class LiveMigrationTask(base.TaskBase):
    def __init__(self, context, instance, destination,
                 block_migration, disk_over_commit, migration, compute_rpcapi,
                 servicegroup_api, scheduler_client):
        super(LiveMigrationTask, self).__init__(context, instance)
    ... 

    def _execute(self):
        self._check_instance_is_active()
        self._check_host_is_up(self.source)

        if not self.destination:
            self.destination = self._find_destination()
            self.migration.dest_compute = self.destination
            self.migration.save()
        else:
            self._check_requested_destination()

        # TODO(johngarbutt) need to move complexity out of compute manager
        # TODO(johngarbutt) disk_over_commit?

        #调用 ComputeAPI 类中的 live_migration() RPC接口
        return self.compute_rpcapi.live_migration(self.context,
                host=self.source,
                instance=self.instance,
                dest=self.destination,
                block_migration=self.block_migration,
                migration=self.migration,
                migrate_data=self.migrate_data)

class ComputeAPI(object):

    # 这是一个RPC远程调用的方法
    def live_migration(self, ctxt, instance, dest, block_migration, host,
                       migration, migrate_data=None):
        args = {'migration': migration}
        version = '4.2'
        if not self.client.can_send_version(version):
            version = '4.0'

        # 获取目标 compute 主机(DEST HOST)的RPC client，即被调用的服务进程的HostIP
        cctxt = self.client.prepare(server=host, version=version)

        # 通过目标主机对象的 RPC cliient 来调用远程过程方法 cast() ，以此来实现远程调用
        cctxt.cast(ctxt, 'live_migration', instance=instance,
                   dest=dest, block_migration=block_migration,
                   migrate_data=migrate_data, **args)
        # cast()异步远程调用，不会阻塞别的进程，适合于需要长时间进行的执行过程
        # cast()的第二个参数是RPC client调用的函数名，case()后面的参数会继续作为参数传入该调用函数
        # cast()函数内的live_migration()函数是 manager.live_migration() 视具体实现迁移功能的函数，在manager.py内实现。

@wrap_exception()
    @wrap_instance_event(prefix='compute')
    @wrap_instance_fault
    def live_migration(self, context, dest, instance, block_migration,
                       migration, migrate_data):
        """执行实时迁移。

        :param context: security context
        :param dest: destination host
        :param instance: a nova.objects.instance.Instance object
        :param block_migration: if true, prepare for block migration
        :param migration: an nova.objects.Migration object
        :param migrate_data: implementation specific params

        """
        self._set_migration_status(migration, 'queued')

        def dispatch_live_migration(*args, **kwargs):
            with self._live_migration_semaphore:
                # 调用_do_live_migration执行迁移
                self._do_live_migration(*args, **kwargs)

        # NOTE(danms): We spawn here to return the RPC worker thread back to
        # the pool. Since what follows could take a really long time, we don't
        # want to tie up RPC workers.
        utils.spawn_n(dispatch_live_migration,
                      context, dest, instance,
                      block_migration, migration,
                      migrate_data)