The migrate_from action can serve one of two purposes:

The example below illustrates a push type migration:

foobar_migrate_from() {
    # exit immediately if configuration is not valid
    foobar_validate_all || exit $?

    # After the resource has been migrated, check whether it resumed
    # correctly. If the resource starts asynchronously, the agent may
    # spin on the monitor function here -- if the resource does not
    # run within the defined timeout, the cluster manager will
    # consider the migrate_from action failed
    while ! foobar_monitor; do
        ocf_log debug "Resource has not yet migrated, waiting"
        sleep 1
    done

    # only return $OCF_SUCCESS if _everything_ succeeded as expected
    return $OCF_SUCCESS
}

In contrast, a freeze/thaw type migration may implement its thaw operation like this:

foobar_migrate_from() {
    # exit immediately if configuration is not valid
    foobar_validate_all || exit $?

    # actually start up the resource here (make sure to immediately
    # exit with an $OCF_ERR_ error code if anything goes seriously
    # wrong)
    ocf_run frobnicate --thaw || exit OCF_ERR_GENERIC

    # After the resource has been migrated, check whether it resumed
    # correctly. If the resource starts asynchronously, the agent may
    # spin on the monitor function here -- if the resource does not
    # run within the defined timeout, the cluster manager will
    # consider the migrate_from action failed
    while ! foobar_monitor; do
        ocf_log debug "Resource has not yet migrated, waiting"
        sleep 1
    done

    # only return $OCF_SUCCESS if _everything_ succeeded as expected
    return $OCF_SUCCESS
}