Tuesday, July 29, 2008

1. Thoughts on a new NDB API, Baseline thoughts

I spent some time during my vacation thinking about some
new ideas. I designed the first version of the NDB API
about 10 years ago and obviously in those days the maximum
number of CPU's in most systems was 2 so it wasn't a big
problem having a single mutex protecting send and receive
in the NDB API (The NDB API is the low level API used by the
storage engine NDB which is the storage engine in MySQL

Another design criteria I made when designing the NDB API
was that most developers want to use a synchronous API.
Thus the asynchronous API was made afterwards and didn't
cover all operations. Most developers still develop using
synchronous API's, however most of the use for the NDB
API is for specialised applications such as Telco servers,
storage engine code, LDAP servers. Also I'm thinking in
even using it inside an operating system kernel to design
a clustered file system.

Thus today it seems like a better idea to use an asynchronous
API as the base and then put the synchronous API on top of

When designing the original NDB API it was sufficient to think
of simple key lookups, later it was advanced with also handling
scans of tables and indexes. However current design problems
are related to parallelising SQL queries and also there are
implementations of things such as BLOB's that actually require
multiple sequential and parallel operations. Thus in the new
design it's necessary to consider the possibility of starting
complex operations involving multiple threads (sometimes even
multiple processes), multiple operations in sequence and in

These ideas will be fed into the existing NDB API. It will also
be used in the iClaustron project where I aim to build something
that can be used as a clustered file system. iClaustron is both
designed with the aim to at some point in time be a useful thing,
but at the same time I use it as my personal playground where I
can test new ideas and see how my ideas turns out when turned
into source code.

The original NDB API was designed in C++ as all the rest of the
MySQL Cluster code. Within the data nodes I think we've found
a good compromise of what to use in the C++ language and what
not to use. However in general I found that debates around what
should be used in C++ tends to take an improportionate amount of
time compared to the value of those discussions. So for that
reason I decided to use C as the language of choice for iClaustron.
Actually there was more reasons for this, it makes it a lot easier
to use the code inside an operating system kernel such as Linux
or FreeBSD and second it makes it easier to write layers to other
languages such as Python, Perl,...

Most of the thoughts on this new NDB API has been in my mind for
more than 2 years (actually some of the thoughts have already been
implemented in NDB already), however during my vacation I had
some fun in designing out all the details I hadn't considered

It's my view of a nice vacation to relax on a beach or walking in the
mountains while inventing some new ideas based on an interesting
problems. I cheated by solving a Sudoku as well this vacation but in
general I like mind games that are related to what I do for a living.
Inventing the idea is the fun part of innovation, then comes the
hard part of actually doling out all the details, writing code and
testing code and selling the ideas. This is the work part.

I will follow this posting with a high level view on the ideas as far
they've been developed so far. In parallel I'll also "dump" the ideas
into code format. I like to think of my coding as a "brain dump", I
have a fairly unusual way of writing code. I think about the problem
for a long time and when I'm satisfied I write the code for it. I then
write all the code with only a very minimal set of compilation and
test cases. The main idea of coding in this phase is still design, so
in principal I write a design in the form of code. This also means
that I try to write as much comments as possible since I know that I
will otherwise forget my ideas. Working for MySQL has made me much
more aware of software engineering issues as well, so today I do also
a bit of thinking on software engineering as well in the design.

An architecture for the design is obviously very important, and the
architecture has borrowed heavily from the way the Linux kernel is
designed with lots of interfaces similar to the VFS interface in
Linux using a struct of a set of function pointers.

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