Events and Metrics in AppScope

When AppScope has interposed a function, and then the application being scoped executes that function, AppScope can emit events and metrics. Both events and metrics follow patterns that are rigorously defined in validatable JSON Schema, and documented in the Schema Reference.

AppScope outputs metrics either in StatsD format or in equivalent JSON. AppScope can also watch for and intercept StatsD-formatted metrics being emitted by a scoped application.

What Events Do

Events describe the action performed by the interposed function. For example, a event could tell you that a network connection was established by Firefox, from the local IP/port to the remote IP/port, using HTTP over TCP.

The process‑start message

If configevent is set to true, AppScope sends a process‑start message upon establishing a new connection. This message – the start.msg event – reports the configs in effect at the time. The element reports the version of AppScope that's being used.

The process‑start message also includes four distinct identifiers:

  • UUID with key uuid and a value in canonical UUID form. UUID is a universally-unique process identifier, meaning that no two processes will ever have the same UUID value on the same machine.
  • Machine ID with key machine_id and a value that AppScope obtains from /etc/machine-id. The machine ID uniquely identifies the host, as described in the man page. When /etc/machine-id is not available (e.g., in Alpine, or in a container), AppScope generates the machine ID using a repeatable MD5 hash of the host's first MAC address. Two containers on the same host can have the same machine ID.
  • Process ID, with key pid and a value that is always unique at any given time, but that the machine can reuse for different processes at different times.
  • ID, with key id and a value that concatenates (and may truncate) the scoped app's hostname, procname, and command. This value is not guaranteed to be unique.

By itself, the UUID process ID is unique for a given machine. In principle, UUID and Machine ID together constitute a tuple ID that is unique across all machine namespaces.

What Metrics Do

Metrics can do any of three things:

  • When verbosity is set to lower values, metrics summarize aspects of the action performed by the interposed function, for a reporting period that defaults to 10 seconds. (See the Metric verbosity level section of the config file.) For example, a metric could tell you that an httpd process has read a total of 320967 bytes from the filesystem, having performed multiple reads over a 10-second period.
  • When verbosity is set to higher values, metrics provide details about the action performed by the interposed function in real time. For example, an metric could tell you that an httpd process has done one read of 8245 bytes from one specific file, /etc/httpd/conf/httpd.conf.
  • When the metric name begins with proc, metrics periodically report information about resource usage at a point in time. For example, a proc.mem metric could tell you that httpd is currently using 62,123 KB of memory.

The table below gives the gory details:

Verbosity Level Cardinality (dimensions) Aggregate Error Metrics? Aggregate DNS Metrics? Filesystem Metrics to Aggregate Network Metrics to Aggregate
0 none yes yes all all
1 data and unit yes yes all all
2 same as 1, plus class and proto yes yes all all
3 same as 2, plus op yes yes all all
4 same as 3, plus pid, host, proc, and http_status yes yes all all
5 same as 4, plus domain and file no yes all all
6 same as 5, plus localip, remoteip, localp, port, and remotep no no all but open/close all
7 same as 6, plus fd and args no no all but open/close and stat all but connect
8 same as 7, plus duration, numops, req_per_sec, req, resp, and protocol no no all but open/close, stat, and seek same as 7
9 same as 8 no no all but open/close, stat, seek, and read/write all but connect and send/recv

How to Interpret Metrics

To interpret a given metric, you must consider its type. There are four possibilities:

Metric Type Value Description
gauge A numeric value that can increase or decrease over time – like a temperature or pressure gauge.
counter A cumulative numeric value – it can only increase over time.
timer Measures how long a given event type takes (duration).
histogram A StatsD histogram distributes sampled observations into buckets. With AppScope, we encounter histograms only in the special case where AppScope intercepts StatsD-formatted metrics whose type is histogram. AppScope merely preserves that labeling: we assume that the scoped application has already aggregated the values into buckets.

For example, proc.fd is a gauge that indicates how many files were open at one point in time. If we're scoping top, that's typically fewer than 10. By contrast, is a count that increments every time a file is opened. When scoping top over a reporting period of 10 seconds, you could see values in the hundreds or thousands.

It's important to take into account whether the application or command you are scoping is short-lived or longer-lasting. For commands that complete very quickly, a gauge will report the value at the moment that AppScope exits the process.

By default, all classes of events and metrics are turned on – but you can turn any class of metric or event data on or off individually. To do this, include or omit the desired watch type(s) from the metric > watch[*] array and/or the event > watch[*] array in the config file. Environment variables can achieve the same effect. For example, the environment variable to turn off metrics of watch type statsd would be SCOPE_METRIC_STATSD=false. To turn on events of watch type logfile, you'd use SCOPE_EVENT_LOGFILE=true.