Client Responses

The org.knora.webapi.KnoraExceptionHandler is brought implicitly into scope of pekko-http, and by doing so registered and used to handle the transformation of all KnoraExceptions into HttpResponses. This handler handles only exceptions thrown inside the route and not the actors. However, the design of reply message passing from actors (by using future2Message), makes sure that any exceptions thrown inside actors, will reach the route, where they will be handled.

API Routing

The API routes in the routing package are defined using the DSL provided by the pekko-http library. A routing function has to do the following:

  1. Authenticate the client.
  2. Figure out what the client is asking for.
  3. Construct an appropriate request message and send it to the appropriate responder.
  4. Return a result to the client.

To simplify the coding of routing functions, they are contained in objects that extend org.knora.webapi.routing.Authenticator. Each routing function performs the following operations:

  1. Authenticator.getUserADM is called to authenticate the user.
  2. The request parameters are interpreted and validated, and a request message is constructed to send to the responder. If the request is invalid, BadRequestException is thrown. If the request message is requesting an update operation, it must include a UUID generated by UUID.randomUUID, so the responder can obtain a write lock on the resource being updated.

The routing function then passes the message to a function in an API-specific routing utility: RouteUtilV2 or RouteUtilADM. This utility function sends the message to ResponderManager (which forwards it to the relevant responder), returns a response to the client in the appropriate format, and handles any errors.

Logging

Logging in DSP-API is configurable through logback.xml, allowing fine grain configuration of what classes / objects should be logged from which level.

The Akka Actors use Akka Logging while logging inside plain Scala Objects and Classes is done through Scala Logging.