I can't see the difference between an object dictionary object and an application object.
mapping of application objects into a PDO is determined by a
corresponding default PDO mapping structure within the object
dictionary
But we don't map such thing as "application object", but an object of the object dictionary.
In SDO service, the exact same objects are avaiable, but the specification don't use the term application object.
What is the property of an application object which distinguishes it from any other object?
CANopen application layer and communication profile
You can only map items located in the object dictionary. When adapting the protocol stack to your application, you have to arrange this relation between the object dictionary and variables in your code.
The mapping parameters are stored at for example 0x1A00 for TPDO1. From there you can map objects stored elsewhere in the object dictionary, such as for example objects in the manufacturer-specific area 0x2000-0x5FFF or device profile-specific areas from 0x6000 and upwards.
For example if you implement the device profile CiA 401 for general-purpose I/O module, you can make 8 bytes for TPDO1 available in 0x1A00, which in turn points at 0x6000:1 to 0x6000:8, which is where the device profile states that digital on/off should be stored. Or you could map something specific to your product from 0x2000.
If you do something like the above, it means that the network can either access the data indirectly through what's mapped to the PDO, or directly by doing a SDO access at for example 0x6000:1.
Also, consider if you need to support dynamic mapping = the user can change mapping themselves, or static mapping = mapping parameters are read-only, always the same.
Related
I have read lot of D-Bus documentation and understood various concepts such as:
1. Object
2. Interface
3. Method
4. Signal
5. Bus Connection
6. System vs Session Daemon
However while reading through ObjectManager::GetManagedObjects I came across this concept of "Object Tree". I know that I when I invoke the above method on a service object, it gives me all the interfaces in the tree with the service object as the root (Since the service object implements ObjectManager interface from D-Bus).
I would like someone to very clearly explain what does it mean exactly when one object comes "under" a root object in an "object tree"
Are children object just properties of parent object?
Do children object subclass the parent object (Just like OOP)
What is D-Bus Object Tree? Please!
The semantics of the object tree are determined by the particular service which is providing it. By convention, the location of objects in a tree is just determined by their object path. For example, /org/freedesktop/Accounts/User1000 is considered to be a ‘child object’ of /org/freedesktop/Accounts. However, there is no formal relationship beyond this.
To answer your specific questions:
Are children object just properties of parent object? No, they are separate objects with their own set of properties. The only thing they have in common with the parent is a prefix on their object path.
Do children object subclass the parent object (Just like OOP)? No. They may implement the same interfaces, different interfaces, or anything inbetween. Typically, they will implement a different interface — it’s quite common to have a parent object implement a ‘manager’ interface, and the child objects implement an ‘item’ interface, for describing collections of items.
Is there a faster way to search data in JavaScript (specifically on V8 via node.js, but without c/c++ modules) than using the JavaScript Object?
This may be outdated but it suggests a new class is dynamically generated for every single property. Which made me wonder if a binary tree implementation might be faster, however this does not appear to be the case.
The binary tree implementation isn't well balanced so it might get better with balancing (only the first 26 values are roughly balanced by hand.)
Does anyone have an idea on why or how it might be improved? On another note: does the dynamic class notion mean there are actually ~260,000 properties (in the jsperf benchmark test of the second link) and subsequently chains of dynamic class definitions held in memory?
V8 uses the concepts of 'maps', which describe the layout of the data in an object.
These maps can be "fast maps" which specify a fixed offset from the start of the object at which a particular property can be found, or they can be "dictionary map", which use a hashtable to provide a lookup mechanism.
Each object has a pointer to the map that describes it.
Generally, objects start off with a fast map. When a property is added to an object with a fast map, the map is transitioned to a new one which describes the location of the new property within the object. The object is re-allocated with enough space for the new data item if necessary, and the object's map pointer is set to the new map.
The old map keeps a record of the transitions from it, including a pointer to the new map and a description of the property whose addition caused the map transition.
If another object which has the old map gets the same property added (which is very common, since objects of the same type tend to get used in the same way), that object will just use the new map - V8 doesn't create a new map in this case.
However, once the number of properties goes over a certain theshold (in fact, the current metric is to do with the storage space used, not the actual number of properties), the object is changed to use a dictionary map. At this point the object is re-written using a hashtable. In general, it won't undergo any more map transitions - any more properties that are added will just go in the hashtable.
Fast maps allow V8 to generate optimized code (using Crankshaft) where the offset of a property within an object is hard-coded into the machine code. This makes it very fast for cases where it can do this - it avoids the need for doing any lookup.
Obviously, the generated machine code is then dependent on the map - if the object's data layout changes, the code has to be discarded and re-optimized when necessary. V8 has a type profiling mechanism which collects information about what the types of various objects are during execution of unoptimized code. It doesn't trigger optimization of the code until certain stability constraints are met - one of these is that the maps of objects used in the function aren't changing frequently.
Here's a more detailed description of how this stuff works.
Here's a video where one of the lead developers of V8 describes stuff like map transitions and lots more.
For your particular test case, I would think that it goes through a few hundred map transitions while properties are being added in the preparation loop, then it will eventually transition to a dictionary based object. It certainly won't go through 260,000 of them.
Regarding your question about binary trees: a properly sized hashtable (with a sensible hash function and a significant number of objects in it) will always outperform a binary tree for a use-case where you're just searching, as your test code seems to do (all of the insertion is done in the setup phase).
I know where all we can define the chef attributes, attribute types and also their precendence levels. I just want to understand how they are stored internally.
Suppose I declare an attribute
default[:app][:install] = "/etc/app"
1) How is it stored internally? Is it using in a tree structure(heirearchy) in the node object or is it as hashmaps or a list of variables in the node object?
2) Also, in most of the cookbooks I see that attributes are declared in 2 or 3 levels something as above I dont understand if it is a standard or is it a best practice? Are there any guidelines for the way the attributes have to be declared? Is it something to do with its internal storage. Can't I declare the attribute as
default[:appinstall]= "/etc/app"
and access it as below in my recipe?
node[:appinstall]
Just four Mashes (subclass of Hash which does the string vs. symbol key fixups). When you access the merged view via node['foo'] it uses a Chef::Node::Attribute object to traverse all four in parallel until it finds a leaf value.
What you have shown is correct for setting and using attributes, though string keys are preferred over symbols. You should also in general scope your attributes with the name of the cookbook like:
default['mycookbook']['appinstall'] = '/etc/app'
This will reduce the chances of collisions with other cookbooks.
I recently read in "Professional C# 4 and .NET 4" that:
You can never instantiate an interface.
But periodically I see things like this:
IQuadrilateral myQuad;
What are the limitations in using interfaces directly (without having a class inherit from the interface)? How could I use such objects (if they can even be called objects)?
For example instead of using a Square class that derives from IQuadrilateral, to what extent could I get away with creating an interface like IQuadrilateral myQuad?
Since interfaces don't implement methods, I don't think I could use any methods with them. I thought interfaces didn't have fields to them (only properties), so I'm not sure how I could store data with them.
The answer is simple, you can't instantiate an interface.
The example you provided is not an example of instantiating an interface, you are just defining a local variable of the type IQuadrilateral
To instantiate the interface, you would have to do this:
IQuadrilateral myQuad = new IQuadrilateral();
And that isn't possible since IQuadrilateral does not have a constructor.
This is perfectly valid:
IQuadrilateral myQuad = new Square();
But you aren't initiating IQuadrilateral, you are initiating Square and assigning it to a variable with the type IQuadrilateral.
The methods available in myQuad would be the methods defined in the interface, but the implementation would be based on the implementation in Square. And any additional methods in Square that are not part of the IQuadrilateral interface would not be available unless you cast myQuad to a Square variable.
You can't create an instance of an interface.
The code you showed defines a variable of type IQuadrilateral. The actual instance this variable points to will always be of a concrete class implementing this interface.
Background Knowledge
Think of an interface as a contract. In a contract between two people, it defines what is capable, what is expected from the parties involved. In programming, it works the same way. The interface defines what to expect, what must exist for you to conform to that interface. Therefore, since it only defines what to expect, it itself, doesn't provide the implementation, the "code under the covers" so to speak, does.
A property behaves like a field, but allows you to intercept when someone assigns a value to it or reads the value. You can also deny reading or writing to it, your choice when you define the property. Interfaces work with properties instead of fields because of this. Since the "contract" is just defining what property should be there (name and type), and if it should allow a read or write capabilities, it leaves it up to the implementer to provide this.
Take for example the IEnumerator interface from the .NET framework. This interface was designed to allow iteration over a collection of objects. The purpose is not to change items, or randomly access them, it's just for getting object A and moving to the next, and the next, and the next, as many times as needed. Many collection type classes implement this: Queue, ArrayList, SortedList, Stack, etc. All these types of objects store many objects and now they all share the common "contract": the ability to iterate one-by-one over them.
However, you can see that the IEnumerator interface has a MoveNext() method declared. Why? This is because the items may not be served in the same manner. For example, people will generally access the ArrayList from the first item to the last. But a Stack was designed opposite, for people to access the last object down to the first.
Questions Answered
With all this knowledge, the limitation of declaring a variable as the interface type as opposed to the class type that implemented the interface is that you only get access to what the interface (the contract) says should be there. The benefit though is that you can assign to this variable any class type that implements the interface.
I have just started reading DDD. I am unable to completely grasp the concept of Entity vs Value objects.. Can someone please explain the problems (maintainability, performance.. etc) a system could face when a Value object is designed as a Entity object? Example would be great...
Reduced to the essential distinction, identity matters for entities, but does not matter for value objects. For example, someone's Name is a value object. A Customer entity might be composed of a customer Name (value object), List<Order> OrderHistory (List of entities), and perhaps a default Address (typically a value object). The Customer Entity would have an ID, and each order would have an ID, but a Name should not; generally, within the object model anyway, the identity of an Address probably does not matter.
Value objects can typically be represented as immutable objects; changing one property of a value object essentially destroys the old object and creates a new one, because you're not as concerned with identity as with content. Properly, the Equals instance method on Name would return "true" as long as the object's properties are identical to the properties of another instance.
However, changing some attribute of an entity like Customer doesn't destroy the customer; a Customer entity is typically mutable. The identity remains the same (at least once the object has been persisted).
You probably create value objects without realizing it; anytime you are representing some aspect of an Entity by creating a fine-grained class, you've got a value object. For example, a class IPAddress, which has some constraints on valid values but is composed of simpler datatypes, would be a value object. An EmailAddress could be a string, or it could be a value object with its own set of behaviors.
It's quite possible that even items that have an identity in your database don't have an identity in your object model. But the simplest case is a composite of some attributes that make sense together. You probably don't want to have Customer.FirstName, Customer.LastName, Customer.MiddleInitial and Customer.Title when you can compose those together as Customer.Name; they'll probably be multiple fields in your database by the time you think about persistence, but your object model doesn't care.
Any object that is collectively defined by all of it attributes is a value object. If any of the attributes change you have a new instance of a value object. This is why value objects are defined as immutable.
If the object is not fully defined by all of its attributes then there are a subset of attributes that make up the identity of the object. The remaining attributes can change without redefining the object. This kind of object cannot be defined at immutable.
A simpler way of making the distinction is to think of value objects as static data that will never change and entities as data that evolves in your application.
Value Types :
Value types do not exist on his own, depends on Entity types.
Value Type object belongs to an Entity Type Object.
The lifespan of a value type instance is bounded by the lifespan of the owning entity instance.
Three Value types: Basic(primitive datatypes), Composite(Address) and Collection(Map, List, Arrays)
Entities:
Entity types can exist on his own (Identity)
An entity has its own life-cycle. It may exist independently of any other entity.
For example: Person, Organisation, College, Mobile, Home etc.. every object has its own identity
I don't know if the following is correct, but I would say that in the case of an Address object, we want to use it as a Value Object instead of an Entity because changes to the entity would be reflected on all linked objects (a Person for instance).
Take this case: You are living in your house with some other people. If we would use Entity for Address, I would argue that there would be one unique Address that all Person objects link to. If one person moves out, you want to update his address. If you would update the properties of the Address Entity, all people would have a different address. In the case of a Value Object, we would not be able to edit the Address (since it is immutable) and we would be forced to provide a new Address for that Person.
Does this sound right? I must say that I was/am also still confused about this difference, after reading the DDD book.
Going one step further, how would this be modelled in the database? Would you have all properties of the Address object as columns in the Person table or would you create a separate Address table that would also have a unique identifier? In the latter case, the people living in the same house would each have a different instance of an Address object, but those objects would be the same except for their ID property.
address can be entity or value object that depends on the busiess process. address object can be entity in courier service application but address can be value object in some other application. in courier application identity matters for address object
3 distinction between Entities and Value Objects
Identifier vs structural equality:
Entities have identifier,entities are the same if they have the same
identifier.
Value Objects on beyond the hand have structural equality, we consider two
value objects equal when all the fields are the same. Value objects cannot
have identifier.
Mutability vs immutability:
Value Objects are immutable data structures whereas entities change during
their life time.
Lifespan: Value Objects Should belong to Entities
In a very simple sentence I can say, we have three types of equality:
Identifier equality: a class has id filed and two objects are compared with their id field value.
Reference equality: if a reference to two objects has a same address in memory.
Structural equality: two objects are equal if all members of them are matched.
Identifier equality refers only to Entity and structural equality refers to Value Object only. In fact Value Objects do not have id and we can use them interchangeably. also value objects must be immutable and entities can be mutable and value objects will not have nay table in database.
I asked about this in another thread and I think I'm still confused. I may be confusing performance considerations with data modelling. In our Cataloging application, a Customer doesn't change until it needs to. That sounds dumb - but the 'reads' of customer data far outnumber the 'writes' and since many many web requests are all hitting on the 'active set' of objects, I don't want to keep loading Customers time and again. So I was headed down an immutable road for the Customer object - load it, cache it, and serve up the same one to the 99% of (multi-threaded) requests that want to see the Customer. Then, when a customer changes something, get an 'editor' to make a new Customer and invalidate the old one.
My concern is if many threads see the same customer object and it is mutable, then when one thread starts to change it mayhem ensues in the others.
My problems now are, 1) is this reasonable, and 2) how best to do this without duplicating a lot of code about the properties.
Consider the following examples from Wikipedia, in order to better understand the difference between Value Objects and Entities:
Value Object: When people exchange dollar bills, they generally do not
distinguish between each unique bill; they only are concerned about the face
value of the dollar bill. In this context, dollar bills are Value Objects. However,
the Federal Reserve may be concerned about each unique bill; in this context each
bill would be an entity.
Entity: Most airlines distinguish each seat uniquely on every flight. Each seat is
an entity in this context. However, Southwest Airlines, EasyJet and Ryanair do
not distinguish between every seat; all seats are the same. In this context, a seat is
actually a Value Object.