[Spice-devel] [spice-protocol PATCH 2/2] seamless migration support

[Yonit Halperin]

The main difference between semi-seamless and seamless migration is that
while in semi-seamless migration the state of all the channels is
being completely reset after migration is complete, in seamless migration
the essential parts of the state are restored on the server side, and
are left the same on the client side. semi-seamless migration is
equivalent to having the client disconnect from the src and connected
from scratch to the dest, with the exception, that the handshake with
the dest server occurs before the client has disconnected from the src.
In semi-seamless migration in-flight data gets lost, e.g., a file
transfer to a usb device might be disrupted.

=======================
===protocol details====
=======================

Let s1, s2, and c be the src server, dest server and client, respectively.

Semi-Seamless migration protocol
================================

pre-migration phase:
--------------------
(1) s1->c: SPICE_MSG_MAIN_MIGRATE_BEGIN
    In response, c tries to establish a connection to s2. After the connection is
    established, it is inactive (the client doesn't attempt to read or
    write messages from/to it)
(2) c->s1: SPICE_MSGC_MAIN_MIGRATE_CONNECTED or
           SPICE_MSGC_MAIN_MIGRATE_CONNECT_ERROR

post migration phase:
---------------------
(1) s1->c: SPICE_MSG_MAIN_MIGRATE_END or
           SPICE_MSG_MAIN_MIGRATE_CANCEL
    In case of the former, c disconnects from s1, resets all its
    channels states and switches to an active connection with s2.
(2) c->s2: SPICE_MSGC_MAIN_MIGRATE_END
    The msg signals that all the channels have been migrated successfully to s2.

Seamless migration protocol
===========================

pre-migration phase:
--------------------
In case qemu/libvirt/client do not support seamless migration,
s1 takes the semi-seamless pathway for migration. Otherwise:

(1) s1->c: SPICE_MSG_MAIN_MIGRATE_BEGIN_SEAMLESS (*New*)
    The msg includes the version of the migration protocol
    of s1.
    In response c tries to establish a connection to s2.
(2)
    If the connection fails:
    (2.1) c->s1: SPICE_MSGC_MAIN_MIGRATE_CONNECT_ERROR

    If s2 supports SPICE_MAIN_CAP_SEAMLESS_MIGRATE:
    (2.2) c->s2: SPICE_MSGC_MAIN_MIGRATE_DST_DO_SEAMLESS (*New*)
          The msg includes the version of the migration protocol
          of s1. The msg is used for querying s2 if seamless migration
          is possible, given the migration protocol version of s1.

      (2.2.1) s2->c: SPICE_MSG_MAIN_MIGRATE_DST_SEAMLESS_ACK/NACK (*New*)
      (2.2.2) c->s1: SPICE_MSGC_MAIN_MIGRATE_CONNECTED_SEAMLESS (*New*) or
                     SPICE_MSGC_MAIN_MIGRATE_CONNECTED
              The latter is sent when c receives SEAMLESS_NACK, and
              indicates s1 to apply semi-seamless protocol on post
              migraion phase.

    If s2 does not support SPICE_MAIN_CAP_SEMI_SEAMLESS_MIGRATE:
    (2.3) c->s1: SPICE_MSGC_MAIN_MIGRATE_CONNECTED
          (see 2.2.2)

post migration phase:
---------------------
While the pre migration phase was conducted by the main channel, this
phase's protocol occurs in all the migrated channels.

(1) s1->c: SPICE_MSG_MIGRATE
    The msg marks the client that the connection is paused from s1 side, and
    next to this msg, the only possible msg s1 can send is
    SPICE_MSG_MIGRATE_DATA

    msg optional flags:
    (a) MIGRATE_FLUSH_MARK
        This flag is required for finalizing the channel connection
        without losing any in-flight data.
        This flag indicates that s1 expects SPICE_MSGC_MIGRATE_FLUSH_MARK,
        for signaling that c will pause the connection and not send any more messages
        to s1.
    (b) MIGRATE_DATA
        The flag indicates that c should receive from s1
        SPICE_MSG_MIGRATE_DATA
(2) c->s1: SPICE_MSGC_MIGRATE_FLUSH_MARK (if required)
    c pushes the msg to the head of its output msg queue,
    and sends it before all its other pending msgs - they will be sent to s2
    later.
(3) s1->c: SPICE_MSG_MIGRATE_DATA (if required)
    The msg contains all the data that the server requires for restoring
    the channel's state on s2 side correctly.
(4) c disconnects the channel from s1 and switches to an active connection
    with s2.
(4) c->s2: SPICE_MSGC_MIGRATE_DATA
---
 spice/enums.h    |    5 +++++
 spice/protocol.h |    1 +
 2 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/spice/enums.h b/spice/enums.h
index 2cc7e93..929495f 100644
--- a/spice/enums.h
+++ b/spice/enums.h
@@ -384,6 +384,9 @@ enum {
     SPICE_MSG_MAIN_NAME,
     SPICE_MSG_MAIN_UUID,
     SPICE_MSG_MAIN_AGENT_CONNECTED_TOKENS,
+    SPICE_MSG_MAIN_MIGRATE_BEGIN_SEAMLESS,
+    SPICE_MSG_MAIN_MIGRATE_DST_SEAMLESS_ACK,
+    SPICE_MSG_MAIN_MIGRATE_DST_SEAMLESS_NACK,
 
     SPICE_MSG_END_MAIN
 };
@@ -398,6 +401,8 @@ enum {
     SPICE_MSGC_MAIN_AGENT_DATA,
     SPICE_MSGC_MAIN_AGENT_TOKEN,
     SPICE_MSGC_MAIN_MIGRATE_END,
+    SPICE_MSGC_MAIN_MIGRATE_DST_DO_SEAMLESS,
+    SPICE_MSGC_MAIN_MIGRATE_CONNECTED_SEAMLESS,
 
     SPICE_MSGC_END_MAIN
 };
diff --git a/spice/protocol.h b/spice/protocol.h
index 0408962..b76d80d 100644
--- a/spice/protocol.h
+++ b/spice/protocol.h
@@ -121,6 +121,7 @@ enum {
     SPICE_MAIN_CAP_SEMI_SEAMLESS_MIGRATE,
     SPICE_MAIN_CAP_NAME_AND_UUID,
     SPICE_MAIN_CAP_AGENT_CONNECTED_TOKENS,
+    SPICE_MAIN_CAP_SEAMLESS_MIGRATE,
 };
 
 enum {
-- 
1.7.7.6