Lightweight C library for HTML5 websockets


lws_metrics records and aggregates events at all lws layers.

There are three distinct parts:

  • the architecture inside lws for collecting and aggregating / decimating the events and maintaining statistics about them, these are lws_metric objects
  • an external handler for forwarding aggregated metrics. An lws_system ops interface to pass on the aggregated metrics to an external backend. lws presents its own public metrics objects and leaves it to the external code to have a shim to marry the lws metrics up to whatever is needed in the metrics backend
  • a policy for when to emit each type of aggregated information to the external handler. This can be specified in the generic Secure Streams policy, or a linked-list of lws_metric_policy_t object passed it at context creation in info.metrics_policies.

The external backend interface code may itself make use of lws connectivity apis including Secure Streams itself, and lws metrics are available on that too.

<tt>lws_metrics</tt> policy-based reporting

Normally metrics implementations are fixed at build-time and cannot change without a coordinated reflash of devices along with a change of backend schema.

lws_metrics separates out the objects and code necessary to collect and aggregate the data cheaply, and the reporting policy that controls if, or how often, the results are reported to the external handler.

Metrics are created with a namespace name and the policy applies itself to those by listing the names, with wildcards allowed, the policy applies to, eg if specified in the Secure Streams JSON policy

"metrics": [
"name": "tensecs",
"us_schedule": 10000000,
"report": "cpu.*"
}, {
"name": "30secs",
"us_schedule": 30000000,
"report": "*, n.http.*,*, vh.*"

Metrics that do not have a reporting policy do not report, but continue to aggregate measurements in case they are bound to a policy dynamically later.

Freeform metrics naming

There is no predefined metrics schema, metrics objects, including those created by applications, can independently choose their own name in a namespace like "cpu.srv" or "", and can set a prefix for all metrics names created in a context (by setting info.metrics_prefix at context creation time).

This allows multiple processes in a single device to expose copies of the same metrics in an individually addressable way, eg, if the UI process specifies the prefix "ui", then its lws metrics like "cpu.srv" will actually be created as "ui.cpu.srv".

Applications can freely define their own lws_metrics measurements with their own names in the namespace too, without central registration, and refer to those names in the reporting policy same as any other metric names.

If the metrics backend requires a fixed schema, the mapping between the lws_metrics names and the backend schema indexes will be done in the lws_system external reporting api implementation alone. Metrics objects contain a void * backend_opaque that is ignored by lws and can be set and read by the external reporting handler implementation to facilitate that.

Histogram metrics tagging

Histogram metrics track differently-qualified results in the same metric, for example the metric maintains separate result counts for all variations and kinds of failure.

[2021/03/01 06:34:05:6570] U: my_metric_report:{ss="badcert_selfsigned",hostname="",peer="",tls="invalidca"} 2
[2021/03/01 06:34:05:6573] U: my_metric_report:{hostname="",peer="",tls="invalidca"} 1
[2021/03/01 06:34:05:6576] U: my_metric_report:{ss="badcert_expired",hostname="",peer="",tls="expired"} 2
[2021/03/01 06:34:05:6578] U: my_metric_report:{hostname="",peer="",tls="expired"} 1
[2021/03/01 06:34:05:6580] U: my_metric_report:{ss="badcert_hostname",hostname="",peer="",tls="hostname"} 2
[2021/03/01 06:34:05:6583] U: my_metric_report:{hostname="",peer="",tls="hostname"} 1
[2021/03/01 06:34:05:6585] U: my_metric_report:{dns="nores -2"} 8

The user handler for metrics is expected to iterate these, in the provided examples (eg, minimal-secure-streams-testsfail)

static int
my_metric_report(lws_metric_pub_t *mp)
lws_metric_bucket_t *sub = mp->u.hist.head;
char buf[192];
do {
if (lws_metrics_format(mp, &sub, buf, sizeof(buf)))
lwsl_user("%s: %s\n", __func__, buf);
} while ((mp->flags & LWSMTFL_REPORT_HIST) && sub);
/* 0 = leave metric to accumulate, 1 = reset the metric */
return 1;
static const lws_system_ops_t system_ops = {
.metric_report = my_metric_report,

<tt>lws_metrics</tt> decimation

Event information can easily be produced faster than it can be transmitted, or is useful to record if everything is working. In the case that things are not working, then eventually the number of events that are unable to be forwarded to the backend would overwhelm the local storage.

For that reason, the metrics objects are designed to absorb and summarize a potentially large number of events cheaply by aggregating them, so even extreme situations can be tracked meaningfully inbetween dumps to the backend.

There are two approaches:

  • "aggregation": decimate keeping a uint64 mean + sum, along with a max and min
  • "histogram": keep a linked-list of different named buckets, with a 64-bit counter for the number of times an event in each bucket was observed

A single metric aggregation object has separate "go / no-go" counters, since most operations can fail, and failing operations act differently.

lws_metrics 'aggregation' supports decimation by

  • a mean of a 64-bit event metric, separate for go and no-go events
  • counters of go and no-go events
  • a min and max of the metric
  • keeping track of when the sample period started

In addition, the policy defines a percentage variance from the mean that optionally qualifies events to be reported individually.

The lws_metrics 'histogram' allows monitoring of different outcomes to produce counts of each outcome in the "bucket".

<tt>lws_metrics</tt> flags

When the metrics object is created, flags are used to control how it will be used and consumed.

For example to create a histogram metrics object rather than the default aggregation type, you would give the flag LWSMTFL_REPORT_HIST at creation time.

Flag Meaning
LWSMTFL_REPORT_OUTLIERS track outliers and report them internally
LWSMTFL_REPORT_OUTLIERS_OOB report each outlier externally as they happen
LWSMTFL_REPORT_INACTIVITY_AT_PERIODIC explicitly externally report no activity at periodic cb, by default no events in the period is just not reported
LWSMTFL_REPORT_MEAN the mean is interesting for this metric
LWSMTFL_REPORT_ONLY_GO no-go pieces invalid and should be ignored, used for simple counters
LWSMTFL_REPORT_DUTY_WALLCLOCK_US the aggregated sum or mean can be compared to wallclock time
LWSMTFL_REPORT_HIST object is a histogram (else aggregator)

Built-in lws-layer metrics

lws creates and maintains various well-known metrics when you enable build with cmake -DLWS_WITH_SYS_METRICS=1:

Aggregation metrics

metric name scope type meaning
cpu.svc context monotonic over time time spent servicing, outside of event loop wait context go/no-go mean duration of blocking libc DNS lookup context go/no-go mean duration of SYS_ASYNC_DNS lws DNS lookup context go/no-go mean duration of tcp connection until accept context go/no-go mean duration of tls connection until accept
n.http.txn context go (2xx)/no-go mean duration of lws http transaction context go/no-go mean duration of Secure Stream transaction context go/no-go mean time taken for client -> proxy connection
vh.[vh-name].rx vhost go/no-go sum received data on the vhost
vh.[vh-name].tx vhost go/no-go sum transmitted data on the vhost

Histogram metrics

metric name scope type meaning context histogram Histogram of connection attempt failure reasons

Connection failure histogram buckets

Bucket name Meaning
tls/invalidca Peer certificate CA signature missing or not trusted
tls/hostname Peer certificate CN or SAN doesn't match the endpoint we asked for
tls/notyetvalid Peer certificate start date is in the future (time wrong?)
tls/expired Peer certificate is expiry date is in the past
dns/badsrv No DNS result because couldn't talk to the server
dns/nxdomain No DNS result because server says no result

The lws-minimal-secure-streams example is able to report the aggregated metrics at the end of execution, eg

[2021/01/13 11:47:19:9145] U: my_metric_report: cpu.svc: 137.045ms / 884.563ms (15%)
[2021/01/13 11:47:19:9145] U: my_metric_report: Go: 4, mean: 3.792ms, min: 2.470ms, max: 5.426ms
[2021/01/13 11:47:19:9145] U: my_metric_report: Go: 4, mean: 40.633ms, min: 17.107ms, max: 94.560ms
[2021/01/13 11:47:19:9145] U: my_metric_report: Go: 3, mean: 91.232ms, min: 30.322ms, max: 204.635ms
[2021/01/13 11:47:19:9145] U: my_metric_report: n.http.txn: Go: 4, mean: 63.089ms, min: 20.184ms, max: 125.474ms
[2021/01/13 11:47:19:9145] U: my_metric_report: Go: 4, mean: 161.740ms, min: 42.937ms, max: 429.510ms
[2021/01/13 11:47:19:9145] U: my_metric_report: vh._ss_default.rx: Go: (1) 102, NoGo: (1) 0
[2021/01/13 11:47:19:9145] U: my_metric_report: vh.le_via_isrg.rx: Go: (22) 28.165Ki
[2021/01/13 11:47:19:9145] U: my_metric_report: vh.le_via_isrg.tx: Go: (1) 267
[2021/01/13 11:47:19:9145] U: my_metric_report: vh.api_amazon_com.rx: Go: (1) 1.611Ki, NoGo: (1) 0
[2021/01/13 11:47:19:9145] U: my_metric_report: vh.api_amazon_com.tx: Go: (3) 1.505Ki

lws-minimal-secure-stream-testsfail which tests various kinds of connection failure reports histogram results like this

[2021/01/15 13:10:16:0933] U: my_metric_report: tot: 36, [ tls/invalidca: 5, tls/expired: 5, tls/hostname: 5, dns/nxdomain: 21 ]

Support for openmetrics

Openmetrics defines a textual metrics export format comaptible with Prometheus. Lws provides a protocol plugin in ./plugins/protocol_lws_openmetrics_export that enables direct export for prometheus scraping, and also protocols to proxy openmetrics export for unreachable servers.