contents/understanding-lttng/plumbing/lttng-sessiond.md

   1 ---
   2 id: lttng-sessiond
   3 ---
   4
   5 At the heart of LTTng's plumbing is the _session daemon_, often called
   6 by its command name, `lttng-sessiond`.
   7
   8 The session daemon is responsible for managing tracing sessions and
   9 what they logically contain (channel properties, enabled/disabled
  10 events, etc.). By communicating locally with instrumented applications
  11 (using LTTng-UST) and with the LTTng Linux kernel modules
  12 (LTTng-modules), it oversees all tracing activities.
  13
  14 One of the many things that `lttng-sessiond` does is to keep
  15 track of the available event types. User space applications and
  16 libraries actively connect and register to the session daemon when they
  17 start. By contrast, `lttng-sessiond` seeks out and loads the appropriate
  18 LTTng kernel modules as part of its own initialization. Kernel event
  19 types are _pulled_ by `lttng-sessiond`, whereas user space event types
  20 are _pushed_ to it by the various user space tracepoint providers.
  21
  22 Using a specific inter-process communication protocol with Linux kernel
  23 and user space tracers, the session daemon can send channel information
  24 so that they are initialized, enable/disable specific probes based on
  25 enabled/disabled events by the user, send event filters information to
  26 LTTng tracers so that filtering actually happens at the tracer site,
  27 start/stop tracing a specific application or the Linux kernel, etc.
  28
  29 The session daemon is not useful without some user controlling it,
  30 because it's only a sophisticated control interchange and thus
  31 doesn't make any decision on its own. `lttng-sessiond` opens a local
  32 socket for controlling it, albeit the preferred way to control it is
  33 using `liblttng-ctl`, an installed C library hiding the communication
  34 protocol behind an easy-to-use API. The `lttng` tool makes use of
  35 `liblttng-ctl` to implement a user-friendly command line interface.
  36
  37 `lttng-sessiond` does not receive any trace data from instrumented
  38 applications; the _consumer daemons_ are the programs responsible for
  39 collecting trace data using shared ring buffers. However, the session
  40 daemon is the one that must spawn a consumer daemon and establish
  41 a control communication with it.
  42
  43 Session daemons run on a per-user basis. Knowing this, multiple
  44 instances of `lttng-sessiond` may run simultaneously, each belonging
  45 to a different user and each operating independently of the others.
  46 Only `root`'s session daemon, however, may control LTTng kernel modules
  47 (that is, the kernel tracer). With that in mind, if a user has no root
  48 access on the target system, he cannot trace the system's kernel, but
  49 should still be able to trace its own instrumented applications.
  50
  51 It has to be noted that, although only `root`'s session daemon may
  52 control the kernel tracer, the `lttng-sessiond` command has a `--group`
  53 option which may be used to specify the name of a special user group
  54 allowed to communicate with `root`'s session daemon and thus record
  55 kernel traces. By default, this group is named `tracing`.
  56
  57 If not done yet, the `lttng` tool, by default, automatically starts a
  58 session daemon. `lttng-sessiond` may also be started manually:
  59
  60 <pre class="term">
  61 lttng-sessiond
  62 </pre>
  63
  64 This starts the session daemon in foreground. Use
  65
  66 <pre class="term">
  67 lttng-sessiond --daemonize
  68 </pre>
  69
  70 to start it as a true daemon.
  71
  72 To kill the current user's session daemon, `pkill` may be used:
  73
  74 <pre class="term">
  75 pkill lttng-sessiond
  76 </pre>
  77
  78 The default `SIGTERM` signal terminates it cleanly.
  79
  80 Several other options are available and described in
  81 <a href="/man/8/lttng-sessiond" class="ext"><code>lttng-sessiond</code>'s manpage</a>
  82 or by running `lttng-sessiond --help`.