Doing Full-Text Searches in .NET with Sphinx - Part 3: Searching

by Dennis 9. July 2013 16:38

Now that we’ve successfully imported our data into Sphinx in the previous part of this series, we can finally start to implement searching! As before, the current state of the sample is available for download. Before we start though, I want to make an addendum to the last post.

Word Forms


In the last post I forgot to setup a file containing word forms. With the help of word forms, you can remap one or more words to another. This feature can be used to normalize different word forms to one normal form or to map words to commonly used abbreviations. If you index text such as that from Stack Overflow, it practically screams for a list of word forms that maps words to their abbreviations, because there are many that are commonly used in the programming world, e.g. VS for Visual Studio, NH for NHibernate, HG for Mercurial and so on. I’ve therefore added a small word forms file to the sample that contains a few of these abbreviations.



To create and execute our search queries we'll use SphinxConnector.NET's fluent query API which is similar to LINQ. It translates the queries you create in code to SphinxQL and maps the query results back to an object of your choice.

Query Preprocessing and Match Clause Creation

Upon receiving a search string from the client, you usually don’t want to send it to Sphinx as is. The very least you should do, is to escape certain characters in the search string that would otherwise be interpreted as operators or modifiers of Sphinx’ extended query syntax (instead of escaping them, you might also choose to completely remove them from incoming search strings). To escape the incoming query string, we’ll use SphinxConnector.NET’s SphinxHelper.EscapeString() method.

What your match clause will look like depends on how you want Sphinx to determine what results are the most relevant to your users. As the definition of what a good result is, can vary greatly between use cases, there is no one size fits all solution and we’re only going to do some basic stuff here.  But this should be enough to get you started. To see a complete list of what Sphinx query syntax offers, please refer to the relevant section in the Sphinx manual.

The first thing we do after escaping, is to split our incoming string into an array of strings by using ‘space’ as the separator. If we only have one search term, we’ll just send it to Sphinx as is. If on the other hand, there is more than one search term, we tell Sphinx to:

  1. match the terms as a phrase (by wrapping them in ") OR
  2. match the terms as is (with an implicit AND, so documents have to contain all terms to match)  OR
  3. match a least one term wrapping the query in “ again and specifying a quorum of 1

This way, documents that contain the given keywords as a phrase or at least all keywords somewhere in the text, will be ranked higher than documents that only contain one of the keywords.

private static string BuildMatchClause(string escapedQuery)
    var terms = escapedQuery.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);

    string matchClause = String.Empty;

    if (terms.Length > 1)
        matchClause = escapedQuery.ToPhrase() + " | "; 
} matchClause += "(" + escapedQuery + ")";

if (terms.Length > 1) { matchClause += " | " + escapedQuery.ToPhrase(1);
} return matchClause; }

ToPhrase is an extension method for String, that I've defined to make the code more readable. It also takes the quorum count as an optional parameter.

Now that we’ve created our match clause we’re ready to send our query to Sphinx. The complete query looks like this:

Lazy<QueryMetadata> searchMetaData;
var searchResults = FulltextSession.Query<Post>().
                                    Where(x => x.PostType == PostType.Question).
                                    Select(x => new SearchResult {
                                        Id = x.Id,
                                        Title = x.Title,
                                        CreationDate = x.CreationDate,
                                        Tags = x.Tags,
                                        Snippet = x.Body.GetSnippet(matchClause),
                                        Weight=x.CreationDate >= DateTime.Today.AddDays(-90) ?
				x.Weight * 1.1 : x.Weight //Boost weight of recent posts }).
                                    Options(options => options.Ranker(SphinxRankMode.SPH04)).
                                    ToFutureList(out searchMetaData);

Let's walk through it: we first create a query by invoking the Query method of the IFulltextSession interface that SphinxConnector.NET provides and specify our Post document model (see part 2 for details on this) as the document type parameter. We then pass the previously created match clause to the Match method. Next, we add a filter on PostType and tell Sphinx to only search through documents that are questions. We now use the Select method to project the results into a class named SearchResult. This is a view model class that we use to provide our search result view with the necessary data to display the results. Note how we’re boosting the weight of results that aren’t older than 90 days (though these aren’t necessarily more relevant than older posts, but I wanted to demonstrate how this can be done). As not all search results are displayed on a single page, I’ve defined an extension method named Page(), as a wrapper around Limit() to improve readability. Last, we set the ranker to SPH04, which uses phrase proximity and BM25 to rank the results, but also boosts the rank of a result if it contains a search term at the very beginning or end of a text field.

Finally, we invoke ToFutureList with an out parameter to retrieve the query meta data which, among other data, contains the time Sphinx needed to execute the search. We will display this on our search result page, to brag about our fast full-text search Winking smile.

In case you’re wondering what the call to ToFutureList means (compared to calling ToList), here’s an explanation: when we invoke this method, SphinxConnector.NET will not immediately execute the query, but rather wait until the results are actually accessed. This enables SphinxConnector.NET to send more than one query to Sphinx, thus saving some network overhead. In the next paragraph we’ll be creating two queries for facets that can be send along with our first query.


Now, that we’ve created our full-text query, we can move on to creating facets that are displayed along with the search result. We will be showing two facets of our query: the number of questions asked per month and the number of questions with an accepted answer .

These are probably not the most interesting facets one could display, but they are good enough to demonstrate the creation of facets with Sphinx and SphinxConnector.NET:

var resultsPerMonth = FulltextSession.Query<Post>().
                                      Where(x => x.PostType == PostType.Question).
                                      Select(x => new {
                                          Count = Projection.Count(),
                                      GroupBy(x => x.Month).

var acceptedAnswers = FulltextSession.Query<Post>().
                                      Where(x => x.PostType == PostType.Question).
                                      Select(x => new {
                                          Count = Projection.Count(),
                                          HasAcceptedAnswer = x.AcceptedAnswerId > 0
                                      GroupBy(x => x.HasAcceptedAnswer).



In this post we looked at how to create a basic Sphinx full-text query with SphinxConnector.NET. We did some preprocessing on the search term(s) we received from the client and created a match clause for our query. We then created queries for two facets, namely the number of questions with an accepted answer and the number of questions per month for our query. Finally, we also made use of SphinxConnector.NET’s future query capabilities to execute the queries as efficiently as possible. The complete working example is available for download here. In the next post, we'll be looking at how to implement autocomplete and correction suggestions.



Doing Full-Text Searches in .NET with Sphinx - Part 2: Data Import

by Dennis 18. April 2013 11:52

This is the second post in a series that is intended as an introduction to Sphinx for .NET developers who have not yet heard of Sphinx and are looking for a powerful full-text search engine for their websites or applications. The first part of this series served as an introduction to Sphinx, in this part we’ll get our hands dirty and start working on our ASP.NET MVC based sample application.

We’ll be creating a simple website that will allow us to search through Stackoverflow data (or rather the posts made by Stackoverflow users). The Stackoverflow team kindly provides this data under a Creative Commons license in an XML format. As the dataset is pretty large, just the posts are 7 GB in size (uncompressed), we’ll only use about 10,000 posts for demonstration purposes. A file with only these posts is included in the download so you won’t have to download the whole archive.

Having an existing set of data that needs to be imported into Sphinx is probably the most common scenario when people start to work with Sphinx. We will begin by creating the document model class and the Sphinx configuration file. We will be storing the documents in a real-time index, so we also need to create a small program to read the input data from the XML file.

The current state of this sample application is available for download, it uses .NET 4.0, ASP.NET MVC3, Sphinx 2.1.1, SphinxConnector.NET and contains a Visual Studio 2010 solution. It contains the console application for importing data and the web application which will be described in detail in the next part. Please note that the layout of the website is simple and might have some display quirks as I don’t want to spend too much time on making it look pretty. 

Creating the Document Model

The first step is to determine which part of the data should be stored in the index. One approach is to store only the bare minimum that is needed for full-text searches. Often, this requires that after the search the original data is retrieved from the data source, most likely a database, to display a meaningful search result to the user. As this requires one or more additional network round trips, I personally like to store as much data in the index as is needed (and feasible) to avoid this overhead.

In this example we’ll be storing everything in the index, because it reduces the complexity of the example. For instance, we use Sphinx to display the questions on the front page, which does not involve searching at all. But, storing your complete dataset in Sphinx is usually not a good idea because it can drastically increase the resources needed by Sphinx. Also Sphinx isn’t a database and shouldn’t be treated as such, for instance it doesn’t execute certain types of queries as efficient as a database would (though there are a some tricks for that). We are doing this here for the sake of simplicity, and it might even be feasible in some cases, but should be done with care!

Let’s take a look at our input data. The Stackoverflow data archive contains a readme file with a description of the input data. Here is the relevant part for the posts.xml file which we are interested in:

       - Id
       - PostTypeId
          - 1: Question
          - 2: Answer
       - ParentID (only present if PostTypeId is 2)
       - AcceptedAnswerId (only present if PostTypeId is 1)
       - CreationDate
       - Score
       - ViewCount
       - Body
       - OwnerUserId
       - LastEditorUserId
       - LastEditorDisplayName="Jeff Atwood"
       - LastEditDate="2009-03-05T22:28:34.823"
       - LastActivityDate="2009-03-11T12:51:01.480"
       - CommunityOwnedDate="2009-03-11T12:51:01.480"
       - ClosedDate="2009-03-11T12:51:01.480"
       - Title=
       - Tags=
       - AnswerCount
       - CommentCount
       - FavoriteCount

We’ll leave out some attributes that are not needed in our example which leads to the following model class:

public class Post
    public int Id { get; set; }

    public PostType PostType { get; set; }
    public long ParentId { get; set; }
    public long AcceptedAnswerId { get; set; }
    public string Body { get; set; }
    public string Title { get; set; }
    public string Tags { get; set; }
    public long Score { get; set; }
    public long ViewCount { get; set; }
    public long AnswerCount { get; set; }
    public DateTime CreationDate { get; set; }
    public int Weight { get; set; }

You might have already noticed that we’ve added a property named ‘Weight’ to our document class which is not present in the data source. When Sphinx searches the index for results it assigns each match a weight depending on its relevancy to the search query. The higher the weight, the more relevant it is for the query, and because we might want to use the weight in our own relevancy calculations, we need to add a property for it to our model.

Configuring Sphinx

Next we need to create the configuration for the Sphinx server. By default Sphinx will look for a file named sphinx.conf in the directory where the executable resides so we’ll name it just that. The configuration file consists of several sections: one for the server itself, one for the indexer program which is not relevant for our sample, and the index configuration. For our documents the index configuration looks like this:

index posts
    type                    = rt
    path                    = posts
    rt_field                = body 
    rt_attr_string          = body 
    rt_field                = title     
    rt_attr_string          = title
    rt_field                = tags          
    rt_attr_string          = tags 

    rt_attr_uint            = parentid
    rt_attr_uint            = score 
    rt_attr_uint            = viewcount
    rt_attr_uint            = posttype
    rt_attr_uint            = answercount
    rt_attr_timestamp       = creationdate    
    rt_attr_uint            = acceptedanswerid
    rt_attr_uint            = votecount
    charset_type            = utf-8
    min_word_len            = 1    
    dict                    = keywords                    
    expand_keywords         = 1                            
    rt_mem_limit            = 1024M
    charset_table           = 0..9, a..z, A..Z->a..z, U+DF, \
                              U+FC->u, U+DC->u, U+FC,U+DC, \
                              U+F6->o, U+D6->o, U+F6,U+D6, \
                              U+E4->a, U+C4->a, U+C4,U+E4, \
                              U+E1->a, U+C9->a, U+E9->e, U+C9->e, \
                              U+410..U+42F->U+430..U+44F, U+430..U+44F, U+00E6, \
                              U+00C6->U+00E6, U+01E2->U+00E6, U+01E3->U+00E6, \
                              U+01FC->U+00E6, U+01FD->U+00E6, U+1D01->U+00E6, \
                              U+1D02->U+00E6, U+1D2D->U+00E6, U+1D46->U+00E6
    blend_chars             = U+23, U+2B, -, ., @, &

stopwords               = stopwords.txt }

Let’s look through the different configuration options: first we specify the index type and the path where the index files should be stored. Next we declare the fields and attributes; it is not necessary to explicitly declare the id attribute, Sphinx does that automatically for us.

The next thing to note is the declaration of the full-text fields which are the fields that are searched by Sphinx when it processes a full-text query. In order to be able to retrieve the contents of these fields, so that we can display them to the user, we also declare string attributes for each of them. Why is it necessary to declare both? When the keywords are inserted into the index, they are hashed. This implies that there is no way to retrieve the original data that was inserted. This is where string attributes come into play. They enable us to store and retrieve the original content, which we then use to display the search result to the user, which allows us to avoid accessing the database altogether.

Next we specify the charset to use for our documents which is utf8 in our case. For the minimum word length we use one, the keywords dictionary type is set to ‘keywords’, we also enable Sphinx’ internal keyword expansion to allow for partial word matches and set the memory limit for RT index RAM chunks to 1024MB.

The charset table is up next. It defines which characters are recognized by Sphinx and also allows to remap a character to another. It is important to understand that if a character is not in the charset table, Sphinx will treat that character as a separator during indexing. To configure the charset table we can use both the character itself and/or its Unicode number. In our charset table we first add numbers from 0 to 9, lower case chars and remap upper case chars to lower case chars. Additionally we define some more mappings e.g. German umlauts are mapped to their non-umlaut equivalent like ü –> u. When defining your own charset table you can refer to the Sphinx wiki which contains a page with charset tables.

Lastly, we define the blend chars and configure a file with stop words. Blend chars are characters that are treated by Sphinx as both regular characters and separators. Suppose that you’re indexing email addresses and configure the ‘@’ symbol as a blend char, an email address such as ‘’ will be indexed as a whole and as ‘foo’ and ‘’. This enables users to find an email address even if they don’t remember the domain or search for addresses that belong to a particular domain.

The stop words file contains a list of words that should be ignored by Sphinx during indexing. While any word can be configured as a stop word, most of the time frequently occurring words such as ‘the’, ‘is’, etc. are configured as stop words because it reduces the size of the full-text index and thereby improves query times.

Importing the Documents

To import the Stackoverflow data we create a simple console application that reads the input data from the XML file and uses SphinxConnector.NET’s fluent API to save the data into the index. Here’s the ImportPosts method:

public static void ImportPosts()
    IFulltextStore fulltextStore = new FulltextStore().Initialize();

    using (IFulltextSession session = fulltextStore.StartSession())

        int count = 0;

        foreach (var post in GetPosts())

            if (count++ % fulltextStore.Settings.SaveBatchSize == 0)


The FulltextStore class is the entry point to the fluent API. It is used to configure the connection string, mapping conventions and other settings. It also serves as a factory for creating instances of the IFulltextSession interface which provides the methods we need to execute full-text queries and to save and delete documents from RT-indexes. In the above code we’re creating an instance of the IFulltextSession interface by calling StartSession and then proceed to insert the documents. The GetPosts method is responsible for reading the documents from the XML file.


In the second installment of this series we created our document model and a Sphinx real-time index setup for these documents. We then created a small console application that performs the import of the documents from the source XML file. In the next part, we’ll create a website that’ll allow us to search the index we just created.

Tags: , ,


Indexing Office and PDF Files With Sphinx and .NET

by Dennis 6. February 2013 13:19

Sphinx is a great full-text search engine with many amazing features, but there is one feature missing that would make it even better: the ability to directly index Word, Excel, PowerPoint, PDF files. How one can index these kinds of documents with Sphinx is something that is often asked in the Sphinx forum. Today I’d like to show you an easy way to extract text from these document types and store them in a Sphinx real-time index from your .NET application.

There are a bunch of tools and libraries out there that claim to be able to extract text from various document formats. As it is a pretty hard task to support many formats and extract text reliably, the quality of each library varies greatly. One tool that stands out is the Apache Tika™ toolkit. It is a Java library that

“detects and extracts metadata and structured text content from various documents using existing parser libraries.”

And it is really very good at it. Amongst others, it supports Microsoft Office, Open Document (ODF), and PDF files. But wait, I said Java library, didn’t I? “What am I supposed to do with a Java library in my .NET application?”, you might ask. Well, we’ll just convert it from Java to .NET using IKVM.NET. IKVM.NET is a .NET implementation of a Java Virtual Machine (JVM) which can be used as a drop-in replacement for Java. And it comes with a Java bytecode to CIL translator named ikvmc that we can use to build a .NET version of Apache Tika. In the next section, I’ll walk through the steps required to do this. At the end of this article you can download a complete sample application that uses Tika to extract text from some files and stores them in a Sphinx real-time index via SphinxConnector.NET.

Creating a .NET Version of Apache Tika

To create your own .NET version of Apacha Tika you need to:

  1. Download IKVM.NET
  2. Download the .jar file from the Tika project page (the current version at the time of writing is 1.3)
  3. Extract IKVM.NET to a folder of your choice
  4. Optional: Add the bin folder of IKVM.NET to your %PATH% variable
  5. Execute the following command (add/change the paths to ikvmc.exe and tika-app-1.3.jar if needed):
ikvmc.exe -target:library -version: -out:Apache.Tika.dll tika-app-1.3.jar

Let’s walk through the command line parameters: With –target:library we tell ikvmc to convert the jar to a class library. This is needed because the jar file is also usable as a standalone console/gui application, i.e. contains a main() method, which by default would cause ikvmc to generate an exe file. Next, we specify the version for our output DLL because otherwise ikvmc would set the version to Finally we specify the output file name via –out: and the path to the Tika jar file.

After hitting Enter, ikvmc starts to translate the Java library to .NET. It’ll output a lot of warning messages, but will eventually finish and produce a working DLL. Note that if you want to sign the output assembly you can do so by specifying a key file via the -keyfile: command line option.

Extracting Text from Documents

Now that we've created a .NET library of Tika, we can start extracting text from documents. I’ve created small wrapper that provides methods to perform the extraction. To build the wrapper DLL we need to add references to a couple of IKVM.NET libraries:


Note that you need to reference more of IKVM.NET’s DLL’s in an application that uses Tika, these are just the required files to compile the wrapper project.

The AutoTextExtractor class which handles the extraction of text from files and binary data (useful if your documents are stored in a DB) and the TextExtractionResult class are based on these by Kevin Miller:

public class AutoTextExtractor
  public TextExtractionResult Extract(string filePath,OutputType outputType = OutputType.Text)
      return Extract(System.IO.File.ReadAllBytes(filePath), outputType);

  public TextExtractionResult Extract(byte[] data, OutputType outputType = OutputType.Text)
      var parser = new AutoDetectParser();
      var metadata = new Metadata();

      using (Writer outputWriter = new StringWriter())
      using (InputStream inputStream = TikaInputStream.get(data, metadata))
          parser.parse(inputStream, GetTransformerHandler(outputWriter, outputType), 
              metadata, new ParseContext());
          return AssembleExtractionResult(outputWriter.toString(), metadata);

  private static TextExtractionResult AssembleExtractionResult(string text, Metadata metadata)
      Dictionary<string, string> metaDataResult = metadata.names().
          ToDictionary(name => name, name => String.Join(", ", metadata.getValues(name)));

      string contentType = metaDataResult["Content-Type"];

      return new TextExtractionResult
          Text = text,
          ContentType = contentType,
          Metadata = metaDataResult

  private TransformerHandler GetTransformerHandler(Writer outputWriter, OutputType outputType)
      var factory = (SAXTransformerFactory)TransformerFactory.newInstance();
      TransformerHandler handler = factory.newTransformerHandler();
      handler.getTransformer().setOutputProperty(OutputKeys.METHOD, outputType.ToString());
      handler.setResult(new StreamResult(outputWriter));

      return handler;

Here’s the TextExtractionResult class:

public class TextExtractionResult
    public string Text { get; set; }
    public string ContentType { get; set; }
    public IDictionary<string, string> Metadata { get; set; }

And the OutputType enumeration:

public enum OutputType

Demo Application

I’ve created a small demo application that contains a DLL that wraps Tika with the help of the aforementioned classes, and a console application that demonstrates how to extract and store the contents of some files in a Sphinx real-time index with SphinxConnector.NET. The code that does the extraction is pretty simple:

private static SphinxDocumentModel[] GetDocuments()
    AutoTextExtractor textExtractor = new AutoTextExtractor();

    int id = 1;

    return (from filePath in Directory.EnumerateFiles(@"..\..\..\..\testfiles")
            select new SphinxDocumentModel
                Id = id++,
                FileContent = textExtractor.Extract(filePath).Text,
                FilePath = Path.GetFullPath(filePath)

I’ll omit the code that saves the documents to the index, as it is straightforward. Tip: if you are working with big files you might have to increase Sphinx’ max_packet_size setting.

The archive contains all required libraries so it’s pretty big (IKVM.NET and Tika alone take up about 50 MB in space).


[1] Zip-Package (22 MB)

[2] 7z-Package (18 MB)

Tags: , , , ,

How-to | Tutorial

Importing Data into Sphinx RT-Indexes with SphinxConnector.NET’s Fluent API

by Dennis 19. November 2012 12:04

If you are facing the task of importing data into a Sphinx RT-index, SphinxConnector.NET’s fluent API makes this really easy with just a couple lines of code (the document model class is omitted for brevity):

void Import()
    IFulltextStore fulltextStore = new FulltextStore().Initialize();

    int count = 0;

    using (IFulltextSession session = fulltextStore.StartSession())
        foreach (var document in GetDocuments())

            if (++count % fulltextStore.Settings.SaveBatchSize == 0)


The important part is the call to FlushChanges each time a batch of documents has been passed to Save. This avoids high memory usage when importing many documents, because SphinxConnector.NET has to keep each document in memory until FlushChanges is called (though for smaller datasets it might be acceptable to flush all changes at the end of the import process).

Not only is this much simpler than writing SphinxQL by hand, it’s also faster because of SphinxConnector.NET’s automatic batching. The default value for SaveBatchSize is 16, which provides good performance, but can of course be adjusted for environments where a higher batch size leads to even more performance.

Tags: , ,

Tutorial | How-to

Doing Full-Text Searches in .NET with Sphinx - Part 1: Introduction

by Dennis 25. October 2012 11:16

This is the first post in a series (part 2, part 3) that is intended as an introduction to Sphinx for .NET developers who have not yet heard of Sphinx and are looking for a powerful full-text search engine for their websites or applications. The first part of this series serves as an introduction to Sphinx, and over the next parts we’ll build an ASP.NET MVC site that uses Sphinx.

What is Sphinx?


Sphinx is an open source full text search server, designed from the ground up with performance, relevance (aka search quality), and integration simplicity in mind. It's written in C++ and works on Linux (RedHat, Ubuntu, etc), Windows, MacOS, Solaris, FreeBSD, and a few other systems.


As we can see, Sphinx runs a stand-alone server (as opposed to embedded solutions) on pretty much any important operating system. Its key benefits are high search and indexing performance, scalability and providing high quality search results. It is used by a variety of sites from different domains like news, ecommerce and social media e.g. Slashdot, Craigslist and tumblr. Before you can use Sphinx for full-text searches you need to import your data into Sphinx.

How to get your data into Sphinx


First, let’s talk about some terminology: Sphinx stores our data in an index and each entry in an index is called a document. An index is comparable to a table in a database and a document is analogous to a row in database table. A document can contain text and attributes and must have a unique id. The text is the data that we want to search through while attributes contain additional data that is stored along with our text. Attributes can be used to hold data like dates, numbers, booleans and strings. The latter is especially interesting and useful as we’ll see in the next part.

So, how do we get our documents into Sphinx? There are several answers to that and it can sometimes be confusing for the beginner but once you get behind it, it’s not that difficult.

Index Types

As we already mentioned, Sphinx stores documents in an index. Let’s first take a look at the index types that Sphinx provides. Sphinx comes with two different index types: disk-based indexes (also referred to as batch indexes) and real-time (RT) indexes. The name disk-based index can be a bit confusing, because RT indexes are of course also stored on disk. RT indexes consist of so called disk-chunks which in turn are just regular disk-based indexes. The difference between these two index types is the way they are filled with data.

Disk-Based or Batch Indexes

With disk-based indexes, there are two ways to add documents: from a database or from a XML pipe. A disk-based index always needs to have a data source configured. As stated before, this source can be either a database or a stream of XML data. Sphinx can talk to many databases directly; it supports MSSQL, MySQL and PostgreSQL natively, other databases can be accessed via ODBC. If a database isn’t supported or the data is not stored in one, it can be exported to XML in a schema that Sphinx understands, and be processed from there.

The indexing for disk-based indexes is not done by the Sphinx search server itself, but by a separate program aptly called indexer. It fetches the data from a data source and creates the index files which are then served to clients by a program called searchd (the actual search server). Once a disk-based index is created, no new documents can be added to it, though attributes of existing documents can be updated. So how does one add new documents to an index? The easiest way is to just run the indexer again and re-create the index from the ground up. Schedule an indexer run every 20 minutes or so and new documents are available in the index with a maximum delay of 20 minutes plus the time it takes to create the index. While this seems crude, it can be a valid approach for smaller datasets because of its simplicity.

The approach commonly used for larger datasets, is a so called main-delta scheme, where one has a big index that is created once (main) and a smaller one that holds new documents (delta). Since the delta index is small it can re-build regularly. As the delta index grows it can be merged with the main index to keep indexing time short. Next, let’s take a look at how RT-indexes work:

Real-Time Indexes

The most notable difference between disk-based indexes and RT-indexes is the fact that the latter are not created by the indexer program. Then how do my documents get in there, you ask. Simple, we just INSERT them. With RT-indexes we can insert, update and delete documents in real-time (hence the name), all changes to an RT-index are made nearly instantaneously. These operations are done via Sphinx’ own query language named SphinxQL which supports INSERT, UPDATE, and DELETE statements similar to SQL. To reiterate (because sometimes new Sphinx users are confused by this): unlike disk-based indexes, real-time indexes are independent of the data source! It does not matter where your data comes from, you retrieve it from any source and then insert it into a real-time index via an INSERT INTO… statement, just as you would insert data into a database.

So which index type should I use?

Unfortunately, that is not an easy question to answer. It really depends on each project’s specific requirements but here are some general guidelines:

  • RT-indexes are missing some features (infix indexing for example) that disk-based indexes have. So if you can’t live without that, disk-based indexes are the way to go (Update: Infixes are supported as of Sphinx 2.1.1)
  • If you need instantaneous index updates, RT-indexes are the way to go (not entirely true, see below)
  • The indexing speed of disk-based indexes is often higher than that of RT-indexes

It is also possible to use both index types e.g. disk-based indexes for existing data and RT-indexes for new documents. Also, if you start out with disk-based indexes and decide to switch to RT indexes later, Sphinx provides an easy way to convert your existing indexes via the ATTACH INDEX statement.



In the first part of this series we took a high-level look at Sphinx - what index types it provides, how indexing works and different strategies for indexing data. In the next part we’ll get our hands dirty and do some actual coding!

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