Wednesday, October 16, 2013

MariaDB Dynamic Columns client API

I have blogged on using MariaDB Dynamic Columns already, and I hope this was useful and introduction. I have a few more things on this subject though, but one so far little known and used feature is the Client API for Dynamic Columns, see the MariaDB Knowledge Base for details. What this is all about is that Dynamic Columns were originally envisioned as a means of managing the "dynamic columns" used by Cassandra when using the MariaDB Cassandra Storage Engine. But as we realize, this is the server side of things (the Storage Engine) but there is a corresponding client library also, that is part of the MariaDB Client API.

As you have seen if you have read my previous blog on this subject, or whatever is written about MariaDB Dynamic Columns elsewhere, which is not much, MariaDB actually has no external representation of the dynamic column: Either you get the binary representation or you parse the binary representation using one of the supplied functions (like COLUMN_GET to get the value of an attribute in a dynamic column) or you get the binary. The only exception is the function COLUMN_JSON that retrieves the whole dynamic column and converts it to JSON, supporting nested objects also. Regrettably, there is no built-in means of adding a dynamic column in JSON format (Ulf Wendel has provided some JSON User Designed Functions though, but they don't line up with Dynamic Columns on the other hand).

Now, if we assume we are not using the COLUMN_GET function or something like that, can I programmatically parse a dynamic_column on the client? Well, I could sure use the server functions again, calling COLUMN_LIST and functions like that from a SELECT statement or in a Storage Procedure (I'll show this later), but what is real handy (if you are a C-developer like myself) is the DynamicColumns API functions in the MariaDB Client Library.

To begin with, these functions are not as easy to use as it might seem in the Knowledge Base article mentioned above. There are some bugs in what files are required that currently makes it kind of messy to build programs that use the Dynamic Columns API (which brings up another question: How many times can you write Dynamic Columns in one single blogpost?) This will be fixed eventually (it is known, reported bugs we talk about here), but until it is, let me show you some examples, so you get the point of this API.

To begin with, the API assumes that data passed to it is in the form of a DYNAMIC_COLUMN struct, which is a string with a specified length, i.e. it is NOT a null-terminated string but instead it is really, truly binary. This might be silly, but it is just how things are. So the DYNAMIC_COLUMN struct is set up with the data you got from a MariaDB Dynamic Column binary data. Still, this might not be valid Dynamic Column data, so to check that you should call the mariadb_dyncol_check() referencing this data to make sure it is valid Dynamic Column data.

The function that you then want to use is by far the most powerful one, mariadb_dyncol_unpack(). This will unpack a MariaDB dynamic column are retrieve the basic information on it:
  • How many attributes there are (how many "Dynamic Columns")
  • What the names of those attributes are.
  • The the values of those attributes are.
The values are returned as DYNAMIC_COLUMN_VALUE structs and the names as MYSQL_LEX_STRING structs. Most of this is described in the knowledge base article mentioned above.

I spent some time playing with this, most of that time was spent understanding how the API was meant to work and how to build an application on it. One night, after some heavy drinking, I decided that the best way to show this would be to write some code, instead of just blog about it on and on, so still severely drunk I wrote a simple tool for extracting Dynamic Column data from MariaDB and show it in some different ways (JSON, pretty JSON, Indented dynamic columns etc). To allow you to build it yourself I used autotools, but fact is to use that, you have to also copy some includefiles from the MariaDB source distribution (the reason is those bugs I mention above).

So there is a pre-built executable (I built this on an Ubuntu 10.10) that assumes you use the MariaDB 10.x Client Library (I used 10.0.4). If this doesn't work, as I said, you can always build it yourself (which isn't a throughly tested procedure). Also, for any of this to work, you need the Jansson JSON library.

I'll develop some more Dynamic Columns related tools eventually, but for now this is what I have in the dyncoltools toolset, the dyncoldebug tool available on sourceforge.



Ulf Wendel said...

Ahoi Anders,

I have not developed the JSON UDFs, I just happen to be a guy who's blog post on the JSON UDFs spread widely.


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