Fledge Tasks¶

Tasks are part of the Fledge IoT platform. They are like services, but with a clear distinction:

services are started at a certain point (usually at startup) and they are likely to continue to work until Fledge stops.
tasks are started when required, they execute a job and then they terminate.

In simple terms, a service is meant to always listen and react to requests, while a task is triggered by an event and then when job is terminated, the tasks ends.

That said, tasks and services shared these same features:

They are both started by the Fledge scheduler. It is likely that services are started at startup, while tasks can start at a given time or interval.
They both use the internal API to communicate with other services.
They both use the same pluggable architecture to separate a common logic, usually associated to the internal features of Fledge, from a more generic logic, usually closer to the type of operations that must be performed.

In this chapter we present a set of tasks that are commonly available in Fledge.

Purge¶

The Purge task is triggered by the scheduler to purge old data that is still stored (buffered) in Fledge. The logic applied to the task is relatively simple:

The task is called exclusively (i.e. there cannot be more than one Purge task running at any given time) by the Fledge scheduler every hour (by default).
Data that is older than a certain date/time is removed.
Optionally, data is removed if the total size of the stored objects is bigger than 1GByte (default)
Optionally, data is not removed if it has not been extracted and used by any North task or service yet.
All purge operations are stored in the audit log.

Purge Schedule¶

Purge is one of the tasks launched by the Fledge scheduler. You can retrieve information about the scheduling by calling the GET method of the schedule call. The name and the process name of the task are both purge:

$ curl -sX GET http://localhost:8081/fledge/schedule
...
{ "id"          : "cea17db8-6ccc-11e7-907b-a6006ad3dba0",
  "name"        : "purge",
  "time"        : 0,
  "enabled"     : true,
  "repeat"      : 3600,
  "type"        : "INTERVAL",
  "exclusive"   : true,
  "processName" : "purge",
  "day"         : null },
...
$

As you can see from the JSON output, the task is scheduled to be executed every hour (3,600 seconds). In order to change the interval between Purge tasks, you can call the PUT method of the schedule call by passing the associated id. For example, in order to change the task to be executed any 5 minutes (i.e. 300 seconds) you should call:

$ curl -sX PUT http://localhost:8081/fledge/schedule/cea17db8-6ccc-11e7-907b-a6006ad3dba0 -d '{"repeat": 300}'
{ "schedule": { "id": "cea17db8-6ccc-11e7-907b-a6006ad3dba0",
                "name"        : "purge",
                "time"        : 0,
                "enabled"     : true,
                "repeat"      : 300,
                "type"        : "INTERVAL",
                "exclusive"   : true,
                "processName" : "purge",
                "day"         : null }
}
$

Purge Configuration¶

The configuration of the Purge task is stored in the metadata structures of Fledge and it can be retrieve using the GET method of the category/PURGE_READ call. This is the command used to retrieve the configuration in JSON format:

$ curl -sX GET http://localhost:8081/fledge/category/PURGE_READ
{ "retainUnsent" : { "type": "boolean",
                     "default": "False",
                     "description": "Retain data that has not been sent to any historian yet.",
                     "value": "False" },
  "age"          : { "type": "integer",
                     "default": "72",
                     "description": "Age of data to be retained, all data that is older than this value will be removed,unless retained. (in Hours)",
                     "value": "72" },
  "size"         : { "type": "integer",
                     "default": "1000000",
                     "description": "Maximum size of data to be retained, the oldest data will be removed to keep below this size, unless retained. (in Kbytes)",
                     "value": "1000000" } }
$

Changes can be applied using the PUT method for each parameter call. For example, in order to change the retention policy for data that has not been sent to historians yet, you can use this call:

$ curl -sX PUT http://locahost:8081/fledge/category/PURGE_READ/retainUnsent -d '{"value": "True"}'
{ "type": "boolean",
  "default": "False",
  "description": "Retain data that has not been sent to any historian yet.",
  "value": "True" }
$

The following table shows the list of parameters that can be changed in the Purge task:

Item	Type	Default	Description
retainUnsent	boolean	False	Retain data that has not been sent to “North” yet When True, data that has not yet been retrieved by any North service or task, will not be purged. When False, data is purged without checking whether it has been sent to a North destination yet or not.
age	integer	72	Age in hours of the data to be retained. Data that is older than this value, will be purged.
size	integer	1000000	Size in KBytes of data that will be retained in Fledge. Older data will be removed to keep the data stored in Fledge below this size.