Added a PHP script and some instructions to create an AirPi datacenter

Author: Niccolo Rigacci

extras/datacenter/README.md

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+# How to Create an AirPi Datacenter
+
+Do you have several AirPi stations? Do you want to collect all the data
+into a **single database** and access them from a **single web server**?
+
+* Get an Apache + PHP + Database host. The PHP code uses the MDB2 library.
+* Create a database and initialize it with the required tables and indexes.
+* Populate the "stations" table with name, login and password for each
+station.
+* Install the update.php script into the datacenter host; it will receive
+data from the AirPi stations.
+* On each station configure the HTTP_REQUEST_URL, login and password.
+
+## Asyncronous and Incremental Data Sumbission
+
+Each station is not required to be on-line 24h/24h to submit data.
+To make an example: you can give internet connection to a station only once
+a day, by using a WiFi adapter and a mobile phone with tethering enabled.
+When the station tries to upload its data to the datacenter (default
+is every 5 minutes) it will flush all the data measured an stored locally.
+
+## Files
+
+* **update.php** This is the script which will collect the data from the
+stations. The **db_dsn.php** snippet contains the database credentials.
+The same script is called by the station to know how much of past data 
+is to be uploaded. The station must point its configuration parameter
+**HTTP_REQUEST_URL** to the URL for this script; e.g. https://host/update.php.
+* **db-initialize-pgsql.sql** Use this SQL script to create the tables
+and the indexes into the database. This is for a PostgreSQL database, you
+should adapt for other engines, like MySQL, etc.

extras/datacenter/db-initialize-pgsql.sql

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+-- CREATE USER "airpi" PASSWORD '****';
+-- CREATE DATABASE airpi OWNER airpi ENCODING 'UTF8';
+
+CREATE TABLE stations2 (
+    id       SERIAL PRIMARY KEY,
+    name     VARCHAR UNIQUE NOT NULL,
+    login    VARCHAR UNIQUE NOT NULL,
+    password VARCHAR NOT NULL,
+    lat      DOUBLE PRECISION,
+    lon      DOUBLE PRECISION
+);
+
+CREATE TABLE data (
+    station_id INTEGER NOT NULL,
+    time_stamp TIMESTAMP NOT NULL,
+    type       VARCHAR NOT NULL,
+    value      FLOAT
+);
+ALTER TABLE data ADD CONSTRAINT data_station_id_fkey FOREIGN KEY (station_id) REFERENCES stations(id);
+CREATE UNIQUE INDEX data_time_stamp_type_idx ON data (station_id, time_stamp, type);
+CREATE INDEX data_time_stamp_idx ON data (time_stamp);
+CREATE INDEX data_type_idx ON data (type);

extras/datacenter/db_dsn.php

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+<?php
+
+// Connection to the database, using the MDB2 abstraction.
+require_once 'MDB2.php';
+$dsn = 'pgsql://airpi:MySecretPass@localhost:5432/airpi';
+
+?>

extras/datacenter/update.php

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+<?php
+
+// Store sensors data from remote stations into a datacenter database.
+
+require_once('db_dsn.php');
+
+// ISO 8601 UTC timestamp format. See strftime() PHP function.
+define('TIMESTAMP_FORMAT', '%Y-%m-%dT%H:%M:%SZ');  
+
+$login = (isset($_REQUEST['login'])) ? $_REQUEST['login'] : NULL;
+$password = (isset($_REQUEST['password'])) ? $_REQUEST['password'] : NULL;
+if ($login == NULL) exit('Invalid login');
+
+// Connect to the database, use the MDB2 abstraction.
+$mdb2 = MDB2::connect($dsn);
+if (PEAR::isError($mdb2)) exit($mdb2->getMessage());
+$mdb2->setFetchMode(MDB2_FETCHMODE_ASSOC);
+
+// Authenticate the client.
+$sql = 'SELECT id, password FROM stations WHERE login = ?';
+$query = $mdb2->prepare($sql, array('text'), array('integer', 'text'));
+$result = $query->execute($login);
+if (PEAR::isError($result)) exit($result->getMessage());
+$station = $result->fetchRow();
+$query->free();
+$result->free();
+if (! isset($station['password'])) exit('Login failed');
+if ($station['password'] != md5($password)) exit('Login failed');
+
+// Received some data, INSERT into the database.
+if (isset($_REQUEST['sensors_data'])) {
+    $sensors_data = json_decode($_REQUEST['sensors_data']);
+    //error_log("JSON decoded sensors_data = " . print_r($sensors_data, true));
+    // Open a transaction
+    $res = $mdb2->beginTransaction();
+    $sql = 'INSERT INTO data (station_id, time_stamp, type, value) VALUES (?, ?, ?, ?)';
+    $query = $mdb2->prepare($sql, array('integer', 'text', 'text', 'float'));
+    if (PEAR::isError($query)) exit($query->getMessage());
+    // TODO: Insert the whole array with a single query?
+    // TODO: Enclose in BEGIN/COMMIT
+    foreach($sensors_data as $row) {
+        array_unshift($row, $station['id']);
+        $result = $query->execute($row);
+        if (PEAR::isError($result)) {
+            $mdb2->rollback();
+            exit($result->getMessage());
+        }
+    }
+    $mdb2->commit();
+    $query->free();
+}
+
+// Get the timestamp of last inserted data.
+$sql = 'SELECT max(time_stamp) AS time_stamp FROM data WHERE station_id = ?';
+$query = $mdb2->prepare($sql, array('integer'), array('timestamp'));
+if (PEAR::isError($query)) exit($query->getMessage());
+$result = $query->execute($station['id']);
+if (PEAR::isError($result)) exit($result->getMessage());
+$row = $result->fetchRow();
+$query->free();
+$result->free();
+if ($row['time_stamp'] == NULL) {
+    // Unix Epoch (January 1 1970 00:00:00 GMT)
+    $utc_timestamp = gmstrftime(TIMESTAMP_FORMAT, 0);
+} else {
+    $utc_timestamp = substr($row['time_stamp'], 0, 10) . 'T' . substr($row['time_stamp'], 11, 8) . 'Z';
+    if (!preg_match('/^\d{4}-\d\d-\d\dT\d\d:\d\d:\d\dZ$/', $utc_timestamp)) {
+        exit('Invalid database timestamp format');
+    }
+}
+
+$mdb2->disconnect();
+
+header('Content-type: application/json');
+echo json_encode(array(
+    'name' => $login,
+    'latest_timestamp' => $utc_timestamp
+));
+
+?>