Method MigrateConnection

src/server/worker.cc:393–423

MigrateConnection moves the connection to another worker when reducing the number of workers. To make it simple, we do not migrate connections that are blocked on a key or stream, or that are paused (CLIENT PAUSE). Such connections stay on the worker being shut down and will be closed when it stops.

// or stream, or that are paused (CLIENT PAUSE). Such connections stay on the
// worker being shut down and will be closed when it stops.
void Worker::MigrateConnection(Worker *target, redis::Connection *conn) {
  if (!target || !conn) return;

  auto bev = conn->GetBufferEvent();
  // disable read/write event to prevent the connection from being processed during migration
  bufferevent_disable(bev, EV_READ | EV_WRITE);
  // We cannot migrate the connection if it has a running command
  // since it will cause data race since the old worker may still process the command.
  if (!conn->CanMigrate()) {
    bufferevent_enable(bev, EV_READ | EV_WRITE);
    return;
  }
  // Paused connections are not migrated; they will be closed when the worker stops.
  // Only re-enable WRITE here; READ must remain disabled to preserve the paused state.
  if (conn->IsPaused()) {
    bufferevent_enable(bev, EV_WRITE);
    return;
  }

  // remove the connection from current worker
  DetachConnection(conn);
  if (!target->AddConnection(conn).IsOK()) {
    conn->Close();
    return;
  }
  bufferevent_base_set(target->base_, bev);
  conn->SetCB(bev);
  // SetOwner before bufferevent_enable so callbacks see the correct owner.
  conn->SetOwner(target);
  bufferevent_enable(bev, EV_READ | EV_WRITE);
}

void Worker::DetachConnection(redis::Connection *conn) {
  if (!conn) return;