Intro

• REST service using MongoDB as backend

• Two basic operations (to simplify)– Update attributes in entities

(e.g. update the “speed” attribute in the “Car1” entity)

– Subscribe to changes in a given attribute on a given entity, so when an update of it occurs, then a notification to a given URL is sent

MongoDB

REST sesrvice

Update

MongoDB

REST sesrvice

“Update the Car1 speed to 80 km/h”

Entities are stored in the ‘entities’ collection:{ "_id": "Car1", "attrs": [ { "name": "speed", "value": "80" } ]}

Subscription

MongoDB

REST sesrvice

“I want to receive a notification on

http://notify.me each time any Car speed changes”

Subscriptions are stored in the ‘csubs’ collection:{ "_id": ObjectId("5149fd46f0075f83a4ca0300"), "reference": "http://notify.me", "entity": "Car.*", "attr": "speed" }

Update triggering subscription (“the problem”)

MongoDB

REST sesrvice

“Update the Car1 speed to 85 km/h”

In order to know which subscriptions are triggered by this update, a query on the “csubs” collection has to be done using “Car1” for the lookup. However, we

have the regular expression stored in csubs (regex is not in the query!)

Currently we use {$where: “\"Car1\".match(this.entity)" }, which is not recommended by MongoDB

documentation.

Reverse regex will solve this problem in a smart way: {entity: {$regexApply: {“Car1”}}}

(See https://jira.mongodb.org/browse/SERVER-13902 for more details on $regexApply)