With Docker, you can easily do things like:

• rename container

docker container rename foo old-foo

• tell tomcat to shutdown ASAP

docker kill --signal TERM old-foo

• start new tomcat

docker run --network=host ...

This works easily for PHP-FPM. But what about Tomcat?

Related pages: https://iximiuz.com/en/posts/multiple-containers-same-port-reverse-proxy/ https://innovation.ebayinc.com/tech/engineering/zero-downtime-instant-deployment-and-rollback/

• on startup:
  • bind HTTP port
  • start webapps
• on shutdown:
  • handle remaining requests, it waits for "gracefulStopAwaitMillis" / "unloadDelay" (only 2 seconds by default)
  • save sessions
  • unbind HTTP port

Issues:

• during "gracefulStopAwaitMillis", new requests are waiting and will be aborted

• on startup:
  • start webapps
  • bind HTTP port
• on shutdown:
  • unbind HTTP port
  • handle remaining requests
  • save sessions

Issues:

• when the second tomcat is starting, the sessions file may not be available yet
• during the second tomcat startup, the requests will return an error (HTTP 502 if rev proxied). This is especially bad for POST like requests

• on startup:
  • bind HTTP port
  • start webapps
• on shutdown:
  • save sessions
  • unbind HTTP port
  • handle remaining requests

Issues: none!

An alternative to EarlyCloseHttp11NioProtocol is to use unixDomainSocketPath (if you have a apache2/nginx on the same host) since you can easily rm it.

<Connector protocol="HTTP/1.1" ...

with

<Connector protocol="fr.univparis1.tomcat.EarlyCloseHttp11NioProtocol" ...

<Context>
    <Manager className="fr.univparis1.tomcat.EarlySessionsUnloadManager" pathname="/var/lib/sessions/" />
</Context>

It requires a listener:

<Server port="-1">
  <Listener className="fr.univparis1.tomcat.EarlySessionsUnloadListener" />
  ...

Compared to StandardManager, it features storing sessions in per-webapp files when "pathname" is a directory.