Old notifications pushed immediately when subscription is created - fiware-orion

Question

I'm using Fiware Orion Context Broker version 0.20.
I notice that when I create a context subscription, my provided endpoint immediately gets notified about changes to the corresponding context elements that happened before I created the subscription.
To clarify: (note: I used these steps with a clean database)
I started the accumulator from the test package /usr/share/contextBroker/tests/accumulator-server.py 1028 /accumulate on
Created a context element using http://localhost:1026/v1/updateContext
{
"contextElements": [
{
"type": "Room",
"isPattern": "false",
"id": "Room1",
"attributes": [
{
"name": "temperature",
"value": "20"
}
]
}
],
"updateAction": "APPEND"
}
Then I created the subscription using http://localhost:1026/v1/subscribeContext
{
"entities": [
{
"type": "Room",
"isPattern": "true",
"id": ".*"
}
],
"attributes": [
"temperature"
],
"reference": "http://localhost:1028/accumulate",
"duration": "P1M",
"notifyConditions": [
{
"type": "ONCHANGE",
"condValues": [
"temperature"
]
}
],
"throttling": "PT5S"
}
I immediately receive the following content in the accumulator
POST http://localhost:1028/accumulate
Content-Length: 472
User-Agent: orion/0.20.0 libcurl/7.19.7
Host: localhost:1028
Accept: application/xml, application/json
Content-Type: application/json
{
"subscriptionId" : "55521671985dc3976b879780",
"originator" : "localhost",
"contextResponses" : [
{
"contextElement" : {
"type" : "Room",
"isPattern" : "false",
"id" : "Room1",
"attributes" : [
{
"name" : "temperature",
"type" : "",
"value" : "20"
}
]
},
"statusCode" : {
"code" : "200",
"reasonPhrase" : "OK"
}
}
]
}
Furthermore, if I create multiple contextElements before adding the subscription, they are all part to the contextResponses in the notification.
For my use case, this behavior is undesirable. The subscriptions are very dynamic (they come and go often throughout the lifecycle of the application) and I do not want the entire history every time I create a subscription. I only want to be notified about changes starting from the moment T created the subscription. (Not a history)
Did I overlook something in the documentation and can I resolve this by changing the contents of the subscription request? If not, is this generally accepted behavior for the context broker or just a plain bug?

Answer 1

It is the expected behaviour, as described in the manual:
You may wonder why accumulator-server.py is getting this message if you don't actually do any update. This is because the Orion Context Broker considers the transition from "non existing subscription" to "subscribed" as a change.
We understand that for some uses cases this is not convenient. However, behaving in the opossite way ruins another uses cases which need to know the "inicial state" before starting getting notifications corresponding to actual changes. The best solution to make everybody happy is to make this configurable, so each client can chose what it prefers. This feature is currently in our roadmap (see this issue in github.com).
While this gets implemented in Orion, in your case maybe a possible workaround is just ignore the first received nofitication belonging to a subscription (you can identify the subscription to which one notification belongs by the subscriptionId field in the notification payload). All the following notifications beloning to that subscription will correspond to actual changes.
EDIT: the posibility of avoiding initial notification has been finally implemented at Orion. Details are at this section of the documentation. It is now in the master branch (so if you use fiware/orion:latest docker you will get it) and will be include in next Orion version (2.2.0).