Expose your dev machine to the public via reverse SSH tunnel

Scenario: you are creating a REST service which needs to be exposed to the public even in early stage of development due to an upstream provider which sends back feedback data from a webhook API.

You are also behind a NAT so you’d have to port forward yourself out but you can’t do that for whatever reason. Or maybe you are behind a firewall and 7 proxies.

All you need is an external server with a public IP. Then, on your machine:

ssh -R 0.0.0.0:8080:localhost:8080 server_user@public_server_ip

In the above example, I used port 8080 which my REST service uses when I develop. On your public server, make sure you have

GatewayPorts yes

in /etc/ssh/sshd_config. If it is missing, add it.

And that’s it.. your local REST service is now publicly accessible via public_server_ip:8080.

a-m-a-z-i-n-g

Cen
GitHub
Eurobattle.net
Lagabuse.com
Bnetdocs

OJDBC7 in a Docker container? Prepare for trouble

Scenario: A JDK8 Docker container using OJDBC7 to connect to the database. Sounds simple enough, what could go wrong?

Simptoms: Connecting to the database randomly takes several minutes, fails with a weird SqlRecoverableException: no more data to read from socket or just works fine as if there is no problem.

The same Docker image also works fine on some machine but fails consistently on other.

The reason is this. Docker is not good at /dev/random. Probably even more so if you run it in a VM, since it’s double isolated from actual entropy sources (my non scientific observation). For whatever reason, OJDBC defaults to /dev/random and this causes a block when connecting to the database due to high probability of /dev/random depletion.

Simple solution is to just mount /dev/urandom to /dev/random inside the Docker, in docker run command:

So.. if you ever want to use OJDBC inside Docker, remember this flag. It will save lives or at least spare you hours of useless debugging.

 

Cen
GitHub
Eurobattle.net
Lagabuse.com
Bnetdocs

JPQL: getting whole entities on a distinct column

If you want distinct entities in JPQL you would normally write something like this:

But this will do a DISTINCT on the whole Entity. This will also fail on Oracle DB if your Entity contains a CLOB. What if you really want to do a DISTINCT on a field, for example:

Unfortunately, this only returns an array of ID fields. If you want to retrieve the full entities and do a DISTINCT on a field the final query looks like:

Cen
GitHub
Eurobattle.net
Lagabuse.com
Bnetdocs