SqlStore not thread-safe because of Closed Statements
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Key: GORA-84
URL: https://issues.apache.org/jira/browse/GORA-84
Project: Gora
Issue Type: Bug
Components: storage-sql
Reporter: Ferdy Galema
In some multithreaded test code (actually Nutchgora TestGoraStorage) the
current trunk of Gora SqlStore fails because of Closed Statements:
Caused by: java.sql.SQLException: statement is closed
at org.hsqldb.jdbc.Util.sqlException(Unknown Source)
at org.hsqldb.jdbc.JDBCPreparedStatement.executeBatch(Unknown Source)
at org.apache.gora.sql.store.SqlStore.flush(SqlStore.java:329)
... 9 more
Caused by: org.hsqldb.HsqlException: statement is closed
at org.hsqldb.error.Error.error(Unknown Source)
at org.hsqldb.error.Error.error(Unknown Source)
at org.hsqldb.Session.executeCompiledBatchStatement(Unknown Source)
at org.hsqldb.Session.execute(Unknown Source)
... 11 more
I am not sure why this problem suddenly arises as I could not find relevant
changes in the SqlStore related code.
The problem is caused of incorrectly synchronized code when buffering and
flushing put requests. Some statements are accidently shared between threads
because of the shared write buffer (the HashSet writeCache). So it may happen
that some closed statements are trying to be executed, raising the above
exception. When I disabled SqlUtils.close() by making the method empty, the
tests worked again. So that is the lazy man's solution: Disable the writeCache.
I suggest that somebody who is more into maintaining this store fixes it user a
better solution (proper synchronization). When there is no interest in doing
so, I can still always implement the lazy man's solution :)
By the way, the fact that this error was not detected in Gora's own tests
suggests that we might want to expand it's coverage. This is in line what was
already mentioned a few days ago on the dev list ("inconsistencies in
TestXXXStore").
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