API

An application programming interface (API) is a protocol intended to be used as an interface by software components to communicate with each other. Wikipedia:API

The API is implemented as a RESTful API, so it uses HTTP requests to store and read data. Thus, REST uses HTTP for all four CRUD (Create / Read / Update / Delete) operations.

PVLng supports all of them:

Request method	Action
PUT	create data set
GET	read data set(s)
POST	update data set
DELETE	delete data set

Data storage

For data storage we have two possibilities:

• Web front end for the master data, like channels and their relationships
• HTTP REST API for the operational channel data with HTTP PUT requests

The storage API accepts only one parameter data and routes it to the requested channel.

By default (load live data) system date and time will be used.

The model behind the channel represents the interface to the database.

Data readout

This section describes only the readout of measuring data via API, see the detailed API help for more supported actions.

The data readout from the system must be done with HTTP GET requests.

The readout API analyzes the requests, identifies the channel and returns the extracted data.

The model behind the channel provides a defined interface to query data.

Requested format

The returned content type is detected by the requested file extension

Extension	Format	Content type
.json	JSON (default, if no extension is set)	application/json
.csv	Semicolon separated values	application/csv
.tsv	TAB separated values	application/tsv
.txt	Space separated values (proposed for single attribute requests)	text/plain

Parameters

All parameters are optional.

Parameter name	Description	Format	default	Examples
start	Start timestamp for readout	string	00:00	YYYY-mm-dd HH:ii:ss seconds since 1970 relative from now sunrise
end	End timestamp for readout	string	24:00	YYYY-mm-dd HH:ii:ss seconds since 1970 relative from now sunset
period	Example	string	<empty>	[1-9][0-9]*minutes [1-9][0-9]*hours [1-9][0-9]*days [1-9][0-9]*weeks [1-9][0-9]*month [1-9][0-9]*quarters [1-9][0-9]*years last readlast all
attributes	Return channel attributes as 1st data set	bool	0	yes
full	Return all data, not only timestamp and value	bool	0	X
short	JSON: Return data as numeric indexed array, not as object with named keys	bool	0	On

Result

If parameter attributes=1 is set, the 1^st data set will contain the channel attributes and further rows the following fields.

Data are not rounded at all, they are delivered as stored or calculated.

By default, only the Timestamp and the Data column are returned. If you set the request parameter full=1, all the following data are returned.

Field	Description
datetime	A readable timestamp; mostly used for debugging
timestamp	UTC Timestamp, good for direct use in graphing
data	Reading value for this timestamp
min	Minimal value of consolidated data
max	Maximal value of consolidated data
count	Count of consolidated data sets for returned row
timediff	Time range of the consolidation in seconds

Return codes

The status code will be doubled into the response header and the response body.

This offers the possibility to analyze either the header or the body to check for success or errors.

For every request the response body will contain further information, especially in case of an error.

The following subset of HTTP status codes are used by the API.

200 - OK

The request has succeeded

• Read of data was successful
• Data was error free, but not saved
  • Insert data within 5 seconds interval will be ignored (adjustable in database table pvlng_config -> DoubleRead)
  • Not changed Switcher states will be ignored

201 - Created

The request has been fulfilled and resulted in a new resource being created

• Data was successful saved
• On batch loads the response body contains the count of inserted data sets.

204 - No Response

The server has fulfilled the request but does not need to return an entity-body

• Response for a DELETE request

400 - Bad Request

The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications.

• There was mostly a problem during data readout
• See response body for details

401 - Unauthorized

The request requires user authentication

• Missing or wrong API key for PUT requests
• Missing or wrong API key for GET requests for private channels, they are only accessible with API key.

405 - Method Not Allowed

The method specified in the Request-Line is not allowed for the resource identified by the Request-URI

• Only PUT / GET / POST / DELETE are allowed

500 - Internal Server Error

The server encountered an unexpected condition which prevented it from fulfilling the request.

• See response body for details