I have a cloud server, where a block device (/dev/xvdb1 with filesystem ext3) is attached .
I want to create a persistent symlink (/dev/test_link) which should always target to (/dev/xvdb1) using udev rules.
OS -> 16.04 ubuntu
You should start with using
udevadm monitor when attaching your target device, or
udevadm info --name=/dev/xvdb1 --attribute-walk, when already attached, to have a look at how udev sees your device. Than you can create udev rules which match only the special device.
blkid would be a good starter, where you can see the ID of your device, for which you can than create udev rules.
(Examples for rules can e.g. be found at https://wiki.debian.org/udev )