--- /dev/null
+RFC - LTTng session and daemon configuration save and restore
+
+Author: Jérémie Galarneau <jeremie.galarneau@efficios.com>
+
+Version:
+ - v0.2: 04/12/2013
+ * Reorganized the text following David Goulet's recommendations
+ - v0.1: 03/12/2013
+ * Initial proposal
+
+Introduction
+-------------
+
+This proposal details the file formats, LTTng APIs and command line interfaces
+used to save and restore session and daemon configurations.
+
+
+Daemon Configuration File Format
+--------------------------------
+
+The LTTng session and relay daemons shall be configurable using INI
+configuration files. The set of configuration options accepted by LTTng daemons
+corresponds to the command line options available for each daemon.
+
+The sections in the files must match the daemons' names: [sessiond] and
+[relayd]. The entries under each section must match the long option names
+supported by the daemons. Options specified on the command line shall always
+have precedence over options set by the configuration files.
+
+An example demonstrating the use of such configuration
+options follows.
+
+[sessiond]
+consumerd32-path=/lib/lttng/libexec/lttng-consumerd
+consumerd64-path=/lib64/lttng/libexec/lttng-consumerd
+no-kernel=1
+
+[relayd]
+data-port=tcp://0.0.0.0:5343
+output=/tmp/MyTraces
+
+Daemon configuration files are, by default, either stored system-wide under
+/etc/lttng/*.conf or in the user's home directory under $HOME/.lttng/*.conf.
+
+New options may be added to the daemon configuration file as features are
+added to the relay and session daemons. The daemons shall ignore unrecognized
+options to ensure forward compatibility.
+
+
+LTTng Command Line - Daemon Configuration
+-----------------------------------------
+
+On launch, the session, relay and consumer daemons will search for files with
+the .conf extension in the following folders, in order of precedence:
+ 1) Any path specified using a new --load-config option
+ 2) $HOME/.lttng/*.conf
+ 3) /etc/lttng/*.conf
+
+
+Session Configuration File Format
+---------------------------------
+
+The LTTng session configuration file format must enable the serialization of the
+session, channel and event structures of the LTTng API. Since these concepts
+are eminently hierarchical, it only makes sense to use a file format that can
+express such relationships.
+
+While an INI based file format was the first considered, for its ease of use
+and human readability, it appears that INI provides no standard way of
+expressing hierarchies. It is, of course, possible to flatten session/channel
+hierarchies using prefixed section names, but this is both error prone and
+unsupported by most INI file reading libraries. This means that we would have
+to code our own INI handling code and produce a specification to document our
+additions to the INI format. INI also provides no standard versioning mechanism.
+
+In keeping with the desire to use a human-readable format, we have considered
+rolling our own format. This format could express hierarchy using tabulations
+to delimit scopes, not unlike the Python language. Such a format would be both
+relatively easy to parse, but also readable. However, this both requires
+documenting the format in a specification, rolling our own parsing and
+validation code, thus losing the benefit of reusing external libraries. This
+option has the highest maintenance cost considering the amount of code and
+documentation to be produced while presenting marginal benefits over the INI
+approach.
+
+Finally, it seems that using a standard hierarchical format such as JSON or
+XML would be the most appropriate choice. Both of these formats have intrinsic
+support for representation of hierarchical relationships. They also benefit from
+having a lot of hardened external libraries that will provide parsing,
+validation and write support. This is a huge advantage from both
+maintainability and interoperability standpoints. However, both formats
+present the disadvantage of being harder, although minimally, to edit manually.
+It remains to be seen if manual editing of session configurations really is an
+important use-case. Using lttng's save feature would probably prove more
+convenient than editing the files manually. Furthermore, the addition of a
+"dry-run" option to the lttng client would significantly alleviate this pain
+point.
+
+XML seems like a better option than JSON since it makes it possible to have
+robust file validation using a pre-defined schema. The use of version-specific
+schemas could also be beneficial for backward compatibility as the format
+moves forward. Finally, character encoding handling is already a solved
+problem for most XML libraries.
+
+
+LTTng ABI/API
+-------------
+
+Two new functions are added to the lttng.h API.
+
+int lttng_load_sessions(const char *url, const char *session_name,
+ int flags/struct lttng_session_load_attr *attr);
+
+Load sessions. If url is a directory, all .lttng files it contains will be
+loaded. If session_name is NULL, all sessions found in url are restored.
+If a session_name is provided, only this session will be restored.
+
+A supplementary argument, either a "flags" argument or an attribute structure,
+is used to indicate whether or not the sessions currently known to the
+session daemon should be replaced by the ones found in the configuration
+file(s), provided that their names are the same.
+Even though this is the only current use-case for this argument, a structure
+with a reasonable amount of padding may be more suitable than a primary
+type to accommodate new features. Thoughts?
+
+
+int lttng_save_sessions(const char *url, const char *session_name,
+ int flags/struct lttng_session_save_attr *attr);
+
+Save sessions. If url is a directory, the session configuration will be saved
+as session_name.lttng. If a complete file name is provided, the session(s) to be
+saved will be written to this file. If session_name is NULL, all sessions
+currently known to the session daemon will be saved. If a name is provided, only
+that session will be saved.
+
+The reasoning for the flags/attr argument is the same as for the
+lttng_load_sessions() function, but for a configuration file overwrite
+option.
+
+
+LTTng Command Line - Session Configuration
+------------------------------------------
+
+Tracing session configurations can be saved and restored using the lttng command
+line client. This capability introduces two new command line options.
+
+
+-- Load session configurations --
+
+$ lttng load -s SESSION_NAME [OPTIONS]
+
+Loads tracing session configurations.
+
+.lttng files will be searched in the following default paths, in order of
+precedence:
+ 1) $(HOME)/.lttng/sessions
+ 2) /etc/lttng/sessions
+
+A session name or the -a option must be passed as the SESSION_NAME argument.
+Like some other lttng commands, such as enable-event, the -a option stands for
+"all". If this option is used, every session found in the paths will be loaded.
+
+If a session was saved as active, the tracing for that session will be activated
+automatically on load. The current session configuration of the session daemon
+is also preserved when a configuration is loaded. The new session configurations
+are added to the current session set.
+
+Tracing sessions saved in an active state will be resumed on load.
+
+--input-path, -i
+Path in which to search for SESSION_NAME's description. If the path is a
+directory, all .lttng files it contains will be opened and searched for
+SESSION_NAME. If an input path is provided, the -a option will load all
+session configurations found in this path. The default paths are ignored when
+this option is used.
+
+--force, -f
+If a restored session's name conflicts with a currently existing session, the
+original session configuration is preserved. However, if the --force option is
+used, the restored session takes precedence. The original session will be
+destroyed.
+
+
+-- Save session configurations --
+
+$ lttng save -s SESSION_NAME [OPTIONS]
+
+Saves tracing configurations.
+
+The selected tracing session's configuration will be saved in the
+$HOME/.lttng/sessions folder. If the -a option is used, every session known
+to the session daemon will be saved.
+
+--output-path, -o
+If the provided output path is a directory, the session will be saved in a
+file named "SESSION_NAME.lttng". However, if the path is a file, the session(s)
+will all be saved in the same file.
projects / lttng-tools.git / blobdiff