When I run the following Java program on an AWS EC2 Linux virtual machine connecting to an Oracle database in my company’s internal network it hangs forever.
When I run it on a Linux machine on our internal network it runs fine.
Evidently my company uses an AWS feature called “Gateway Load Balancer” to connect our AWS network to our internal on premises network. Evidently the GLB has a 350 second timeout. See this document:
Here is a quote of the relevant paragraph:
Some applications or API requests, such as synchronous API calls to databases, have long periods of inactivity. GWLB has a fixed idle timeout of 350 seconds for TCP flows and 120 seconds for non-TCP flows. Once the idle timeout is reached for a flow, it is removed from GWLB’s connection state table. As a result, the subsequent packets for that flow are treated as a new flow and may be sent to a different healthy firewall instance. This can result in the flow timing out on the client side. Some firewalls have a default timeout of 3600 seconds (1 hour). In this case, GWLB’s idle timeout is lower than the timeout value on the firewall, which causes GWLB to remove the flow without the firewall or client being aware it was dropped.Best practices for deploying Gateway Load Balancer
This means that my JDBC call using the thin driver will work fine if I sleep for 349 seconds but will hang forever if I try to sleep for 350 seconds. The solution is to update a Linux operating system parameter and to update the JDBC connect string.
sysctl -w net.ipv4.tcp_keepalive_time=60
add this line to /etc/sysctl.conf:
Evidently our default tcp_keepalive_time value was 7200 seconds which is longer than the 350 second timeout so we had to lower it to 60 seconds to that the Gateway Load Balancer would know that our JDBC call was actually doing something.
You have to add (ENABLE=broken) to the jdbc connect string like this:
Once I did this my Java test program worked fine. It ran for about 350 seconds and finished cleanly.
If you are working in AWS and connecting to a on premises database using JDBC and you have a SQL statement that should run for 350 seconds or more and hangs forever you might check whether you are being affected by this timeout.
p.s. I forgot to mention that the Oracle database session goes away after 350 seconds. It is just the client side JDBC call that hangs apparently forever.
p.p.s. We have a related issue with Putty sessions connecting to Amazon EC2 Linux VMs timing out after 350 seconds. A coworker offered this article as a solution:
The Putty keepalives setting works great!
Another coworker of mine was saying that certain types of firewalls work this way with timeouts. The problem is that the GWLB times out our on-premises side but not our AWS side. So, in the case of using Putty to ssh into an EC2 that does not have keepalives configured my Putty session, which also does not have keepalives configured, times out after 350 seconds of idle time. When I hit enter, I get “Network error: Software caused connection abort” but if I check my BASH shell process id, I see that my shell process was never terminated. So, old processes hang around forever on my EC2 if the ssh connection times out due to the GWLB 350 second timeout.
Maybe it is normal for connections on one side of a firewall to time out and the other side to hang forever? I am not sure.