MySQLBlobResource always returns text 'content' as value of content column

[paulcwarren]

MySQLBlobResource always returns text 'content' as value of content column

[paulcwarren]

Thanks @inngvar. That isn't much of an embarrassing bug, is it?!?

I wonder how that was missed by our integration tests. Anyways, will fix.

[paulcwarren]

I looked at this a bit today. In fact it is working as designed; i.e. although a little weird the SQL is correct. When it comes to blob streaming the devil is in the detail as this blog post explains. On the surface JDBC provides several ways to get an input stream from a result set but when you go look at how they are implemented you often find they read the whole BLOB into memory.

This, in fact, is what MySQL does unless you use a feature called "locator emulation". With "locator emulation" turned on then the getBlob() jdbc api will use a different implementation that streams the content properly (i.e. in reasonable size chunks) through the memory space of the JVM running the mysql connector client. Without this the risk of OOMs are much greater and increase with the size of the content being stored.

To use "location emulation" you have to do a couple of things.

1. You have to specify emulateLocators=true on the end of the jdbc connection string URL
2. The SQL you use to fetch the result set have to specify the name of the column that stores the blob/"locator". This is because behind the scenes when you call getBlob and then getBinaryStream on that blob object it creates and executes its own queries fetching chunks of the blob at a time and it uses the column name encoded in the sql statement to help create these queries. Presumably because otherwise it wouldnt know which column the blob was stored in.

So, it is not clear if this is working for you, or not. If it isnt then I suspect you might not have emulateLocators=true on the end of your JDBC connection string. Admittedly, this isn't in our docs. So perhaps that is the bug here?

Anyways, let me know.

[inngvar]

Thank you, solution you suggest ( add emulateLocators=true) works fine.
And also thanks for the detailed description of the problem. As I thought, not everything is as obvious as it seemed.

[paulcwarren]

No problem. I'm going to close this as I think we are resolved. However, feel free to re-open or raise another issue if you find anything else.