Thursday, January 21, 2016

Docker: Handling multiple copies of the same database/container

Inspired by Frits Hoogland's excellent article on Oracle running in Docker, I started building a lot of Oracle containers. It's nice to have multiple different Oracle versions available at your fingertips for research, product testing and so on.

However, one thing annoys me with Docker: if you want any usable IPC, you need to use --ipc=host. This means that all the images share the same namespace and, furthermore, when a container exits it sometimes does not clean up the IPC entries.

As you probably know the IPC is used by Oracle for SGA memory and semaphore sets. It identifies which belong to which instance, by combining SID and ORACLE_HOME.

This in turn means that you cannot run two databases with the same SID and ORACLE_HOME at the same time... which is usually fine, but not so with Docker and --ipc=host. In this case we do want to run multiple containers built off the same image, or perhaps have multiple similar images with the same ORACLE_HOME, differing in minor details only, such as patchset level.

Fortunately it is actually pretty easy to change the ORACLE_SID, without altering the name of the database. The only thing you really need to change is the name of the spfile (or you can specify the name explicitly when starting the database). You should also change the name of the password file, if you use one, and add an entry to /etc/oratab for convenience.

This has to happen when the container is started, not in the image. And you also have to decide how you handle container stop/start: do you want to generate a new name, or do you remember the new names? (Because, as you know, you need the SID to startup the database in the start scripts.)

I decided to go with the first approach, so that on every start I generate a new name. And I just copy the scripts, so that the previous name is always there and the copy scripts always find it, even when executed repeatedly.

export OLD_SID=SRC
export NEW_SID=`perl -e 'my @c=("A".."Z","a".."z","0".."9");my $s; $s.=$c[rand @c] for 1..8;print $s;'`
. oraenv #get ORACLE_HOME
cp spfile$OLD_SID.ora spfile$NEW_SID.ora
cp orapw$OLD_SID orapw$NEW_SID.ora
echo "$NEW_SID:$ORACLE_HOME:N" >> /etc/oratab
echo "Generated: $NEW_SID:$ORACLE_HOME:N"
. oraenv
cd -

You can also see that the names of some files will change, for example, alert log changed from, for example alert log changed from diag/rdbms/src/SRC/trace/altert_SRC.log to diag/rdbms/src/081b59ce/trace/alert_081b59ce.log.

So, to conclude, note that the purpose of this script is to have a quick and easy way to spin up multiple containers - and it leaves much room for improvement. There are other possibilities, such as statically registering the new SIDs in listener.ora so you can connect to start the instances without knowing the SID, or writing the new SIDs to disk and using them on container restart.