Cassandra is one of the most interesting NoSQL databases which resolves plenty of complex problems with extremely simple solutions. They are not the easiest options, but can be deduced from this db foundations.
Cassandra uses Sorted String Tables as its store for rows values. When queried, it simply finds the value offset with the index file and searched the data file for this offset. New files are flushed once in a while to disc and a new memory representation of SST is started again. The files, once stored on disc are no longer modified (the compactation is another scenario). How would you backup them? Here comes the simplicity and elegance of this solution. Cassandra stores hard links to each SST flushed from memory in a special directory. Hard links preserves removing of a file system inodes, allowing to backup your data to another media. Once once backup them, they can be removed and it’d be the file system responsibility to count whether it was the last hard link and all the inodes can be set free. Having your data written once into not modified files gives you this power and provides great simplicity. That’s one of the reasons I like Cassandra’s design so much.
The docs for Cassandra backups are available here.
The previous post contained an information about lazy loading of group of properties, let’s call them families as it is called in the Cassandra. What about the following code. How many db hits you’d like to get by default?
using (var s = sessionFactory.Open())
var user = s.Load<IUser>(5);
foreach(var post in user.Posts)
I’ll tell you how many you’ll get. The answer is two: first hit will occur, when a collection of posts is accessed in the foreach loop, the second – when a title is printed on the console. During the second hit all the posts loaded in the session will have their titles loaded. In some cases it may drive to a small overhead, but it simplifies batching and working with your entities in the majority of cases. Would anyone like to set FetchMode, like it was done in the NHibernate?
As i it is stated in the official documentation, the column family can be compared to a table in the relational database and as with table, the main target of creating one is to hold the same type of objects together to create a better model, allow querying, etc. Speaking about Cassandra it has one more advantage: the whole column family is stored on one server (the data are consistently hashed by key), so you may consider a column family as a collection of items frequently used together, for instance: the surname and the name, or the user name and the password.
In NHibernate you can embed those values within component to make it look like a whole, but they’re still stored in the same table. Speaking about Deiphobus, it can be easily configured to match the needs of grouping properties (simple properties, and referencing other entities in many-to-one or one-to-one way) by implementing a convention interface:
public interface IPropertyFamilyConvention
/// Gets the Cassandra family name.
/// <param name="mappedType">The mapped type.</param>
/// <param name="propertyInfo">The info of a mapped property.</param>
/// <returns>The family name.</returns>
FamilyName GetFamilyName(Type mappedType, PropertyInfo propertyInfo);
It’s worth to mention, that by default all the properties are mapped to one column family called ‘Entity’. Ok, you know how it influences the storage from the Cassandra point of view, but what about Deiphobus? As it was implemented, every time you access previously not loaded property of an entity mapped with Deiphobus, the whole column family, containing the specified property is loaded from the database (actually a bit more data is retrieved, but for the sake of simplicity it can be omitted in here). It means, that once you started using a property which is strongly connected with others, all the needed properties will be loaded in one db hit. Simple and powerful, isn’t it?