RavenDB on the NoSQL Podcast
Code reviewThe bounded queue
The following code has just been written (never run, never tested).
It’s purpose is to serve as a high speed, no locking transport between two threads, once of them producing information, the other consuming it, in a bounded, non blocking manner.
Note that this is done because the default usage of BlockingCollection<T> here generated roughly 80% of the load, which is not ideal
reWhy you can't be a good .NET developer
This post is in reply to Rob’s post, go ahead and read it, I’ll wait.
My answer to Rob’s post can be summarize in a single word:
In particular, this statement:
it is impossible to be a good .NET developer. To work in a development shop with a team is to continually cater for the lowest common denominator of that team and the vast majority of software shops using .NET have a whole lot of lowest common denominator to choose their bad development decisions for.
Um, nope. That only apply to places that are going for the lowest common denominator. To go from there to all .NET shops is quite misleading. I’ll give our own example, of building a high performance database in managed code, which has very little lowest common denominator anything anywhere, but that would actually be too easy.
Looking at the landscape, I can see quite a lot of people doing quite a lot of interesting things at the bleeding edge. Now, it may be that this blog is a self selecting crowd, but when you issue statements as “you can’t be a good .NET developers”, that is a pretty big statement to stand behind.
Personally, I think that I’m pretty good developer, and while I dislike the term “XYZ developer”, I do 99% of my coding in C#.
Now, some shops have different metrics, they care about predictability of results, so they will be extremely conservative in their tooling and language usage, the old “they can’t handle that, so we can’t use it” approach. This has nothing to do with the platform you are using, and all to do with the type of culture of the company you are at.
I can certainly find good reasons for that behavior, by the way, when your typical product lifespan is measured in 5 – 10 years, you have a very different mindset than if you aim at most a year or two away. Making decisions on brand new stuff is dangerous, we lost a lot when we decided to use Silverlight, for example. And the decision to go with CoreCLR for RavenDB was made with explicit back off strategy in case that was sunk too.
Looking at the kind of directions that people leave .NET for, it traditionally have been to the green green hills of Rails, then it was Node.JS, not I think it is Elixir, although I’m not really paying attention. That means that in the time a .NET developer (assuming that they investing in themselves and continuously learning) invested in their platform, learned a lot on how to make it work properly, the person who left for greener pastures has had to learn multiple new frameworks and platforms. If you think that this doesn’t have an impact on productivity, you are kidding yourself.
The reason you see backlash against certain changes (project.json coming, going and then doing disappearing acts worthy of Houdini) is that there is value in all of that experience.
Sure, sometimes change is worth it, but it needs to be measured against its costs. And sometimes there are non trivial.
Proposed solution to the low level interview question
For the actual question, see the original post.
So the first thing that we need to decide is what will be the data format on the tire. Since we have only 32KB to work with, we need to consider the actual storage.
32KB is small enough to fit in a unsigned short, so all the references we’ll used will be shorts. We also need to store a bit of metadata, so we’ll use the first 4 bytes as the header for just that.
- ushort SpaceUsed;
- ushort LastAllocation;
Now that we have this, we need to decide how to store the actual data. To make things easy, we are going to define the following way to allocate memory:
This is about the simplest way that you can go about doing things, note that we use a length prefix value, and we limit allocations to a max of 127 bytes each. We use a negative size to indicate a delete marker.
So basically, now we have a pretty trivial way to allocate memory, and we can implement the trie as we would normally do. There are a few wrinkles, however.
Deleting the memory doesn’t actually make it eligible for reuse, and it is quite likely to get fragmented easily. In order to handle that, we will track the amount of space that is used, and if we got to the end of the space, we’ll check the UsedSpace value. If this is still too little, we can abort, there is no available space here. However, if we go to the end of the buffer, but we have free space available, we can do the following:
- Scan the buffer for available spots (find available locations that have negative size).
- Failing that, we will copy the data to a temporary buffer, then re-add everything to the buffer from scratch. In other words, we defrag it.
Another issue we have is that the maximum size we can allocate is 127. This value is big enough so most actual strings can fit into it nicely, but a trie already has the property that a large string might be broken into pieces, we’ll just cut each node in the trie to a max size of 127. Actually, the max size is likely to be less than that, because there is also some information that we need to keep track per entry.
- byte NumberOfChildren;
- byte Flags; // node type, (internal, leaf or both)
- ushort ChildrenPosition;
So in practice we have about 123 bytes to work with for the length. Note that we don’t store the string value of the node’s length (we can get that from the allocation information), and that we store the actual children in an array that is stored separately. This allows us to easily add items to the trie as child nodes. If the node is a leaf node, we also need to store the actual value (which is 8 bytes), we store that information at the end of the value (giving us 115 bytes for that section of the value).
All in all, there is going to be a bit of pointer arithmetic and bit counting, but is likely to be a pretty simple implementation.
Note that additional optimizations would be to try align everything so it would fit into a cache line, trying to place nodes near their children (which are more likely to be followed), etc.
RavenDB Conference 2016–Slides
The slides from the RavenDB Conference are now available, we’ll post videos in a few weeks, once we are done post processing them.
- RavenDB 3.5 – Oren Eini
- Modern Web Dev with RavenDB – Judah Himango
- RavenDB Embedded at Massive Scale – Rodrigo Rosaruo, RDI
- Lessons from the Trenches – Dan Bishop, RavenHQ
- Know Thy Cost (or where performance bugs lurk) – Federico Lois, Corvalius
- Should I use a Document Database? – Elemar Junior
- RavenDB 4.0 – Oren Eini
- Zapping Ever Faster – Hagay Albo, ZAP Group
- Implementing CQRS and Event Sourcing with RavenDB – Elemar Junior
- Building Codealike – a journey into the developers analytics world – Federico Lois Corvalius
- Delving into Data Subscriptions – Kijana Woodard
- Replication Changes in RavenDB 3.5 – Jonathan Matheus, RavenHQ
Early bird offer for RavenDB Conference
Early bird pricing for RavenDB Conference in Texas in available until the end of the month.
RavenDB in VS Live!
Our booth in VS Live!
And you can tell that it isn't me who is taking the picture because it looks awesome:
Announcing: RavenConf 2016
Our second RavenDB conference is now open. You can visit the conference site to see our speakers and topic, but in general, we are going to talk about the new RavenDB 3.5 release, including some extremely exciting features that we kept as a surprise. We are going to hear from customers and users about their experiences in utilizing RavenDB in demanding and high performance environments and we are going to unveil our plans for RavenDB 4.0.
Come and join us.
RavenDB at the Basta Conference
This is from today, at the RavenDB booth in the Basta conference.