Ayende @ Rahien

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RavenDB 2.5: What is new here?

Well, this is still just a high level list only, but there is a lot of stuff there. In many cases, I posted full blog entries about each new feature, but I’ll post a few words about those that I didn’t.

Features:

Indexing:

Improvements:

Studio:

Operations - As usual, we have a few goodies for the ops people. Some of them aren’t really interesting to devs, but they are part of creating production quality software. We now allow Backup Operators (and not just admins) to initiate backups, and if you are restoring a db from another machine with different settings, RavenDB will automatically set things up so you don’t have to manually do anything to get things working. We also added a bunch more of endpoints for debug & analysis and added some more information to our existing endpoints.

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The joy of schema less development

imageAs the author of a schema less database, I find myself in the strange position of the barefoot shoemaker. I need to explain a bit. Our current storage engines, Esent and Munin (which was designed mostly to be like Esent) have rigid schemas. There are tables, and indexes, etc. This means that features that touch the storage layer tend to be much more complex. They require migrations, adding new features that require storage means that we have to create new storage tables, or modify indexes, or any number of a bunch of stuff that we developed RavenDB so our users wouldn’t have to.

I have been working with our managed implementation of LevelDB quite a lot lately. In order to do more than merely write tests for this, I tried to create a feature complete feature, an aggregation engine. The code is not production worthy (yet!), but what struck me quite hard was the fact that except for the fact that the storage layer is full of bugs (well, that is why I was writing stuff on top of it, to expose it), I had a blast actually working with it.

I could make modifications & changes with hardly any friction, and it was a real pleasure to start working with things in this fashion.

Why LevelDB, all of a sudden?

Something that is no one really seems to be asking is why we started doing all this work with LevelDB all of a sudden. We already have an industry grade solution for storage with Esent. Sure, it doesn’t work with Linux, and that is somewhat important for us. But you would be wrong if you thought that this was why we pursue this.

To be perfectly honest, we want a storage solution that is on our control. Something that we can work with, tweak and understand all on our own. Just about every other tidbit of RavenDB is something that we have full transparency into. Sure, I’ll be the first to admit that I haven’t been through every single line of code in the Lucene codebase, but when we run into problems there, I was able to go into the code and fix stuff.

Esent has been incredibly useful for us, and it has been very stable. But to be able to tweak every last erg of performance and speed, we really need to have a lot more control over the storage layer than we currently have. And no, that doesn’t mean that we are going to abandon Esent. It merely means that we want to have options. And even in our early spikes ,the sort of things our managed implementation of LevelDB provides make implementing things a real breeze.

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RavenDB in Action: MEAP edition

This is probably the RavenDB book that I am most looking forward to. Written by Itamar Syn-Hershko, who worked for Hibernating Rhinos and is mainly responsible for the fact that I understand how lucene works.

You can get it here: http://www.manning.com/synhershko/ 

Use coupon code ravdbco to get 50% off the original price

image

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A face lift for the RavenDB Management Studio

New in RavenDB 2.5, we have merged the documents & collection views, so no you can move between them more easily.

image

Speaking of views, we also added a way to detach a document view and have it follow us around. This give us the ability to look it us when doing work on something like the indexes that operate on it.

image

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Some thoughts about compression & storage

One of the advantages that keeps showing up with leveldb is the notion that it compresses the data on disk by default. Since reading data from disk is way more expensive than the CPU cost of compression & decompression, that is a net benefit.

Or is it? In the managed implementation we are currently working on, we chose to avoid this for now. For a very simple reason. By storing the compressed data on disk, it means that you cannot just give a user the memory mapped buffer and be done with it, you actually have to decompress the data yourself, then hand the user the buffer to the decompressed memory. In other words, instead of having a single read only buffer that the OS will manage / page / optimize for you, you are going to have to allocate memory over & over again, and you’ll pay the cost of decompressing again and again.

I think that it would be better to have the client make that decision. They can send us data that is already compressed, so we won’t need to do anything else, and we would still be able to just hand them a buffer of data. Sure, it sounds like we are just moving the cost around, isn’t it? But what actually happens is that you have a better chance to do optimizations. For example, if I am storing the data compressing via gzip. And I’m exposing the data over the wire, I can just stream the results from the storage directly to the HTTP stream, without having to do anything about it. It can be decompressed on the client.

On the other hand, if I have storage level decompression, I am paying for the cost of reading the compressed data from disk, then allocating new buffer, decompressing the data, then going right ahead and compressing it again for sending over the wire.

Reviewing Basho’s Leveldb

After taking a look at HyperLevelDB, it is time to see what Basho has changed in leveldb. They were kind enough to write a blog post detailing those changes, unfortunately, unlike HyperLevelDB, they have been pretty general and focused on their own product (which makes total sense). They have called out the reduction of “stalls”, which may or may not be related to issues with the write delay that leveldb intentionally introduce under load.

Okay, no choice about it, I am going to go over the commit log and see if I can find interesting stuff. The first tidbit that caught my eye is improving the compaction process when you have on disk corruption. Instead of stopping, it would move the bad data to the “lost” directory and move on. Note that there is some data loss associated with this, of course, but that won’t necessarily be felt by the users.

As a note, I dislike this code formatting:

image

Like HyperLevelDB, Basho made a lot of changes to compaction, it appears that this is the case for performance reasons:

  • No compactions triggered by reads, that is too slow.
  • There are multiple threads now handling compactions, with various levels of priorities between them. For example, flushing the immutable mem table is high priority, as is level 0 compaction, but standard compactions can wait.
  • Interestingly, when flushing data from memory to level 0, no compression is used.
  • After those were done, they also added additional logic to enforce locks that would give flushing from memory to disk and from level 0 downward much higher priority than everything else.

As an aide, another interesting thing I noticed, Basho also moved closing files and unmmaping memory to a background thread. I am not quite sure why that is the case, I wouldn’t expect that to be very expensive.

Next on the list, improving caching. Mostly by taking into account actual file sizes and by introducing a reader/writer lock.

Like HyperLevelDB, they also went for larger files, although I think that in this case, they went for significantly larger files than even HyperLevelDB did. Throttling, unlike with HyperLevelDB, where they did away with write throttling altogether in favor of concurrent writes, Basho’s leveldb went into a much more complex system of write throttling base on the current load, pending work, etc. The idea is to gain better load distribution overall. (Or maybe they didn’t think about the concurrent write strategy).

I wonder (but didn’t check) if some of the changes were pulled back into the leveldb project. Because there is some code here that I am pretty sure duplicate work already done in leveldb. In this case, the retiring of data that has already been superseded.

There is a lot of stuff that appears to relate to maintenance. Scanning SST files for errors, perf counters, etc. It also look like the decided to go to assembly for actually implementing CRC32. In fact, I am pretty sure that the asm is for calling hardware CRC inside the CPU. But I am unable to decipher that.

What I find funny is that another change I just run into is the introduction of a way to avoid copying data when Get()ing data from leveldb. If you’ll recall, I pointed that out as an issue a while ago in my first review of leveldb.

And here is another pretty drastic change. In leveldb, only level 0 can have overalapping files, but Basho’s changed things so the first 3 levels would have overlapping files. The idea is that you can do cheaper compactions this way, I am guessing.

I am aware that this is a bit of a mess, with regards to the review, but I just went over the code and wrote down the notes as I saw them. Overall, I think that I like HyperLevelDB changes better, but they have the advantage of using a much later codebase.

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Trivial Lru Cache impl

It has been a while since I actually posted some code here, and I thought that this implementation was quite nice, in that it is simple & works for what it needs to do.

 

   1: public class LruCache<TKey, TValue>
   2: {
   3:     private readonly int _capacity;
   4:     private readonly Stopwatch _stopwatch = Stopwatch.StartNew();
   5:  
   6:     private class Node
   7:     {
   8:         public TValue Value;
   9:         public volatile Reference<long> Ticks;
  10:     }
  11:  
  12:     private readonly ConcurrentDictionary<TKey, Node> _nodes = new ConcurrentDictionary<TKey, Node>();
  13:  
  14:     public LruCache(int capacity)
  15:     {
  16:         Debug.Assert(capacity > 10);
  17:         _capacity = capacity;
  18:     }
  19:  
  20:     public void Set(TKey key, TValue value)
  21:     {
  22:         var node = new Node
  23:         {
  24:             Value = value,
  25:             Ticks = new Reference<long> { Value = _stopwatch.ElapsedTicks }
  26:         };
  27:  
  28:         _nodes.AddOrUpdate(key, node, (_, __) => node);
  29:         if (_nodes.Count > _capacity)
  30:         {
  31:             foreach (var source in _nodes.OrderBy(x => x.Value.Ticks).Take(_nodes.Count / 10))
  32:             {
  33:                 Node _;
  34:                 _nodes.TryRemove(source.Key, out _);
  35:             }
  36:         }
  37:     }
  38:  
  39:     public bool TryGet(TKey key, out TValue value)
  40:     {
  41:         Node node;
  42:         if (_nodes.TryGetValue(key, out node))
  43:         {
  44:             node.Ticks = new Reference<long> {Value = _stopwatch.ElapsedTicks};
  45:             value = node.Value;
  46:             return true;
  47:         }
  48:         value = default(TValue);
  49:         return false;
  50:     }
  51: }
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TPL and the case of the !@#(*@! hung process

So, here I am writing some really fun code, when I found out that I am running into dead locks in the code. I activate emergency protocols and went into deep debugging mode.

After being really through in figuring out several possible causes, I was still left with what is effectively a WTF @!(*!@ DAMN !(@*#!@* YOU !@*!@( outburst and a sudden longing for something to repeatedly hit.

Eventually, however, I figure out what was going on.

I have the following method: Aggregator.AggregateAsync(), inside which we have a call to the PulseAll method. That method will then go and execute the following code:

   1: public void PulseAll()
   2: {
   3:     Interlocked.Increment(ref state);
   4:     TaskCompletionSource<object> result;
   5:     while (waiters.TryDequeue(out result))
   6:     {
   7:         result.SetResult(null);
   8:     }
   9: }

After that, I return from the method. In another piece of the code (Aggregator.Dispose) I am waiting for the task that is running the AggregateAsync method to complete.

Nothing worked! It took me a while before I figured out that I wanted to check the stack, where I found this:

image

Basically, I had a dead lock because when I called SetResult on the completion source (which freed the Dispose code to run), I actually switched over to that task and allowed it to run. Still in the same thread, but in a different task, I run through the rest of the code and eventually got to the Aggregator.Dispose(). Now, I could only get to it if it the PulseAll() method was called. But, because we are on the same thread, that task hasn’t been completed yet!

In the end, I “solved” that by introducing a DisposeAsync() method, which allowed us to yield the thread, and then the AggregateAsync task was completed, and then we could move on.

But I am really not very happy about this. Any ideas about proper way to handle async & IDisposable?

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Deadlocking with the TPL, how to

As I mentioned, I run into a very nasty issue with the TPL. I am not sure if it is me doing things wrong, or an actual issue.

Let us look at the code, shall we?

We start with a very simple code:

   1: public class AsyncEvent
   2: {
   3:     private volatile TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
   4:     
   5:     public Task WaitAsync()
   6:     {
   7:         return tcs.Task;
   8:     }
   9:  
  10:     public void PulseAll()
  11:     {
  12:         var taskCompletionSource = tcs;
  13:         tcs = new TaskCompletionSource<object>();
  14:         taskCompletionSource.SetResult(null);
  15:     }
  16: }

This is effectively an auto reset event. All the waiters will be released when the PulseAll it called. Then we have this runner, which just execute work:

   1: public class Runner : IDisposable
   2: {
   3:     private readonly ConcurrentQueue<TaskCompletionSource<object>> items =
   4:         new ConcurrentQueue<TaskCompletionSource<object>>();
   5:     private readonly Task<Task> _bg;
   6:     private readonly AsyncEvent _event = new AsyncEvent();
   7:     private volatile bool _done;
   8:  
   9:     public Runner()
  10:     {
  11:         _bg = Task.Factory.StartNew(() => Background());
  12:     }
  13:  
  14:     private async Task Background()
  15:     {
  16:         while (_done == false)
  17:         {
  18:             TaskCompletionSource<object> result;
  19:             if (items.TryDequeue(out result) == false)
  20:             {
  21:                 await _event.WaitAsync();
  22:                 continue;
  23:             }
  24:  
  25:             //work here, note that we do NOT use await!
  26:  
  27:             result.SetResult(null);
  28:         }
  29:     }
  30:  
  31:     public Task AddWork()
  32:     {
  33:         var tcs = new TaskCompletionSource<object>();
  34:         items.Enqueue(tcs);
  35:  
  36:         _event.PulseAll();
  37:  
  38:         return tcs.Task;
  39:     }
  40:  
  41:     public void Dispose()
  42:     {
  43:         _done = true;
  44:         _event.PulseAll();
  45:         _bg.Wait();
  46:     }
  47: }

And finally, the code that causes the problem:

   1: public static async Task Run()
   2: {
   3:     using (var runner = new Runner())
   4:     {
   5:         await runner.AddWork();
   6:     }
   7: }

So far, it is all pretty innocent, I think you would agree. But this cause hangs with a dead lock. Here is why:

image

Because tasks can share threads, we are in the Background task thread, and we are trying to wait on that background task completion.

Result, deadlock.

If I add:

   1: await Task.Yield();

Because that forces this method to be completed in another thread, but that looks more like something that you add after you discover the bug, to be honest.

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Aggregation, they work!

And… this test just passed!

image

Just to give you some idea, this is sitting on top of RavenDB’s implementation of leveldb. In fact, I have been using this code to test out the leveldb implementation.

But this actually store all the events, run the aggregation over them and give you the aggregated results. And the entire things works, quite nicely, even if I say so myself.

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RavenDB 2.5 Release Candidate is out

Starting with build 2603, we are now considering 2.5 to be a release candidate. It is feature frozen, and only bug fixes are going in.

You can download the new version here: http://hibernatingrhinos.com/builds/ravendb-unstable-v2.5, and it is also available on nuget as an unstable release.

There have been a lot of changes, most of which were recorded on this blog. Next week I’ll do a proper post about what is new, but in the meantime, you can check the release notes.

I urge you to download it and take it for a spin. We really need feedback from users about how it works, and any problems that you might run into.

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Update on my HyperLevelDB review

It appears that in my previous post I have had an issue with how I read the code. In particular, I looked at the commit log and didn’t look at the most recent changes with regards to how HyperLevelDB does the writes. Robert Escriva has been kind enough to point me in the right direction.

The way that this works is a lot more elegant, I think.

When you want to make a write to a file, you ask for a segment at a particular offset. If we have that offset already mapped, we give it to the caller. Otherwise, we increase the file size if needed, then map the next segment. That part is done under a lock, so there isn’t an issue of contention over the end of the file. That is much nicer than the pwrite method.

That said, however, I am still not sure about the issue with the two concurrent transactions. What actually happens here is that while we gained concurrency in the IO, there is still some serialization going on. In other words, even though transaction B was actually flushed to disk before transaction A, it would still wait for transaction A to complete, alleviating the concern that I have had about it.

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Reviewing HyperLevelDB–Concurrent Writes

As mentioned earlier, HyperDex has made some changes to LevelDB to make it work faster for their scenarios. I was curious to see what changed, so I took a look at the code. In my previous post, I dealt with compaction, but now I want to deal exclusively with the changes that were made to leveldb to make writes more concurrent.

Update: it appears that I made a mistake in reading the code (I didn't check the final version). See this post for the correction.

 

Another change that was made that I am really not sure that I am following is the notion of concurrent log writer. This relies on the pwrite() method, which allows you to write a buffer to a file at a specified position. I have not been able to figure out what is going on if you have concurrent writes to that file. The HyperDex modifications include synchronization on the offset where they will actually  make the write, but after that, they make concurrent calls. It make sense, I guess, but I have several problems with that. I was unable to find any details about the behavior of the system when making concurrent calls to pwrite() at the end of the file, especially since your position might be well beyond the current end of file.

I couldn’t figure out what the behavior was supposed to be under those conditions, so I fired up a linux machine and wrote the following code:

   1: int main() {
   2:   char* str1 = "1234\n";
   3:   char* str2 = "5678\n";
   4:   int  file_descriptor;
   5:   int  ret;
   6:   char fn[]="test";
   7:  
   8:   if ((file_descriptor = creat(fn, S_IRUSR | S_IWUSR)) < 0)
   9:   {
  10:     perror("creat() error");
  11:     return -1;
  12:   }
  13:   else {
  14:     ret = pwrite(file_descriptor, str2, 5, 5);
  15:     printf("Wrote %d\n", ret);
  16:     
  17:     ret = pwrite(file_descriptor, str1, 5, 10);
  18:     printf("Wrote %d\n", ret);
  19:     
  20:     if (close(file_descriptor)!= 0)
  21:        perror("close() error");
  22:     
  23:   }
  24:   
  25:   return 0;
  26: }

I’ll be the first to admit that this is ugly code, but it gets the job done, and it told me that you could issues those sort of writes, and it would do the expected thing. I am going to assume that it would still work when used concurrently on the same file.

That said, there is still a problem, let us assume the following sequence of events:

  • Write A
  • Write B
  • Write A is allocated range [5 – 10] in the log file
  • Write B is allocated range [10 – 5] in the log file
  • Write B is done and returns to the user
  • Write A is NOT done, and we have a crash

Basically, we have here a violation of durability, because when we read from the log, we will get to the A write, see it is corrupted and stop processing the rest of the log. Effectively, you have just lost a committed transaction. Now, the log format actually allows that to happen, and a sophisticated reader can recover from that, but I haven’t seen yet any signs that that was implemented.

To be fair, I think that the log reader should be able to handle zero'ed data and continue forward. There is some sort of a comment about that. But that isn't supported by the brief glance that I saw, and more importantly, it isn't won't help if you crashed midway through writing A, so you have corrupted (not zero'ed) data on the fie. This would also cause the B transaction to be lost.

The rest appears to be just thread safety and then allowing concurrent writes to the log, which I have issue with, as I mentioned. But I am assuming that this will generate a high rate of writes, since there is a lot less waiting. That said, I am not sure how useful that would be. There is still just one physical needle that writes to the disk. I am guessing that it really depends on whatever or not you need the call to be synced or not. If you do, there are going to be a lot more fsync() than before, when it was merged into a single call.

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Reviewing HyperLevelDB–Compactions

As mentioned earlier, HyperDex has made some changes to LevelDB to make it work faster for their scenarios. I was curious to see what changed, so I took a look at the code.

The fun part is that I can easily check just the things that were changed by HyperDex, I don’t need to go read and understand a completely new engine. I’m currently just looking at the logs, and noting important stuff.

A lot of the work appears to have been concentrated on the notion of compaction. In particular, compaction means that we need to merge multiple files into a single file at a higher level. That is great, until you realize that this may mean that you’ll be writing a lot more data than you though you would, because you keep copying the data from one file to another, as it bubbles deeper into the levels.

The first thing that seemed to have happened is that when you get to a deep enough level, the file sizes that you are allowed to have become much larger, that means that you’ll have more data at higher level and likely reduce the number of times that you need to compact things.

The next step in the process was to create two parallel processes. One to handle the common compaction from memory to level-0 and the other to handle level-0 and down. I assume that this was done to avoid contention between the two. It is far more common to have a memory to level-0 compaction than level-0 down, and having to sync between the two is likely causing some slow down. The next change I noticed was tightening locks. Instead of having a single lock that was used for more writes, there now appears to be more granular locks, so you gain more concurrency under parallel load.

Next, the change was made to the compaction heuristics. I am not sure that I understand yet the changes, especially since I am just going through the commit log now. Once thing I noticed right away is that they removed the seek budget that you had for files. It appears that compactions triggered by reads were a source of too much pain for HyperDex, so it was removed. Note that HyperDex appears to be very interested in a consistent performance for high write scenarios.

Looking at the logs, it appears that there were a lot of strategies attempted for getting a better compaction strategy for what HyperDex is doing. Interestingly enough (and good for them) I see a lot of back & forth, trying something out, then backing out of it, etc. Sometimes over the courses of several weeks or months. A lot of that appears to be more like heuristically setting various things, like number of compacting threads, the various config options, etc. Trying to find the best match.

Another thing that I guess improved performance is that leveldb will intentionally slow down writes when is is doing compaction, or just about to, to reduce the load when heavy background operations are running. HyperLevelDB removed that, so you will have more load on the system, but no artificial governors on the system performance.

But coming back to compactions, it appears that what they ended up doing is to have three levels of background compactions.

  • Memory Table –> Level0 files.
  • Optimistic Compaction
  • Background Compaction

To be honest, I think that at least between the last two there is a lot of code that can be shared, so it is a bit hard to read, but basically. We have an optimistic compaction thread running that waits for enough data to comes through to level 0, at which point it will try to do a low cost, high yield compaction. If that isn’t enough, we have the background compaction that will kick in, but that seems to be for there is really no other alternative.

Given the importance of compaction to performance, it would probably make more sense to split them into a separate concept, but I guess that they didn’t want to diverge too much from the leveldb codebase.

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Raft Consensus

After having struggled with understanding Paxos for a while, I run into the Raft Draft paper, and I was very impressed. To start with, I have read at least four or five papers about Paxos, read 2 – 5 implementations of the algorithm in different languages, implemented it myself, and I am still not really comfortable about it.

On the other hand, after doing a single pass through the Raft paper, I have a much greater sense of understanding what it is about, how it works and even how I can implement that, if I want to. Hell, I fully expect to be able to hand that paper to a passerby CS student and get a working implementation without needing to get Sheldon Cooper involved.  One thing to note, this paper and algorithm were heavily focused on making this understandable, and I think that they were quite successful in doing that. For that matter, I wish that other papers were this easy to read and follow.

Very interesting, and unlike the Paxos paper, immediately and almost painfully obvious how you would actually make use of something like that.

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What comes after leveldb?

Kellabyte rightfully points out that leveldb was designed primarily for mobile devices, and that there have been many things that can be done that weren’t because of that design goal. I was already pointed out to HyperDex’s port of leveldb, and I am also looking into what Basho is doing with it.

You can read a bit about what HyperDex did to improve things here. And Basho challenges are detailed in the following presentation. The interesting thing about this is why people want to go to leveldb to start with.

To put it simply, it is a really good product, in the sense that it is doing what it needs to do, and it is small enough so you can understand it in a reasonably short order. The fun starts when you disagree with the decisions made for you by leveldb, it is actually quite easy to make changes to the root behavior of the project without making significant changes to the overall architecture. Looking over the HyperDex changes overview, it appears that they were able to make very targeted changes. All the hard problems (storing, searching, atomicity, etc) were already solved, now the problem is dealing with optimizing them.

I’m going to review the changes made by both Basho & HyperDex to leveldb, because I think that it would be a facinating object of study. In particular, I was impressed with how leveldb merged transactions, and I am looking forward at how HyperDex parallelized them.

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On friction in software

I was pointed out to this article about friction in software, in particular, because I talk a lot about zero friction development. Yet the post show a totally different aspect of friction.

I would actually agree with that definition of friction. The problem is that there are many types of frictions. There is the friction that you have to deal with when you build a new project, in a new domain or using new technologies, so you need to figure out how to make those things work, and you spend a lot of your time just getting things working. Then there is the friction of the process you are using. If you need to have a work item associated with each commit (and only one commit), it means that you are going to either commit more slowly, or spend a lot more time just writing effectively useless work items. Then you have the friction of the common stuff. If your users require you fairly elaborate validation, even a simple date entry form can become an effort taking multiple days.

All of those are various types of frictions. And all of those adds up to the time & cost of building software. I tend to divide them in my head to friction & taxes. Friction is everything that gets in the way unnecessarily. For example, if I need to spend a day making a release, that is friction that I can easily automate. If I need to spend a lot of time on the structure of my software, that is friction. If I have checklists that I need to go through, that is usually friction.

Then there are the taxes, stuff that you do because you have to. In my case, this is usually error handling and production readiness, failure analysis and recovery, etc. In other cases this can be i18n, validation or the like. Those are things that you can streamline, but you can’t really reduce. If you need to run in a HA environment, you are going to be needing to write the code to do that, and test that. And that ain’t friction, even though it slows you down. It is just part of the cost of doing business.

And, of course, we have the toll trolls. That is what I call to those things that you don’t have to do, but are usually forced upon you. Sometimes it is for a valid reason, but a lot of the time it is Just Because. Example of tolls that are being laid upon software include such things as having to integrate with the CRM / ERP / TLD of the day. Being forced to use a specific infrastructure, even though it is widely inappropriate. And, possibly the best thing ever: “I went golfing with Andrew this weekend, and I have great news. We are a Java shop now!”

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Raven Streams: What to do with the data?

So, we have events sitting in a stream, and you can append to them as long as you like. And you can read from them, too. And you can also generate aggregations and look at those. In the previous post, I discussed the example of billing for a phone company as the example.

Let us consider this for a second. You’ll probably have aggregation per customer per month. So loading them would probably look something like:

eventStream.AggregationFor<MonthlyBilling>(“customers/1234/2013-05”);

And that is nice & all, but what about the more complex things? What happen if I want to search for high billing statement? Or all statements for the past year? What happen if I want to do a lookup by the customer name, not its id?

Well, I already know how to use Lucene, so we can plug this into the system and allow you to sear…

Wait a second! I already wrote all of that. As it turns out, a lot of the stuff that we want to do is already done. RavenDB has it implemented. For that matter, storing the aggregation results in RavenDB is going to introduce a lot of additional options. For example, we can do another map/reduce (inside RavenDB) to give you additional aggregation. We can do full text search, enrich the data and a lot more.

So that is what will probably happen. We will be writing the aggregated results from Raven Streams into RavenDB as standard documents, and you could then process them further using RavenDB’s standard tools.

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Raven Streams: Aggregations–from the system point of view

Previously we introduced the following index (not sure if that would be a good name, maybe aggregation or projection?):

   1: from msg in messages
   2: select new
   3: {
   4:     Customer = msg.From,
   5:     Count = 1
   6: }
   7:  
   8: from result in results
   9: group result by result.Customer
  10: into g
  11: select new
  12: {
  13:     Customer = g.Key,
  14:     Count = g.Sum(x=>x.Count)
  15: }

Let us consider how it works from the point of view of the system. The easy case is when we have just one event to work on. We are going to run it through the map and then through the reduce, and that would be about it.

What about the next step? Well, on the second event, all we actually need to do is run it through the map, then run it and the previous result through the reduce. The only thing we need to remember is the final result. No need to remember all the pesky details in the middle, because we don’t have the notion of updates / deletes to require them.

This make the entire process so simple it is ridiculous. I am actually looking forward to doing this, if only because I have to dig through a lot of complexity to get RavenDB’s map/reduce’s indexes to where they are now.

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Raven Streams: aggregations–how the user sees them

The major reason for streams is the idea that you don’t really care about each individual item on its own. What you care about a lot more is some sort of aggregation over those values. And sure, you do want to be able to access the values, but you generally don’t.

Let us say that you are a phone company, and you want to use Raven Streams to record all the events that happened, so you can bill on them. Let us imagine that we are interested in just SMS for the moment, so we append each sms to the stream.

Then we are going to write something like:

   1: from msg in messages
   2: select new
   3: {
   4:     Customer = msg.From,
   5:     Count = 1
   6: }
   7:  
   8: from result in results
   9: group result by result.Customer
  10: into g
  11: select new
  12: {
  13:     Customer = g.Key,
  14:     Count = g.Sum(x=>x.Count)
  15: }

If you ever did RavenDB map/reduce indexes, this should be very familiar to you. However, unlike RavenDB, here we don’t need to handle any pesky updates or deletes. That means that the implementation is much simpler, but I’ll discuss that on my next post.

In the meantime, let us consider what is the result of this would be. It would generate a result, which we would persist and allow you to lookup. One can imagine that you can do this via the customer id, and get the sum total as it is right now.

But you’ll probably want to do additional operations, so we need to consider this as well.

For that matter, imagine the scenario where we want to get the data about SMS, MMS, phone calls, etc. How would you expect that to look like?

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Raven Streams architecture thoughts: Storage

Raven Streams is a working title.

So far, I have been working mostly on getting things working from an infrastructure perspective. That is, I want to be able to successfully write & read from the streams, and I have a general architecture for it.

Basically, we have something like this:

image

This is a single stream (of which we expect to have more than one, but not a very high number). The stream makes all writes to a mem table, backed by a log. When the mem table is full, we switch to a new memtable and write all the data to a sorted string table (SST). This is very much like the design of leveldb.

Internally, however, we represent all data as etags. That is, you don’t have an actual key that you can use, the stream will generate an etag for you. And the SST are sorted in order, so the oldest events are in the oldest file, etc.

On startup, if we haven’t flushed properly, we will translate the log file into a new SST and start from scratch. I am not sure yet if we want / need to implement compaction. Maybe we will do that for very small files only.

Right now there are two methods that you actually care about:

  • AppendAsync(data)
  • ReadFrom(etag)

This is how you use them:

   1: for (int i = 0; i < 15; i++)
   2: {
   3:     eventStream.AppendAsync(new RavenJObject { { "counter", i } }).Wait();
   4: }
   5:  
   6: int x = 0;
   7: foreach (var item in eventStream.ReadFrom(Etag.Empty))
   8: {
   9:     var value = item.Value<int>("counter");
  10:     Assert.Equal(x++, value);
  11: }

This is all working pretty nicely right now. It isn’t nearly ready for production or even test usage, but this is encouraging. Next, I’ll discuss the intended implementation of map/reduce against this.

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