How to Install and Configure Chrony

It is essential to keep the correct time on a server. This is especially true when it comes to processing financial transactions or other vital functions which need to be handled in a specific order. Using the Network Time Protocol (or NTP), computers can synchronize their internal clock times with the internet standard reference clocks. In essence, NTP is a hierarchy of servers. The higher the Stratum number of a server, the more accurate the timing is and the lower the Stratum number of a server is, the lower the accuracy and time stability. Stratus are defined by the distance from the initial reference clock.

#tutorials #atomic clock #centos #chrony #clock #drift #internal time clock #network time protocol #ntp #offset #peers #pool.ntp.org #server time #stratum #system clock #time #time drift #timekeeping #ubuntu #utc

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React Native Dev

1652543820

Background Fetch for React Native Apps

react-native-background-fetch

Background Fetch is a very simple plugin which attempts to awaken an app in the background about every 15 minutes, providing a short period of background running-time. This plugin will execute your provided callbackFn whenever a background-fetch event occurs.

There is no way to increase the rate which a fetch-event occurs and this plugin sets the rate to the most frequent possible — you will never receive an event faster than 15 minutes. The operating-system will automatically throttle the rate the background-fetch events occur based upon usage patterns. Eg: if user hasn't turned on their phone for a long period of time, fetch events will occur less frequently or if an iOS user disables background refresh they may not happen at all.

:new: Background Fetch now provides a scheduleTask method for scheduling arbitrary "one-shot" or periodic tasks.

iOS

There is no way to increase the rate which a fetch-event occurs and this plugin sets the rate to the most frequent possible — you will never receive an event faster than 15 minutes. The operating-system will automatically throttle the rate the background-fetch events occur based upon usage patterns. Eg: if user hasn't turned on their phone for a long period of time, fetch events will occur less frequently.
scheduleTask seems only to fire when the device is plugged into power.
⚠️ When your app is terminated, iOS no longer fires events — There is no such thing as stopOnTerminate: false for iOS.
iOS can take days before Apple's machine-learning algorithm settles in and begins regularly firing events. Do not sit staring at your logs waiting for an event to fire. If your simulated events work, that's all you need to know that everything is correctly configured.
If the user doesn't open your iOS app for long periods of time, iOS will stop firing events.

Android

The Android plugin provides a HeadlessJS implementation allowing you to continue handling events even after app-termination (see @config enableHeadless)

Installing the plugin

⚠️ If you have a previous version of react-native-background-fetch < 2.7.0 installed into react-native >= 0.60, you should first unlink your previous version as react-native link is no longer required.

$ react-native unlink react-native-background-fetch

With `yarn`

$ yarn add react-native-background-fetch

With `npm`

$ npm install --save react-native-background-fetch

Setup Guides

iOS Setup

`react-native >= 0.60`

Auto-linking Setup

Android Setup

`react-native >= 0.60`

Auto-linking Setup

Example

ℹ️ This repo contains its own Example App. See /example

import React from 'react';
import {
  SafeAreaView,
  StyleSheet,
  ScrollView,
  View,
  Text,
  FlatList,
  StatusBar,
} from 'react-native';

import {
  Header,
  Colors
} from 'react-native/Libraries/NewAppScreen';

import BackgroundFetch from "react-native-background-fetch";

class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      events: []
    };
  }

  componentDidMount() {
    // Initialize BackgroundFetch ONLY ONCE when component mounts.
    this.initBackgroundFetch();
  }

  async initBackgroundFetch() {
    // BackgroundFetch event handler.
    const onEvent = async (taskId) => {
      console.log('[BackgroundFetch] task: ', taskId);
      // Do your background work...
      await this.addEvent(taskId);
      // IMPORTANT:  You must signal to the OS that your task is complete.
      BackgroundFetch.finish(taskId);
    }

    // Timeout callback is executed when your Task has exceeded its allowed running-time.
    // You must stop what you're doing immediately BackgroundFetch.finish(taskId)
    const onTimeout = async (taskId) => {
      console.warn('[BackgroundFetch] TIMEOUT task: ', taskId);
      BackgroundFetch.finish(taskId);
    }

    // Initialize BackgroundFetch only once when component mounts.
    let status = await BackgroundFetch.configure({minimumFetchInterval: 15}, onEvent, onTimeout);

    console.log('[BackgroundFetch] configure status: ', status);
  }

  // Add a BackgroundFetch event to <FlatList>
  addEvent(taskId) {
    // Simulate a possibly long-running asynchronous task with a Promise.
    return new Promise((resolve, reject) => {
      this.setState(state => ({
        events: [...state.events, {
          taskId: taskId,
          timestamp: (new Date()).toString()
        }]
      }));
      resolve();
    });
  }

  render() {
    return (
      <>
        <StatusBar barStyle="dark-content" />
        <SafeAreaView>
          <ScrollView
            contentInsetAdjustmentBehavior="automatic"
            style={styles.scrollView}>
            <Header />

            <View style={styles.body}>
              <View style={styles.sectionContainer}>
                <Text style={styles.sectionTitle}>BackgroundFetch Demo</Text>
              </View>
            </View>
          </ScrollView>
          <View style={styles.sectionContainer}>
            <FlatList
              data={this.state.events}
              renderItem={({item}) => (<Text>[{item.taskId}]: {item.timestamp}</Text>)}
              keyExtractor={item => item.timestamp}
            />
          </View>
        </SafeAreaView>
      </>
    );
  }
}

const styles = StyleSheet.create({
  scrollView: {
    backgroundColor: Colors.lighter,
  },
  body: {
    backgroundColor: Colors.white,
  },
  sectionContainer: {
    marginTop: 32,
    paddingHorizontal: 24,
  },
  sectionTitle: {
    fontSize: 24,
    fontWeight: '600',
    color: Colors.black,
  },
  sectionDescription: {
    marginTop: 8,
    fontSize: 18,
    fontWeight: '400',
    color: Colors.dark,
  },
});

export default App;

Executing Custom Tasks

In addition to the default background-fetch task defined by BackgroundFetch.configure, you may also execute your own arbitrary "oneshot" or periodic tasks (iOS requires additional Setup Instructions). However, all events will be fired into the Callback provided to BackgroundFetch#configure:

⚠️ iOS:

scheduleTask on iOS seems only to run when the device is plugged into power.
scheduleTask on iOS are designed for low-priority tasks, such as purging cache files — they tend to be unreliable for mission-critical tasks. scheduleTask will never run as frequently as you want.
The default fetch event is much more reliable and fires far more often.
scheduleTask on iOS stop when the user terminates the app. There is no such thing as stopOnTerminate: false for iOS.

// Step 1:  Configure BackgroundFetch as usual.
let status = await BackgroundFetch.configure({
  minimumFetchInterval: 15
}, async (taskId) => {  // <-- Event callback
  // This is the fetch-event callback.
  console.log("[BackgroundFetch] taskId: ", taskId);

  // Use a switch statement to route task-handling.
  switch (taskId) {
    case 'com.foo.customtask':
      print("Received custom task");
      break;
    default:
      print("Default fetch task");
  }
  // Finish, providing received taskId.
  BackgroundFetch.finish(taskId);
}, async (taskId) => {  // <-- Task timeout callback
  // This task has exceeded its allowed running-time.
  // You must stop what you're doing and immediately .finish(taskId)
  BackgroundFetch.finish(taskId);
});

// Step 2:  Schedule a custom "oneshot" task "com.foo.customtask" to execute 5000ms from now.
BackgroundFetch.scheduleTask({
  taskId: "com.foo.customtask",
  forceAlarmManager: true,
  delay: 5000  // <-- milliseconds
});

API Documentation

Config

Common Options

`@param {Integer} minimumFetchInterval [15]`

The minimum interval in minutes to execute background fetch events. Defaults to 15 minutes. Note: Background-fetch events will never occur at a frequency higher than every 15 minutes. Apple uses a secret algorithm to adjust the frequency of fetch events, presumably based upon usage patterns of the app. Fetch events can occur less often than your configured minimumFetchInterval.

`@param {Integer} delay (milliseconds)`

ℹ️ Valid only for BackgroundFetch.scheduleTask. The minimum number of milliseconds in future that task should execute.

`@param {Boolean} periodic [false]`

ℹ️ Valid only for BackgroundFetch.scheduleTask. Defaults to false. Set true to execute the task repeatedly. When false, the task will execute just once.

Android Options

`@config {Boolean} stopOnTerminate [true]`

Set false to continue background-fetch events after user terminates the app. Default to true.

`@config {Boolean} startOnBoot [false]`

Set true to initiate background-fetch events when the device is rebooted. Defaults to false.

❗ NOTE: startOnBoot requires stopOnTerminate: false.

`@config {Boolean} forceAlarmManager [false]`

By default, the plugin will use Android's JobScheduler when possible. The JobScheduler API prioritizes for battery-life, throttling task-execution based upon device usage and battery level.

Configuring forceAlarmManager: true will bypass JobScheduler to use Android's older AlarmManager API, resulting in more accurate task-execution at the cost of higher battery usage.

let status = await BackgroundFetch.configure({
  minimumFetchInterval: 15,
  forceAlarmManager: true
}, async (taskId) => {  // <-- Event callback
  console.log("[BackgroundFetch] taskId: ", taskId);
  BackgroundFetch.finish(taskId);
}, async (taskId) => {  // <-- Task timeout callback
  // This task has exceeded its allowed running-time.
  // You must stop what you're doing and immediately .finish(taskId)
  BackgroundFetch.finish(taskId);
});
.
.
.
// And with with #scheduleTask
BackgroundFetch.scheduleTask({
  taskId: 'com.foo.customtask',
  delay: 5000,       // milliseconds
  forceAlarmManager: true,
  periodic: false
});

`@config {Boolean} enableHeadless [false]`

Set true to enable React Native's Headless JS mechanism, for handling fetch events after app termination.

📂 index.js (MUST BE IN index.js):

import BackgroundFetch from "react-native-background-fetch";

let MyHeadlessTask = async (event) => {
  // Get task id from event {}:
  let taskId = event.taskId;
  let isTimeout = event.timeout;  // <-- true when your background-time has expired.
  if (isTimeout) {
    // This task has exceeded its allowed running-time.
    // You must stop what you're doing immediately finish(taskId)
    console.log('[BackgroundFetch] Headless TIMEOUT:', taskId);
    BackgroundFetch.finish(taskId);
    return;
  }
  console.log('[BackgroundFetch HeadlessTask] start: ', taskId);

  // Perform an example HTTP request.
  // Important:  await asychronous tasks when using HeadlessJS.
  let response = await fetch('https://reactnative.dev/movies.json');
  let responseJson = await response.json();
  console.log('[BackgroundFetch HeadlessTask] response: ', responseJson);

  // Required:  Signal to native code that your task is complete.
  // If you don't do this, your app could be terminated and/or assigned
  // battery-blame for consuming too much time in background.
  BackgroundFetch.finish(taskId);
}

// Register your BackgroundFetch HeadlessTask
BackgroundFetch.registerHeadlessTask(MyHeadlessTask);

`@config {integer} requiredNetworkType [BackgroundFetch.NETWORK_TYPE_NONE]`

Set basic description of the kind of network your job requires.

If your job doesn't need a network connection, you don't need to use this option as the default value is BackgroundFetch.NETWORK_TYPE_NONE.

NetworkType	Description
`BackgroundFetch.NETWORK_TYPE_NONE`	This job doesn't care about network constraints, either any or none.
`BackgroundFetch.NETWORK_TYPE_ANY`	This job requires network connectivity.
`BackgroundFetch.NETWORK_TYPE_CELLULAR`	This job requires network connectivity that is a cellular network.
`BackgroundFetch.NETWORK_TYPE_UNMETERED`	This job requires network connectivity that is unmetered. Most WiFi networks are unmetered, as in "you can upload as much as you like".
`BackgroundFetch.NETWORK_TYPE_NOT_ROAMING`	This job requires network connectivity that is not roaming (being outside the country of origin)

`@config {Boolean} requiresBatteryNotLow [false]`

Specify that to run this job, the device's battery level must not be low.

This defaults to false. If true, the job will only run when the battery level is not low, which is generally the point where the user is given a "low battery" warning.

`@config {Boolean} requiresStorageNotLow [false]`

Specify that to run this job, the device's available storage must not be low.

This defaults to false. If true, the job will only run when the device is not in a low storage state, which is generally the point where the user is given a "low storage" warning.

`@config {Boolean} requiresCharging [false]`

Specify that to run this job, the device must be charging (or be a non-battery-powered device connected to permanent power, such as Android TV devices). This defaults to false.

`@config {Boolean} requiresDeviceIdle [false]`

When set true, ensure that this job will not run if the device is in active use.

The default state is false: that is, the for the job to be runnable even when someone is interacting with the device.

This state is a loose definition provided by the system. In general, it means that the device is not currently being used interactively, and has not been in use for some time. As such, it is a good time to perform resource heavy jobs. Bear in mind that battery usage will still be attributed to your application, and shown to the user in battery stats.

Methods

Method Name	Arguments	Returns	Notes
`configure`	`{FetchConfig}`, `callbackFn`, `timeoutFn`	`Promise<BackgroundFetchStatus>`	Configures the plugin's `callbackFn` and `timeoutFn`. This callback will fire each time a background-fetch event occurs in addition to events from `#scheduleTask`. The `timeoutFn` will be called when the OS reports your task is nearing the end of its allowed background-time.
`scheduleTask`	`{TaskConfig}`	`Promise<boolean>`	Executes a custom task. The task will be executed in the same `Callback` function provided to `#configure`.
`status`	`callbackFn`	`Promise<BackgroundFetchStatus>`	Your callback will be executed with the current `status (Integer)` `0: Restricted`, `1: Denied`, `2: Available`. These constants are defined as `BackgroundFetch.STATUS_RESTRICTED`, `BackgroundFetch.STATUS_DENIED`, `BackgroundFetch.STATUS_AVAILABLE` (NOTE: Android will always return `STATUS_AVAILABLE`)
`finish`	`String taskId`	`Void`	You MUST call this method in your `callbackFn` provided to `#configure` in order to signal to the OS that your task is complete. iOS provides only 30s of background-time for a fetch-event -- if you exceed this 30s, iOS will kill your app.
`start`	`none`	`Promise<BackgroundFetchStatus>`	Start the background-fetch API. Your `callbackFn` provided to `#configure` will be executed each time a background-fetch event occurs. NOTE the `#configure` method automatically calls `#start`. You do not have to call this method after you `#configure` the plugin
`stop`	`[taskId:String]`	`Promise<boolean>`	Stop the background-fetch API and all `#scheduleTask` from firing events. Your `callbackFn` provided to `#configure` will no longer be executed. If you provide an optional `taskId`, only that `#scheduleTask` will be stopped.

Debugging

iOS

🆕 `BGTaskScheduler` API for iOS 13+

⚠️ At the time of writing, the new task simulator does not yet work in Simulator; Only real devices.
See Apple docs Starting and Terminating Tasks During Development
After running your app in XCode, Click the [||] button to initiate a Breakpoint.
In the console (lldb), paste the following command (Note: use cursor up/down keys to cycle through previously run commands):

e -l objc -- (void)[[BGTaskScheduler sharedScheduler] _simulateLaunchForTaskWithIdentifier:@"com.transistorsoft.fetch"]

Click the [ > ] button to continue. The task will execute and the Callback function provided to BackgroundFetch.configure will receive the event.

Simulating task-timeout events

Only the new BGTaskScheduler api supports simulated task-timeout events. To simulate a task-timeout, your fetchCallback must not call BackgroundFetch.finish(taskId):

let status = await BackgroundFetch.configure({
  minimumFetchInterval: 15
}, async (taskId) => {  // <-- Event callback.
  // This is the task callback.
  console.log("[BackgroundFetch] taskId", taskId);
  //BackgroundFetch.finish(taskId); // <-- Disable .finish(taskId) when simulating an iOS task timeout
}, async (taskId) => {  // <-- Event timeout callback
  // This task has exceeded its allowed running-time.
  // You must stop what you're doing and immediately .finish(taskId)
  print("[BackgroundFetch] TIMEOUT taskId:", taskId);
  BackgroundFetch.finish(taskId);
});

Now simulate an iOS task timeout as follows, in the same manner as simulating an event above:

e -l objc -- (void)[[BGTaskScheduler sharedScheduler] _simulateExpirationForTaskWithIdentifier:@"com.transistorsoft.fetch"]

Old `BackgroundFetch` API

Simulate background fetch events in XCode using Debug->Simulate Background Fetch
iOS can take some hours or even days to start a consistently scheduling background-fetch events since iOS schedules fetch events based upon the user's patterns of activity. If Simulate Background Fetch works, your can be sure that everything is working fine. You just need to wait.

Android

Observe plugin logs in $ adb logcat:

$ adb logcat *:S ReactNative:V ReactNativeJS:V TSBackgroundFetch:V

Simulate a background-fetch event on a device (insert <your.application.id>) (only works for sdk 21+:

$ adb shell cmd jobscheduler run -f <your.application.id> 999

For devices with sdk <21, simulate a "Headless JS" event with (insert <your.application.id>)

$ adb shell am broadcast -a <your.application.id>.event.BACKGROUND_FETCH

Download Details:
Author: transistorsoft
Source Code: https://github.com/transistorsoft/react-native-background-fetch
License: MIT license

#react #reactnative #mobileapp #javascript

Chet Lubowitz

1595855400

How to install PgAdmin 4 on CentOS 8

pgAdmin is the leading graphical Open Source management, development and administration tool for PostgreSQL. pgAdmin4 is a rewrite of the popular pgAdmin3 management tool for the PostgreSQL database.

In this tutorial, we are going to show you how to install pgAdmin4 in Server Mode as a web application using httpd and Wsgi module on CentOS 8.

Install pgAdmin4 on CentOS 8

**01-**To install pgAdmin4 on CentOS 8 we need to add an external repository, so execute the following command:

$ sudo rpm -i https://ftp.postgresql.org/pub/pgadmin/pgadmin4/yum/pgadmin4-redhat-repo-1-1.noarch.rpm

02- After we add the pgAdmin4 repository, let’s use the below command to install pgAdmin4 as server mode:

$ sudo dnf install pgadmin4-web

03- Before proceeding with the configuration of pgAdmin4, we need to install policycoreutils tool:

$ dnf install policycoreutils-python-utils

04- Once we done installing pgAdmin4, we need to configure the pgAdmin4 by setting up the initial pgAdmin user account

#databases #linux #ubuntu #install pgadmin4 #install pgadmin4 centos #pgadmin #pgadmin 4 install #pgadmin 4 install centos #pgadmin4 #pgadmin4 install centos

Brain Crist

1595372400

How to Install Android Studio on Ubuntu 20.04

1./ Install Android Studio Using APT

Method 1./ Install Android Studio Using APT

1- We need to add official repository to the sources list. Therefore, type the below command to add the Android Studio repository:

$ sudo add-apt-repository ppa:maarten-fonville/android-studio

02- Now let’s install Android studio and all the software dependencies as below.

$ sudo apt update
$ sudo apt install android-studio

Method 2./ Install Android Studio Using snap

We can install using the snap tool. So, use the below command to install Android Studio:

$ sudo snap install android-studio --classic

2./ Starting Android Studio

01- You can start Android Studio either by typing the command android-studio in your terminal or by clicking on the Android Studio icon (Activities -> Android Studio).