no doc procs registered to process a batch with priority 0

I came across this error message recently and I was able to figure out the problem.

WARNING    Could not send batch to ESP content distributor, will retry automatically. Reason given: process() failed: exception (no_resources) no doc procs registered to process a batch with priority 0

The problem was found in the document processor log files. (var\log\procserver\)

It turned out that a file was missing in esp\var\procserver\\\xxx.

What we did in order to solve the problem was:

1. stop the indexingdispather

2. stop all proc servers

3. delete all files and folders under esp\var\procserver\

4. start the indexingdispatcher

5. start proc servers