Triage codelab

Contributors: cphoenix@

This codelab explains the Triage utility:

  • What it's for.
  • How to run it, including command line options.
  • How to add and test configuration rules to detect problems in Fuchsia snapshots.

The source files and examples to which this document refers are available at:

What is Triage?

Triage allows you to scan Fuchsia snapshots ( files) for predefined conditions.

The Triage system makes it easy to configure new conditions, increasing the usefulness of Triage for everyone.


Before you start on this codelab, make sure you have completed the following:

Running Triage from the command line

  • To run Triage:
fx triage

This command downloads a fresh file using the fx snapshot command. This command runs the default rules, which are located in the source tree:

  • //src/diagnostics/config/triage/*.triage

To analyze a specific file, use --data.

  • You can specify at most one --data argument.
  • The argument to --data can also be a path to a directory containing an unzipped snapshot.
  • If you run fx triage without specifying a --data option, it runs a fresh fx snapshot and analyzes its files.
fx triage --data my/foo/

To use a specific configuration file or all *.triage files in a specific directory, use --config.

  • You can use multiple --config arguments.
  • If a --config argument is used, the default rules will not be automatically loaded.
fx triage --config my/directory --config my/file.triage

Adding Triage rules

The rest of this codelab explains how to configure new behavior in Triage.


Triage condenses the mass of Diagnostic data into useful information, through the following steps:

  1. Select values from the inspect.json file using selector strings in the select section of the config file.
  2. Perform computations and comparisons to generate new values, as specified in the eval section of the config file.
  3. Take actions according to entries in the act section of the config file.
    1. Warn if an error condition (Boolean expression) is true.
    2. Display a value to the user.
  4. Support unit-testing the actions and computation via entries in the test section.

Find the codelab's sample files

Navigate to the examples/diagnostics/triage directory in your source tree. The following command is intended to run from that directory:

fx triage --config . --data snapshot

Running this command in the sample directory with the unmodified codelab files prints a line from a pre-supplied action:

Warning: 'always_triggered' in 'rules' detected 'Triage is running': 'always_true' was true


This codelab includes an inspect.json file with Inspect data to make the exercises work predictably. This file is in the sample directory under snapshot/inspect.json.

Note: inspect.json files are normally packaged in the file produced by fx snapshot. Either use unzip to unpack these files or give the .zip file as the argument to --data. For this codelab the snapshot has already been unzipped.


The Triage program uses configuration loaded from one or more .triage files. This codelab uses the rules.triage file located in the sample directory.

.triage files use JSON5 which is easier to write and read than JSON:

  • It's good style to put a comma after the last list item.
  • Most keys (including all valid Triage names) don't need to be wrapped in quotation marks.
  • /* Multiline */ and // single-line comments can be used.

Add selectors for the Inspect values

The inspect.json file in the sample directory indicates a couple of problems with the system. You're going to configure the triage system to detect those problems.

This step configures Triage to extract values from the data in the inspect.json file.

The rules.triage file contains a key-value section called select. Each key (name) can be used in the body of other config entries. Each value is a selector string. In effect, each entry in the select section (and the eval section, described below) defines a variable.

The selector string is a colon-separated string that tells where in the Inspect data to find the value you need.

select: {
    disk_total: "INSPECT:bootstrap/fshost:root/data_stats/stats:total_bytes",
    // "data_stat" is an intentional typo to fix later in the codelab.
    disk_used: "INSPECT:bootstrap/fshost:root/data_stat/stats:used_bytes",

Inspect data published by a component is organized as a tree of nodes with values (properties) at the leaves. The inspect.json file is an array of these trees, each with a moniker that identifies the source component.

The portion of the selector string between the INSPECT: and the second colon should match one of the moniker strings in the inspect.json file.

The portion between the second and third colons is a /-separated list of node names.

The portion after the last colon is the property name.

The above selector string indicates a component whose moniker matches the string bootstrap/fshost and whose inspect tree contains the path root/data_stat/stats. It also indicates the used_bytes property from the stats subnode of the root node of that component's Inspect Tree.

Put the above selectors into the “select” section of the rules.triage file.

Generating selectors

Entering selectors by hand is error-prone, so fx triage --select can be used to output valid selectors.

fx triage --data snapshot --select total_bytes

Multiple --select parameters can be used. The program will generate all possible selectors for the snapshot's Diagnostic data, then filter (grep) through all --select parameters.

Add a computation

After selecting values from the inspect.json file, you need to do some logic, and probably some arithmetic, to see whether those values indicate a condition worth flagging.

Copy and add the following line to the eval section of the rules.triage file to calculate how full the disk is:

eval: {
    disk_percentage: "disk_used / disk_total",

eval entries use ordinary infix math expressions. See the Details section for more information.

Add an action

In the “act” part of the config file, add an action that prints a warning when the disk is 98% full. Use the following lines:

act: {
    disk_full: {
        type: "Warning",
        trigger: "disk_percentage > 0.98",
        print: "Disk reached 98% full",

Note the following:

  • The “trigger” is an expression that evaluates to a Boolean value. This may be the name of a Boolean-type selector or computation, or any suitable math expression.
  • See the Details section for more information about comparisons.
  • See the config reference for other supported actions.

Add a gauge

act: {
    disk_display: {
        type: "Gauge",
        value: "disk_percentage",
        format: "percentage",

Gauges always display the supplied value.

The format field is optional. A value of “percentage” tells the output formatter to expect a value between 0 and 1 and display it as a percentage.

Try it out

The following command will run Triage against the local config file.

fx triage --config . --data snapshot

You will get an error that looks like the following:

[ERROR] In config 'rules': No value found matching selector

There was a typo in the selector rules. Triage could not find values needed to evaluate a rule. In fact, the correct selector is “data_stats” not “data_stat.” Fix it in your selector rules and try again.

fx triage --config . --data snapshot

Now what happened? Nothing, right? So, how do you know whether there was no problem in the inspect.json file, or a bug in your rule?

Test your rule

You can (and should!) add tests for your actions. For each test, specify values and whether or not those values should trigger your rule.

To test the rule you've added, add the following to the test section of the rules.triage file:

test: {
    is_full: {
        yes: ["disk_full"],
        values: {
            disk_used: 98,
            disk_total: 100,

Note: Unlike the right hand side of eval entries, the values entries are parsed as JSON values, not expression strings. Numbers should not be quoted.

The keys in the values section should be the names of eval or select entries. The values supplied will override the value that the entry would have selected or calculated.

You can also test conditions in which actions should not trigger:

test: {
    not_full: {
        no: ["disk_full"],
        values: {
            disk_used: 97,
            disk_total: 100,

To run the test, just run Triage. It automatically self-tests each time it's run.

fx triage --config . --data snapshot

Whoops! That should signal an error:

Test is_full failed: trigger 'disk_percentage > 0.98' of action disk_full returned Bool(false), expected true

Fix your rule

You want to trigger when the disk is 98% or more full, but that's not quite what you wrote, and your test caught the problem. Modify the > in your action to be a >=:

        trigger: "disk_percentage >= 0.98",

Run Triage again. The error should disappear, replaced by a warning that your inspect.json file does in fact indicate a full disk.

Warning: 'disk_full' in 'rules' detected 'Disk is 98% full': 'disk98' was true

Log file scanning

You can write expressions to test whether a line of a log file (syslog.txt, klog.txt, bootlog.txt) is matched by a regular expression. It looks like this:

    eval: {
        syslog_has_not_found: "SyslogHas('ERROR.*not found')",
    act: {
        something_not_found: {
            trigger: "SyslogHas('ERROR.*not found')",

Note: To nest quotation marks you can use either single quote ' or escaped double quote \".

The functions are SyslogHas(), KlogHas(), BootlogHas(). If a log file is missing (for example, some snapshots contain no bootlog.txt) it is treated the same as an empty file.

To test this, you can include entries in the test like this:

test: {
    test_error: {
        yes: [error_scan],
        syslog: "ERROR: file Foo not found\nSecond line OK",


The snapshot contains a file annotations.json, which contains information on the build, board, uptime, and so on.

Values can be fetched from this file by using the function Annotation() with a single string parameter, which is a key of the JSON object in the file. For example,

eval: {
    using_chromebook: "Annotation('build.board') == 'chromebook-x64'",

Use multiple configuration files

You can add any number of Triage configuration files, and even use variables defined in one file in another file. This has lots of applications:

  • One file for disk-related variables and actions, and another for network-related variables and actions.
  • A file to define product-specific numbers.
  • Separate files for particular engineers or teams.

Add a file “product.triage” containing the following:

    eval: {
        max_components: "4",

Note the following:

  • Empty sections may be omitted from .triage files. This file contains no select, act, or test entries.
  • Although numeric values in JSON are not quoted, 4 is a math expression string so it does need to be quoted.

Add the following entries to the rules.triage file:

select: {
    actual_components: "INSPECT:bootstrap/archivist:root/event_stats:components_started",

That will extract how many components were active in the device.

eval: {
    too_many_components: "actual_components > product::max_components",

That compares the actual components with the theoretical maximum for the product.

Note: To use variable names from another file, combine the file name, two colons, and the variable name.

Finally, add an action:

act: {
    component_overflow: {
        type: "Warning",
        trigger: "too_many_components",
        print: "Too many components!",

Unfortunately, this device tried to use too many components, so this warning should trigger when “fx triage” is run.

Note: The trigger of an action can also use file::name syntax to refer to a variable from another file.

In a production environment, several “product.triage” files could be maintained in different directories, and Triage could be directed to use any of them with the “--config” command line argument.

Tests and namespaces

Tests use only the metrics within the file where the test occurs, plus the values supplied by the test. An expression (eval or test trigger) that uses namespaced values like “a::b” must have those values supplied by an “a::b” entry in the test's values.

Note: Unlike most keys in .triage files, namespaced names must be double-quoted when used as keys.

test: {
    component_max_ok: {
        no: [
        values: {
            actual_components: 17,
            "product::max_components": 17,



Names (of selectors, expressions, actions, and tests, as well as the basenames of config files) can be any letter or underscore, followed by any number of letters, numbers, or underscores.

Names beginning with underscores may have special meaning in future versions of Triage. They‘re not forbidden, but it’s best to avoid them.

The name of each .triage file establishes its namespace. Loading two .triage files with the same name from different directories is not allowed.

Math expressions

  • Variables can be 64-bit float, signed 64-bit int, or Boolean.
  • Arithmetic expressions use + - * / // operators, with ordinary order and precedence of operations.
  • The division operator / produces a float value.
  • The division operator // produces an int value, truncating the result toward 0, even with float arguments. (Note this is different from Python 3 where // truncates downward.)
  • + - * preserve the type of their operands (mixed promotes to float).
  • Comparison operators are > >= < <= == !=
  • Comparisons have Boolean result type and can be used to trigger actions.
  • You can combine computations and comparisons in a single eval rule.
  • You can use parentheses.
  • You can use the key names of eval and select entries as variables.
  • Spaces are optional everywhere, and allowed everywhere except inside filename::variable namespaced variables.

Predefined functions

Triage provides predefined functions for use in eval expressions:

  • Max(value1, value2, value3...) returns the largest value, with type promotion to float.
  • Min(value1, value2, value3...) returns the smallest value, with type promotion to float.
  • And(value1, value2, value3...) takes Boolean arguments and returns the logical AND of the values.
  • Or(value1, value2, value3...) takes Boolean arguments and returns the logical OR of the values.
  • Not(value) takes one Boolean argument and returns the logical NOT of it.
  • SyslogHas(matcher), KlogHas(matcher), BootlogHas(matcher) return true if the corresponding log file has a line matching matcher, which is a string containing a regex expression.
  • Annotation(key) returns the corresponding value from the annotations.json file.
  • Option(value1, value2, value3...) returns the first useful value, to support selector migrations and defaults: the first non-empty-list, non-Missing value if any; or empty list if one was given; or Missing.
  • Missing(value) returns true if the value is an error indication.
  • Days(), Hours(), Minutes(), Seconds(), Millis(), Micros(), and Nanos() calculate values for comparison with monotonic timestamps.
  • Now() returns the approximate timestamp when the Diagnostic data was created.
  • StringMatches(value, regex) applies the given regex to the given value and returns true if there is a match. The regex syntax is that supported by the Rust regex crate.

Note: Since logs are not structured, selectors can't be applied to them, so we supply regex matching functions instead.

Functional programming

Triage can apply functions to vectors of values. Vectors have the format "[expr, expr, expr...]". Some selectors return multi-element vectors.

Triage provides the functions Map(), Fold(), Filter(), and Count() to process vectors, Fn() to define functions or lambdas for Map, Fold, and Filter to apply, and Apply() to apply a Fn() to arguments.

For more information see Configuring fx triage.

Further Reading

See fx triage for the latest features and options - Triage will keep improving!