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[lttng-tools.git] / doc / proposals / 0001-session-daemon-architecture.txt

RFC - LTTng session daemon architecture

WARNING:
  Parts of the proposal are obsolete but we keep this version for historical
  purposes.

Author: David Goulet <david.goulet@polymtl.ca>

Contributors:
    * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    * Yannick Brosseau <yannick.brosseau@polymtl.ca>
    * Nils Carlson <nils.carlson@ericsson.com>
    * Michel Dagenais <michel.dagenais@polymtl.ca>
    * Stefan Hajnoczi <stefanha@gmail.com>

Version:
    - v0.1: 17/01/2011
        * Initial proposal

    - v0.2: 19/01/2011
        After multiple reply from all the contributors above, here is the list
        of what has changed:
        * Change/Add Terminology elements from the initial model
        * New figures for four new scenarios
        * Add inprocess library section
        * LTTng kernel tracer support proposition
        * More details for the Model and Components
        * Improve the basic model. Quite different from the last one

    - v0.3: 28/01/2011
        In response from Michel Dagenais and Nils Carlson comments:
        * Add scaling to reasons of this re-engineering
        * Purpose of the session ID
        * Explain why ltt-sessiond creates the tracing buffers
        * ust-consumerd interaction schema
        * Clarify inprocess library behavior

    - v0.4: 01/02/2011
        After Mathieu Desnoyers and Michel Dagenais comments:
        * Add section Introduction
        * Define the global and per-user ltt-sessiond
        * Add details for ltt-sessiond in the inprocess lib section
        * Session ID are now UUID
        * Add buffer snapshot schema for ust-consumerd
        * ltt-sessiond validate inprocess lib version
        * ltt-sessiond socket validation by the inprocess lib.
        * Add lttng definition
        * Add consumer to the Model section
	- v0.5: 23/07/2012
		* Please refer to the thesis of David Goulet for the complete
		and up to date specifications of the architecture and algorithms used.
		(http://publications.polymtl.ca/842/1/2012_DavidGoulet.pdf)

Terminology
-----------------

ltt-sessiond - Main daemon for trace session registry for UST and LTTng
NOTE: Changed to lttng-sessiond in the git tree

ust-consumerd - Daemon that consume UST buffers for a specific application

ltt-consumerd - Daemon that consume LTTng buffers

tracing session - A trace linked to a set of specific tracepoints and to a set
                  of tracing buffers

tracing buffers - Buffers containing tracing data

tracing data - Data created by tracing an application

inprocess library - UST library linked with the application

shared memory - system V shared memory

application common named pipe - Global named pipe that triggers application
                                registration, on pipe event, to ltt-sessiond

lttng - New command line tool for LTTng and UST tracing control

Introduction
-----------------

This RFC propose a brand new UST and LTTng daemon model. This re-engineering
was mostly driven by the need of:

    * Better security in terms of access rights on tracing data
    * Manage tracing session
    * Scaling in terms of thread/processes needed to perform tracing
    * LTTng and UST integration in terms of merging traces and session control
    * Networking such as streaming and remote control over different traces

The new model follows the basic principles of having a session registry
(ltt-sessiond) and consumers for each tracing session (ust-consumerd and
ltt-consumerd).

With this proposal, LTTng and UST will share the same tracing session, be
managed by the same tool and bring a complete integration between these two
powerful tools.

NOTE: This proposal does NOT makes UST dependent on LTTng and vice versa.

Model
-----------------

A global and/or per-user registry keeps track of all tracing sessions. Any user
that wants to manage either a kernel trace using LTTng or an application trace
with UST must interact with that registry for any possible actions.

The model address multiple tracing use cases based on the fact that we
introduce a tracing Unix group (tracing group). Only users in that group or
root can use the global registry. Other users will create a local registry
(per-user registry) that will be completely independent from the global one.

Two cases:

    1) Users in the tracing group, it's tracing session can consume all tracing
       buffers from all applications and the kernel.

    2) Users NOT in the tracing group, it's tracing session can only consume
       data from its own applications' buffers hence tracing his applications.

A session stored by the registry consist of:

    * Session name (given by the user or automatically assigned)
    * List of traces (LTTng or UST)
    * Tracepoints/markers associated to a trace of that session
    * UUID
    * Associated user (UID)

Then, consumers are used to extract data from tracing buffers. These consumers
are daemon consuming either UST or/and LTTng buffers. For a single session,
only one UST consumer and one LTTng consumer is necessary. The daemon CAN
handle multiple tracing buffers for network streaming by example or for quick
snapshot. These consumers are told by the inprocess library or the kernel to
start getting out data on disk or network.

For the next subsections, every components of this new proposal is explained
from the global and per-user registry perspective.

LTT-SESSIOND:

The ltt-sessiond daemon acts as a session registry i.e. by keeping reference to
all active session and, by active, it means a session in any state other than
destroyed. Each entity we are keeping track of, here session, will have a
universal unique identifier (UUID) assigned to it. The purpose of this UUID is
to track a session in order to apply any kind of actions (Ex: Attach, Destroy).
A human readable version SHOULD be consider in order to facilitate the session
identification when listed by lttng.

The daemon creates two local Unix sockets (AF_UNIX). The first one is for what
we call client communication i.e. interaction with lttng (or any other
compatible tools). That socket is set with the ltt-sessiond credentials with
read-write mode for both user and group. The second one is a global socket for
application registration for the UST case (see inprocess lib subsection below).

This daemon is also responsible for tracing buffers creation. Two main reasons
motivate this design:

    * The ltt-sessiond needs to keep track of all the shared memory segments in
    order to be able to give reference to any other possible consumer.

    * For the case of sharing tracing buffers between all userspace
    applications, having the registry allocating them will allow that but, if
    the inprocess library was allocating them, we will need to redesign the
    whole model.

For all tracing actions either to interact with a session or a specific trace,
the lttng client MUST go through ltt-sessiond. The daemon will take care of
routing the command to the write inprocess library or the kernel.

Global registry:

A global registry SHOULD be started, idealy at boot, with credentials UID root
and GID of the tracing group. Only user within the tracing group will be able
to interact with that registry. All applications will try to register to that
registry using the global socket (second one discuss above).

Per-user registry:

This type of registry address two use cases. The first one is when a session
creation is requested from lttng but no global ltt-sessiond exist. So, a
ltt-sessiond will be spawned in order to manage the tracing of that user. The
second use case is when a user is not in the tracing group thus he cannot
communication with the global registry.

However, care MUST be put in order to manage the socket's daemon. They are not
global anymore so they should be created in the home directory of the user
requesting tracing.

In both cases, for global and per-user registry, all applications MUST try to
register to both ltt-sessiond. (see inprocess library subsection for details)

The trace roles of ltt-sessiond:

    Trace interaction - Create, Destroy, Pause, Stop, Start, Set options

    Registry - keep track of trace's information:
        * shared memory location (only the keyid)
        * application PID (UST)
        * type (kernel or UST)
        * session name
        * UID

    Buffers creation - creates shared memory for the tracing buffers.

UST-CONSUMERD:

The purpose of this daemon is to consume the UST trace buffers for only a
specific session. The session MAY have several traces for example two different
applications. The client tool, lttng has to create the ust-consumerd if NONE
is available for that session. It is very important to understand that for a
tracing session, there is only one ust-consumerd for all the traced
applications.

This daemon basically empty the tracing buffers when asked for and writes that
data to disk for future analysis using LTTv or/and TMF (Tracing Monitoring
Frameworks). The inprocess library is the one that tells the ust-consumerd
daemon that the buffers are ready for consumption.

Here is a flow of action to illustrate the ust-consumerd life span:

1)
+-----------+    ops     +--------------+
| lttng A   |<---------->| ltt-sessiond |
+-----------+            +--------------+

lttng ask for tracing an application using the PID and the session UUID. The
shared memory reference is given to lttng and the ust-consumerd communication
socket if ust-consumerd already exist.

2a) If ust-consumerd EXIST

+-----------+
| lttng A   |
+-----------+
    | mem ref.
    |   +---------------+   read   +------------+
    +-->| ust-consumerd |--------->| shared mem |
        +---------------+          +------------+

In that case, lttng only ask ust-consumerd to consume the buffers using
the reference it previously got from ltt-sessiond.

2b) If ust-consumerd DOES NOT EXIST

+-----------+              +--------------+
| lttng A   |        +---->| ltt-sessiond |
+-----------+        |     +--------------+
    | ID             |
    | mem ref.       | register
    |   +---------------+
    +-->| ust-consumerd |
        +---------------+

lttng spawns the ust-consumerd for the session using the session UUID in
order for the daemon to register as a consumer to ltt-sessiond for that
session.

Quick buffer snapshot:

1) Here, lttng will request a buffer snapshot for an already running session.

+-----------+                     +--------------+
| lttng A   |-------- ops ------->| ltt-sessiond |
+-----------+                     +--------------+
 |                                      | command
 |  +-----------------+     +-------+<--+
 |  | ust-consumerd 1 |<----| app_1 |-+
 |  +-----------------+     +-------+ | write
 |    1   |                           v
 |        |               +-------------+
 |        +--- read ----->| shared mem. |
 |                        +-------------+
 |                               ^
 |  +-----------------+          |
 +->| ust-consumerd 2 |----------+
    +-----------------+  snapshot
            | write
            |
            +---> disk/network

The first ust-consumerd (1) was already consuming buffers for the current
session. So, lttng ask for a live snapshot. A new ust-consumerd (2) is
spawned, snapshot the buffers using the shared memory reference from
ltt-sessiond, writes date to disk and die after all.

On the security side, the ust-consumerd gets UID/GID from the lttng
credentials since it was spawned by lttng and so the files containing the
tracing data will also be set to UID/GID of the lttng client. No setuid or
setgid is used, we only use the credentials of the user.

The roles of ust-consumerd:

    Register to ltt-sessiond - Using a session UUID and credentials (UID/GID)

    Consume buffers - Write data to a file descriptor (on disk, network, ...)

Buffer consumption is triggered by the inprocess library which tells
ust-consumerd when to consume.

LTT-CONSUMERD:

The purpose of this daemon is to consume the LTTng trace buffers for only a
specific session.

For that kernel consumer, ltt-sessiond will pass different anonymous file
descriptors to the ltt-consumerd using a Unix socket. From these file
desriptors, it will be able to get the data from a special function export by
the LTTng kernel.

ltt-consumerd will be managed by the exact same way as ust-consumerd. However,
in order to trace the kernel, you are either root (UID=0) or in the tracing
group.

The roles of ltt-consumerd:

    Register to ltt-sessiond - Using a session UUID and credentials (UID/GID)

    Consume buffers - Write data to a file descriptor (on disk, network, ...)

Kernel triggers ltt-consumerd for buffer consumption.

UST INPROCESS LIBRARY:

When the application starts, this library will check for the global named pipe
of ltt-sessiond. If present, it MUST validate that root is the owner. This
check is very important to prevent ltt-sessiond spoofing. If the pipe is root,
we are certain that it's the privileged user that operates tracing. Then, using
it's UID, the application will try to register to the per-user ltt-sessiond
again verifying before the owner ship of the named pipe that should match the
UID.

Before registration, the inprocess library MUST validate with the ltt-sessiond
the library version for compatibility reason. This is mechanism is useful for
library compatibility but also to see if ltt-sessiond socket is valid (means
that an actual ltt-sessiond is listening on the other side). Having no response
for over 10 seconds, the application will cut communication on that socket and
fallback to the application common named pipe (explain below).

If the socket is valid, it will register as a traceable application using the
apps credentials and will open a local Unix socket, passed to ltt-sessiond, in
order to receive an eventual shared memory reference. It will then wait on it
if any other command are given by the lttng client. This socket becomes the
only channel of communication between the registry and the application.

If no ltt-sessiond is present at registration, the application tries to open
the application common named pipe or create it if it does not exist and wait on
it (using poll or epoll Linux API). Having any type of event on that pipe, the
inprocess library will then try to register to the global and per-user
ltt-sessiond. If it fails again, it goes back again to wait on that pipe.

SHARED MEMORY

For UST, this is the memory area where the tracing buffers will be held and
given access in read-write mode for the inprocess library of the application.

On the LTTng side (for ltt-consumerd), these buffers are in the kernel space
and given access by opening a file in the debugfs file system. With an
anonymous file desriptor, this consumer will be able to extract the data.

This memory is ONLY used for the tracing data. No communication between
components is done using that memory.

A shared memory segment for tracing MUST be set with the tracing group GID for
the UST buffers. This is the job of ltt-sessiond.

PREREQUISITES:

The global ltt-sessiond daemon MUST always be running as "root" or an
equivalent user having the same privilege as root (UID = 0).

The ltt-sessiond daemon SHOULD be up and running at all time in order to trace
a tracable application.

The new lttng library API MUST be used to interact with the
ltt-sessiond registry daemon for every trace action needed by the user.

A tracing group MUST be created. Whoever is in that group is able to access the
tracing data of any buffers and is able to trace any application or the kernel.

WARNING: The tracing group name COULD interfere with other already existing
groups. Care should be put at install time for that (from source and packages)

The next section illustrates different use cases using that new model.

Use Cases
-----------------

Each case considers these :

* user A - UID: A; GID: A, tracing
* user B - UID: B; GID: B, tracing

Scenario 1 - Single user tracing app_1
------

This first scenario shows how user A will start a trace for application app_1
that is not running.

1) lttng ask ltt-sessiond for a new session through a Unix socket. If
allowed, ltt-sessiond returns a session UUID to the client.
(Ex: ops --> new session)

+-----------+    ops     +--------------+
| lttng A   |<---------->| ltt-sessiond |
+-----------+            +--------------+

2) The app_1 is spawned by lttng having the user A credentials. Then, app_1
automatically register to ltt-sessiond has a "tracable apps" through the global
named pipe of ltt-sessiond using the UID/GID and session UUID.

The shared memory is created with the app_1 UID (rw-) and tracing group GID
(r--) and a reference is given back to app_1

+-----------+            +--------------+
| lttng A   |            | ltt-sessiond |
+-----------+            +--------------+
    |                     ^    |
    |   +-------+         |    |   +-------------+
    +-->| app_1 |<--------+    +-->| shared mem. |
        +-------+                  +-------------+

3) app_1 connect to the shared memory and ust-consumerd is spawned with the
session UUID and lttng credentials (user A). It then register to ltt-sessiond
for a valid session to consume using the previous session UUID and credentials.

+-----------+            +--------------+
| lttng A   |        +-->| ltt-sessiond |----------+
+-----------+        |   +--------------+          |
    |                |                             |
    |   +---------------+    read                  | commands
    +-->| ust-consumerd |---------+                |   and
        +---------------+         v                | options
          ^  |                  +-------------+    |
          |  v          +------>| shared mem. |    |
        +-------+       |       +-------------+    |
        | app_1 |--------                          |
        +-------+     write                        |
            ^                                      |
            +---------------------------------------

Scenario 2 - Single user tracing already running app_1
------

1) lttng ask ltt-sessiond for a new session through a Unix socket. If allowed
(able to write on socket), ltt-sessiond returns a session UUID to the client.

+-----------+    ops     +--------------+
| lttng A   |<---------->| ltt-sessiond |
+-----------+            +--------------+
                           ^
        +-------+   read   |
        | app_1 |----------+
        +-------+

NOTE: At this stage, since app_1 is already running, the registration of app_1
to ltt-sessiond has already been done. However, the shared memory segment is
not allocated yet until a trace session is initiated. Having no shared memory,
the inprocess library of app_1 will wait on the local Unix socket connected to
ltt-sessiond for the reference.

+-----------+            +--------------+
| lttng A   |            | ltt-sessiond |
+-----------+            +--------------+
                          ^    |
        +-------+         |    |   +-------------+
        | app_1 |<--------+    +-->| shared mem. |
        +-------+    commands      +-------------+
            |                           ^
            +---------- write ----------+

2) lttng spawns a ust-consumerd for the session. We get the same figure as
step 3 in the first scenario.

There is a small difference though. The application MAY NOT be using the same
credentials as user A (lttng).  However, the shared memory is always GID of
the tracing group. So, in order for user A to trace app_1, is MUST be in the
tracing group otherwise, if the application is not set with the user
credentials, user A will not be able to trace app_1

Scenario 3 - Multiple users tracing the same running application
------

1) Session are created for the two users. Using the same exact mechanism as
before, the shared memory and consumers are created. Two users, two sessions,
two consumers and two shared memories for the same application.

+-----------+                     +--------------+
| lttng A   |-------- ops ------->| ltt-sessiond |
+-----------+      ^              +--------------+
                   |                           ^ commands
+-----------+      |               +-------+<--+
| lttng B   |------+          +--->| app_1 |------- write -----+
+-----------+                 |    +-------+                   |
                              |                                |
         +-----------------+  |               +-------------+  |
         | ust-consumerd A |--O--- read ----->| shared mem. |<-+
         +-----------------+  |               +-------------+  |
                              |                                |
         +-----------------+  v               +-------------+  |
         | ust-consumerd B |--+--- read ----->| shared mem. |<-+
         +-----------------+                  +-------------+

ust-consumerd A - UID: user A (rw-), GID: tracing (r--)
ust-consumerd B - UID: user B (rw-), GID: tracing (r--)

Scenario 4 - User not in the tracing group
------

For this particular case, it's all goes back to the first scenario. The user
MUST start the application using his credentials. The session will be created
by the per-user ltt-sessiond but he will not be able to trace anything that the
user does not owned.