In a typical client server model, what does it mean to subscribe or unsusbscribe to a feed? Is there a generic codebase or boilerplate model or set of standard procedures or class design and functionalities involved? This is all C++ based. There's no other info other than the client is attempting to connect to the server to retrieve data based on some sort of signature. I know it's somewhat vague, but I guess this is really a question of what are things to keep in mind and what a typical subscribe or unsubscribe method might entail. Maybe something along the lines of extending a client server model like http://www.linuxhowtos.org/C_C++/socket.htm.
This is primarily an information architecture question. "Subscribing to feeds" implies that the server offers a lot of information, which may not be uniformly relevant to all clients. Feeds are a mechanism by which clients can select relevant information.
Concretely, you first need to identify the atoms of information that you have. What are the smallest chunks of data ? What properties to they have? Can new atoms replace older atoms, and if so, what identifies their relation? Are there other atom relations besides replacement?
Next, there's the mapping of those atoms to particular feeds. What are the possible combinations of atoms needed by a client? How can these combinations be bundled in two ore more feeds? It is possible to map each atom uniquely to a single feed? Or must atoms be shared between feeds? If so, is that rare enough that you can ignore it and just send duplicates?
If a client connects, how do you figure out which atoms need to be shared? Is it just live streaming (atoms are sent only when they're generated on the server), do you have a set of current atoms (sent when a client connects), or do you need some history as well? Is there client caching?
It's clear that you can't have a single off-the-shelf solution when the business side is so diverse.
Related
According to the document (https://arrow.apache.org/docs/dev/format/Flight.html), an Apache Arrow Flight client cannot get sorted data from multiple endpoints. It seems that this is by design.
In the introduction document (https://arrow.apache.org/blog/2019/10/13/introducing-arrow-flight/), they say "While Flight streams are not necessarily ordered, we provide for application-defined metadata which can be used to serialize ordering information.". But I think the application-defined metadata is not very useful since a general client (like a BI application) that uses a wrapper - for example, Apache Arrow Flight SQL, let alone a wrapper of wrapper: Apache Arrow Flight SQL JDBC driver - does not know it.
Is there any standard way to get sorted data from multiple Apache Arrow Flight endpoints? If not, why did the designers choose not to support that feature?
Thanks.
It was not considered at the time, but you are right: it would be useful to have a way to indicate this so that various wrappers and projects building on top have a standardized way to know how to handle this.
The main idea is that if data is sorted, you should return a single endpoint. I believe the reasoning was that it would be rare to have an implementation capable of doing sorting across multiple endpoints, since that would be expensive to implement. Of course, that isn't very useful if your backend can actually sort data across multiple workers!
I (as one of the contributors to the project) am planning to put up a proposal to handle this case. If you are interested, please keep a watch on the mailing list: dev#arrow.apache.org.
I did a search on the board and there were some threads related to what I will ask but the other questions were not exactly like my situation.
I want to implement a service (ejbs) and different clients (rest api, webservice, jsf managed beans and maybe some other client) are going to use this service. My question is: in this scenario, where should data validation occur?
Seems reasonable to me to do it inside my business control (ejbs),- since I don't want to implement one validator type for each client- but I don't see people doing it...
best regards,
Oliver
The general advice would be: Every component, which exposes functionality to the outside should validate the input it receives. It should not hope for the best that it will in all cases receive valid input. Additionally, as you said, it keeps the validation at one place.
On the other hand it may be a reasonable decision when you have both sides under your control to decide for an early validation on the client and document the expected/required valid input data.
You have a similar problem when designing a relational database structure - you can have all sorts of constraints to ensure valid input data or you can check validity in the component storing the data in the database.
And, not to forget, whenever you validate in a deeper layer, all higher layers have to handle the exceptions or error messages when validation fails.
Regarding your specific question, the usage of the same service from different clients advises to validate within the service.
I'm looking for a possiblity to monitor all messages in a SOA enviroment with an intermediary, who'll be designed to enforce different rule-sets over the message's structure and sequences (e.g., let's say it'll check and ensure that Service A has to be consumed before B).
Obviously the first idea that came to mind is how WS-Adressing might help here, but I'm not sure if it does, as I don't really see any mechanism there to ensure that a message will get delivered via a given intermediary (as it is in WS-Routing, which is an outdated proprietary protocol by Microsoft).
Or maybe there's even a different approach that the monitor wouldn't be part of the route but would be notified on request/responses, which might it then again make somehow harder to actively enforce rules.
I'm looking forward to any suggestions.
You can implement a "service firewall" either by intercepting all the calls in each service as part of your basic servicehost. Alternatively you can use 3rd party solutions and route all your service calls to them (they will do the intercepting and then forward calls to your services).
You can use ESBs to do the routing (and intercepting) or you can use dedicated solutions like IBM's datapower, XML firewall from Layer7 etc.
For all my (technical) services I use messaging and the command processor pattern, which I describe here, without actually calling the pattern name though. I send a message and the framework finds to corresponding class that implements the interface that corresponds to my message. I can create multiple classes that can handle my message, or a single class that handles a multitude of messages. In the article these are classes implementing the IHandleMessages interface.
Either way, as long as I can create multiple classes implementing this interface, and they are all called, I can easily add auditing without adding this logic to my business logic or anything. Just add an additional implementation for every single message, or enhance the framework so it also accepts IHandleMessages implementations. That class can than audit every single message and store all of them centrally.
After doing that, you can find out more information about the messages and the flow. For example, if you put into the header information of your WCF/MSMQ message where it came from and perhaps some unique identifier for that single message, you can track the flow over various components.
NServiceBus also has this functionality for auditing and the team is working on additional tooling for this, called ServiceInsight.
Hope this helps.
Is there a way to track per user data with websocket++ 0.3X?
I want to be able to identify them so to keep track of what they're looking at and what should be sent to them. Take stack as an example: while you're looking at this question, a websocket could (and I think does) keep in memory that you're looking at this question and send you the appropriate updates like votes, new comments & answers, and StackExchange updates in the upper left corner.
Also, the users need to be able to identified. Is there a session id inherent in websockets that are already hiding in websocket++? If not, how does websocket++ track users?
The simplest way is, as you mentioned in your answer, to use connection_hdl as a key for an associative container that stores any other data you need. WebSocket++ does have some other options for cases where that sort of lookup would be too costly.
Since this is a common question, I've written up some more formal examples & documentation on how to do this here: http://www.zaphoyd.com/websocketpp/manual/common-patterns/storing-connection-specificsession-information.
Ah, OK. It was hiding in plain sight all along. http://www.zaphoyd.com/websocketpp/manual/building-program-websocket
The connection handle is a token that uniquely identifies the
connection that received the message. It can be used to identify where
to send reply messages or stored and used to push messages later. The
type of the connection handle is websocketpp::connection_hdl.
I need to document an application that serves as a facade for a set of webservices. The application accepts SOAP requests and transforms these requests into a format understandable by the underlying web service. There are several such services, each with its own interface. Some accept SOAP, some HTTP POST, some... other formats not mentioned in polite society.
I need to document how we map the fields from our SOAP calls to the fields for these other formats. Before everyone cries "XSLT" I must mention that the notation must be human-friendly. Ideally it would be something Excel-able.
Has anyone encountered this sort of problems before? How did you solve it? Is there a human-friendly notation for tree-to-tree transformations that can fit on a spreadsheet?
I've had to do just this. The way I did it was to just start writing, following the hierarchical structure.
I eventually would find that I was repeating myself. An example was that certain elements had a common set of attributes. I would pull the documentation of that common set up before the sections on the specific elements. Same thing with documentation of handling of specific simpleTypes.
Eventually, there was even some high level discussion on the overall flow and "philosophy" of the transformation. But I let it all happen bit by bit, fixing it as I became bored with repetition.
That said, I'm a developer, not a tech writer.
I haven't really found anything so far, but I've found pointers to many libraries that help transform objects of one type to another in Java. For reference, I'm listing the most promising ones here, all doing some kind of JavaBean to JavaBean conversion:
Transmorph
EZMorph
Dozer