When we build a new feature in RavenDB, we either have at least some idea about what we want to build or we are doing something that is pure speculation. In either case, we will usually spend only a short amount of time trying to plan ahead.

A good example of that can be found in my RavenDB 7.1 I/O posts, which cover about 6+ months of work for a major overhaul of the system. That was done mostly as a series of discussions between team members, guidance from the profiler, and our experience, seeing where the path would lead us. In that case, it led us to a five-fold performance improvement (and we’ll do better still by the time we are done there).

That particular set of changes is one of the more complex and hard-to-execute changes we have made in RavenDB over the past 5 years or so. It touched a lot of code, it changed a lot of stuff, and it was done without any real upfront design. There wasn’t much point in designing, we knew what we wanted to do (get things faster), and the way forward was to remove obstacles until we were fast enough or ran out of time.

I re-read the last couple of paragraphs, and it may look like cowboy coding, but that is very much not the case. There is a process there, it is just not something we would find valuable to put down as a formal design document. The key here is that we have both a good understanding of what we are doing and what needs to be done.

RavenDB 4.0 design document

The design document we created for RavenDB 4.0 is probably the most important one in the project’s history. I just went through it again, it is over 20 pages of notes and details that discuss the current state of RavenDB at the time (written in 2015) and ideas about how to move forward.

It is interesting because I remember writing this document. And then we set out to actually make it happen, that wasn’t a minor update. It took close to three years to complete the process, to give you some context about the complexity and scale of the task.

To give some further context, here is an image from that document:

And here is the sharding feature in RavenDB right now:

This feature is called prefixed sharding in our documentation. It is the direct descendant of the image from the original 4.0 design document. We shipped that feature sometime last year. So we are talking about 10 years from “design” to implementation.

I’m using “design” in quotes here because when I go through this v4.0 design document, I can tell you that pretty much nothing that ended up in that document was implemented as envisioned. In fact, most of the things there were abandoned because we found much better ways to do the same thing, or we narrowed the scope so we could actually ship on time.

Comparing the design document to what RavenDB 4.0 ended up being is really interesting, but it is very notable that there isn’t much similarity between the two. And yet that design document was a fundamental part of the process of moving to v4.0.

What Are Design Documents?

A classic design document details the architecture, workflows, and technical approach for a software project before any code is written. It is the roadmap that guides the development process.

For RavenDB, we use them as both a sounding board and a way to lay the foundation for our understanding of the actual task we are trying to accomplish. The idea is not so much to build the design for a particular feature, but to have a good understanding of the problem space and map out various things that could work.

Recent design documents in RavenDB

I’m writing this post because I found myself writing multiple design documents in the past 6 months. More than I have written in years. Now that RavenDB 7.0 is out, most of those are already implemented and available to you. That gives me the chance to compare the design process and the implementation with recent work.

Vector Search & AI Integration for RavenDB

This was written in November 2024. It outlines what we want to achieve at a very high level. Most importantly, it starts by discussing what we won’t be trying to do, rather than what we will. Limiting the scope of the problem can be a huge force multiplier in such cases, especially when dealing with new concepts.

Reading throughout that document, it lays out the external-facing aspect of vector search in RavenDB. You have the vector.search() method in RQL, a discussion on how it works in other systems, and some ideas about vector generation and usage.

It doesn’t cover implementation details or how it will look from the perspective of RavenDB. This is at the level of the API consumer, what we want to achieve, not how we’ll achieve it.

AI Integration with RavenDB

Given that we have vector search, the next step is how to actually get and use it. This design document was a collaborative process, mostly written during and shortly after a big design discussion we had (which lasted for hours).

The idea there was to iron out the overall understanding of everyone about what we want to achieve. We considered things like caching and how it plays into the overall system, there are notes there at the level of what should be the field names.

That work has already been implemented. You can access it through the new AI button in the Studio. Check out this icon on the sidebar:

That was a much smaller task in scope, but you can see how even something that seemed pretty clear changed as we sat down and actually built it. Concepts we didn’t even think to consider were raised, handled, and implemented (without needing another design).

Voron HSNW Design Notes

This design document details our initial approach to building the HSNW implementation inside Voron, the basis for RavenDB’s new vector search capabilities.

That one is really interesting because it is a pure algorithmic implementation, completely internal to our usage (so no external API is needed), and I wrote it after extensive research.

The end result is similar to what I planned, but there are still significant changes.  In fact, pretty much all the actual implementation details are different from the design document. That is both expected and a good thing because it means that once we dove in, we were able to do things in a better way.

Interestingly, this is often the result of other constraints forcing you to do things differently. And then everything rolls down from there.

“If you have a problem, you have a problem. If you have two problems, you have a path for a solution.”

In the case of HSNW, a really complex part of the algorithm is handling deletions. In our implementation, there is a vector, and it has an associated posting list attached to it with all the index entries. That means we can implement deletion simply by emptying the associated posting list. An entire section in the design document (and hours spent pondering) is gone, just like that.

If the design document doesn’t reflect the end result of the system, are they useful?

I would unequivocally state that they are tremendously useful. In fact, they are crucial for us to be able to tackle complex problems. The most important aspect of design documents is that they capture our view of what the problem space is.

Beyond their role in planning, design documents serve another critical purpose: they act as a historical record. They capture the team’s thought process, documenting why certain decisions were made and how challenges were addressed. This is especially valuable for a long-lived project like RavenDB, where future developers may need context to understand the system’s evolution.

Imagine a design document that explores a feature in detail—outlining options, discussing trade-offs, and addressing edge cases like caching or system integrations. The end result may be different, but the design document, the feature documentation (both public and internal), and the issue & commit logs serve to capture the entire process very well.

Sometimes, looking at the road not taken can give you a lot more information than looking at what you did.

I consider design documents to be a very important part of the way we design our software. At the same time, I don’t find them binding, we’ll write the software and see where it leads us in the end.

What are your expectations and experience with writing design documents? I would love to hear additional feedback.

RavenDB is meant to be a self-managing database, one that is able to take care of itself without constant hand-holding from the database administrator. That has been one of our core tenets from the get-go. Today I checked the current state of the codebase and we have roughly 500 configuration options that are available to control various aspects of RavenDB’s behavior.

These two statements are seemingly contradictory, because if we have so many configuration options, how can we even try to be self-managing? And how can a database administrator expect to juggle all of those options?

Database configuration is a really finicky topic. For example, RocksDB’s authors flat-out admit that out loud:

Even we as RocksDB developers don't fully understand the effect of each configuration change. If you want to fully optimize RocksDB for your workload, we recommend experiments and benchmarking.

And indeed, efforts were made to tune RocksDB using deep-learning models because it is that complex.

RavenDB doesn’t take that approach, tuning is something that should work out of the box, managed directly by RavenDB itself. Much of that is achieved by not doing things and carefully arranging that the environment will balance itself out in an optimal fashion. But I’ll talk about the Zen of RavenDB another time.

Today, I want to talk about why we have so many configuration options, the vast majority of which you, as a user, should neither use, care about, nor even know of.

The idea is very simple, deploying a database engine is a Big Deal, and as such, something that users are quite reluctant to do. When we hit a problem and a support call is raised, we need to provide some mechanism for the user to fix things until we can ensure that this behavior is accounted for in the default manner of RavenDB.

I treat the configuration options more as escape hatches that allow me to muddle through stuff than explicit options that an administrator is expected to monitor and manage. Some of those configuration options control whether RavenDB will utilize vectored instructions or the compression algorithm to use over the wire. If you need to touch them, it is amazing that they exist. If you have to deal with them on a regular basis, we need to go back to the drawing board.

I wanted to test low-level file-system behavior in preparation for a new feature for RavenDB. Specifically, I wanted to look into hole punching - where you can give low-level instructions to the file system to indicate that you’re giving up disk space, but without actually reducing the size of the file.

This can be very helpful in space management. If I have a section in the file that is full of zeroes, I can just tell the file system that, and it can skip storing that range of zeros on the disk entirely. This is an advanced feature for file systems. I haven't actually used that in the past, so I needed to gain some expertise with it.

I wrote the following code for Linux:

int fd = open("test.file", O_CREAT | O_WRONLY, 0644);
lseek(fd, 128 * 1024 * 1024 - 1, SEEK_SET); // 128MB file
write(fd, "", 1);
fallocate(fd, // 32 MB hole from the 16MB..48MB range
    FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, 
    16 * 1024 * 1024, 32 * 1024 * 1024); 
close(fd);

The code for Windows is here if you want to see it. I tested the feature on both Windows & Linux, and it worked. I could see that while the file size was 128MB, I was able to give back 16MB to the operating system without any issues. I turned the code above into a test and called it a day.

And then the CI build broke. But that wasn’t possible since I tested that. And there had been CI runs that did work on Linux. So I did the obvious thing and started running the code above in a loop.

I found something really annoying. This code worked, sometimes. And sometimes it just didn’t.

In order to get the size, I need to run this code:

struct stat st;
fstat(fd, &st);
printf("Total size: %lld bytes\n",
    (long long)st.st_size);
printf("Actual size on disk: %lld bytes\n", 
    (long long)st.st_blocks * 512);

I’m used to weirdness from file systems at this point, but this is really simple. All the data is 4KB aligned (in fact, all the data is 16MB aligned). There shouldn’t be any weirdness here.

As you can see, I’m already working at the level of Linux syscalls, but I used strace to check if there is something funky going on. Nope, there was a 1:1 mapping between the code and the actual system calls issued.

That means that I have to debug deeper if I want to understand what is going on. This involves debugging the Linux Kernel, which is a Big Task. Take a look at the code in the relevant link. I’m fairly certain that the issue is in those lines. The problem is that this cannot be, since both offset & length are aligned to 4KB.

I got out my crystal ball and thinking hat and meditated on this. If you’ll note, the difference between the expected and actual values is exactly 4KB. It almost looks like the file itself is not aligned on a 4KB boundary, but the holes must be.

Given that I just want to release this space to the operating system and 4KB is really small, I can adjust that as a fudge factor for the test. I would love to understand exactly what is going on, but so far the “file itself is not 4KB aligned, but holes are” is a good working hypothesis (even though my gut tells me it might be wrong).

If you know the actual reason for this, I would love to hear it.

And don't get me started on what happened with sparse files in macOS. There, the OS will randomly decide to mark some parts of your file as holes, making any deterministic testing really hard.

RavenDB has a hidden feature, enabled by default and not something that you usually need to be aware of. It has built-in support for caching. Consider the following code:

async Task<Dictionary<string, int>> HowMuchWorkToDo(string userId)
{
    using var session = _documentStore.OpenAsyncSession();
    var results = await session.Query<Item>()
        .GroupBy(x =>new { x.Status, x.AssignedTo })
        .Where(g => g.Key.AssignedTo == userId && g.Key.Status != "Closed")
        .Select(g => new 
        {
            Status = g.Key.Status,
            Count = g.Count()
        })
        .ToListAsync();


    return results.ToDictionary(x => x.Status, x => x.Count);
}

What happens if I call it twice with the same user? The first time, RavenDB will send the query to the server, where it will be evaluated and executed. The server will also send an ETag header with the response. The client will remember the response and its ETag in its own memory.

The next time this is called on the same user, the client will again send a request to the server. This time, however, it will also inform the server that it has a previous response to this query, with the specified ETag. The server, when realizing the client has a cached response, will do a (very cheap) check to see if the cached response matches the current state of the server. If so, it can inform the client (using 304 Not Modified) that it can use its cache.

In this way, we benefit twice:

• First, on the server side, we avoid the need to compute the actual query.
• Second, on the network side, we aren’t sending a full response back, just a very small notification to use the cached version.

You’ll note, however, that there is still an issue. We have to go to the server to check. That means that we still pay the network costs. So far, this feature is completely transparent to the user. It works behind the scenes to optimize server query costs and network bandwidth costs.

We have a full-blown article on caching in RavenDB if you care to know more details instead of just “it makes things work faster for me”.

Aggressive Caching in RavenDB

The next stage is to involve the user. Enter the AggressiveCache() feature (see the full documentation here), which allows the user to specify an additional aspect. Now, when the client has the value in the cache, it will skip going to the server entirely and serve the request directly from the cache.

What about cache invalidation? Instead of having the client check on each request if things have changed, we invert the process. The client asks the server to notify it when things change, and until it gets notice from the server, it can serve responses completely from the local cache.

I really love this feature, that was the Good part, now let’s talk about the other pieces:

There are only two hard things in Computer Science: cache invalidation and naming things.

-- Phil Karlton

The bad part of caching is that this introduces more complexity to the system. Consider a system with two clients that are using the same database. An update from one of them may show up at different times in each. Cache invalidation will not happen instantly, and it is possible to get into situations where the server fails to notify the client about the update, meaning that we didn’t clear the cache.

We have a good set of solutions around all of those, I think. But it is important to understand that the problem space itself is a problem.

In particular, let’s talk about dealing with the following query:

var emps = session.Query<Employee>()
    .Include(x => x.Department)
    .Where(x => x.Location.City == "London")
    .ToListAsync();

When an employee is changed on the server, it will send a notice to the client, which can evict the item from the cache, right? But what about when a department is changed?

For that matter, what happens if a new employee is added to London? How do we detect that we need to refresh this query?

There are solutions to those problems, but they are super complicated and have various failure modes that often require more computing power than actually running the query. For that reason, RavenDB uses a much simpler model. If the server notifies us about any change, we’ll mark the entire cache as suspect.

The next request will have to go to the server (again with an ETag, etc) to verify that the response hasn’t changed. Note that if the specific query results haven’t changed, we’ll get OK (304 Not Modified) from the server, and the client will use the cached response.

Conservatively aggressive approach

In other words, even when using aggressive caching, RavenDB still has to go to the server sometimes. What is the impact of this approach when you have a system under load?

We’ll still use aggressive caching, but you’ll see brief periods where we aren’t checking with the server (usually be able to cache for about a second or so), followed by queries to the server to check for any changes.

In most cases, this is what you want. We still benefit from the cache while reducing the number of remote calls by about 50%, and we don’t have to worry about missing updates. The downside is that, as application developers, we know that this particular document and query are independent, so we want to cache them until we get notice about that particular document being changed.

The default aggressive caching in RavenDB will not be of major help here, I’m afraid. But there are a few things you can do.

You can use Aggressive Caching in the NoTracking mode. In that mode, the client will not ask the server for notifications on changes, and will cache the responses in memory until they expire (clock expiration or size expiration only).

There is also a feature suggestion that calls for updating the aggressive cache in a background manner, I would love to hear more feedback on this proposal.

Another option is to take this feature higher than RavenDB directly, but still use its capabilities. Since we have a scenario where we know that we want to cache a specific set of documents and refresh the cache only when those documents are updated, let’s write it.

Here is the code:

public class RecordCache<T>
{
    private ConcurrentLru<string, T> _items = 
        new(256, StringComparer.OrdinalIgnoreCase);
    private readonly IDocumentStore _documentStore;


    public RecordCache(IDocumentStore documentStore)
    {
        const BindingFlags Flags = BindingFlags.Instance | 
            BindingFlags.NonPublic | BindingFlags.Public;
        var violation = typeof(T).GetFields(Flags)
            .FirstOrDefault(f => f.IsInitOnly is false);
        if (violation != null)
        {
            throw new InvalidOperationException(
                "You should cache *only* immutable records, but got: " + 
                typeof(T).FullName + " with " + violation.Name + 
                " which is not read only!");
        }


        var changes = documentStore.Changes();
        changes.ConnectionStatusChanged += (_, args) =>
        {
            _items = new(256, StringComparer.OrdinalIgnoreCase);
        };
        changes.ForDocumentsInCollection<T>()
            .Subscribe(e =>
            {
                _items.TryRemove(e.Id, out _);
            })
            ;
        _documentStore = documentStore;
    }


    public ValueTask<T> Get(string id)
    {
        if (_items.TryGetValue(id, out var result))
        {
            return ValueTask.FromResult(result);
        }
        return new ValueTask<T>(GetFromServer(id));


    }


    private async Task<T> GetFromServer(string id)
    {
        using var session = _documentStore.OpenAsyncSession();
        var item = await session.LoadAsync<T>(id);
        _items.Set(id, item);
        return item;
    }
}

There are a few things to note about this code. We are holding live instances, so we ensure that the values we keep are immutable records. Otherwise, we may hand the same instance to two threads which can be… fun.

Note that document IDs in RavenDB are case insensitive, so we pass the right string comparer.

Finally,  the magic happens in the constructor. We register for two important events. Whenever the connection status of the Changes() connection is modified, we clear the cache. This handles any lost updates scenarios that occurred while we were disconnected.

In practice, the subscription to events on that particular collection is where we ensure that after the server notification, we can evict the document from the cache so that the next request will load a fresh version.

Caching + Distributed Systems = 🤯🤯🤯

I’m afraid this isn’t an easy topic once you dive into the specifics and constraints we operate under. As I mentioned, I would love your feedback on the background cache refresh feature, or maybe you have better insight into other ways to address the topic.

I got into an interesting discussion on LinkedIn about my previous post, talking about Code Rot. I was asked about Legacy Code defined as code without tests and how I reconcile code rot with having tests.

I started to reply there, but it really got out of hand and became its own post.

“To me, legacy code is simply code without tests.” Michael Feathers, Working Effectively with Legacy Code

I read Working Effectively with Legacy Code for the first time in 2005 or thereabout, I think. It left a massive impression on me and on the industry at large. The book is one of the reasons I started rigorously writing tests for my code, it got me interested in mocking and eventually led me to writing Rhino Mocks.

It is ironic that the point of this post is that I disagree with this statement by Michael because of Rhino Mocks. Let’s start with numbers, last commit to the Rhino Mocks repository was about a decade ago. It has just under 1,000 tests and code coverage that ranges between 95% - 100%.

I can modify this codebase with confidence, knowing that I will not break stuff unintentionally. The design of the code is very explicitly meant to aid in testing and the entire project was developed with a Test First mindset.

I haven’t touched the codebase in a decade (and it has been close to 15 years since I really delved into it). The code itself was written in .NET 1.1 around the 2006 timeframe. It literally predates generics in .NET.

It compiles and runs all tests when I try to run it, which is great. But it is still very much a legacy codebase.

It is a legacy codebase because changing this code is a big undertaking. This code will not run on modern systems. We need to address issues related to dynamic code generation between .NET Framework and .NET.

That in turn requires a high level of expertise and knowledge. I’m fairly certain that given enough time and effort, it is possible to do so. The problem is that this will now require me to reconstitute my understanding of the code.

The tests are going to be invaluable for actually making those changes, but the core issue is that a lot of knowledge has been lost. It will be a Project just to get it back to a normative state.

This scenario is pretty interesting because I am actually looking back at my own project. Thinking about having to do the same to a similar project from someone else’s code is an even bigger challenge.

Legacy code, in this context, means that there is a huge amount of effort required to start moving the project along. Note that if we had kept the knowledge and information within the same codebase, the same process would be far cheaper and easier.

Legacy code isn’t about the state of the codebase in my eyes, it is about the state of the team maintaining it. The team, their knowledge, and expertise, are far more important than the code itself.

An orphaned codebase, one that has no one to take care of, is a legacy project even if it has tests. Conversely, a project with no tests but with an actively knowledgeable team operating on it is not.

Note that I absolutely agree that tests are crucial regardless. The distinction that I make between legacy projects and non-legacy projects is whether we can deliver a change to the system.

Reminder: A codebase that isn’t being actively maintained and has no tests is the worst thing of all. If you are in that situation, go read Working Effectively with Legacy Code, it will be a lifesaver.

I need a feature with an ideal cost of X (time, materials, effort, cost, etc). A project with no tests but people familiar with it will be able to deliver it at a cost of 2-3X. A legacy project will need 10X or more. The second feature may still require 2X from the maintained project, but only 5X from the legacy system. However, that initial cost to get things started is the killer.

In other words, what matters here is the inertia, the ability to actually deliver updates to the system.

A customer called us about some pretty weird-looking numbers in their system:

You’ll note that the total number of entries in the index across all the nodes does not match. Notice that node C has 1 less entry than the rest of the system.

At the same time, all the indicators are green. As far as the administrator can tell, there is no issue, except for the number discrepancy. Why is it behaving in this manner?

Well, let’s zoom out a bit. What are we actually looking at here? We are looking at the state of a particular index in a single database within a cluster of machines. When examining the index, there is no apparent problem. Indexing is running properly, after all.

The actual problem was a replication issue, which prevented replication from proceeding to the third node. When looking at the index status, you can only see that the entry count is different.

When we zoom out and look at the state of the cluster, we can see this:

There are a few things that I want to point out in this scenario. The problem here is a pretty nasty one. All nodes are alive and well, they are communicating with each other, and any simple health check you run will give good results.

However, there is a problem that prevents replication from properly flowing to node C. The actual details aren’t relevant (a bug that we fixed, to tell the complete story). The most important aspect is how RavenDB behaves in such a scenario.

The cluster detected this as a problem, marked the node as problematic, and raised the appropriate alerts. As a result of this, clients would automatically be turned away from node C and use only the healthy nodes.

From the customer’s perspective, the issue was never user-visible since the cluster isolated the problematic node. I had a hand in the design of this, and I wrote some of the relevant code. And I’m still looking at these screenshots with a big sense of accomplishment.

This stuff isn’t easy or simple. But to an outside observer, the problem started from: why am I looking at funny numbers in the index state in the admin panel? And not at: why am I serving the wrong data to my users.

The design of RavenDB is inherently paranoid. We go to a lot of trouble to ensure that even if you run into problems, even if you encounter outright bugs (as in this case), the system as a whole would know how to deal with them and either recover or work around the issue.

As you can see, live in production, it actually works and does the Right Thing for you. Thus, I can end this post by saying that this behavior makes me truly happy.

I was talking to a colleague about a particular problem we are trying to solve. He suggested that we solve the problem using a particular data structure from a recently published paper. As we were talking, he explained how this data structure works and how that should handle our problem.

The solution was complex and it took me a while to understand what it was trying to achieve and how it would fit our scenario. And then something clicked in my head and I said something like:

Oh, that is just epoch-based, copy-on-write B+Tree with single-producer/ concurrent-readers?

If this sounds like nonsense to you, it is fine. Those are very specific terms that we are using here. The point of such a discussion is that this sort of jargon serves a very important purpose. It allows us to talk with clarity and intent about fairly complex topics, knowing that both sides have the same understanding of what we are actually talking about.

The idea is that we can elevate the conversation and focus on the differences between what the jargon specifies and the topic at hand. This is abstraction at the logic level, where we can basically zoom out a lot of details and still keep high intent accuracy.

Being able to discuss something at this level is hugely important because we can convey complex ideas easily. Once I managed to put what he was suggesting in context that I could understand, we were able to discuss the pros and cons of this data structure for the scenario.

I do appreciate that the conversation basically stopped making sense to anyone who isn’t already well-versed in the topic as soon as we were able to (from my perspective) clearly and effectively communicate.

“When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.”

Clarity of communication is a really important aspect of software engineering. Being able to explain, hopefully in a coherent fashion, why the software is built the way it is and why the code is structured just so can be really complex. Leaning on existing knowledge and understanding can make that a lot simpler.

There is also another aspect. When using jargon like that, it is clear when you don’t know something. You can go and research it. The mere fact that you can’t understand the text tells you both that you are missing information and where you can find it.

For software, you need to consider two scenarios. Writing code today and explaining how it works to your colleagues, and looking at code that you wrote ten years ago and trying to figure out what was going on there.

In both cases, I think that this sort of approach is a really useful way to convey information.

I usually talk about the things that I do that were successful. Today I want to discuss something that I tried but failed at. Documenting failed approaches is just as important, though less enjoyable, as documenting what we excel at.

In order to explain the failure, I need to get a bit deeper into how computers handle memory. There is physical memory, the RAM sticks that you have in your machine, and then there is how the OS and CPU present that memory to your code. Usually, the abstraction is quite seamless, and we don’t need to pay attention to it.

Occasionally, we can take advantage of this model. Consider the following memory setup, showing a single physical memory page that was mapped in two different locations:

In this case, it means that you can do things like this:

*page1 = '*';
printf("Same: %d - Val: %c\n", (page1 == page2), *page2); 
// output is:
// Same: 0 - Val: *

In other words, because the two virtual pages point to the same physical page in memory, we can modify memory in one location and see the changes in another. This isn’t spooky action at a distance, it is simply the fact that the memory addresses we use are virtual and they point to the same place.

Note that in the image above, I modified the data using the pointer to Page 1 and then read it from Page 2. The Memory Management Unit (MMU) in the CPU can do a bunch of really interesting things because of this. You’ll note that each virtual page is annotated with an access permission.

In this case, the second page is marked as Copy on Write. That means that when we read from this page, the MMU will happily read the data from the physical page it is pointed to. But when we write, the situation is different.

The MMU will raise an exception to the operating system, telling it that a write was attempted on this page, which is forbidden. At this point, the OS will allocate a new physical page, copy the data to it, and then update the virtual address to point to the new page. Here is what this looks like:

Now we have two distinct mappings. A write to either one of them will not be reflected on the other. Here is what this looks like in code:

*page1 = '1'; // now 
printf("Page1: %c, Page2: %c\n", *page1, *page2); 
// output: Page1: 1, Page2: 1
*page2 = '2'; // force the copy on write to occur
printf("Page1: %c, Page2: %c\n", *page1, *page2); 
// output: Page1: 1, Page2: 2

As long as the modifications happened through the first page address (the orange one in the image), there was no issue and any change would be reflected in both pages. When we make a modification to the second page (the green one in the image), the OS will create a new physical page and effectively split them forever.

Changes made to either page will only be reflected in that page, not both, since they aren’t sharing the same page.

Note that this behavior applies at a page boundary. What happens if I have a buffer, 1GB in size, and I use this technique on it? Let’s assume that we have a buffer that is 1GB in size and I created a copy-on-write mapping on top of it.

The amount of physical memory that I would consume is still just 1GB.

In fact, I would effectively memcpy()very quickly, since I’m not actually copying anything. And for all intents and purposes, it works. I can change the data through the second buffer, and it would not show up in the first buffer. Of particular note is that when I modify the data on the second buffer, only a single page is changed. Here is what this looks like:

So instead of having to copy 1GB all at once, we map the buffer again as copy on write, and we can get a new page whenever we actually modify our “copy” of the data.

So far, this is great, and it is heavily used for many optimizations. It is also something that I want to use to implement cheap snapshots of a potentially large data structure. The idea that I have is that I can use this technique to implement it.

Here is the kind of code that I want to write:

var list = new CopyOnWriteList();
list.Put(1);
list.Put(2);


var snapshot1 = list.CreateSnapshot();


list.Put(3)




var snapshot2 = list.CreateSnapshot();


list.Put(4);

And the idea is that I’ll have (at the same time) the following:

I want to have effectively unlimited snapshots, and the map may contain a large amount of data. In graphical form, you can see it here:

We started with Page 1, created a Copy of Write for Page 2, modified Page 2 (breaking the Copy on Write), and then attempted to create a Copy on Write for Page 2. That turns out to be a problem.

Let’s see the code that we need in order to create a copy using copy-on-write mapping on Linux:

int shm_fd = shm_open("/third", O_CREAT | O_RDWR, 0666);
ftruncate(shm_fd, 4096);
char *page1 = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);
page1[0] = 'A'; page1[1] = 'B';
// pages1 = 'AB'
char *page2 = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE, shm_fd, 0);
// pages2 = 'AB'
page1[0]= 'a';
// pages1 = 'aB'
// pages2 = 'aB' (same pagee)
page2[2] = 'C'; // force a private copy creation
// pages1 = 'aB'
// pages2 = 'aBC'
page1[1] = 'b';
// pages1 = 'ab'
// pages2 = 'aBC' (no change here)

The code in Windows is pretty similar and behaves in the same manner:

HANDLE hMapFile = CreateFileMapping(INVALID_HANDLE_VALUE,
    NULL,PAGE_READWRITE,0,4096, TEXT("Local\\MySharedMemory"));
char* page1 = MapViewOfFile(hMapFile,
    FILE_MAP_READ | FILE_MAP_WRITE, 0, 0, 4096);
page1[0] = 'A'; page1[1] = 'B';
// pages1 = 'AB'
char* page2 = MapViewOfFile(hMapFile,
    FILE_MAP_COPY, 0, 0, 4096);
// pages2 = 'AB'
page1[0] = 'a';
// pages1 = 'aB'
// pages2 = 'aB' (same pagee)
page2[2] = 'C'; // force a copy on write 
// pages1 = 'aB'
// pages2 = 'aBC'
page1[1] = 'b';
// pages1 = 'ab'
// pages2 = 'aBC' (no change here)

Take a look at the API we have for creating a copy-on-write:

MapViewOfFile(hMapFile, FILE_MAP_COPY, 0, 0, 4096); // windows
mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE, shm_fd, 0); // linux

A key aspect of the API is that we need to provide a source for the Copy-on-Write operation. That means that we can only create a Copy-on-Write from a single source. We cannot perform a Copy-on-Write on top of a page that was marked as copy-on-write. This is because we cannot refer to it. Basically, I don’t have a source that I can use for this sort of mapping.

I tried being clever and wrote the following code on Linux:

int selfmem = open("/proc/self/mem", O_RDWR);
char *page2 = mmap(NULL, 4096, PROT_READ | PROT_WRITE, 
                   MAP_PRIVATE, selfmem, (off_t)page1);

On Linux, you can use the special file /proc/self/mem to refer to your memory using file I/O. That means that I can get a file descriptor for my own memory, which provides a source for my copy-on-write operation.

I was really excited when I realized that this was a possibility. I spent a lot of time trying to figure out how I could do the same on Windows. However, when I actually ran the code on Linux, I realized that this doesn’t work.

The mmap() call will return ENODEV when I try that. It looks like this isn’t a supported action.

Linux has another call that looks almost right, which is mremap(), but that either zeros out or sets up a userfaulfdhandler for the region. So it can’t serve my needs.

Looking around, I’m not the first person to try this, but it doesn’t seem like there is an actual solution.

This is quite annoying since we are almost there. All the relevant pieces are available, if we had a way to tell the kernel to create the mapping, everything else should just work from there.

Anyway, this is my tale of woe, trying (and failing) to create a snapshot-based system using the Memory Manager Unit. Hopefully, you’ll either learn something from my failure or let me know that there is a way to do this…

Reading code is a Skill (with a capital letter, yes) that is really important for developers. You cannot be a good developer without it.

Today I want to talk about one aspect of this. The ability to go into an unfamiliar codebase and extract one piece of information out. The idea is that we don’t need to understand the entire system, grok the architecture, etc. I want to understand one thing about it and get away as soon as I can.

For example, you know that project Xyz is doing some operation, and you want to figure out how this is done. So you need to look at the code and figure that out, then you can go your merry way.

Today, I’m interested in understanding how the LMDB project writes data to the disk on Windows. This is because LMDB is based around a memory-mapped model, and Windows doesn’t keep the data between file I/O and mmap I/O coherent.

LMDB is an embedded database engine (similar to Voron, and in fact, Voron is based on some ideas from LMDB) written in C. If you are interested in it, I wrote 11 posts going through every line of code in the project.

So I’m familiar with the project, but the last time I read the code was over a decade ago. From what I recall, the code is dense. There are about 11.5K lines of code in a single file, implementing the entire thing.

I’m using the code from here.

The first thing to do is find the relevant section in the code. I started by searching for the WriteFile() function, the Win32 API to write. The first occurrence of a call to this method is in the mdb_page_flush function.

I look at this code, and… there isn’t really anything there. It is fairly obvious and straightforward code (to be clear, that is a compliment). I was expecting to see a trick there. I couldn’t find it.

That meant either the code had a gaping hole and potential data corruption (highly unlikely) or I was missing something. That led me to a long trip of trying to distinguish between documented guarantees and actual behavior.

The documentation for MapViewOfFile is pretty clear:

A mapped view of a file is not guaranteed to be coherent with a file that is being accessed by the ReadFile or WriteFile function.

I have my own run-ins with this behavior, which was super confusing. This means that I had experimental evidence to say that this is broken. But it didn’t make sense, there was no code in LMDB to handle it, and this is pretty easy to trigger.

It turns out that while the documentation is pretty broad about not guaranteeing the behavior, the actual issue only occurs if you are working with remote files or using unbuffered I/O.

If you are working with local files and buffered I/O (which is 99.99% of the cases), then you can rely on this behavior. I found some vaguereferences to this, but that wasn’t enough. There is this post that is really interesting, though.

I pinged Howard Chu, the author of LMDB, for clarification, and he was quick enough to assure me that yes, my understanding was (now) correct. On Windows, you can mix memory map operations with file I/O and get the right results.

The documentation appears to be a holdover from Windows 9x, with the NT line always being able to ensure coherency for local files. This is a guess about the history of documentation, to be honest. Not something that I can verify.

I had the wrong information in my head for over a decade. I did not expect this result when I started this post, I was sure I would be discussing navigating complex codebases. I’m going to stand in the corner and feel upset about this for a while now.

Today I got in my car to drive to work and realized that Waze suggested “Work” as the primary destination to select. I had noticed that before, and it is a really nice feature. Today, I got to thinking about how I would implement something like that.

That was a nice drive since I kept thinking about algorithms and data flow. When I got to the office, I decided to write about how we can implement something like that. Based on historical information, let’s suggest the likely destinations.

Here is the information we have:

The Lat & Lng coordinates represent the start location, the time is the start time for the trip, and the destination is obvious. In the data set above, we have trips to and from work, to the gym once a week, and to our parents over the weekends.

Based on this data, I would like to build recommendations for destinations. I could try to analyze the data and figure out all sorts of details. The prediction that I want to make is, given a location & time, to find where my likely destination is going to be.

I could try to analyze the data on a deep level, drawings on patterns, etc. Or I can take a very different approach and just throw some computing power at the problem.

Let’s talk in code since this is easier. I have a list of trips that look like this:

public record Trip(double Lat, double Lng, string Destination, DateTime Time);
Trip[] trips = RecentTrips(TimeSpan.FromDays(90));

Given that, I want to be able to write this function:

string[] SuggestDestination((double Lat, double Lng) location, DateTime now)

I’m going to start by processing the trips data, to extract the relevant information:

var historyByDest = new Dictionary<string, List<double[]>>();
foreach (var trip in trips)
{
    if (historyByDest.TryGetValue(trip.Destination, out var list) is false)
    {
        historyByDest[trip.Destination] = list = new();
    }
    list.Add([
        trip.Lat,
        trip.Lng,
        trip.Time.Hour * 100 + trip.Time.Minute, // minutes after midnight
        trip.Time.DayOfYear,
        (int)trip.Time.DayOfWeek
    ]);
}

What this code does is extract details (location, day of the week, time of day, etc.) from the trip information and store them in an array. For each trip, we basically break apart the trip across multiple dimensions.

The next step is to make the actual prediction we want, which will begin by extracting the same dimensions from the inputs we get, like so:

double[] compare = [
    location.Lat, 
    location.Lng, 
    now.Hour * 100 + now.Minute, 
    now.DayOfYear, 
    (int)now.DayOfWeek
];

Now we basically have an array of values from which we want to predict, and for each destination, an array that represents the same dimensions of historical trips. Here is the actual computation:

List<(string Dest, double Score)> scores = new();


foreach (var (dest, items) in historyByDest)
{
    double score = 0;
    foreach (var cur in items)
    {
        for (var i = 0; i < cur.Length; i++)
        {
            score += Math.Abs(cur[i] - compare[i]);
        }
    }
    score /= items.Count;
    scores.Add((dest, score));
}


scores.Sort((x, y) => x.Score.CompareTo(y.Score));

What we do here is compute the difference between the two arrays: the current start location & time compared to the start location & time of historical trips. We do that not only on the raw data but also extract additional features from the information.

For example, one dimension is the day of the week, and the other is the time of day. It is not sufficient to compare just the date itself.

The end result is the distance between the current trip start and previous trips for each of the destinations I have. Then I can return the destinations that most closely match my current location & time.

Running this over a few tests shows that this is remarkably effective. For example, if I’m at home on a Saturday, I’m very likely to visit either set of grandparents. On Sunday morning, I head to the Gym or Work, but on Monday morning, it is more likely to be Work.

All of those were mostly fixed, with the day of the week and the time being different. But If I’m at my parents’ house on a weekday (which is unusual), the location would have a far greater weight on the decision, etc. Note that the code is really trivial (I spent more time generating the actual data), but we can extract some nice information from this.

The entire code is here, admittedly it’s pretty dirty code since I wanted to test how this would actually work. At this point, I’m going to update my Curriculum Vitae and call myself a senior AI developer.

Joking aside, this approach provides a good (although highly simplified) overview of how modern AI systems work. Given a data item (image, text, etc.), you run that through the engine that outputs the embedding (those arrays we saw earlier, with values for each dimension) and then try to find its nearest neighbors across multiple dimensions.

In the example above, I explicitly defined the dimensions to use, whereas LLMs would have their“secret sauce” for this. The concept, at a sufficiently high level, is the same.