contents/understanding-lttng/core-concepts/tracing-session.md

   1 ---
   2 id: tracing-session
   3 ---
   4
   5 A _tracing session_ is&mdash;like any session&mdash;a container of
   6 state. Anything that is done when tracing using LTTng happens in the
   7 scope of a tracing session. In this regard, it is analogous to a bank
   8 website's session: you can't interact online with your bank account
   9 unless you are logged in a session, except for reading a few static
  10 webpages (LTTng, too, can report some static information that does not
  11 need a created tracing session).
  12
  13 A tracing session holds the following attributes and objects (some of
  14 which are described in the following sections):
  15
  16   * a name
  17   * the tracing state (tracing started or stopped)
  18   * the trace data output path/URL (local path or sent over the network)
  19   * a mode (normal, snapshot or live)
  20   * the snapshot output paths/URLs (if applicable)
  21   * for each [domain](#doc-domain), a list of [channels](#doc-channel)
  22   * for each channel:
  23     * a name
  24     * the channel state (enabled or disabled)
  25     * its parameters (event loss mode, sub-buffers size and count,
  26       timer periods, output type, trace files size and count, and the rest)
  27     * a list of added context information
  28     * a list of [events](#doc-event)
  29   * for each event:
  30     * its state (enabled or disabled)
  31     * a list of instrumentation points (tracepoints, system calls,
  32       dynamic probes, other types of probes)
  33     * associated log levels
  34     * a filter expression
  35
  36 All this information is completely isolated between tracing sessions.
  37 As you can see in the list above, even the tracing state
  38 is a per-tracing session attribute, so that you may trace your target
  39 system/application in a given tracing session with a specific
  40 configuration while another one stays inactive.
  41
  42 <div class="img img-100">
  43 <img src="/images/docs26/concepts.png" alt="LTTng concepts">
  44 </div>
  45
  46 Conceptually, a tracing session is a per-user object; the
  47 [Plumbing](#doc-plumbing) section shows how this is actually
  48 implemented. Any user may create as many concurrent tracing sessions
  49 as desired.
  50
  51 <div class="img img-100">
  52 <img src="/images/docs26/many-sessions.png" alt="Tracing sessions per user">
  53 </div>
  54
  55 The trace data generated in a tracing session may be either saved
  56 to disk, sent over the network or not saved at all (in which case
  57 snapshots may still be saved to disk or sent to a remote machine).