Hello Mr. Masahiro
I found the Mongo Plugin can not work well with the log that forwarded from other server. (I have test it for one month, and get this conclusion)
I forward log from one server (with fluentd out_forward) to another server (with fluentd in_forward), but the mongo plugin sometime failed to transfer the log to mongoDB. Then I must restart the fluentd to get it work again.
2013-05-31 03:16:19 +0900 fluent.error: {"message":"forward error: can't convert String into Integer"}
2013-05-31 03:16:19 +0900 [error]: forward error: can't convert String into Integer
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/mixin.rb:176:in `[]='
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/mixin.rb:176:in `filter_record'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/mixin.rb:96:in `block in format_stream'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/event.rb:127:in `feed_each'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/event.rb:127:in `each'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/mixin.rb:95:in `format_stream'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/output.rb:224:in `emit'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluent-plugin-mongo-0.7.0/lib/fluent/plugin/out_mongo.rb:100:in `emit'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/match.rb:38:in `emit'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/engine.rb:120:in `emit_stream'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/plugin/in_forward.rb:118:in `on_message'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/plugin/in_forward.rb:180:in `feed_each'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/plugin/in_forward.rb:180:in `on_read_msgpack'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/cool.io-1.1.0/lib/
cool.io/io.rb:108:in `on_readable'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/cool.io-1.1.0/lib/
cool.io/io.rb:170:in `on_readable'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/cool.io-1.1.0/lib/
cool.io/loop.rb:96:in `run_once'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/cool.io-1.1.0/lib/
cool.io/loop.rb:96:in `run'
2013-05-31 03:16:19 +0900 [error]: /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.33/lib/fluent/plugin/in_forward.rb:78:in `run'