Add captions to figures

[lttng-docs.git] / contents / understanding-lttng / core-concepts / tracing-session.md

---
id: tracing-session
---

A _tracing session_ is—like any session—a container of
state. Anything that is done when tracing using LTTng happens in the
scope of a tracing session. In this regard, it is analogous to a bank
website's session: you can't interact online with your bank account
unless you are logged in a session, except for reading a few static
webpages (LTTng, too, can report some static information that does not
need a created tracing session).

A tracing session holds the following attributes and objects (some of
which are described in the following sections):

  * a name
  * the tracing state (tracing started or stopped)
  * the trace data output path/URL (local path or sent over the network)
  * a mode (normal, snapshot or live)
  * the snapshot output paths/URLs (if applicable)
  * for each [domain](#doc-domain), a list of [channels](#doc-channel)
  * for each channel:
    * a name
    * the channel state (enabled or disabled)
    * its parameters (event loss mode, sub-buffers size and count,
      timer periods, output type, trace files size and count, and the rest)
    * a list of added context information
    * a list of [events](#doc-event)
  * for each event:
    * its state (enabled or disabled)
    * a list of instrumentation points (tracepoints, system calls,
      dynamic probes, other types of probes)
    * associated log levels
    * a filter expression

All this information is completely isolated between tracing sessions.
As you can see in the list above, even the tracing state
is a per-tracing session attribute, so that you may trace your target
system/application in a given tracing session with a specific
configuration while another one stays inactive.

<figure class="img img-100">
<img src="/images/docs26/concepts.png" alt="LTTng concepts">
<figcaption>
    A <em>tracing session</em> is a container of domains,
    channels, and events.
</figcaption>
</figure>

Conceptually, a tracing session is a per-user object; the
[Plumbing](#doc-plumbing) section shows how this is actually
implemented. Any user may create as many concurrent tracing sessions
as desired.

<figure class="img img-100">
<img src="/images/docs26/many-sessions.png" alt="Tracing sessions per user">
<figcaption>
    Each user may create as many tracing sessions as desired.
</figcaption>
</figure>

The trace data generated in a tracing session may be either saved
to disk, sent over the network or not saved at all (in which case
snapshots may still be saved to disk or sent to a remote machine).