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libQtCassandra  0.4.7
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-- libQtCassandra

A C++ library
The libQtCassandra organization
Communication between objects
Multi-thread support
Why Qt?

-- The Cassandra System

Terminology Explained
Changes between versions

-- libQtCassandra copyright and license


A C++ library

It looks like there was a need for a C++ library for the Cassandra system. Looking around the only one available was indeed in C++ and helping a bit by hiding some of the Thrift implementation, but in the end you still had to know how the Thrift structure had to be used to make it all work.

So, I decided to write my own version, especially, I was thinking that it should be possible to access the data with the following simple array like syntax:

   cluster[context][table][row][column] = value;
   value = cluster[context][table][row][column];

In other words, you should be able to create a cluster object and then from that object access the contexts (keyspaces), the tables (column families), the rows and columns (actually, the values are saved in cells in this library, but the name used in the cell is the name of the column.)

Looking at the implementation of a system I will create soon after having done this library it very much feels like this is going to be the basis for many accesses to the Cassandra database.

The one thing still missing is a clean way of handling the exception. At this time the libQtCassandra library generate runtime_error and logic_error when you missuse a function. It also captures some Thrift exception in a few cases (although we try to avoid doing so, the connect() is the main one that we capture so you can attempt to connect and instead of throwing you get a return value of false when the connect failed.) But... at this time, if you get a Cassandra Thrift exception, you cannot do anything with it unless you include the Cassandra Thrift header files (otherwise you don't get the declaration of these exceptions.) Thus, we should overload all the exceptions and have try/catch around all the calls to the Cassandra Thrift interface and convert those exception in a libQtCassandra exception.

The libQtCassandra organization

  • Cluster

The cluster is defined in a QCassandra object. At the time you call connect() you get a connection to the Cassandra server and you can start using the other functions.

Clusters are used to manage contexts and to access the Cassandra server.

Note that internally all accesses to the Cassandra system is done with one f_client pointer.

  • Contexts

A cluster can have many contexts. It has the default System contexts that comes with the Cassandra environment. Others can be create()'d, and opened on later connections.

Contexts can be used to manage tables.

  • Tables

A table has a name and it includes one to many rows. The number of rows can be really large and not impact the Cassandra cluster much.

The readRows() function is used to read a range of rows as determined by the row predicate.

  • Column Definitions

A table may have a list of column definitions. You generally do not need a column definition unless you want to force a certain type of data in a column. In that case a column definition is required (Assuming that the default type is not what you want for that column.)

We do not use this information internally. Only to forward to the Cassandra server (and also we read the current status from the Cassandra server.)

At a later time, we may check the type defined here and in the row (if not defined as a column type) and check the data supplied to verify that the data passed to the row is valid before sending it to the server. This is probably not necessary because if you have bad data your application won't work, no matter what.

  • Rows

A row is named using a key, which means the name can include any character including '\0' (it just cannot be empty.) The key can actually be an integer, or even a complete structure. Rows include cells.

Contrary to an SQL engine where the number of columns is generally relatively restrained, the number of cells in a row can be very large and accessing a specific cell remains fast (assuming the cell names remain reasonably small.)

For this reason, when you read a row, you do not have to read all the cells on that row. Instead, you may read just one or two cells.

Row names can use UUIDs, in that case use a QUuid object and directly call the different table functions that accept a row key name.

  • Cells

A cell is named using a column key, which means the name can include any character including '\0' (it just cannot be empty.) Just like for rows, the key can be an integer, or a structure. With a well organized structure you can actually sort your data in some pretty advanced manner.

Cell keys are attached to a value. This value is limited to 2Gb in size, although remember that sending 2Gb over a network connection is going to be slow and the entire data of one cell must be transferred when written or read.

Note that when you create an index, you may use a default value (including the Null value.) For example, if you have a Page table with a row numbered 3 and that row has a cell named Path with a value X. You may add this row like this (somewhat simplified for this example):

 // add the value "X" to row "3" in table "Page"
 int64_t row_number = 3;
 QCassandraValue row_id;
 row_id = row_number;
 QCassandraValue value;
 value = "X";
 context["Page"][row_id]["Path"] = value;

 // "simultaneously," create an index from value to row identifier
 QCassandraValue empty;
 context["Page_Path_Index"][value.binaryValue()][row_id] = empty;

 // search rows in pages that have a path set to value

As you can see, the second write uses the empty value so you do not waste any space in the database (note however that at this time there is a problem with the QRow::exists() function which creates a cell when it doesn't exist so an empty value may not always be practical.)

Note that when writing a value to a cell, the consistency level of the value is used. When reading a cell, the value is returned so its consistency level cannot be changed by you before the call. By default the value as defined by the setDefaultConsistencyLevel() function is used (the default being ONE.) You may change that default if you are expected to use that consistency level in the majority of places. Otherwise, you can also set the consistency level on a cell. Remember that this is used for reads only. There are additional details in the QCassandraCell::setConsistencyLevel() function

Cell names can use UUIDs, in that case use a QUuid object and directly call the different row functions that accept a cell key name.

  • Values

The cells are set to a specific value using the QCassandraValue class. Beside their binary data, values have a timestamp that represents the time and date when they were created and a TTL (time to live) value in seconds. By default the timestamp is set to gettimeofday() and the TTL is set to 0 (which means permanent.)

The values are cached in memory by the libQtCassandra library. Not only that, multiple write of the same value to the same cell will generate a single write to the Cassandra database (i.e. the timestamp is ignored in this case, see the QCassandraCell class for more info.)

Values also include a consistency level. By default this is set to ONE which may not be what you want to have... (in many cases QUORUM is better for writes.) The consistency level is defined here because it can then easily be propagated when using the array syntax.

 QCassandraValue v;
 v.setTtl(60 * 60 * 24);  // live for 1 day

Communication between objects

In general, the objects communicate between parent and child. However, some times the children need to access the QCassandraPrivate functions to send an order to the Cassandra server. This is done by implementing functions in the parents and calling functions in cascade.

The simplest message sent to the Cassandra server comes from the connect() which is the first time happen. It starts from the QCassandra object and looks something like this:


When you save a QCassandraValue in a QCassandraCell, then the cell calls the row, which calls the table, which calls the context which has access to the QCassandraPrivate:


As you can see the context makes itself current as required. This ensures that the correct context is current when a Cassandra RPC event is sent. This is completely automatic so you do not need to know whether the context is current. Note that the function is optimized (i.e. the pointer of the current context is saved in the QCassandra object so if it doesn't change we avoid the set_keyspace() call.)

Messages from rows, tables, and contexts work the same way, only they do not include calls from the lower layers.

The drop() calls have one additional call which deletes the children that were just dropped. (i.e. if you drop a row, then its cells are also dropped.)

Multi-thread support

The libQtCassandra library is NOT multi-thread safe. This is for several reasons, the main one being that we are not currently looking into writing multi-threaded applications (on a server with a heavy load, you have many processes running in parallel and having multiple threads in each doesn't save you anything.)

If you plan to have multiple threads, I currently suggest you create one QCassandra object per thread. The results will be similar, although it will make use of more memory and more accesses to the Cassandra server (assuming each thread accesses the common data, in that case you probably want to manage your own cache of the data.)

Why Qt?

First of all, calm down, you won't need to run X or some other graphical environment. We only use QtCore.

Qt has many objects ready made that work. It seems to make sense to use them. More specifically, it has a QString which supports UTF-8 out of the box. So instead of using iconv and std::string, Qt seems like a good choice to me.

The main objects we use are the QString, QByteArray, QMap, QVector, and QObject. The QByteArray is used to transport binary data. The QMap is used to sort out the list of contexts, tables, rows, cells, and column definitions.

Note that the QByteArray has a great advantage over having your own buffer: it knows how to share its data pointer between multiple instances. Only a write will require a copy of the data buffer.

Many of the classes in the libQtCassandra derive from the QObject class. This means they cannot be copied. Especially, the QCassandra object which includes the socket connection to the Cassandra server (on top of the contexts, tables, rows, cells, column definitions... and since they all have a parent/child scheme!)

In regard to QObject's, we want to make use of the QObject name field to save smaller, QString names in objects. That way you could look for an object in memory with the find<>() algorithm.
The QObject is capable of managing parent/children. We want to make use of that capability for the cluster's contexts, the context's tables, the table's rows, the row's cells. That way the data is managed 100% like a Qt tree which would allow some algorithms to work on the tree without having to know anything of the libQtCassandra library.

The Cassandra System

Terminology Explained

The Cassandra System comes with a terminology that can easily throw off people who are used to more conventional database systems (most of that terminology comes from the Big Table document by Google.)

This library attempts to hide some of the Cassandra terminology by offering objects that seem to be a little closer to what you'd otherwise expect in a database environment.

One Cassandra server instance runs against one cluster. We kept the term cluster as it is the usual term for a set of databases. Writing this in terms of C++ array syntax, the system looks like a multi-layer array as in (you can use this syntax with libQtCassandra, btw):

   cluster[context][table][row][column] = value;
   value = cluster[context][table][row][column];

Note that in Cassandra terms, it would look like this instead:

   cluster[keyspace][column_family][key][column] = value;
   value = cluster[keyspace][column_family][key][column];

One cluster is composed of multiple contexts, what Cassandra calls a keyspace. One context corresponds to one database. A context can be setup to replicate or not and it manages memory caches (it includes many replication and cache parameters.) We call these the context because once a cluster connection is up, you can only have one active context at a time. (If you worked with OpenGL, then this is very similar to the glMakeCurrent() function call.)

Although the libQtCassandra library 100% hides the current context calls since it knows when a context or another needs to be curent, switching between contexts can be costly. Instead you may want to look into using two QCassandra objects each with a different context.

Different contexts are useful in case you want to use one context for statistic data or other data that are not required to be read as quickly as your main data and possibly needs much less replication (i.e. ONE for writes and ALL for reads on a statistic table would work greatly.)

One context is composed of tables, what Cassandra calls a column family. By default, all the tables are expected to get replicated as defined in this context. However, some data may be marked as temporary with a time to live (TTL). Data with a very small TTL is likely to only live in the memory cache and never make it to the disk.

Note that the libQtCassandra library let you create table objects that do not exist in the Cassandra system. These are memory only tables (when you quite they're gone!) These can be used to manage run-time globals via the libQtCassandra system. Obviously, it would most certainly be more effective (faster) to just use globals. However, it can be useful to call a function that usually accesses a Cassandra table, but in that case you dynamically generate said data.

A table is identified by a name. At this time, we only offer QString for table names. Table names must be letters, digits and the underscore. This limitation comes from the fact that it is used to represent a filename. Similarly, it may be limited in length (OS dependent I guess, the Cassandra system does not say from what I've seen. Anyway it should be easy to keep the table names small.)

Tables are composed of rows. Here the scheme somewhat breaks from the usual SQL database as rows are independent from each others. This is because one row may have 10 "columns," and the other may have just 1. Each row is identified by what Cassandra calls a key. The key can either be a string or a binary identifier (i.e. an int64_t for example.)

The name of a row can be typed. In most cases, the default binary type is enough (assuming you save integers in big endians, which is what the libQtCassandra does.) This is important when you want to use a Row Predicate.

Rows are composed of cells. Cassandra calls them columns, but in practice, the name/value pair is just a Cell. Although some tables may define column types and those cells (with the same name) will then be typed and part of that column.

A column is a name and a value pair. It is possible to change the characteristics of a column on how it gets duplicated, cached, and how the column gets compared with a Column Predicate.

The name of a column can be a QString or binary data. It is often a QString as it looks like the name of a variable (var=<value>).

The row and column names are both limited to 64Kb. The value of a column is currently limited to 2Gb, however, you'll need a HUGE amount of memory (~6Gb+) to be able to handle such large values and not only that, you need to do it serially (i.e. one process at a time can send that much data or the memory will quickly exhaust and the processes will fail.) It is strongly advised that you limit your values to Mb instead.

By default, the QCassandra object checks the size with a much small limit (64Mb) to prevent problems. At a later time, we may offer a blob handling that will save large files by breaking them up in small parts saved in the Cassandra database.


In general, the libQtCassandra documentation will tell you how things work in the function you're trying to use. This means you should not really need to know all the details about the Cassandra system up front to start using it in C++ with this library.

For example, the QCassandraTable::dropRow() function works. The row doesn't disappear immediately from the database, but all the cells are. The QCassandraTable::readRows() function will correctly skip the row as long as you include at least one column in your predicate. All of these details are found in the corresponding function documentation.

Actual Cassandra's FAQ:

Changes between versions

  • version 0.4.7

. Added the QCassandraLock object to lock any number of processes using the Cassandra database system and the Lamport bakery algorithm.

. Fixed some documentation.

. Added support for specifying the consistency level of a read via the f_value of a cell.

. Added a synchronization function so we can wait and have all the nodes running with the same schema version.

. Fixed the QCassandra::Snitch() so it returns the snitch, not the protocol version.

. Added a test to verify that locks work when processes run on any number of clients.

. Updated the tests to properly work in a cluster environment.

. Added a unit test for the QCassandraValue class which checks a large number of cases, this allowed me to apply several fixes.

. The <, <=, >, >= QCassandraValue operators were invalid

. Added Bool support in the QCassandraValue object

. Added an index to the QCassandraValue data to read any value in an array or structure; and a size for strings and binaries (although a size of a UTF-8 string...)

. Moved the read of values to global functions so they are accessible from other functions without having to use QCassandraValue variables (only the writes were accessible)

. Fixed the findContext() so it loads the Cassandra contexts first if not yet loaded.

. Fixed the disconnect() function so it clears all the variables and the same Cassandra object can safely be reused.

. Fixed the QCassandraPrivate::getColumnSlice() and QCassandraPrivate::getRowSlices() functions which would not first transform the column key to a QByteArray and thus binary keys with null characters would invariably fail.

. Fixed the QCassandraRow::dropCell() as it would emit a read of the cell before actually dropping it. The read would retrive the consistency value and/or the timestamp. These values are retrieved from memory only now, if defined.

. Fixed the CMakeLists.txt so the thrift library is linked against the libQtCassandra library itself; that way it is not required in the binaries that link against the libQtCassandra library.

. Removed all references to the boost_system library.

. Added some #include of <unistd.h> for Ubuntu 12.10 and over.

. Reviewed the SSL connection capability. It is still not considered to be working but the password can now be specified from your application.

. Included 2013 in copyright notices.

  • version 0.4.6

. Added direct support for QUuid as row and column keys.

. Added direct support for char * and wchar_t * so we do not have to first cast strings to QString everywhere.

. Fixed bug testing row key size to limit of 64535 instead of 65535.

. Added a test as row and column keys cannot be empty. It will now throw an error immediately if so.

. Updated some documentation accordingly and with enhancements.

  • version 0.4.5

. Added a first_char and last_char variables (QChar) in column predicate which can be used to define "[nearly] All column names".

. Fixed the names of two functions: setFinishColumnName() and setFinishColumnKey() are now setEndColumnName() and setEndColumnKey() respectively (as documented and so it matches the getters.)

. Added support for indexes defined with columns. The column predicate now has a setIndex() function and that allows you to call readCells() repititively until all the columns matching the predicate were returned (very similar to reading a large set of rows.)

. Fixed a few things in the documentation.

  • Version 0.4.4

. Added support for composite columns. It was functional before but with knowledge on how to build the column key which is actually quite complicated (okay, not that hard, but libQtCassandra is here to hide that sort of thing!) Use the compositeCell() function of your QCassandraRow objects.

  • Version 0.4.3

. Added support for counters.

. Fixed several usage of keys so 0 bytes works as expected. (in getValue() and insertValue())

. Small fixes to documentation.

  • Version 0.4.2 (see also)

. Fixed the QCassandraTable::readRows() so it automatically updates the row predicate with the last row as the new start key. This is very important because the rows returned to you get sorted by key in the table, whereas, in Cassandra they are not sorted that way at all. (At least not by default when you use the RandomPartitioner which is very likely.)

. Fixed the QCassandraContext::descriptionOption() which would create empty options when the sought option did not exist in the context.

. Upgraded the version of Thrift to 0.8.0. There are some problems with the output of the thrift command line option (some missing #include and invalid references.) I fixed the generated code as required so it compiles and the result works as expected.

. Made updates to the code so it works with version 1.1 of Cassandra. This includes proper support for the replication factor which was deprecated as a direct field in the KsDef structure. The other deprecated fields are simply ignored at this point (those are in Tables, see CfDef in interface/cassandra.thrift of Cassandra 1.1)

. Fixed replicateOnWrite() which now returns the expected value.

. Fixed all the context and table get...() functions so if the value is marked as unset, empty or zero is returned instead of the current value saved in the object (which may not reflect what the database is defined as.)

. Added the million_rows test to ensure we can create over 1 million rows and read them back. At this time, in my environment, it often crashes the Cassandra server... Java problems?

. Added functions that return the partitioner and snitch information from the cluster.

. Fixed QCassandraContext::prepareContextDefinition() which would force the replication factor to 1 instead of the user defined value.

. The CMakeLists.txt now properly defines the folder where the compiled thrift library lies so it can link with it in the standalone version of the library.

  • Version 0.4.1

. Fixed the size of the buffer used to save 64 bit integers.

. Fixed the size of integers used to handle floating points.

. Fixed the double being read as 8 bytes and somehow converted to a float instead of a double.

. Fixed the test of the string set in a value to limit the UTF-8 version of the string to 64Mb (instead of the number of UCS-2 characters held by a QString.)

. Enhanced documentation about the findRow() and findCell() which do not look for a row or cell in the Cassandra system, it only checks in memory!

. Better support older versions of g++ (4.1 cannot properly cast the controlled variables for enumerations) -- thank you to John Griswold for reporting the problem.

. Added some missing documentation.

  • Version 0.4.0

. Enhanced the cmake scripts

. Enhanced documentation of different functions

. Removed all the Qt sub-folders from all the #include

. Removed all unwanted files from the source package using a CPACK variable

. Added some try/catch to avoid a certain number of exceptions from Cassandra

. Fixed the column and row predicates to make use the whole key instead of just the string up to the first null terminator

. Updated the copyright version to include 2012

  • Version 0.3.2

. Fixed the creation of a row predicate as it wasn't defining a column predicate which is necessary when we call readRows() with the default parameters on a table.

  • Version 0.3.1

. Added support for installation targets and generation of binary packages.

  • Version 0.3.0

. Added a dropContext() in the QCassandra object.

. Added proper unparenting of the context and table classes.

. Started to make use of the Controlled Variables (requires 1.3.0 or better.)

libQtCassandra copyright and license

Copyright (c) 2011-2013 Made to Order Software Corp.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.


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