DEV Community: Zarmin

Datadog PHP APM filtering

Zarmin — Wed, 21 Feb 2024 11:59:54 +0000

A client needed a solution to filter PHP APM traces before sending them to the Datadog server to reduce ingestion fees.

The filter rules were to exclude everything except requests exceeding 1 second and errors (HTTP code >= 400, exceptions, PHP errors).

They still wanted to utilize automatic instrumentation for several libraries (e.g., Laravel, Curl, PDO).

(The client is still using PHP 7.4, which is end-of-life; I know - they will upgrade to 8.x later.)

This auto-prepend script will filter out the specified request traces (returning from the function will retain the trace).

<?php

// dd-filter.php

register_shutdown_function(function() {
  // config
  $TIME_LIMIT_SEC = 1.0;

  // exit if Datadog (DDTrace) is NOT loaded
  if (!class_exists('\DDTrace\GlobalTracer') || !class_exists('\DDTrace\Tag')) {
    return;
  }

  // get HTTP status code
  $httpResponseCode = http_response_code();
  if ($httpResponseCode >= 400) {
    return; // keep DD trace
  }

  // is the script exited with an error?
  $lastError = error_get_last();
  if ($lastError) {
    return; // keep DD trace
  }

  // get current request time
  if (empty($_SERVER['REQUEST_TIME_FLOAT'])) {
    return; // keep DD trace 
  }
  $startTime = $_SERVER['REQUEST_TIME_FLOAT'];
  $currentTime = microtime(true);
  $elapsedTime = max($currentTime - $startTime, 0.0);

  if ($elapsedTime > $TIME_LIMIT_SEC) {
    return; // keep DD trace
  }

  // drop DD trace
  $tracer = \DDTrace\GlobalTracer::get();
  $span = $tracer->getActiveSpan();
  if (null !== $span) {
    $span->setTag(\DDTrace\Tag::MANUAL_DROP, true);
  }
});

(Yes, I know it would be better if the configuration, for example, TIME_LIMIT_SEC, were kept in a separate file.)

This can be set up in the following way:

Place this script to /etc/php/7.4/datadog-filter/dd-filter.php
Set the script for auto-prepending in /etc/php/7.4/fpm/php.ini: auto_prepend_file = /etc/php/7.4/datadog-filter/dd-filter.php
Restart PHP-FPM: systemctl restart php7.4-fpm

(The client is using their app on a Debian 12 Linux VM.)

Electronic load repair

Zarmin — Tue, 16 Jan 2024 14:48:06 +0000

I wanted my first post to be interesting, but it was a facepalm instead.

While using my ~~aliexpress~~ cheap electronic load to test a 12V lead-acid battery, I smelled some ~~burnt smell~~ magic smoke and it showed 0A of current regardless of the position of the pot meters.

Original Aliexpress photo about the electronic load

I removed the big heatsink and before measuring / inspecting anything, I quickly replaced the IRFP260N power mosfet, because I had one in the drawer.

Electronic load without heatsink (and with the replaced MOSFET)

After that it showed the same symptoms, 0A of current.

I went through some systematic analysis steps:

measuring the old and new power MOSFET (old was bad, new was good), so the replacement was neccessary, but not sufficient.
inspecting the whole board under a microscope checking for burnt parts: found nothing.
measuring a few components for damage, such as shorted caps and shorted/open resistors, but found nothing
taking photos of both sides of the PCB, merging them as semi-transparent images and try to follow the traces to reconstruct the schematics

Reconstructing the traces

During the reconstruction it looked like a standard electronic load design using a power MOSFET and a comparator op-amp, but I had issues drawing the schematics, because the MOSFET pins were weird...

My wrong schematics reconstruction vs a good simple electronic load reference circuit... Check for the MOSFET pins

It took me half a minute to understand I could fix the whole issue under 2 minutes, if I was aware of the orientation of the power MOSFET...

After flipping it, everything was fine...

First test post

Zarmin — Thu, 04 Jan 2024 17:14:02 +0000

markdown test ~post~