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

   1 ---
   2 id: lttng-relayd
   3 ---
   4
   5 When a tracing session is configured to send its trace data over the
   6 network, an LTTng _relay daemon_ must be used at the other end to
   7 receive trace packets and serialize them to trace files. This setup
   8 makes it possible to trace a target system without ever committing trace
   9 data to its local storage, a feature which is useful for embedded
  10 systems, amongst others. The command implementing the relay daemon
  11 is `lttng-relayd`.
  12
  13 The basic use case of `lttng-relayd` is to transfer trace data received
  14 over the network to trace files on the local file system. The relay
  15 daemon must listen on two TCP ports to achieve this: one control port,
  16 used by the target session daemon, and one data port, used by the
  17 target consumer daemon. The relay and session daemons agree on common
  18 default ports when custom ones are not specified.
  19
  20 Since the communication transport protocol for both ports is standard
  21 TCP, the relay daemon may be started either remotely or locally (on the
  22 target system).
  23
  24 While two instances of consumer daemons (32-bit and 64-bit) may run
  25 concurrently for a given user, `lttng-relayd` needs only be of its
  26 host operating system's bitness.
  27
  28 The other important feature of LTTng's relay daemon is the support of
  29 _LTTng live_. LTTng live is an application protocol to view events as
  30 they arrive. The relay daemon still records events in trace files,
  31 but a _tee_ allows to inspect incoming events.
  32
  33 <figure class="img img-100">
  34 <img src="/images/docs26/lttng-live.png" alt="LTTng live">
  35 <figcaption>
  36     The relay daemon creates a <em>tee</em>, forwarding the trace data
  37     to both trace files and a live viewer.
  38 </figcaption>
  39 </figure>
  40
  41 Using LTTng live
  42 locally thus requires to run a local relay daemon.