WSO2 EI Dataservice as Rest API with sequence calls - rest

Question

I'm new to wso2 and currently I'm trying to expose a mongodb data service as REST API. I followed this tutorial and exposed my data service as Rest resource like this:
<data>
<config>...</config>
<query id="count" useConfig="MongoDB">
<expression>myCollection.count()</expression>
<result outputType="json">{
"Documents": {
"Document": [
{
"Data": "$document"
}
]
}
}</result>
</query>
<operation name="count_op">
<call-query href="count"/>
</operation>
<resource method="GET" path="users/count">
<call-query href="count"/>
</resource>
</data>
Works fine, however when I tried to access the resource from my angular2 project, I stumbled upon the CORS problem. I read up some posts about that and figured I need to modify my Rest API with something like this:
<resource methods="OPTIONS" url-mapping="/*">
<inSequence>
<property action="set" name="HTTP_SC" scope="axis2"
type="STRING" value="200"/>
<property action="set" name="messageType" scope="axis2"
type="STRING" value="application/json"/>
<sequence key="rest_add_access_control_headers"/>
<respond/>
</inSequence>
<outSequence/>
<faultSequence>
<sequence key="rest_add_access_control_headers"/>
<respond/>
</faultSequence>
</resource>
I tried to integrate this code with my data service definition (the one above) but I can't get it working and it makes me wonder if data service exposed as REST resource are the same thing as Rest APIs? It seems I cannot invoke any elements or other mediators in the data service definition. Do I need to create an actual Rest API for this? If so, how can I use my mongo dataservice as a resource for my Rest API?
EDIT: The CORS error I'm getting from my angular2 project when trying to access the service is "No 'Access-Control-Allow-Origin' header is present on the requested resource." which is another topic. But it led me to edit my data service so I edited my data service resource element and added a new one:
<resource method="GET" path="users/count">
<inSequence>
<property action="set" name="HTTP_SC" scope="axis2" type="STRING" value="200"/>
<property action="set" name="messageType" scope="axis2" type="STRING" value="application/json"/>
<sequence key="rest_add_access_control_headers"/>
<respond/>
</inSequence>
<outSequence/>
<faultSequence>
<sequence key="rest_add_access_control_headers"/>
<respond/>
</faultSequence>
<call-query href="count"/>
</resource>
<resource method="OPTIONS" path="users/count">
<inSequence>
<property action="set" name="HTTP_SC" scope="axis2"
type="STRING" value="200"/>
<property action="set" name="messageType" scope="axis2"
type="STRING" value="application/json"/>
<sequence key="rest_add_access_control_headers"/>
<respond/>
</inSequence>
<outSequence/>
<faultSequence>
<sequence key="rest_add_access_control_headers"/>
<respond/>
</faultSequence>
</resource>
rest_add_access_control_headers sequence looks like this:
<?xml version="1.0" encoding="UTF-8"?>
<sequence name="rest_add_access_control_headers" xmlns="http://ws.apache.org/ns/synapse">
<property name="Access-Control-Allow-Origin" scope="transport"
type="STRING" value="*"/>
<property name="Access-Control-Allow-Methods" scope="transport"
type="STRING" value="GET,POST,PUT,DELETE,OPTIONS"/>
<property name="Access-Control-Allow-Headers" scope="transport"
type="STRING" value="origin, content-type, accept, Authorization"/>
</sequence>
The first seems to be fine but the second one makes my dataservice faulty, giving this error in managemenet console:
DS Code: UNKNOWN_ERROR Source Data Service:- Name: mongodb_dataservice Location: \MongoDB.dbs Description: N/A Default Namespace: http://ws.wso2.org/dataservice Nested Exception:- java.lang.NullPointerException at org.wso2.carbon.dataservices.core.DataServiceFactory.createDataService(DataServiceFactory.java:207) at org.wso2.carbon.dataservices.core.DBDeployer.createDBService(DBDeployer.java:797) at org.wso2.carbon.dataservices.core.DBDeployer.processService(DBDeployer.java:1152) at org.wso2.carbon.dataservices.core.DBDeployer.deploy(DBDeployer.java:201) at org.apache.axis2.deployment.repository.util.DeploymentFileData.deploy(DeploymentFileData.java:136) at org.apache.axis2.deployment.DeploymentEngine.doDeploy(DeploymentEngine.java:807) at org.apache.axis2.deployment.repository.util.WSInfoList.update(WSInfoList.java:144) at org.apache.axis2.deployment.RepositoryListener.update(RepositoryListener.java:377) at org.apache.axis2.deployment.RepositoryListener.checkServices(RepositoryListener.java:254) at org.apache.axis2.deployment.RepositoryListener.startListener(RepositoryListener.java:371) at org.apache.axis2.deployment.scheduler.SchedulerTask.checkRepository(SchedulerTask.java:59) at org.apache.axis2.deployment.scheduler.SchedulerTask.run(SchedulerTask.java:67) at org.wso2.carbon.core.deployment.CarbonDeploymentSchedulerTask.runAxisDeployment(CarbonDeploymentSchedulerTask.java:93) at org.wso2.carbon.core.deployment.CarbonDeploymentSchedulerTask.run(CarbonDeploymentSchedulerTask.java:138) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) Caused by: java.lang.NullPointerException at org.wso2.carbon.dataservices.core.engine.DataService.init(DataService.java:352) at org.wso2.carbon.dataservices.core.DataServiceFactory.createDataService(DataServiceFactory.java:190) ... 20 more
Again, I'm not sure if I'm even supposed to put such code in data service definition since the code was meant to be for Rest API definition, the one where you use sequences, mediators etc...
How can I consume my dataservice with an ESB API? Would you please provide an example on how to do that ? I did some research but couldn't find anything.

Answer 1

May I know why you are unable to invoke the data service? Are you getting any error while invocation? if you could provide the logs it will be helpful.
Moreover DSS is used to fetch data from DB ,The best practice is to provide an ESB API which will consume this service rather than exposing DSS service to outside world directly