jdbc:h2:mem: threadsafe?

[Laird Nelson]

The documentation says that connecting to the JDBC URL "jdbc:h2:mem:" creates a "private" in-memory database.

Is the database so created private to the thread that creates it?

If threads t1 and t2 both attach to jdbc:h2:mem:, do they or do they not share the database?  I hope that it is intended that they do not.

Best,

Laird

[Laird Nelson]

On Monday, October 7, 2013 2:41:21 PM UTC-7, Laird Nelson wrote:

The documentation says that connecting to the JDBC URL "jdbc:h2:mem:" creates a "private" in-memory database.

Is the database so created private to the thread that creates it?

To be clear, I'm aware of this documentation paragraph as well:

"In some cases, only one connection to a in-memory database is required. This means the database to be opened is private. In this case, the database URL is jdbc:h2:mem: Opening two connections within the same virtual machine means opening two different (private) databases."

I just want to make sure that two connections in two threads in the same VM each get a database scoped to that thread.

Best,

Laird 

[Thomas Mueller]

Hi,

I think the documentation is quite clear.

Of course in Java, you can pass around objects to any thread you like. If your application has a handle on an object, you can also call methods on it concurrently in multiple threads. H2 is thread-safe (as documented), but there are parts of the JDBC API that are problematic, for example it doesn't make sense to call PreparedStatement.setString + execute concurrently on the same object. But that has nothing to do with H2.

Regards,

Thomas

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[Laird Nelson]

On Tuesday, October 8, 2013 10:24:12 AM UTC-7, Thomas Mueller wrote:

I think the documentation is quite clear.

Let me rephrase the question (and sorry for just now noticing the timely reply):

Suppose I have two threads, t1 and t2.  Suppose each of them opens a connection to jdbc:h2:mem:  Suppose that each of them now issues a CREATE TABLE Fred statement.  Suppose further that each of them now inserts one row into the Fred table.

If t1 then issues a SELECT COUNT(*) FROM Fred, and if no other database activity takes place, will the result be guaranteed to be 1?  Or will it be 2?

Thanks,

Best,

Laird

[Ryan How]

It will be 2. and the second create table will fail because the table already exists. But I don't see how that has anything to do with being thread safe? The in memory database is per process, so all threads access the same database. If you want a separate database per thread then give the databases different names.

[Laird Nelson]

On Monday, December 30, 2013 5:38:46 PM UTC-8, Kartweel wrote:

It will be 2.

Thanks.

 

and the second create table will fail because the table already exists.

Yep; thanks.

 

But I don't see how that has anything to do with being thread safe?

I'm sure I worded it badly.  I am trying to see how best to run parallel unit tests using the maven-surefire-plugin, where you can control parallelism at the process and thread level.  I didn't see anything in the H2 documentation about whether a "mem" database was one-per-process or one-per-thread.

 

The in memory database is per process, so all threads access the same database.

Thank you; this is what I was looking for.  Is there a spot in the documentation that says this explicitly?

 

If you want a separate database per thread then give the databases different names.

That has been my approach so far; it dawned on me (wrongly, as it turns out) that perhaps I was overengineering something—that perhaps jdbc:h2:mem was enough for a private per-thread database, but as you point out it is not.  Thanks for your time and the clarity of your response.

Best,

Laird

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[Thomas Mueller]

Hi,

> a private per-thread database

There is nothing like a "per-thread database". My guess is what you mean is "in-memory private".

Each time you open a connection using the exact database URL "jdbc:h2:mem:", you will get a new database ("in-memory private"). It doesn't matter if that's within the same process or thread or not.

Each time you open a connection using the database URL "jdbc:h2:mem:test" or similar (that is, something after "mem:"), you will get _the same_ database within the same classloader if you use the same name. This again doesn't depend on the thread.

For details, see: 

http://h2database.com/html/features.html#database_url

http://h2database.com/html/features.html#in_memory_databases

Regards,

Thomas

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