I am trying to understand Stuart Sierra's component, specifically the naming convention for the components in order to structure a Clojure application.
If I look into system for instance, I see several components mapped to the :server key :
aleph
immutant
Since both use the same key :server, does that mean that I can only use one of them if I use this library ?
Similarly, I use onyx. Several components are already defined inside onyx system.clj.
Does that mean that some keys are effectively reserved by onyx ?
What will happen to the :port parameter, which seems to be used by many components in the wild ?
Questions
What is get the difference between the keys used when associng in the start method and the keys used in component/system-map ?
Is there a naming convention for those keys, how do we avoid collisions between those keys ?
In which cases (if any) does it make sense to have several systems and can they run at the same time ?
Keys in the system map identify specific components (instances) in that system. You can use whatever key you like for whatever component you need.
Keys in a specific component record can be one of three things:
a configuration value set up at creation time
some internal value that is irrelevant to the user of the component
a dependency (which will refer to another component when the system is started)
1 and 2 are generally set up by the component constructor and users do not need care about the actual key used in the record.
Dependencies are configured by setting a mapping on the depending component from the dependency key (3) to the key in the system map referring to the dependancy component. This is done with the component/using function and giving it a map of component keys to system-map keys as the second argument. That way you can always map any expected key to any actually used key. You can use the short-hand form of component/using with a vector of keys, but only if the keys in the system-map are the same as the keys in the component you're configuring.
I hope that answers the first two questions
The third question I think I'd like to see an example of what you're looking for as a separate post
The last question: yes you can have multiple systems running at the same time. That may or may not make sense depending on what you want to do, but running a test system as well as a development system seems like a fairly obvious setup.
Related
I am trying to figure out whether an edge/lane is internal. When SuMO creates internal edges/lanes, it prefixes them with a colon [1]. Currently, I am exploiting this information, however, it seems that you also can annotate arbitrary other edges as internal using the tag function. This is also set for internal edges created by SuMO [1]. Therefore, I want to retrieve the information via TraCI.
To my knowledge, there is no TraCI command to retrieve this information (i.e. either the value of function or whether the edge/lane is internal).
The classes MSEdge and MSLane in the microsim directory have methods to retrieve both of those values, however, the classes Edge and Lane from libsumo do not.
I also checked whether the value of the function tag might get added to the parameter map during initialization, which I could access via TraCI's getParameter. This also does not seem to be the case. I checked some files from the netimport directory but could not find anything satisfactory.
Is there any other way to retrieve the function/isInternal information via TraCI without adding a new TraCI command (and the aforementioned missing methods in libsumo)?
This is a static property of the network so the easiest way of retrieving the information is to parse the network. In Python you can use sumolib for that:
import sumolib
net = sumolib.net.readNet("my.net.xml")
function = {}
for e in net.getEdges():
function[e.getID()] = e.getFunction()
There is currently no TraCI call for that but the colon thing is a very good indicator. The main developers are also a bit reluctant to add all static information retrieval to the TraCI API in order not to overload it.
There's a pattern I haven't figured out for Component yet:
I have some "live" configuration that requires disk IO (a component) on system-start, and has a dependency on a map of static config (.edn), and after this "live" configuration is instantiated, it won't change or side-effect anything anymore.
For ex: I need to set this up once, and it depends on static config:
(buddy.core.backends/jws
{:secret (buddy.core.keys/public-key
path-to-public-key-from-static-config)})
I would then reuse that backend ad-infinitum, (ex: in buddy.auth.middleware/wrap-authentication), and it doesn't change, nor side-effect.
Possible Solutions
I could make a component that stores this backend at system-start. But this gives up generality, because when I want to add similar "live config", it would have to be explicitly written into the component, and that gives up the spirit of generality that I think Component champions (eg Duct says components define side-effects, and create boundaries to access them)
I could pass a generic component a map of keys - [fn & args] and the fn+args get evaluated and stored in the component. But this feels like it offloads computation to my configuration .edn, and is an anti-pattern. For example:
(private-key priv-path-from-static
(slurp :password-path-from-static))
Should I encode the notion of slurping in my .edn config? I don't want to offload computation to a config file...
The backend and keys can be instantiated on a per-need basis within each component that requires them. IMO, that's too much of computing the exact same thing, when I'd rather it be stored in memory once.
I could have an atom component that holds a map of these "live" config objects, but then they get destructively added in, and my code has lost it's declarative nature.
TL;DR
What's the best way to create configuration at system-start, possibly needing dependencies, and then becoming available to other components as a component, while not giving up the generality which components should have?
In an ideal world, I think the component itself should describe what type of configuration data it needs. (This could be done with a protocol extending the component in question.). When the config component is started, it should look at all other components, get the list of config requirements and resolve it somehow (from a file, a database, etc.).
I've not seen such a library, but Aviso's config library comes close, maybe you can adapt it to your needs.
I need a way for ember router to route to a recursive path.
For Example:
/:module
/:module/:submodule
/:module/:submodule/:submodule
/:module/:submodule/:submodule/...
Can this be done with Embers router, and if so, how?
I've been looking for examples, tearing apart the source, and I've pretty much come to the conclusion, it's not possible.
In a previous question, someone had pointed me to a way to get the url manually and split it, but I'm stuck at creating the state for the router to resolve to.
As of now, in my project, I currently just use the Ember.HashLocation to setup my own state manager.
The reason for the need of this, is because the module definitions are stored in a database, and at any given point a submodule of a submodule, recursively, could be added. So I'm trying to make the Application Engine handle the change.
Do your submodules in the database not have unique IDs? It seems to me that rather than representing your hierarchy in the path, you should just go straight to the appropriate module or submodule. Of course the hierarchy is still in your data model, but it shouldn't have to be represented in your routing scheme. Just use:
/module/:moduleId
/submodule/:submoduleId
And don't encode the hierarchy in the routes. I understand it might be natural to do so, but there's probably not a technical reason to.
If your submodules don't have unique ids, it's maybe a little tougher...you could build a unique ID by concatenating the ancestor ids together (say, with underscores), which is similar to splitting the URL, but a little cleaner probably. I will say that Ember/Ember Data doesn't seem to be too easy to use with entities with composite keys--if everything has a simple numeric key everything becomes easier (anyone want to argue with me on this, please explain to me how!).
DO you mean like this:
App.Router.map(function(match) {
match('/posts').to('blogPosts');
match('/posts/:blog_post_id').to('showBlogPost');
});
I've got a quick question regarding the use of repositories. But the best way to ask is to show a bit of pseudocode and you guys tell me what the result should be
Get a record from the repository with ID of 1 (assume it exists)
Edit a couple of properties
Query the repository again for an item with ID of 1
Result = ??
Should I get the object with updated values or the object without (original state), bearing in mind that since updating the values of properties (step 2) I have not told the repository to update this record.
I think I should get a copy of the original item and not a reference to the edited version.
Please tell me what is correct.
Cheers
The repository pattern is suppose to act like a collection of your objects, so ideally I think it should return the same object instance which would have the updates in it.
Generally there is an identity map somewhere so your repositories can keep track of what has already been loaded. With an identity map, when you fetch an object with the same Id you should always get the already loaded object back regardless of how many times. This is how all more sophisticated ORMs work and is generally a good practice. An identity map helps keep things in sync while you are in the same transaction and saves you some data access.
NHibernate's session has an identity map it keeps track of so you don't have to worry about trying to implement your own in your repositories. Also I believe you can use NHibernate's stateless session if you want to load another instance without change tracking, but I'm not positive on that.
Judging from your past questions I'm assuming you are using LINQ/C#?
If you are using a DataContext and you haven't called SubmitChanges() then you should get back the original unchanged object.
Just tested it. I was wrong, you get back the changed object.
If you set ObjectTrackingEnabled = false on the DataContext you will get the unchanged object.
This is one of those little detail (and possibly religious) questions. Let's assume we're constructing a REST architecture, and for definiteness lets assume the service needs three parameters, x, y, and z. Reading the various works about REST, it would seem that this should be expressed as a URI like
http://myservice.example.com/service/ x / y / z
Having written a lot of CGIs in the past, it seems about as natural to express this
http://myservice.example.com/service?x=val,y=val,z=val
Is there any particular reason to prefer the all-slashes form?
The reason is small but here it is.
Cool URI's Don't Change.
The http://myservice.example.com/resource/x/y/z/ form makes a claim in front of God and everybody that this is the path to a specific resource.
Note that I changed the name. There may be a service involved, but the REST principle is that you're describing a specific web resource, named /x/y/z/.
The http://myservice.example.com/service?x=val,y=val,z=val form doesn't make as strong a claim. It says there's a piece of code named service that will try to do some sort of query. No guarantees.
Query parameters are rarely "cool". Take a look at the Google Chart API. Should that use a /full/path/notation for all of the fields? Would each URL be cool if it did?
Query parameters are useful. Optional fields can be omitted. New keys can be added to support new functionality. Over time, old fields can be deprecated and removed. Doing this is clumsier with a /path/notation .
Quoting from http://www.xml.com/pub/a/2004/08/11/rest.html
URI Opacity [BP]
The creator of a URI decides the encoding
of the URI, and users should not derive
metadata from the URI itself. URI opacity
only applies to the path of a URI. The
query string and fragment have special
meaning that can be understood by users.
There must be a shared vocabulary between
a service and its consumers.
This sounds like query strings are what you want.
One downside to query strings is that the are unordered. The GET ending with "?x=1&y=2" is different than that ending with "?y=2&x=1". This means the browser and any other intermediate systems won't be able to cache it, because caching is done based on the full URL. If this is a concern, then generate the query string in a well-defined order.
While constructing URIs this is the priniciple I follow. I don't know whether it is perfectly acceptable in all cases
Say for instance, that I have to get the details of an employee, then the URI will be of the form:
GET /employees/1/ and not GET /employees?id=1 since I treat every employee as a resource and the whole URI "employees/{id}" is used in identification of the resource.
On the other hand, if I have algorithmic operations that do not identify a specific resource as such,but merely require inputs to the algorithm which in turn identify the resource, then I use query strings.
For instance GET /employees?empname='%Bob%'&maxResults=100 might give me all employees whose names have the word Bob in them, with the maximum results returned by the query limited to 100.
Hope this answers your question
URIs are strictly split into a hierarchical part (the path) and a non-hierarchical path (the query), and both serve to identify the resource
Tthe URI spec itself (RFC 3986) clearly sets the path and the query portion of a URI as equal.
Section 3.3:
The path component contains data [...] that along with [the] query component
serves to identify a resource.
Section 3.4:
The query component contains [...] data that, along with
[...] the path component serves to identify a resource
So your choice in using x/y/z versus x=val&y=val&z=val has mainly to do if x, y or z are hierarchical in nature or if they're non-hierarchical, and if you can perceive them as always being hierarchical or non-hierarchical for the foreseeable future, along with any technical limitations you might be having on selecting one over the other.
But to answer your question, as others have noted: Neither is more RESTful than the other, since they both end up identifying a resource.
If the resource is the service, independent of parameters, it should be
http://myservice.example.com/service?x=val&y=val&z=val
This is a GET query. One of the principles behind REST is that you GET to read (but not modify!) the resource; you can POST to modify a resource & get a response; you can PUT to write to a resource; and you can DELETE to remove a resource.
If the resource specific with those parameters is a persistent resource, it needs a name. You could (if you organized your webservice this way) POST to http://myservice.example.com/service?x=val&y=val&z=val to create a particular instance of the service and have it return an ID to name this instance, e.g.
http://myservice.example.com/service/12312549
then use GET/POST/PUT/DELETE to interact with that instance.
First of all, defining URIs as part of your API violates a constraint of the REST architecture. You cannot do that and call your API RESTful.
Secondly, the reason query parameters are bad for non-query resource access is that they are generally not cached. It is also a violation of HTTP standards.
A URL with slashes like /x/y/z/ would impose a hierarchy and is not suited for the exact case of just passing three parameters.
If, like you said, x y z are indeed just parameters and the order is not important, it would be more RESTful to use semicolons:
http://myservice.example.com/service/x;y;z/
If your "service" however is just an algorithm that works the same with different parameters, there would also be nothing unRESTful with using ?x=val format.