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+# Statistics
+
+To facilitate debugging and support situations, the _NNG_ library
+provides for collection and reporting of numerous {{i:statistics}}.
+
+These statistics are organized in a tree, and include both values,
+and metadata describing the statistics. In order to be efficient and
+minimize the impact of maintaining statistics, an explicit snapshot
+of statistics must be taken, and that snapshot can then be processed.
+
+> [!NOTE]
+> Statistics may be disabled by build-time configuration options,
+> in order to reduce program size and run-time overheads.
+
+## Statistic Structure
+
+```c
+typedef struct nng_stat nng_stat;
+```
+
+The {{i:`nng_stat`}} structure represents a statistic, which is a single value
+collected at a specific point in time.
+
+This structure has meta-data describing the value, the value itself, and links
+to any sibling or child statistics.
+
+> [!NOTE]
+> The presence, name, and semantics of any given statistic are
+> subject to change at any time and without notice.
+
+## Collecting a Snapshot
+
+```c
+int nng_stats_get(nng_stat **statsp);
+```
+
+The {{i:`nng_stats_get`}} function takes a snapshot of the statistics for
+the system and returns it through the pointer _statsp_.
+This function may return `NNG_ENOMEM` if memory is exhausted, or `NNG_ENOTSUP` if the statistics
+support is not enabled in the build, but is otherwise expected to return zero.
+
+## Freeing a Snapshot
+
+```c
+void nng_stats_free(nng_stat *stat);
+```
+
+The {{i:`nng_stats_free`}} function deallocates the snapshot referenced by _stat_.
+
+> [!IMPORTANT]
+> The _stat_ must be root of the statistics tree, i.e. the value that was returned
+> through _statsp_ using the function `nng_stats_get`.
+
+## Traversing the Tree
+
+```c
+nng_stat *nng_stat_child(nng_stat *stat);
+nng_stat *nng_stat_next(nng_stat *stat);
+```
+
+Traversing the tree of statistics is done using the {{i:`nng_stat_child`}} and
+{{i:`nng_stat_next`}} functions.
+
+The `nng_stat_child` function returns either the first child of _stat_,
+or `NULL` if the _stat_ has no children.
+
+The `nng_stat_next` function returns the nearest sibling to the right of _stat_,
+or `NULL` if _stat_ has no more siblings to the right.
+
+## Finding a Statistic
+
+```c
+nng_stat *nng_stat_find(nng_stat *stat, const char *name);
+nng_stat *nng_stat_find_dialer(nng_stat *stat, nng_dialer dialer);
+nng_stat *nng_stat_find_listener(nng_stat *stat, nng_dialer listener);
+nng_stat *nng_stat_find_socket(nng_stat *stat, nng_dialer socket);
+```
+
+Sometimes it is easiest to search for a specific statistic, matching by name,
+or possibly to find the tree of statistics associated iwth a specific [socket][nng_socket],
+[dialer][nng_dialer], or [listener][nng_listener].
+
+The `nng_stat_find` functions are provided for this purpose.
+
+The {{i:`nng_stat_find`}} function returns the first statistic within the subtree of
+statistics _stat_, with the given _name_. If no such statistic can be found, `NULL`
+is returned.
+
+The {{i:`nng_stat_find_dialer`}}, {{i:`nng_stat_find_listener`}}, and {{i:`nng_stat_find_socket`}}
+return the statistics subtree for the given dialer, listener, or socket object. If no such
+statistic can be found, then they return `NULL`.
+These functions should be provided the root of the statistic tree, in order to ensure
+that they can find the desired object.
+
+## Statistic Identification
+
+```c
+const char *nng_stat_name(nng_stat *stat);
+const char *nng_stat_desc(nng_stat *stat);
+```
+
+Every statistic has a name, returned by {{i:`nng_stat_name`}}, and a description, returned by
+{{i:`nng_stat_desc`}}. Descriptions are human-readable text, which might be useful for display.
+
+## Statistic Type
+
+```c
+int nng_stat_type(nng_stat *stat);
+```
+
+The function {{i:`nng_stat_type`}} returns the type of the statistic.
+The type of a statistic determines the nature of the value, and which
+function can be used to obtain that value.
+
+- {{i:`NNG_STAT_SCOPE`}}: <a name="NNG_STAT_SCOPE"></a>
+  The statistic does not carry any real value, but is
+  used for grouping related statistics together. This is a nexus in the
+  statistics tree.
+
+- {{i:`NNG_STAT_COUNTER`}}: <a name="NNG_STAT_COUNTER"></a>
+  The statistic is a counter that only increments.
+  Usually the change in the value of the statistic is more interesting
+  (as a rate) than the absolute value at any given time. The value should
+  be obtained using [`nng_stat_value`][nng_stat_value].
+  The units will be given by the value returned from [`nng_stat_unit`][nng_stat_unit].
+
+- {{i:`NNG_STAT_LEVEL`}}: <a name="NNG_STAT_LEVEL"></a>
+  The statistic represnts a measured value which corresponds
+  to a specific value at a specific time. For example, this may represent the
+  number of messages currently queued for some operation, or the link speed
+  of a network interface. Most often the absolute value is more interesting
+  than the change in the value over time. Again the value can be obtained with
+  [`nng_stat_value`][nng_stat_value], and any appropriate unit of measurement
+  with [`nng_stat_unit`][nng_stat_unit].
+
+- {{i:`NNG_STAT_STRING`}}: <a name="NNG_STAT_STRING"></a>
+  The statistic is a string, such as a name. The value
+  of the string can be obtained with [`nng_stat_string`][nng_stat_string].
+  The value of this string
+  will remain valid until the snapshot is deallocated with [`nng_stats_free`][nng_stats_free].
+
+- {{i:`NNG_STAT_BOOLEAN`}}: <a name="NNG_STAT_BOOLEAN"></a>
+  The value of the statistic is a truth value (either `true`
+  or `false`) and can be obtained with [`nng_stat_bool`][nng_stat_bool].
+
+- {{i:`NNG_STAT_ID`}}: <a name="NNG_STAT_ID"></a>
+  The value of the statistic is a numeric identifier, such as a socket
+  identifier. The value can be obtained with [`nng_stat_value`][nng_stat_value],
+  and will be fixed for the life of the statistic.
+
+## Statistic Value
+
+```c
+uint64_t nng_stat_value(nng_stat *stat);
+const char *nng_stat_string(nng_stat *stat);
+bool nng_stat_bool(nng_stat *stat);
+```
+
+These functions return the value associated with the statistic.
+
+The {{i:`nng_stat_value`}} function returns the the numeric value for the statistic _stat_
+of type [`NNG_STAT_COUNTER`][NNG_STAT_COUNTER], [`NNG_STAT_LEVEL`][NNG_STAT_LEVEL], or [`NNG_STAT_ID`][NNG_STAT_ID].
+If _stat_ is not one of these types, then it returns zero.
+
+The {{i:`nng_stat_bool`}} function returns the Boolean value (either `true` or `false`) for the statistic _stat_ of
+type [`NNG_STAT_BOOLEAN`][NNG_STAT_BOOLEAN]. If the statistics is not of this type, then it returns `false`.
+
+The {{i:`nng_stat_string`}} function returns a pointer to a string value for the statistic _stat_,
+of type [`NNG_STAT_STRING`][NNG_STAT_STRING]. This string will remain valud until the snapshot that
+_stat_ was collected with is deallocated with [`nng_stats_free`][nng_stats_free]. If the statistic
+is not of type `NNG_STAT_STRING`, then `NULL` is returned.
+
+## Statistic Units
+
+```c
+int nng_stat_unit(nng_stat *stat);
+```
+
+For statistics of type [`NNG_STAT_COUNTER`][NNG_STAT_COUNTER] or [`NNG_STAT_LEVEL`][NNG_STAT_LEVEL],
+it is often useful to know what that quantity being reported measures.
+The following units may be returned from {{i:`nng_stat_unit`}} for such a statistic:
+
+- {{i:`NNG_UNIT_NONE`}}: No unit is known or applies.
+- {{i:`NNG_UNIT_BYTES`}}: A count of bytes.
+- {{i:`NNG_UNIT_MESSAGES`}}: A count of messages.
+- {{i:`NNG_UNIT_MILLIS`}}: A count of milliseconds.
+- {{i:`NNG_UNIT_EVENTS`}}: A count of events of some type.
+
+## Statistic Timestamp
+
+```c
+uint64_t nng_stat_timestamp(nng_stat *stat);
+```
+
+Statistics have a timestamp indicating when the value was sampled,
+obtained via {{i:`nng_stat_timestamp`}}. The timestamp is given in
+in milliseconds since a reference time, and the reference time used
+here is the same reference time used for [`nng_clock`][nng_clock].
+
+## See Also
+
+[nng_clock][nng_clock]
+
+[nng_stat_type]: #statistic-type
+[nng_stats_free]: #freeing-a-snapshot
+[nng_stat_value]: #statistic-value
+[nng_stat_bool]: #statistic-value
+[nng_stat_string]: #statistic-value
+[nng_stat_unit]: #statistic-units
+[NNG_STAT_ID]: #NNG_STAT_ID
+[NNG_STAT_COUNTER]: #NNG_STAT_COUNTER
+[NNG_STAT_LEVEL]: #NNG_STAT_LEVEL
+[NNG_STAT_SCOPE]: #NNG_STAT_SCOPE
+[NNG_STAT_STRING]: #NNG_STAT_STRING
+[NNG_STAT_BOOLEAN]: #NNG_STAT_BOOLEAN
+[nng_clock]: ../util/nng_clock.md
+[nng_socket]: TODO.md
+[nng_dialer]: TODO.md
+[nng_listener]: TODO.md