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Saga Pattern Realization With Camunda

This project includes an example implementation of the Saga pattern using Camunda. The example application represents a travel application that consists of three backend services: TravelService, HotelService and FlightService. For simplicity reasons, only the workflow for booking a trip has been implemented.

Start the Application

  1. Run ./gradlew clean build

  2. Execute docker-compose up --no-start

  3. Execute docker-compose start mysql

  4. Execute docker-compose start travelservice

  5. Execute docker-compose up

  6. Requesting trip bookings is now possible. Either use curl commands, the provided TravelApplication.json insomnia file, which includes different trip booking requests, or access the Swagger UI of the different services:

Service
URL to Swagger UI
TravelService http://localhost:8090/swagger-ui.html
HotelService http://localhost:8081/swagger-ui.html
FlightService http://localhost:8082/swagger-ui.html

An example for such a request:

TravelRequest
{
    "duration":
    {
        "start":"2021-12-01",
        "end":"2021-12-12"
    },
    "start":
    {
        "country":"Scotland",
        "city":"Stirling"
    },
    "destination":
    {
        "country":"Sweden",
        "city":"Stockholm"
    },
    "travellerName": "Max Mustermann",
    "boardType":"breakfast",
    "customerId":"1"
}

To simulate a Saga that fails because no hotel or no flight is available, use one of the following Strings as destination country in the trip booking request: 

"Provoke hotel failure"

"Provoke flight failure"

The services also provide a health and an info endpoint that show some information about the system like that the DB is up and running. These endpoints can be accessed via:

Service
URL to health endpoint
URL to info endpoint
TravelService http://localhost:8090/api/travel/monitor/health http://localhost:8090/api/travel/monitor/info
HotelService http://localhost:8081/api/hotels/monitor/health http://localhost:8081/api/hotels/monitor/info
FlightService http://localhost:8082/api/flights/monitor/health http://localhost:8082/api/flights/monitor/info

If you are on Windows or Mac, you sometimes have to replace localhost with the default IP of your docker machine (use docker-machine ip default to get this default IP).

Stop the Application

To stop the application and remove the created containers, execute the following command: 

docker-compose down --remove-orphans

General Saga Characteristics

External Compensation Trigger

In order to start the compensation of a currently running Saga externally, a BPMN error has to be created for a scheduled or still unfinished task. Before the BPMN error can be created, the task has to be fetched and locked if not an already existing worker has done that. To achieve that, the following request, supplemented with the missing information, has to be sent as POST request to the engine at: http://localhost:8090/engine-rest/external-task/fetchAndLock. Since the implementation uses mainly external tasks, the BPMN error will be created for an external task which means the request is sent to the engine's /external-task endpoint. If an existing worker has already fetched the respective task, this request can be skipped.

 {
      "workerId":"Any name, e.g. compensationProvoker",
      "maxTasks":2,
      "usePriority":true,
      "topics":
      [
         {
            "topicName": "Name of the topic where the task will be sent to, e.g. bookFlight",
            "lockDuration": 10000
         }
      ]
 }

Now that the respective task is locked and fetched, a BPMN error can be created. Therefore, the following request supplemented with the missing information has to be sent as POST request to the engine at: http://localhost:8090/engine-rest/external-task/{taskId}/bpmnError

{
      "workerId": "Name specified in previous request: compensationProvoker",
      "errorCode": "Error Code that triggers the compensation event (check the bpmn definition for that), e.g. FLIGHT_ERROR",
      "errorMessage": "Any message: Provoke compensation externally"
}

The TravelApplication.json insomnia file also includes these requests within the ExternalCompensationTrigger directory.

Monitor the Application

MySQL Database

The camunda database, with its different tables, can be accessed with the following information, which is also included in the docker-compose.yaml file:
User: mysqluser | Password: mysqlpw

Camunda's Cockpit

The cockpit can be accessed either just via http://localhost:8090/ or by using the whole path http://localhost:8090/camunda/app/welcome/default/#!/welcome.

The following credentials are needed in order to be able to access the cockpit:
Username: admin | Password: admin

If the respective values have been changed in the application.properties file of the TravelService the new values have to be used for the username and the password.

Log Files

Each service provides a log that contains some information about it. The logs can be accessed using the name of the relevant container. The different logs can be accessed using the following commands:

Log of Command to execute
TravelService docker logs travelservice_camunda
HotelService docker logs hotelservice_camunda
FlightService docker logs flightservice_camunda

By using the --follow supplement, it will be continued to stream the service's output to the console.

The logging level can be changed in the respective application.properties file.

Camunda_Implementations/Camunda

Last update: 2022-02-15
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