Extendible hash table–iteration
by Oren Eini, CEO RavenDB

I run perf tests and memory utilization tests on my implementation and finally got it to the right place. But the API I have is pretty poor. I can put a key and value, or get the value by key. But we probably want a few more features.

I changed the put implementation to be:

This allows me to do an atomic replace and get the old value from the table. That is a nice low hanging fruit. But the key feature that I want to talk about today is iteration, as you might have figured out from the post title Smile.

I’m writing this code in C, because I find it interesting to practice in different environments, and C doesn’t really have an iteration API. So here is what I came up with:

If this was a public API I was building, I would probably want to hide the implementation details of the hash_iteration_state. Right now, I get a allocation and failure free API, because the caller is responsible for supplying the space for the state.

Here is how we can iterate using this API:

Not too bad, right? And this is basically what you’ll get when you use yield and such in languages that support native iterations. In C, you need to manage this yourself, but I don’t think that I got too lost here.

We store the current state in the state variable, and simply traverse the data in the buckets / pieces as they come.

Looking at this code, what is missing? Error handling…

But wait, I can hear you say, how can there be errors here? There are no moving pieces that can break, surely.

Well, the caller of our API may provide some moving pieces for us. For example, consider this code:

In other words, if we iterate and modify the data, what is going to happen? Well, we may change the position of values, which will lead us to skipping some values, iterating over some values twice, etc. What is worse, this may violate invariants in the code. In particular, the invariant in question is that current_piece_byte_pos always points to the start of a new key. If the data moved because of the put, this doesn’t hold true any longer.

I added protection to that by adding a version field to the directory, which is incremented whenever we call a put / replace on the directory. Then we can check if the value has changed. The issue is how do we report this in? Right now, I wrote:

image

I guess I could have done better by changing the return value to an int and returning better error code directly. This is a perfect case for exception, I think, since this is an edge case that should never be hit in real code. The fact that modifying the hash table will invalidate the iterator and cause it to stop working, on the other hand, might not be immediately obvious to the caller. More likely than not, though, anyone trying to write mutating code such as the one above will quickly figure out that this isn’t working and check exactly why.

Because of that, I decided to keep the bool return value, to simplify the life of our callers.

The full code is here.

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