user should be warned that he needs to restart the server when installer detects that libssl0.9.8 doesn't exist during upgrade on ubuntu 12( installation will be stop and give instructions to install the missing libssl0.9.8)

corresponding bug
http://www.couchbase.com/issues/browse/MB-4862

should user restart manually server after upgrade was stopped?
Answer: yes

ubuntu@ip-10-39-13-185:/tmp$ sudo dpkg -i couchbase-server-community_x86_64_2.0.0-1971-rel.deb
(Reading database ... 31678 files and directories currently installed.)
Preparing to replace couchbase-server 1.8.1 (using couchbase-server-community_x86_64_2.0.0-1971-rel.deb) ...

=INFO REPORT==== 3-Dec-2012::15:56:42 ===
Initiated server shutdown** at node ns_1@127.0.0.1 **

=INFO REPORT==== 3-Dec-2012::15:56:48 ===
Stopped ns_server application** at node ns_1@127.0.0.1 **

• Stopped couchbase-server
  No packages found matching libssl0.9.8.
  Installing cannot proceed since the package 'libssl0.9.8' is missing.
  Please install libssl0.9.8 and try again.

$sudo apt-get install libssl0.9.8

You can also restart couchbase as follows:

$sudo /etc/init.d/couchbase-server start

dpkg: error processing couchbase-server-community_x86_64_2.0.0-1971-rel.deb (--install):
subprocess new pre-installation script returned error exit status 1
Errors were encountered while processing:
couchbase-server-community_x86_64_2.0.0-1971-rel.deb
ubuntu@ip-10-39-13-185:/tmp$ ps -ef| grep couchbase
ubuntu 3687 3502 0 16:41 pts/1 00:00:00 grep --color=auto couchbase
ubuntu@ip-10-39-13-185:/tmp$ ^C
ubuntu@ip-10-39-13-185:/tmp$ sudo /etc/init.d/couchbase-server start

• Started couchbase-server

this information should be added to the documentation/release notes