watch for changes in a log file in a memory efficient way (like tail -f)

watch-log.js

// npm init --yes && npm i -S chalk chokidar split
const fs = require("fs");
const { stat } = require("fs").promises;
const { red, cyan } = require("chalk");
const chokidar = require("chokidar");
const split = require("split");
let config = require("./config.json"); // { okPattern: string, nokPattern: string, logFile: string }
try {
  config = require("./config.user.json");
} catch (err) {}

const statuses = { unchecked: 0, connected: 1, disconnected: 2 };
let lastFileSize = 0;
let lastCumulatedStatus = statuses.unchecked;
const operations = [];
const kilo = (num = 0) => Math.round(num / 1024) + "k";

function getCumulatedStatus() {
  const opSorted = operations.slice().sort((a, b) => a.fileSize - b.fileSize);
  let opStatus = statuses.unchecked;
  opSorted.forEach((op) => {
    if (op.status !== statuses.unchecked) opStatus = op.status;
  });
  return opStatus;
}

function handleStatusChange(current = statuses.unchecked) {
  if (current === statuses.connected) {
    console.log(cyan("CONNECTED"));
  }
  if (current === statuses.disconnected) {
    console.log(red("DISCONNECTED"));
  }
}

async function onChange(event, path) {
  console.log(`event: ${event}`);
  const logStat = await stat(path);
  const fileSize = logStat.size;
  const changeSize = fileSize - lastFileSize;
  lastFileSize = fileSize;
  if (changeSize <= 0) return;
  if (event === "unlink") {
    lastFileSize = 0;
    operations.length = 0;
  }
  console.log(`size: ${kilo(logStat.size)} | diff: ${kilo(changeSize)}`);
  let lineCount = 0;
  const operation = {
    started: Date.now(),
    status: statuses.unchecked,
    finished: null,
    fileSize: fileSize,
  };
  // stream reader and split example is from
  // https://sanori.github.io/2019/03/Line-by-line-Processing-in-node-js/
  fs.createReadStream(path, {
    flags: "rs",
    autoClose: true,
    start: fileSize - changeSize,
    end: fileSize,
    highWaterMark: 1024,
  })
    .pipe(split())
    .on("data", (line) => {
      if (line.includes(config.okPattern))
        operation.status = statuses.connected;
      if (line.includes(config.nokPattern))
        operation.status = statuses.disconnected;
      lineCount++;
    })
    .on("end", () => {
      operation.finished = Date.now();
      operations.push(operation);
      const cumulatedStatus = getCumulatedStatus();
      if (lastCumulatedStatus !== cumulatedStatus)
        handleStatusChange(cumulatedStatus);
      lastCumulatedStatus = cumulatedStatus;
      console.log(
        `lines processed: ${lineCount} | status: ${operation.status} | cumulatedStatus: ${cumulatedStatus}`
      );
    });
}

function main() {
  const fn = config.logFile;
  console.log(`watching: "${fn}"`);
  try {
    // polling is not nice, but (probably using stat) this will trigger a flush
    // (on the OS level) to the checked file, without polling we would have stale data
    chokidar
      .watch(fn, { usePolling: true, interval: 2000 })
      .on("all", onChange);
  } catch (err) {
    console.error(red("unhandled exception:"), error);
  }
}

// ---

main();

Config example:

{
  "logFile": "c:\\Program Files\\Palo Alto Networks\\GlobalProtect\\pan_gp_event.log",
  "changeWallpaper": true,
  "okImage": "ok.png",
  "nokImage": "error.png",
  "okPattern": ["Gateway login finished"],
  "nokPattern": ["Tunnel is down", "User was logged out of Gateway"]
}

error.png

ok.png