Daniel Naber <daniel.na...@languagetool.org> wrote:
> On 2014-02-01 17:08, Xavi Ivars wrote
>
>>> wget -q -T 1 localhost:8081/Languages
>>
>> If it works, then I do nothing, but if it doesn't succeed, then I kill
>> the process (if it's still there) and I restart it again.
>
> That wasn't enough for me, as sometimes the server had a high load but
> still replied to some requests or so, so I'm using a script that checks
> the load the process causes. It's a bad hack, we shouldn't stop fixing
> the actual reason for these problems (I sent you the script anyway via
> email)...
Hi
Regarding the server hanging problem (which I believe is still
happening, but I'd love to be wrong), it's likely the be a
race condition. One thing I noticed in the source code, is the
frequent use of "double-checked locking" pattern (or anti-pattern).
Here is an example taken from
languagetool/languagetool-language-modules/de/src/main/java/org/languagetool/language/German.java:
99 @Override
100 public Tagger getTagger() {
101 if (tagger == null) {
102 synchronized (this) {
103 if (tagger == null) {
104 tagger = new GermanTagger();
105 }
106 }
107 }
108 return tagger;
109 }
This optimization is not safe as far as I know. In fact,
I just looked it up and wikipedia confirms it. See:
http://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java
== BEGIN QUOTE from wikipedia ==
[...] this technique has many subtle problems and should
usually be avoided.
[...]
One of the dangers of using double-checked locking in J2SE 1.4
(and earlier versions) is that it will often appear to work: it is not
easy to distinguish between a correct implementation of the
technique and one that has subtle problems. Depending on
the compiler, the interleaving of threads by the scheduler and
the nature of other concurrent system activity, failures resulting
from an incorrect implementation of double-checked locking
may only occur intermittently. Reproducing the failures can
be difficult.
== END QUOTE ==
Besides the example in...
languagetool/languagetool-language-modules/de/src/main/java/org/languagetool/language/German.java
... I see the double-checked lock anti-pattern being used
at least here:
languagetool-language-modules/ca/src/main/java/org/languagetool/tagging/ca/CatalanTagger.java
languagetool-language-modules/pl/src/main/java/org/languagetool/synthesis/pl/PolishSynthesizer.java
languagetool-language-modules/de/src/main/java/org/languagetool/tagging/de/GermanTagger.java
(this is looks worse, as the check involves multiple variables)
languagetool-core/src/main/java/org/languagetool/synthesis/BaseSynthesizer.java
languagetool-core/src/main/java/org/languagetool/tagging/BaseTagger.java
The wikipedia page proposes Java alternatives. See:
http://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java
Of course, it may not be the root cause of server instabilities,
but it looks worth fixing it.
Regards
Dominique
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