LTTng Live trace reading protocol

Julien Desfossez
February 6th, 2014

This document describes the protocol of live trace reading. It is mainly
intended for trace-viewer developers.

Live trace reading allows a viewer to read safely a LTTng trace while it is
being recorded. The protocol ensures that the viewer is never in a position
where it can read for which it does not have the metadata, and also allows to
viewer to know about inactive streams and skip those up to a certain point in
time.

This feature is implemented starting at lttng-tools 2.4

* General informations
All the data structures required to implement this protocole are provided in
the lttng-viewer-abi.h header. All integer are encoded in big endian during the
transfer.

Just like the streaming protocol, this protocol works always in only one
direction : from the viewer to the relay. After each command, the relay sends a
reply to the viewer (or closes the connection on fatal error).

When the viewer sends a command to the relay, it sends a struct
lttng_viewer_cmd which contains the command (enum lttcomm_relayd_command) and
the size of the actual command if the command requires a payload.
For example, if the viewer wants to list the sessions, it sends a struct
lttng_viewer_cmd with :
cmd = htobe32(VIEWER_CONNECT);
data_size = htobe64(sizeof(struct lttng_viewer_connect));

The cmd_version is currently unused, but might get useful when we extend the
protocol later.

* Protocol sequence
In this section, we describe the normal sequence of event during a live-reading
session. The viewer is abbreviated V and the relay R for conciseness.

Establishing a connection :
- V connects to R on port TCP/5344
- V establishes a session by sending a VIEWER_CONNECT command, payload in
  struct lttng_viewer_connect. In this struct, it sets its major and minor
  version of the protocol it implements, and the type of connection it wants,
  for now only VIEWER_CLIENT_COMMAND is supported.
- If the protocol implemented by V and R are compatible, R sends back the same
  struct with its own version and the newly assigned viewer_session_id.
  Protocols are compatible if they have the same major number. At this point,
  if the communication continues and the minor are not the same, it is implied
  that the two processes will use the min(minor) of the protocol during this
  connection. Protocol versions follow lttng-tools version, so if R implements
  the 2.5 protocol and V implements the 2.4 protocol, R will use the 2.4
  protocol for this connection.

List the sessions :
Once V and R agree on a protocol, V can start interacting with R. The first
thing to do is list the sessions currently active on R.
- V sends the command VIEWER_LIST_SESSIONS with no payload (data_size ignored)
- R sends back a struct lttng_viewer_list_sessions which contains the number of
  sessions to receive, and then for each session (sessions_count), it sends a
  struct lttng_viewer_session
- V must first receive the struct lttng_viewer_list_sessions and then receive
  all the lttng_viewer_session structs. The live_timer corresponds to the value
  set by the user who created the tracing session, if it is 0, the session
  cannot be read in live. This timer in microseconds is the minimum rate at
  which R receives information about the running session. The "clients" field
  contains the number of connected clients to this session, for now only one
  client at a time can attach to session.

Attach to a session :
Now V can select and attach one or multiple session IDs, but first, it needs to
create a viewer_session. Creating a viewer session is done by sending the
command LTTNG_VIEWER_CREATE_SESSION. In the future, this would be the place
where we could specify options global to the viewer session.
Once the session is created, the viewer can issue one or multiple
VIEWER_ATTACH_SESSION commands with the session_id it wants. The "seek"
parameter allows the viewer to attach to a session from its beginning (it will
receive all trace data still on the relayd) or from now (data will be available
to read starting at the next packet received on the relay). The viewer can
issue this command multiple times and at any moment in the process.
R replies with a struct lttng_viewer_attach_session_response with a status and
the number of streams currently active in this session. Then, for each stream,
it sends a struct lttng_viewer_stream. Just like with the session list, V must
receive the first header and then all the stream structs. The ctf_trace_id
parameter in the struct lttng_viewer_stream is particularly important since it
allows the viewer to match all the streams belonging to the same CTF trace (so
one metadata file and multiple stream files). If the stream is a metadata
stream, metadata_flag will be set to 1.
The relay ensures that it sends only ready ctf traces, so once this command is
complete, V knows that it has all the streams ready to be processed.
A quick note about the "sessions": from the relay perspective we see one
session for each domain (kernel, ust32, ust64) of a session as created on the
sessiond. For example, if the user creates a session and adds events in the
kernel and UST and has only 64-bit applications running, the relay sees two
sessions with the same name and hostname. So in order to display a session as
seen by the user, the viewer has to attach to all the sessions with the same
hostname and session name. There might be clashes if two servers on the network
have the same hostname and create the same session name, but the relay cannot
distinguish these cases, so it is currently a known limitation.
During a session, new streams might get added (especially in per-pid tracing)
so the viewer must be ready to add new streams while processing the trace. To
inform V that new streams are available, R sets the
LTTNG_VIEWER_FLAG_NEW_STREAM flag on LTTNG_VIEWER_GET_NEXT_INDEX and
LTTNG_VIEWER_GET_PACKET replies. The viewer must then issue the
LTTNG_VIEWER_GET_NEW_STREAMS command and receive all the streams, just like
with the attach command.

#### below needs to be well written, but the essential is here ###

Get metadata :
A CTF trace cannot be read without the complete metadata.
Send the command VIEWER_GET_METADATA and the struct lttng_viewer_get_metadata.

Once we have all the metadata, we can start processing the trace. In order to
do that, we work with the indexes. Whenever we need to read a new packet from a
stream, we first ask for the next index for this stream and then ask for a
trace packet at a certain offset and length.

Get the next index :
Command VIEWER_GET_NEXT_INDEX
struct lttng_viewer_get_next_index
Receive back a struct lttng_viewer_index 
We might also receive flags :
- LTTNG_VIEWER_FLAG_NEW_METADATA the viewer must ask new metadata
  (LTTNG_VIEWER_GET_METADATA)
- LTTNG_VIEWER_FLAG_NEW_STREAM the viewer must get the new streams
  (LTTNG_VIEWER_GET_NEW_STREAMS)

Get data packet :
Command VIEWER_GET_PACKET
struct lttng_viewer_get_packet
Receive back a struct lttng_viewer_trace_packet
We might also receive a LTTNG_VIEWER_GET_PACKET_ERR and some flags :
- LTTNG_VIEWER_FLAG_NEW_METADATA the viewer must ask new metadata
  (LTTNG_VIEWER_GET_METADATA)
- LTTNG_VIEWER_FLAG_NEW_STREAM the viewer must get the new streams
  (LTTNG_VIEWER_GET_NEW_STREAMS)

For the VIEWER_GET_NEXT_INDEX and VIEWER_GET_PACKET, the viewer must check the
"flags" element of the struct it receives, because it contains important
information such as the information that new metadata must be received before
being able to receive and read the next packet.
When new metadata is added during a session, the GET_NEXT_INDEX will succeed
but it will have the flag LTTNG_VIEWER_FLAG_NEW_METADATA, but the
GET_DATA_PACKET will fail with the same flag as long as the metadata is not
downloaded.

Detach from a session:
Closing the network connection detaches a client from all the sessions it is
currently attached to. It is also possible to detach from a specific session
without disconnecting by sending the LTTNG_VIEWER_DETACH_SESSION command. The
payload of this command is the session ID.
