Collecting Device Metrics

Memfault has support for various built-in AOSP metrics, like battery health, wakelock counts, foregrounded apps and so on.

Aside of the built-in metrics, it is also possible to define custom ones. In this guide we describe step-by-step how to add custom metrics to periodically collect the RSSI (Received Signal Strength Indicator), ambient light sensor measurements and the available disk space of our fictive devices. In Memfault's UI, the data that gets collected from each device over time, is visualized in graphs in the Timeline:

Android Custom Metrics


  1. RECOMMENDED: Define your custom tags and generate helper methods. Define tags with the mflt_ prefix. AOSP-built apps should define an EventLog.logtags file in their root directory and add that entry to their Android.bp.
  2. REQUIRED: Call the EventLog API to log events. If you compiled a custom EventLog.logtags file, use the generated Java APIs. Otherwise use EventLog directly with an integer tag in the range of 1000000-2000000, or a custom mflt_-prefixed tag defined in /system/etc/event-log-tags (which is generated from .logtags files).

When Memfault receives a bug report containing those custom events, they will be displayed in that Bug Report's timeline under EventLog.

Logging custom metrics using EventLog#

The Android SDK contains an API called EventLog. As per its documentation, this API is intended to record system-level diagnostics.

The heart of the API looks like this: EventLog.writeEvent(int tag, ... value) where the dots can be int, float, string, etc. We won't be using this API directly, but a generated helper class. More on that later.


The tag argument is a unique code or "key" that is used to be able to identify the type of event. In this guide, we will be logging RSSI, ambient light level and free disk space. We will use a separate tag for each.

Note that the tag code space is shared with the whole system. Many codes are already used for AOSP-internal purposes. Tags 1000000-2000000 are available for vendor/OEM/3rd party use.

Tags can also be given string names, through ".logtags" files that map the tag code to the tag name, as well as define the expected value type(s). More on that later.

To see what tags are being used on your system, run adb shell cat /system/etc/event-log-tags to dump out the list of known tags.

The Memfault UI will automatically visualize data that is logged for named tags starting with the prefix mflt_.

This requires adding the tag code, tag name to a .logtags file. These .logtags files are merged together to generate the /system/etc/event-log-tags file that is baked into the ROM and is therefore only an option for apps that are built as part of the AOSP build.

Alternatively, for apps that are not built as part of the AOSP build, "anonymous tags" can be used. If a tag code is used, but the tag code is not present in the /system/etc/event-log-tags file, we call it an "anonymous tag". The data will still be visualized, but in the UI, it will be labelled with the tag's code instead of the human-readable tag name.

Creating an EventLog.logtags file#

The AOSP comes with tooling for defining EventLog tags, generating Java helper classes and getting the tags merged into /system/etc/event-log-tags. Create a file called EventLog.logtags in your app's source directory:

option java_package com.acme.myapp
19000000 mflt_rssi (rssi|1)
19000001 mflt_ambient_light (ambient_light|1)
19000002 mflt_disk_space_free (disk_space_free|1)

At the top, specify the Java package of your application. In the next sections, we will get back to this.

The rest of the file contains one tag definition per line. Each starts with the tag code, the tag name and finally the value name and data type. For this guide, we use the int data type (1).

Tag names (minus the mflt_ prefix) show up in the Memfault UI. For example, the tag mflt_disk_space_free will be displayed as "Disk Space Free". See also the screenshot at the top of this page.

Even though values must be named, the value names are currently not used by the Memfault UI.

For more details on the syntax, we recommend reading the commentary in this .logtag file in the AOSP source code.

Building the EventLog.logtags file (for AOSP-built apps)#

The AOSP build system has built-in support for .logtags files, taking care of 2 things:

  1. It will merge our custom tags from the EventLog.logtags file into /system/etc/event-log-tags.

  2. It will generate a .java source file with a helper class com.acme.myapp.EventLog (named based on the logtags file name and option java_package ...).

To build the EventLog.logtags file, add a new java_library to your app's Android.bp file and also add this new target is added to the static_libs list of your app:

java_library {
name: "MyAppLogTags",
srcs: ["EventLog.logtags"],
android_app {
name: "MyApp",
srcs: [
static_libs: [

Generated helper class#

Based on the EventLog.logtags file, the AOSP build system generates a Java class that looks like this:

package com.acme.myapp;
public class EventLog {
/* ... snip ... */
/** 20000000 mflt_rssi (rssi|1) */
public static final int MFLT_RSSI = 20000000;
/** 20000001 mflt_ambient_light (ambient_light|1) */
public static final int MFLT_AMBIENT_LIGHT = 20000001;
/** 20000002 mflt_disk_space_free (disk_space_free|1) */
public static final int MFLT_DISK_SPACE_FREE = 20000002;
public static void writeMfltRssi(int rssi) {
android.util.EventLog.writeEvent(MFLT_RSSI, rssi);
public static void writeMfltAmbientLight(int ambientLight) {
android.util.EventLog.writeEvent(MFLT_AMBIENT_LIGHT, ambientLight);
public static void writeMfltDiskSpaceFree(int diskSpaceFree) {
android.util.EventLog.writeEvent(MFLT_DISK_SPACE_FREE, diskSpaceFree);

The helper class has methods for each of the tags, ensuring the correct tags codes are used and the values are of the expected types.

"Manually" generating the helper class (for non-AOSP-built apps)#

For apps that are not built as part of an AOSP build, we need to run the code generator script to generate a Java helper class from the EventLog.logtags file.

  • Clone the AOSP repo that contains the generator script:
git clone aosp-platform-build
  • Run the code generator script:
cd aosp-platform-build/tools
python2 \
-o /path/to/app/src/com/acme/myapp/ \

As of March 2020, the script was not yet Python 3.x compatible and required Python 2.x to run.

We recommend running the script as part of your app's Gradle build.

Instrumenting your code#

Now that we've defined tags for our metrics and have generated the helper class, let's instrument our app's code.

In our application, we have a long running Service that has access to the data we want to collect. The goal is to periodically collect the metric data for the past hour and log it to the EventLog.

For simplicity, we will use the Handler and Runnable APIs and not the more complicated JobScheduler. Carefully consider what API is best to use in your use case.

package com.acme.myapp;
import android.os.Bundle;
import android.os.Handler;
import java.util.concurrent.TimeUnit;
class MyAppService : Service() {
private final static long MEMFAULT_HEARTBEAT_INTERVAL_MILLIS = TimeUnit.HOURS.toMillis(1)
private Handler handler = new Handler();
private Runnable runnable = new Runnable() {
public void run() {
// Get values for the past hour:
// For aggregates (counters, averages, max, min, etc) ensure to
// reset them after logging the values!
int rssi = getRssi();
int ambientLight = getAmbientLight();
int diskSpaceFree = getDiskSpaceFree();
// Write the values to the EventLog:
// Reschedule:
protected void onCreate(Bundle savedInstanceState) {
// Start collecting metrics:;
protected void onDestroy() {
/* ... snip ... */

Now build AOSP and/or the app, install it and let it run for a while to collect your custom metrics data!

Wrapping Up#

The metrics data is automatically emitted into a Bug Report. For testing purposes, run adb bugreport to generate a Bug Report locally. Upload the resulting bugreport-*.zip file to Memfault, by going into "Issues" => "Manual Upload".

Once the Bug Report is processed, go to "Devices" => Select your Device => "Timeline". A new group called "EventLog" should appear in the Timeline view. Expand the group to see "Rssi", "Ambient Light" and "Disk Space Free".


  • I don't see any rows for my custom metrics in the Memfault UI, how to debug this?

    • Try running adb logcat -b events. This wil print all collected data. If your events are not present there, double check that the EventLog.write... calls actually happen.
    • Make sure the tag names are prefixed with mflt_ (lowercase). Data is only shown for named tags starting with the mflt_ prefix or for anonymous tags that are not added to /system/etc/event-log-tags.
    • Ensure EventLog calls have only 1 value argument. Multiple values per tag are currently not supported.
    • Ensure calls for a given tag must use the same type. Changing the value type from one call to the next for the same tag is not supported.