+[role="since-2.10"]
+[[notif-trigger-api]]
+=== Get notified when a channel's buffer usage is too high or too low
+
+With LTTng's $$C/C++$$ notification and trigger API, your user
+application can get notified when the buffer usage of one or more
+<<channel,channels>> becomes too low or too high. You can use this API
+and enable or disable <<event,event rules>> during tracing to avoid
+<<channel-overwrite-mode-vs-discard-mode,discarded event records>>.
+
+.Have a user application get notified when an LTTng channel's buffer usage is too high.
+====
+In this example, we create and build an application which gets notified
+when the buffer usage of a specific LTTng channel is higher than
+75{nbsp}%. We only print that it is the case in the example, but we
+could as well use the API of <<liblttng-ctl-lttng,`liblttng-ctl`>> to
+disable event rules when this happens.
+
+. Create the application's C source file:
++
+--
+[source,c]
+.path:{notif-app.c}
+----
+#include <stdio.h>
+#include <assert.h>
+#include <lttng/domain.h>
+#include <lttng/action/action.h>
+#include <lttng/action/notify.h>
+#include <lttng/condition/condition.h>
+#include <lttng/condition/buffer-usage.h>
+#include <lttng/condition/evaluation.h>
+#include <lttng/notification/channel.h>
+#include <lttng/notification/notification.h>
+#include <lttng/trigger/trigger.h>
+#include <lttng/endpoint.h>
+
+int main(int argc, char *argv[])
+{
+ int exit_status = 0;
+ struct lttng_notification_channel *notification_channel;
+ struct lttng_condition *condition;
+ struct lttng_action *action;
+ struct lttng_trigger *trigger;
+ const char *tracing_session_name;
+ const char *channel_name;
+
+ assert(argc >= 3);
+ tracing_session_name = argv[1];
+ channel_name = argv[2];
+
+ /*
+ * Create a notification channel. A notification channel
+ * connects the user application to the LTTng session daemon.
+ * This notification channel can be used to listen to various
+ * types of notifications.
+ */
+ notification_channel = lttng_notification_channel_create(
+ lttng_session_daemon_notification_endpoint);
+
+ /*
+ * Create a "high buffer usage" condition. In this case, the
+ * condition is reached when the buffer usage is greater than or
+ * equal to 75 %. We create the condition for a specific tracing
+ * session name, channel name, and for the user space tracing
+ * domain.
+ *
+ * The "low buffer usage" condition type also exists.
+ */
+ condition = lttng_condition_buffer_usage_high_create();
+ lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
+ lttng_condition_buffer_usage_set_session_name(
+ condition, tracing_session_name);
+ lttng_condition_buffer_usage_set_channel_name(condition,
+ channel_name);
+ lttng_condition_buffer_usage_set_domain_type(condition,
+ LTTNG_DOMAIN_UST);
+
+ /*
+ * Create an action (get a notification) to take when the
+ * condition created above is reached.
+ */
+ action = lttng_action_notify_create();
+
+ /*
+ * Create a trigger. A trigger associates a condition to an
+ * action: the action is executed when the condition is reached.
+ */
+ trigger = lttng_trigger_create(condition, action);
+
+ /* Register the trigger to LTTng. */
+ lttng_register_trigger(trigger);
+
+ /*
+ * Now that we have registered a trigger, a notification will be
+ * emitted everytime its condition is met. To receive this
+ * notification, we must subscribe to notifications that match
+ * the same condition.
+ */
+ lttng_notification_channel_subscribe(notification_channel,
+ condition);
+
+ /*
+ * Notification loop. You can put this in a dedicated thread to
+ * avoid blocking the main thread.
+ */
+ for (;;) {
+ struct lttng_notification *notification;
+ enum lttng_notification_channel_status status;
+ const struct lttng_evaluation *notification_evaluation;
+ const struct lttng_condition *notification_condition;
+ double buffer_usage;
+
+ /* Receive the next notification. */
+ status = lttng_notification_channel_get_next_notification(
+ notification_channel, ¬ification);
+
+ switch (status) {
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
+ break;
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
+ /*
+ * The session daemon can drop notifications if
+ * a monitoring application is not consuming the
+ * notifications fast enough.
+ */
+ continue;
+ case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
+ /*
+ * The notification channel has been closed by the
+ * session daemon. This is typically caused by a session
+ * daemon shutting down.
+ */
+ goto end;
+ default:
+ /* Unhandled conditions or errors. */
+ exit_status = 1;
+ goto end;
+ }
+
+ /*
+ * A notification provides, amongst other things:
+ *
+ * * The condition that caused this notification to be
+ * emitted.
+ * * The condition evaluation, which provides more
+ * specific information on the evaluation of the
+ * condition.
+ *
+ * The condition evaluation provides the buffer usage
+ * value at the moment the condition was reached.
+ */
+ notification_condition = lttng_notification_get_condition(
+ notification);
+ notification_evaluation = lttng_notification_get_evaluation(
+ notification);
+
+ /* We're subscribed to only one condition. */
+ assert(lttng_condition_get_type(notification_condition) ==
+ LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);
+
+ /*
+ * Get the exact sampled buffer usage from the
+ * condition evaluation.
+ */
+ lttng_evaluation_buffer_usage_get_usage_ratio(
+ notification_evaluation, &buffer_usage);
+
+ /*
+ * At this point, instead of printing a message, we
+ * could do something to reduce the channel's buffer
+ * usage, like disable specific events.
+ */
+ printf("Buffer usage is %f %% in tracing session \"%s\", "
+ "user space channel \"%s\".\n", buffer_usage * 100,
+ tracing_session_name, channel_name);
+ lttng_notification_destroy(notification);
+ }
+
+end:
+ lttng_action_destroy(action);
+ lttng_condition_destroy(condition);
+ lttng_trigger_destroy(trigger);
+ lttng_notification_channel_destroy(notification_channel);
+ return exit_status;
+}
+----
+--
+
+. Build the `notif-app` application, linking it to `liblttng-ctl`:
++
+--
+[role="term"]
+----
+$ gcc -o notif-app notif-app.c -llttng-ctl
+----
+--
+
+. <<creating-destroying-tracing-sessions,Create a tracing session>>,
+ <<enabling-disabling-events,create an event rule>> matching all the
+ user space tracepoints, and
+ <<basic-tracing-session-control,start tracing>>:
++
+--
+[role="term"]
+----
+$ lttng create my-session
+$ lttng enable-event --userspace --all
+$ lttng start
+----
+--
++
+If you create the channel manually with the man:lttng-enable-channel(1)
+command, you can control how frequently are the current values of the
+channel's properties sampled to evaluate user conditions with the
+opt:lttng-enable-channel(1):--monitor-timer option.
+
+. Run the `notif-app` application. This program accepts the
+ <<tracing-session,tracing session>> name and the user space channel
+ name as its two first arguments. The channel which LTTng automatically
+ creates with the man:lttng-enable-event(1) command above is named
+ `channel0`:
++
+--
+[role="term"]
+----
+$ ./notif-app my-session channel0
+----
+--
+
+. In another terminal, run an application with a very high event
+ throughput so that the 75{nbsp}% buffer usage condition is reached.
++
+In the first terminal, the application should print lines like this:
++
+----
+Buffer usage is 81.45197 % in tracing session "my-session", user space
+channel "channel0".
+----
++
+If you don't see anything, try modifying the condition in
+path:{notif-app.c} to a lower value (0.1, for example), rebuilding it
+(step 2) and running it again (step 4).
+====
+
+
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