ESP8266 disconnected

Please tell me how to solve this problem.
Thank you.
Code:
#include <AsyncMqttClient.h>
#include <ESP8266WiFi.h>
#include <Ticker.h>

#define WIFI_SSID " ******** "
#define WIFI_PASSWORD " *********** "

// Raspberri Pi Mosquitto MQTT Broker
#define MQTT_HOST IPAddress(192, 168, 100, ***)
#define MQTT_PORT 1883

//Temperature MQTT Topics
#define MQTT_PUB_TEMP "mqtt/lm35"

const int LM35sensorPin = A0;
float LM35sensorValue;
float LM35voltageOut;
float LM35temperature;

// Temperature value
float temp;

AsyncMqttClient mqttClient;
Ticker mqttReconnectTimer;

WiFiEventHandler wifiConnectHandler;
WiFiEventHandler wifiDisconnectHandler;
Ticker wifiReconnectTimer;

unsigned long previousMillis = 0; // Stores last time temperature was published
const long interval = 10000; // Interval at which to publish sensor readings

void connectToWifi() {
Serial.println("Connecting to Wi-Fi...");
WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
}

void onWifiConnect(const WiFiEventStationModeGotIP& event) {
Serial.println("Connected to Wi-Fi.");
connectToMqtt();
}

void onWifiDisconnect(const WiFiEventStationModeDisconnected& event) {
Serial.println("Disconnected from Wi-Fi.");
mqttReconnectTimer.detach(); // ensure we don't reconnect to MQTT while reconnecting to Wi-Fi
wifiReconnectTimer.once(2, connectToWifi);
}

void connectToMqtt() {
Serial.println("Connecting to MQTT...");
mqttClient.connect();
}

void onMqttConnect(bool sessionPresent) {
Serial.println("Connected to MQTT.");
Serial.print("Session present: ");
Serial.println(sessionPresent);
}

void onMqttDisconnect(AsyncMqttClientDisconnectReason reason) {
Serial.println("Disconnected from MQTT.");

if (WiFi.isConnected()) {
mqttReconnectTimer.once(2, connectToMqtt);
}
}

void onMqttPublish(uint16_t packetId) {
Serial.print("Publish acknowledged.");
Serial.print(" packetId: ");
Serial.println(packetId);
}

void setup() {
pinMode(LM35sensorPin, INPUT);
Serial.begin(115200);
Serial.println();

wifiConnectHandler = WiFi.onStationModeGotIP(onWifiConnect);
wifiDisconnectHandler = WiFi.onStationModeDisconnected(onWifiDisconnect);

mqttClient.onConnect(onMqttConnect);
mqttClient.onDisconnect(onMqttDisconnect);
//mqttClient.onSubscribe(onMqttSubscribe);
//mqttClient.onUnsubscribe(onMqttUnsubscribe);
mqttClient.onPublish(onMqttPublish);
mqttClient.setServer(MQTT_HOST, MQTT_PORT);
// If your broker requires authentication (username and password), set them below
mqttClient.setCredentials("REPLACE_WITH_YOUR_USER", "REPLACE_WITH_YOUR_PASSWORD");

connectToWifi();
}

void loop() {
//LM35 SENSOR
LM35sensorValue = analogRead(LM35sensorPin);
LM35voltageOut = (LM35sensorValue * 3300) / 1024;

// calculate temperature for LM35 (LM35DZ)
LM35temperature = LM35voltageOut / 10;
//LM35temperatureF = (LM35temperatureC*1.8)+32;
unsigned long currentMillis = millis();
// Every X number of seconds (interval = 10 seconds)
// it publishes a new MQTT message
if (currentMillis - previousMillis >= interval) {
// Save the last time a new reading was published
previousMillis = currentMillis;
// Temperature in Celsius degrees
temp = LM35temperature;

// Publish an MQTT message on topic mqtt/lm35
uint16_t packetIdPub1 = mqttClient.publish(MQTT_PUB_TEMP, 1, true, String(temp).c_str());                            
Serial.printf("Publishing on topic %s at QoS 0, packetId: %i ", MQTT_PUB_TEMP, packetIdPub1);
Serial.printf("Message: %.2f \n", temp);

}
}

Please post your code following the advice given in the link below

You should post code by using code-tags
There is an automatic function for doing this in the Arduino-IDE
just three steps

  1. press Ctrl-T for autoformatting your code
  2. do a rightclick with the mouse and choose "copy for forum"
  3. paste clipboard into write-window of a posting

you should add information where does the code you have posted come from?
Is it a 1 to 1 copy & paste from somewhere?
does the original source use the same Raspberry Pi configuration?
Did you do some basic tests?
could it be normal that the connection is established and disconnected?
I haven't done much with webservers but I remember that for at least a part of "webserving" stuff it is normal to disconnect if the actual dataexchange is done.

What do you see on your raspberry Pi which is running the MQTT-server?
I guess the MQTTserver has some logfiles too. What do you see in these logfiles?

How about using this code-example? It should work with an ESP8266 too

best regards Stefan

Look similar to the problem I have been having and it was down to power. The USB cant deliver enough current when connecting to WiFi.