Bizarre DEA + Spring Behaviour

Daniel Jones

Hi all,

I've witnessed behaviour caused by the combination of a DEA and a Spring application that I can't explain. If you like a good mystery or you happen to know a lot about Java proxies and DEA transient state, please read on!

A particular Spring app was crashing only on specific DEAs in a Cloud Foundry.

All DEAs were from the same CF release (PCF ERT 1.5.2)
All DEAs were up-to-date according to BOSH (ie no outstanding changes waiting to be applied)
All DEAs were deployed with identical BOSH job config
All Warden containers were using the same root FS
The droplet was the same across all DEAs
The droplet version was the same
The droplet tarballs all had the same MD5 checksum
Warden was providing the exact same env and start command to all containers
I saw the same behaviour repeat itself across 5 completely separate Cloud Foundry installations

The crash was Spring not being able to autowire a bean, where it was referenced by implementation rather than interface (yes, I know, but it was not my code!). There was some Javassist/CGLIB action going on, creating proxies for the sake of transaction management.

Rebooting the troublesome DEAs did not fix the problem.

Doing a `bosh recreate` did reliably fix the problem.

Alternatively, changing the Spring code to wire by interface also reliably fixed the problem.

I can't understand why different DEA instances, from the same BOSH release, with the same config, on the same stemcell, running the same version of Warden, with the same droplet, and the same root FS, and the same env, and the same start command, yielded different behaviour. I'm even further confused as to why a `bosh recreate` changed that behaviour. What could possibly have changed? Something on ephemeral disk? But what else is there on ephemeral disk that could have mattered and was likely to have changed? Do CGLIB/Javassist have some native dependencies that weren't in sync between DEAs?

Anyone with a convincing explanation (that does not involve voodoo) will receive one free beer and a high-five at the next CF Summit!


Daniel Jones

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