



Introduction

MGIndex

What is RaptorDB?

This article is the version 2 of my previous article found here ( http://www.codeproject.com/Articles/190504/RaptorDB ), I had to write a new article because in this version I completely redesigned and re-architected the original and so it would not go with the previous article. In this version I have done away with the b+tree and hash index in favor of my ownstructure which for all intents and purposes is superior and the performance numbers speak for themselves.

Here is a brief overview of all the terms used to describe RaptorDB :

Embedded : You can use RaptorDB inside your application as you would any other DLL, and you don't need to install services or run external programs.

: You can use inside your application as you would any other DLL, and you don't need to install services or run external programs. NoSQL : A grass roots movement to replace relational databases with more relevant and specialized storage systems to the application in question. These systems are usually designed for performance.

: A grass roots movement to replace relational databases with more relevant and specialized storage systems to the application in question. These systems are usually designed for performance. Persisted : Any changes made are stored on hard disk, so you never lose data on power outages or crashes.

: Any changes made are stored on hard disk, so you never lose data on power outages or crashes. Dictionary : A key/value storage system much like the implementation in .NET.

: A key/value storage system much like the implementation in .NET. MurMurHash: A non cryptographic hash function created by Austin Appleby in 2008 (http://en.wikipedia.org/wiki/MurmurHash).



Features

RaptorDB

Very fast performance (typically 2x the insert and 4x the read performance of RaptorDB v1)

v1) Extremely small foot print at ~50kb.

No dependencies.



Multi-Threaded support for read and writes.



Data pages are separate from the main tree structure, so can be freed from memory if needed, and loaded on demand.



Automatic index file recovery on non-clean shutdowns.



String Keys are UTF8 encoded and limited to 60 bytes if not specified otherwise (maximum is 255 chars).



Support for long string Keys with the RaptorDBString class.

class. Duplicate keys are stored as a WAH Bitmap Index for optimal storage and speed in access.

Two mode of operation Flush immediate and Deferred ( the latter being faster at the expense of the risk of non-clean shutdown data loss).

Enumerate the index is supported.

Enumerate the Storage file is supported.

Remove Key is supported.



Why another data structure?

MGindex

MGindex

The problem with a b+tree

has the following features :There is always room for improvement, and the ever need for faster systems compels us to create new methods of doing things.is no exception to this rule. Currentlyoutperforms b+tree by a factor ofand, while keeping the main feature of disk friendliness of a b+tree structure.

Theoretically a b+tree is O(N log k N) or log base k of N, now for the typical values of k which are above 200 for example the b+tree should outperform any binary tree because it will use less operations. However I have found the following problems which hinder performance :



Pages in a b+tree are usually implemented as a list or array of child pointers and so while finding and inserting a value is a O(log k) operation the process actually has to move children around in the array or list, and so is time consuming.



Splitting a page in b+tree has to fix parent nodes and children so effectively will lock the tree for the duration, so parallel updates are very very difficult and have spawned a lot of research articles.

Requirements of a good index structure

So what makes a good index structure, here are what I consider essential features of one:



Page-able data structure:



Easy loading and saving to disk.





Free memory on memory constraints.



On-demand loading for optimal memory usage.



Very fast insert and retrieve.

Multi-thread-able and parallel-able usage.



Pages should be linked together so you can do range queries by going to the next page easily.



The MGIndex

MGIndex takes the best features of a b+tree and improves upon on them at the same time removing the impediments. MGIndex is also extremely simple in design as the following diagram shows:





As you can see the page list is a sorted dictionary of first keys from each page along with associated page number and page items count. A page is a dictionary of key and record number pairs.

This format ensures a semi sorted key list, in that within a page the data is not sorted but pages are in sort order relative to each other. So a look-up for a key just compares the first keys in the page list to find the page required and gets the key from the page's dictionary.



MGIndex is O(log M)+O(1), M being N / PageItemCount [ PageItemCount = 10000 in the Globals class]. This means that you do a binary search in the page list in log M time and get the value in O(1) time within a page.



RaptorDB starts off by loading the page list and it is good to go from there and pages are loaded on demand, based on usage.



Page Splits

In the event of page getting full and reaching the PageItemCount ,

MGIndex

will sort the keys in the page's dictionary and split the data in two pages ( similar to a b+tree split) and update the page list by adding the new page and changing the first keys needed. This will ensure the sorted page progression.

Interestingly the processor architecture plays an important role here as you can see in the performance tests as it is directly related to the sorting key time, the Core iX processors seem to be very good in this regard.



Interesting side effects of MGIndex



MGIndex

Because the data pages are separate from the Page List structure, implementing locking is easy and isolated within a page and not the whole index, not so for normal trees.



Splitting a page when full is simple and does not require a tree traversal for node overflow checking as in a b+tree.



Main page list updates are infrequent and hence the locking of the main page list structure does not impact performance.

The above make the MGIndex a really good candidate for parallel updates.

The road not taken / the road taken and doubled back!

Here are some interesting side effects of

Originally I used a AATree found here (http://demakov.com/snippets/aatree.html) for the page structures, for being extremely good and simple structure to understand. After testing and comparing to the internal .net SortedDictionary (which is a Red-Black tree structure) it was slower and so scrapped (see the performance comparisons).



I decided against using SortedDictionary for the pages as it was slower than a normal Dictionary and for the purpose of a key value store the sorted-ness was not need and could be handled in other ways. You can switch to the SortedDictionary in the code at any time if you wish and it makes no difference to the overall code other than you can remove the sorting in the page splits.



I also tried an assorted number of sorting routines like double pivot quick sort, timsort, insertion sort and found that they all were slower than the internal .net quicksort routine in my tests.



Performance Tests

In this version I have compiled a list of computers which I have tested on and below is the results.





As you can see you get a very noticeable performance boost with the new Intel Core iX processors.



Comparing B+tree and MGIndex

For a measure of relative performance of a b+tree, Red/Black tree and MGIndex I have compiled the following results.





Times are in seconds.



B+Tree : is the index code from RaptorDB v1

SortedDictionary : is the internal .net implementation which is said to be a Red/Black tree.



Really big data sets!

To really put the engine under pressure I did the following tests on huge data sets (times are in seconds, memory is in Gb) :





These tests were done on a HP ML120G6 system with 12Gb Ram, 10k raid disk drives running Windows 2008 Server R2 64 bit. For a measure of relative performance to RaptorDb v1 I have included a 20 million test with that engine also.



I deferred from testing the get test over 100 million record as it would require a huge array in memory to store the Guid keys for finding later, that is why there is a NT (not tested) in the table.



Interestingly the read performance is relatively linear.



Index parameter tuning

To get the most out of RaptorDB you can tune some parameters specific to your hardware.



PageItemCount : controls the size of each page.



Here are some of my results:



I have chosen the 10000 number as a good case in both read and writes, you are welcome to tinker with this on your own systems and see what works better for you.

Performance Tests v2.3

In v2.3 a single simple change of converting internal classes to structs rendered huge performance improvements of 2x+ and at least 30% lower memory usage. You are pretty much guaranteed to get 100k+ insert performance on any system.

Some of the test above were run 3 times because the computers were being used at the time (not cold booted for the tests) so the initial results were off. The HP G4 laptop is just astonishing.

I also re-ran the 100 million test on the last server in the above list and here is the results:

As you can see in the above test, the insert time is 4x faster (although the computer specs to not match the HP system tested earlier) and incredibly the memory usage is half than the previous test.

Using the Code

To create or open a database you use the following code :

var guiddb = RaptorDB.RaptorDB<Guid>.Open( " c:\\RaptorDbTest\\multithread" , false ); var strdb = RaptorDB.RaptorDB<string>.Open( " c:\\intdb" , 100 , true );

To insert and retrieve data you use the following code :

Guid g = Guid.NewGuid(); guiddb.Set(g, " somevalue" ); string outstr= " " ; if (guiddb.Get(g, out outstr)) { }

The UnitTests project contains working example codes for different use cases so you can refer to it for more samples.

Differences to v1

The following are a list of differences in v2 opposed to v1 of RaptorDB :



Log Files have been removed and are not needed anymore as the MGIndex is fast enough for in-process indexing.

is fast enough for in-process indexing. Threads have been replaced by timers.

The index will be saved to disk in the background without blocking the engine process.

Messy generic code has been simplified and the need for a RDBDataType has been removed, you can use normal int, long, string and Guid data types.

has been removed, you can use normal int, long, string and Guid data types. RemoveKey has been added.



Other than that existing code should compile as is with the new engine.



Using RaptorDBString and RaptorDBGuid

RaptorDBString is for long string keys (larger than 255 characters) and it is really useful for file paths etc. You can use it in the following way :



var rap = new RaptorDBString( @" c:\raptordbtest\longstringkey" , false ); var db = new RaptorDBGuid( " c:\\RaptorDbTest\\hashedguid" );

RaptorDBGuid is a special engine which will MurMur2 hash the input Guid for lower memory usage (4 bytes opposed to 16 bytes), this is useful if you have a huge number of items which you need to store. You can use it in the following way :



var db = new RaptorDBGuid( " c:\\RaptorDbTest\\hashedguid" );

Global parameters

The following parameters are in the Global.cs file which you can change which control the inner workings of the engine.



Parameter

Default

Description

<code>BitmapOffsetSwitchOverCount

10

Switch over point where duplicates are stored as a WAH bitmap opposed to a list of record numbers

<code>PageItemCount

10,000

The number of items within a page

<code>SaveTimerSeconds

60

Background save index timer seconds ( e.g. save the index to disk every 60 seconds)

<code>DefaultStringKeySize

60

Default string key size in bytes (stored as UTF8)

<code>FlushStorageFileImmetiatley

false

Flush to storage file immediately

<code>FreeBitmapMemoryOnSave

false

Compress and free bitmap index memory on saves

RaptorDB interface

Set(T, byte[]) Set Key and byte array Value, returns void Set(T, string) Set Key and string Value, returns void Get(T, out string) Get the Key and put it in the string output parameter, returns true if key was found Get(T, out byte[]) Get the Key and put it in the byte array output parameter, returns true if key was found <code>RemoveKey(T)

This will remove the key from the index

EnumerateStorageFile() returns all the contents of the main storage file as an IEnumerable< KeyValuePair<T, byte[]> > Enumerate(fromkey)

Enumerate the Index from the key given.

GetDuplicates(T) returns a list of main storage file record numbers as an IEnumerable<int> of the duplicate key specified FetchRecord(int) returns the Value from the main storage file as byte[] , used with GetDuplicates and Enumerate

Count(includeDuplicates) returns the number of items in the database index , counting the duplicates also if specified

SaveIndex() Allows the immediate save to disk of the index (the engine will automatically save in the background on a timer)

Shutdown() This will close all files and stop the engine.

Non-clean shutdowns



In the event of a non clean shutdown RaptorDB will automatically rebuild the index from the last indexed item to the last inserted item in the storage file. This feature also enables you to delete the mgidx file and have RaptorDB rebuild the index from scratch.



Removing Keys

In v2 of RaptorDB removing keys has been added with the following caveats :



Data is not deleted from the storage file.

deleted from the storage file. A special delete record is added to the storage file for tracking deletes and which also help with index rebuilding when needed.

Data is removed from the index.

Unit Tests

The following unit tests are included in the source code (the output folder for all the tests is C:\RaptorDbTest ):

Duplicates_Set_and_Get : This test will generate 100 duplicates of 1000 Guid s and fetch each one (This tests the WAH bitmap subsystem).

: This test will generate 100 duplicates of 1000 s and fetch each one (This tests the WAH bitmap subsystem). Enumerate : This test will generate 100,001 Guid s and enumerate the index from a predetermined Guid and show the result count (the count will differ between runs).

: This test will generate 100,001 s and enumerate the index from a predetermined and show the result count (the count will differ between runs). Multithread_test : This test will create 2 threads inserting 1,000,000 items and a third thread reading 2,000,000 items with a delay of 5 seconds from the start of insert.

: This test will create 2 threads inserting 1,000,000 items and a third thread reading 2,000,000 items with a delay of 5 seconds from the start of insert. One_Million_Set_Get : This test will insert 1,000,000 items and read 1,000,000 items.

: This test will insert 1,000,000 items and read 1,000,000 items. One_Million_Set_Shutdown_Get : This test will do the above but shutdown and restart before reading.

: This test will do the above but shutdown and restart before reading. RaptorDBString_test : This test will create 100,000 1kb string keys and read them from the index.

: This test will create 100,000 1kb string keys and read them from the index. Ten_Million_Optimized_GUID : This test will use the RaptorDBGuid class which will MurMur hash 10,000,000 Guid s writting and reading them.

: This test will use the class which will MurMur hash 10,000,000 s writting and reading them. Ten_Million_Set_Get : The same as 1 million test but with 10 million items.

: The same as 1 million test but with 10 million items. Twenty_Million_Optimized_GUID : The same as 10 million test but with 20 million items.

: The same as 10 million test but with 20 million items. Twenty_Million_Set_Get : The same as 1 million test but with 20 million items.

: The same as 1 million test but with 20 million items. StringKeyTest : A test for normal string keys of max 255 length.

: A test for normal string keys of max 255 length. RemoveKeyTest : A test for removing keys works properly between shutdowns.

File Formats

File Format : *.mgdat

File Format : *.mgbmp

File Format : *.mgidx

File Format : *.mgbmr , *.mgrec

long

These values map the record number to an offset in the BITMAP

index file

and DOCS storage file.

History

Initial Release v2.0 : 19th January 2012

Update v2.1 : 26 th January 2012

: 26 January 2012 lock on safedictionary iterator set, Thanks to igalk474



string default(T) -> "" instead of null, Thanks to Ole Thrane for finding it

for finding it

mgindex string firstkey null fix



added test for normal string keys



fixed the link to the v1 article

Update v2.2 : 8 th February 2012

: 8 February 2012 bug fix removekey, Thanks to syro_pro



removed un-needed initialization in safedictionary, Thanks to Paulo Zemek

Update v2.3 : 1 st March 2012

: 1 March 2012 changed internal classes to structs (2x+ speed, 30% less memory)



added keystore class and code refactoring



added a v2.3 performance section to the article

Update v2.4 : 7 th March 2012

: 7 March 2012 bug fix remove key set page isDirty -> Thanks to Martin van der Geer



Page<T> is a class again to fix keeping it's state



added RemoveKeyTest unit test



removed MemoryStream from StorageFile.CreateRowHeader for speed



current record number is also set in the bitmap index for duplicates

Update v2.5 : 28 th May 2012

: 28 May 2012 added SafeSortedList for access concurrency of the page list



insert performance back to v2.3 speed (removed extra writing to duplicates)

Update v2.6 : 20 th December 2012

: 20 December 2012 post back code from RaptorDB the doc store



added more data types (uint,short,double,float,datetime...)



added locks to the indexfile



updated logger



updated safe dictionary with locks



changed to Path.DirectorySeparatorChar for Mono/MonoDroid support



bug fix edge case in WAHbitarray



updated storage file

Update v2.7.0 : 6 th October 2013

: 6 October 2013 bug fix WAHBitArray



index files are opened in shared mode for the ability of online copy backup



dirty pages are sorted on save for read performance

Update v2.7.5 : 11 th October 2013

: 11 October 2013 bug fix saving page list to disk for counts > 50 million items

Values are stored in the following structure on disk:Bitmap indexes are stored in the following format on disk :The bitmap row is variable in length and will be reused if the new data fits in the record size on disk, if not another record will be created. For this reason a periodic index compaction might be needed to remove unused records left from previous updates.The MGIndex index is saved in the following format as shown below:Rec file is a series ofvalues written to disk with no special formatting.