Generally lot of clients bring down all servers as a part of the APAR\Code installation.
There is an alternate approach that would help a lot with down time.
As most of our APAR’s could be only commerce code changes this would work perfectly, if there are DB changes this approach may or may not work and the APAR might need analysis to see the nature of the DB changes.
1. Bring down the primary node. (Each cluster has only 1 primary node and the rest are federated nodes.)
2. The site is technically still up and should be able to handle the requests.
3. Install APAR using the DM on the primary node. (All the APARs are technically always installed only primary node.)
4. Verify for successfully installation of the APAR.
5. At this point bring down other nodes.
6. Synchronize nodes and restart.
It takes about 15 minutes to do the step 5 and Step 6 and that should be the down time.
Also the same approach is used for code deployment and apparently this helps them to confirm for the code deployment and if there are issues with any code deployment, the site can still be up while the issue is worked upon.
More Information on the topology look up in info center:
IBM has APARs, Fix Packs and Feature packs for Commerce and Fix Packs and iFixes for Application Server.
Topology Background high level: IBM 6.0 Deployment: Cell\Nodes\Node Agent\DM (Network Deployment Manager)