Maciej Laskowski edited this page Sep 27, 2018 · 11 revisions


! Note
Service Knot introduced Service Adapter as an extension point. Service Knot is replaced with a stand-alone module, Knot.x Data Bridge, that defines DataSourceAdapter outside Knot.x Core.
Action Knot introduced Action Adapter as an extension point. It is still a part of Knot Core but we plan to extract it also to dedicated module.
The abstract adapter is removed from Knot.x Core, only Knots are Knot.x Core extension points.
@Deprecated: As of release 1.3.1, replaced by Knot.x Data Bridge.

Adapters are modules which are responsible for communication between Knot.x (exactly Knots) and external services.


How does it work?

Adapters can be thought as extension points where project specific logic appears. With custom Knots they provides very flexible mechanism to inject project specific requirements.

We recommend to create a dedicated Adapter every time some service-level business logic or service response adaption to other format is required. E.g. we need to inject the data directly form the database.

Types of adapters

Knot.x Core by default introduces two types of Adapters connected with Knot implementations:

Knot.x comes with a generic implementation of Service Adapter, that enables communication with external services using HTTP Protocol (only GET requests). This Hello Rest Service Tutorial contains an example of how to integrate external Web based service data into your webpage. See also Http Service Adapter for more information. Please note, that this implementation is very generic and we recommend to create project-specific Adapters for any custom requirements.

Action Adapters are project specific in terms of error handling and redirection mechanisms. Knot.x Core is not going to provide any generic Action Adapters.

For custom Knots we can introduce custom Adapter types. As far as Knots must follow Knot contract, Adapters are coupled with Knot directly so they can define their custom request, response or configuration. The communication between Knot and Adapter can be custom too.

Communication contract between Service Knot and Http Service Adapter is defined by:

  • AdapterRequest input,
  • AdapterResponse output.

Adapter Request

The table below shows all the fields in the AdapterRequest - the communication model between Knot.x Service Knot and Service Adapters.

Name Type Mandatory Description
clientRequest.path String client request url, e.g. /services/mock/first.json
clientRequest.method HttpMethod client request method, e.g. GET, PUT, etc.
clientRequest.headers MultiMap client request headers
clientRequest.params MultiMap client request parameters
params JsonObject JsonObject with additional params that can be passed via configuration file, e.g. "params": { "example": "example-value" }
adapterParams JsonObject JsonObject with additional adapter parameters that can be set in the form of data-knotx-adapter-params in the snippet, e.g. data-knotx-adapter-params='{"myKey":"myValue"}'

Adapter Response

The table below shows all the fields in the AdapterResponse - an object returned by the Adapter to the Service Knot.

Name Type Mandatory Description
clientResponse.statusCode int status code of service response, e.g. 200, 302, 404
clientResponse.headers MultiMap client response headers
clientResponse.body Buffer final response body
signal String defines how original request processing should be handled (currently used only by Action Knot), e.g. next

How to configure?

The Adapter API specifies an abstract class - AdapterConfiguration to handle JSON configuration support. This abstraction can be used while implementing a custom Adapter but it is not required. Every Adapter must be exposed with a unique Event Bus address - that's the only obligation (as is the case with Knots). Please see an example configuration for Http Service Adapter

How to implement your own Adapter?

Knot.x provides the maven archetypes to generate custom Adapters. It is the recommended way to create your own Adapter.

An Adapter logic is executed on a Vert.x event loop. The Vert.x Golden Rule says that the code should never block the event loop. So all time-consuming operations should be coded in an asynchronous way. Default Knot.x Adapters use RxJava which is a popular library for composing asynchronous and event-based programs using observable sequences for the Java VM. RxJava introduce a Reactive Programming that is a development model structured around asynchronous data streams.

Reactive programming code first requires a mind-shift. You are notified of asynchronous events. Then, the API can be hard to grasp (just look at the list of operators). Don’t abuse, write comments, explain, or draw diagrams. RX is powerful, abusing it or not explaining it will make your coworkers grumpy. Read more

Implementation of an Adapter does not require knowledge of how to communicate via the Vert.x event bus. It's wrapped by the Vert.x Service Proxy functionality so any new implementation can focus on the business logic of the Adapter.

In order to implement an Adapter generate a new Adapter module using maven archetype:

mvn archetype:generate -DarchetypeGroupId=io.knotx.archetypes -DarchetypeArtifactId=knotx-adapter-archetype -DarchetypeVersion=X.Y.Z

Note that, the Adapter archetype generates not only a skeleton of your custom Adapter, but also all the configuration files that's required to run a Knot.x instance. More details about the Knot.x deployment can be found in the deployment section.

Archetype generates 3 important java files:

  • ExampleServiceAdapterConfiguration a simple POJO with configuration of the Adapter,
  • ExampleServiceAdapterProxy implement your business logic here in the processRequest() method with the return type of Observable<AdapterResponse> (promise of the AdapterResponse).
  • ExampleServiceAdapter that extends AbstractVerticle. It will simply read the configuration and register your AdapterProxy implementation at the provided address.

The AbstractAdapterProxy class provides the following methods that you can extend in your implementation:

  • Observable<AdapterResponse> processRequest(AdapterRequest message) method that consumes AdapterRequest messages from Knot and returns AdapterResponse object as rx.Observable
  • Optionally, AdapterResponse getErrorResponse(AdapterRequest request, Throwable error) method which handles any Exception thrown during processing, and is responsible for preparing proper AdapterResponse on such situations. By default AbstractAdapterProxy implements this method, and returns AdapterResponse with the ClientResponse object having 500 status code and the error message in response body.
! Note
Please note that while this section focuses on the Java language specifically, it's not the only choice you have. Thanks to the polyglot nature of Vert.x, you can implement your Adapters and Knots using other languages.
! Note
Besides the Verticle implementation itself, a custom implementation of your Adapter must be built as a Knot.x module in order to be deployed as part of Knot.x. Follow the Knot.x Modules documentation in order to learn how to make your Adapter a module.

Building and running example Adapter

To run the extension:

  1. Download the Knot.x fat jar and it to the apps folder.
  2. Build the extension using mvn package
  3. Copy custom Adapter fat jar from the target directory into the apps directory
  4. Execute the bash script. You will see output similar to the following:
2017-08-04 15:10:21 [vert.x-eventloop-thread-2] INFO  i.k.r.FilesystemRepositoryConnectorVerticle - Starting <FilesystemRepositoryConnectorVerticle>
2017-08-04 15:10:21 [vert.x-eventloop-thread-3] INFO  i.k.s.FragmentSplitterVerticle - Starting <FragmentSplitterVerticle>
2017-08-04 15:10:21 [vert.x-eventloop-thread-1] INFO  io.knotx.server.KnotxServerVerticle - Starting <KnotxServerVerticle>
2017-08-04 15:10:21 [vert.x-eventloop-thread-0] INFO  i.k.a.example.ExampleServiceAdapter - Starting <ExampleServiceAdapter>
2017-08-04 15:10:21 [vert.x-eventloop-thread-4] INFO  i.k.k.a.FragmentAssemblerVerticle - Starting <FragmentAssemblerVerticle>
2017-08-04 15:10:21 [vert.x-eventloop-thread-7] INFO  i.k.k.t.HandlebarsKnotVerticle - Starting <HandlebarsKnotVerticle>
2017-08-04 15:10:21 [vert.x-eventloop-thread-6] INFO  i.k.knot.action.ActionKnotVerticle - Starting <ActionKnotVerticle>
2017-08-04 15:10:21 [vert.x-eventloop-thread-5] INFO  i.k.knot.service.ServiceKnotVerticle - Starting <ServiceKnotVerticle>
2017-08-04 15:10:22 [vert.x-eventloop-thread-1] INFO  io.knotx.server.KnotxServerVerticle - Knot.x HTTP Server started. Listening on port 8092
2017-08-04 15:10:22 [vert.x-eventloop-thread-0] INFO  i.k.launcher.KnotxStarterVerticle - Knot.x STARTED

                Deployed 3bd0365c-10ba-4a53-a29c-59b4df06eaff [knotx:io.knotx.FragmentSplitter]
                Deployed defc43bc-ffe0-40cf-8c0c-1be06c5e8739 [knotx:com.example.adapter.example.ExampleServiceAdapter]
                Deployed 6891d9d6-bcfc-42fa-8b21-bd4ea11154da [knotx:io.knotx.FragmentAssembler]
                Deployed 59a2967f-0d87-4dea-ad37-b8fe1dafb898 [knotx:io.knotx.FilesystemRepositoryConnector]
                Deployed b923df1c-f8f9-4615-9b9e-4fba3806a575 [knotx:io.knotx.ActionKnot]
                Deployed c135524e-b4b1-452c-b8e7-628dfc58195d [knotx:io.knotx.ServiceKnot]
                Deployed 93ea2509-e045-49f8-9ddb-fc167e16f020 [knotx:io.knotx.HandlebarsKnot]
                Deployed 880116cf-54a7-4849-954e-dfb4eb536685 [knotx:io.knotx.KnotxServer]
  1. Open a page: http://localhost:8092/content/local/template.html in your browser to validate a page displays value Hello Knot.x.

How to handle blocking code in your own Adapter?

The easiest way to handle a blocking code inside your Adapter is to deploy it as a Vert.x worker. No change in your code is required.

To do so, you need to override configuration of your verticle and set verticle options to be deployed in workers pool via DeploymentOptions.

config.myAdapter {
  config {}
  options.worker: true

Now in your Knot.x instance log file you should see

2018-01-08 10:00:16 [vert.x-worker-thread-0] INFO  com.example.SomeAdapter - Starting <SomeAdapter>

For more information about deployment options of Worker verticles see Vert.x documentation.

How to implement your own Adapter without Rx Java?

Extending AbstractAdapterProxy is the recommended way to implement your custom Adapter. But still you can resign from this approach and implement your custom Adapter with Vert.x handlers (without using RxJava). The only one thing to change is to implement AdapterProxy instead of extending AbstractAdapterProxy. Then you need to implement a method void process(AdapterRequest request, Handler<AsyncResult<AdapterResponse>> result) where you should implement your custom Adapter business logic.

Adapters common library

For many useful and reusable Adapters concept, please check our knotx-adapter-common module. You will find there support for placeholders and http connectivity.

How to run a custom Adapter with Knot.x

Please refer to Deployment section to find out more about deploying and running a custom Adapters with Knot.x.

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