Docs: relayd: document the lifetime of viewer session trace chunks

[lttng-tools.git] / src / bin / lttng-relayd / viewer-session.h

diff --git a/src/bin/lttng-relayd/viewer-session.h b/src/bin/lttng-relayd/viewer-session.h
index 5c8b2e54d31ff07e93dd979a8f921b5fbaaed233..cbf4aa157dcfff4c8071ca3792191a8988a8e199 100644 (file)
--- a/src/bin/lttng-relayd/viewer-session.h
+++ b/src/bin/lttng-relayd/viewer-session.h
@@ -2,22 +2,12 @@
 #define _VIEWER_SESSION_H
 
 /*
- * Copyright (C) 2013 - Julien Desfossez <jdesfossez@efficios.com>
- *                      David Goulet <dgoulet@efficios.com>
- *               2015 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ * Copyright (C) 2013 Julien Desfossez <jdesfossez@efficios.com>
+ * Copyright (C) 2013 David Goulet <dgoulet@efficios.com>
+ * Copyright (C) 2015 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  *
- * This program is free software; you can redistribute it and/or modify it
- * under the terms of the GNU General Public License, version 2 only, as
- * published by the Free Software Foundation.
+ * SPDX-License-Identifier: GPL-2.0-only
  *
- * This program is distributed in the hope that it will be useful, but WITHOUT
- * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
- * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
- * more details.
- *
- * You should have received a copy of the GNU General Public License along with
- * this program; if not, write to the Free Software Foundation, Inc., 51
- * Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
 #include <limits.h>
@@ -40,7 +30,29 @@ struct relay_viewer_session {
         */
        struct cds_list_head session_list;      /* RCU list. */
        pthread_mutex_t session_list_lock;      /* Protects list updates. */
-       /* Once set, the current trace chunk of a viewer must not change. */
+       /*
+        * The viewer session's current trace chunk is initially set, when
+        * a viewer attaches to the viewer session, to a copy the corresponding
+        * relay_session's current trace chunk.
+        *
+        * A live session always attempts to "catch-up" to the newest available
+        * trace chunk. This means that when a viewer reaches the end of a
+        * trace chunk, the viewer session may not transition to the "next" one:
+        * it jumps to the most recent trace chunk available (the one being
+        * produced by the relay_session). Hence, if the producer performs
+        * multiple rotations before a viewer completes the consumption of a
+        * trace chunk, it will skip over those "intermediary" trace chunks.
+        *
+        * A viewer session updates its current trace chunk when:
+        *   1) new viewer streams are created,
+        *   2) a new index is requested,
+        *   3) metadata is requested.
+        *
+        * Hence, as a general principle, the viewer session will reference the
+        * most recent trace chunk available _even if its streams do not point to
+        * it_. It indicates which trace chunk viewer streams should transition
+        * to when the end of their current trace chunk is reached.
+        */
        struct lttng_trace_chunk *current_trace_chunk;
 };
 
@@ -55,7 +67,7 @@ int viewer_session_is_attached(struct relay_viewer_session *vsession,
                struct relay_session *session);
 void viewer_session_close_one_session(struct relay_viewer_session *vsession,
                struct relay_session *session);
-int viewer_session_set_trace_chunk(struct relay_viewer_session *vsession,
+int viewer_session_set_trace_chunk_copy(struct relay_viewer_session *vsession,
                struct lttng_trace_chunk *relay_session_trace_chunk);
 
 #endif /* _VIEWER_SESSION_H */