In React, we can use the useMutation or useQuery inside components. But let's say we want to run the query or the mutation inside a helper file (Let's say we extract the part where we format the data and execute the mutation to a helper function away from the component function). In here, we have two options:
Pass the mutation function obtained from useMutation to the helper function
Call the mutation directly inside the helper function like apolloClient.mutate
What is the most recommended way of doing things and what do you recommend?
The hooks expose additional component state for the returned data, loading state and error state. This is really just a convenience because it means you don't have to call useState yourself. As such, it's perfectly fine to use client.mutate if you don't need to keep track of those states. In a sense, it may be better since you're not needlessly using memory for variables you won't use anyway.
The same could be said for useQuery, which really just uses client.watchQuery under the hood and saves you from having to use useState and useEffect.
We can make some requests to the server using both Query and Mutation. In these queries we can pass some params and we will get some results from the server in both cases. The only one obligatory difference is that we can call the mutation from our props like "this.props.mutation", but it looks like a syntax sugar, because we can wrap our HOC in "withApollo" and we'll receive "query" method in props too. So what is the main difference between these two types of requests?
Strictly speaking there is no difference.
... technically any query could be implemented to cause a data write.
However, it's useful to establish a convention that any operations
that cause writes should be sent explicitly via a mutation.
However, the reference implementation does enforce the following.
While query fields are executed in parallel, mutation fields run in
series, one after the other.
This means that if we send two incrementCredits mutations in one
request, the first is guaranteed to finish before the second begins,
ensuring that we don't end up with a race condition with ourselves.
Both quotes can be found from the links below.
http://graphql.org/learn/queries/#mutations
http://graphql.org/learn/queries/#multiple-fields-in-mutations
I'm looking for guidelines to using a consistent value of the current date and time throughout a transaction.
By transaction I loosely mean an application service method, such methods usually execute a single SQL transaction, at least in my applications.
Ambient Context
One approach described in answers to this question is to put the current date in an ambient context, e.g. DateTimeProvider, and use that instead of DateTime.UtcNow everywhere.
However the purpose of this approach is only to make the design unit-testable, whereas I also want to prevent errors caused by unnecessary multiple querying into DateTime.UtcNow, an example of which is this:
// In an entity constructor:
this.CreatedAt = DateTime.UtcNow;
this.ModifiedAt = DateTime.UtcNow;
This code creates an entity with slightly differing created and modified dates, whereas one expects these properties to be equal right after the entity was created.
Also, an ambient context is difficult to implement correctly in a web application, so I've come up with an alternative approach:
Method Injection + DeterministicTimeProvider
The DeterministicTimeProvider class is registered as an "instance per lifetime scope" AKA "instance per HTTP request in a web app" dependency.
It is constructor-injected to an application service and passed into constructors and methods of entities.
The IDateTimeProvider.UtcNow method is used instead of the usual DateTime.UtcNow / DateTimeOffset.UtcNow everywhere to get the current date and time.
Here is the implementation:
/// <summary>
/// Provides the current date and time.
/// The provided value is fixed when it is requested for the first time.
/// </summary>
public class DeterministicTimeProvider: IDateTimeProvider
{
private readonly Lazy<DateTimeOffset> _lazyUtcNow =
new Lazy<DateTimeOffset>(() => DateTimeOffset.UtcNow);
/// <summary>
/// Gets the current date and time in the UTC time zone.
/// </summary>
public DateTimeOffset UtcNow => _lazyUtcNow.Value;
}
Is this a good approach? What are the disadvantages? Are there better alternatives?
Sorry for the logical fallacy of appeal to authority here, but this is rather interesting:
John Carmack once said:
There are four principle inputs to a game: keystrokes, mouse moves, network packets, and time. (If you don't consider time an input value, think about it until you do -- it is an important concept)"
Source: John Carmack's .plan posts from 1998 (scribd)
(I have always found this quote highly amusing, because the suggestion that if something does not seem right to you, you should think of it really hard until it seems right, is something that only a major geek would say.)
So, here is an idea: consider time as an input. It is probably not included in the xml that makes up the web service request, (you wouldn't want it to anyway,) but in the handler where you convert the xml to an actual request object, obtain the current time and make it part of your request object.
So, as the request object is being passed around your system during the course of processing the transaction, the time to be considered as "the current time" can always be found within the request. So, it is not "the current time" anymore, it is the request time. (The fact that it will be one and the same, or very close to one and the same, is completely irrelevant.)
This way, testing also becomes even easier: you don't have to mock the time provider interface, the time is always in the input parameters.
Also, this way, other fun things become possible, for example servicing requests to be applied retroactively, at a moment in time which is completely unrelated to the actual current moment in time. Think of the possibilities. (Picture of bob squarepants-with-a-rainbow goes here.)
Hmmm.. this feels like a better question for CodeReview.SE than for StackOverflow, but sure - I'll bite.
Is this a good approach?
If used correctly, in the scenario you described, this approach is reasonable. It achieves the two stated goals:
Making your code more testable. This is a common pattern I call "Mock the Clock", and is found in many well-designed apps.
Locking the time to a single value. This is less common, but your code does achieve that goal.
What are the disadvantages?
Since you are creating another new object for each request, it will create a mild amount of additional memory usage and additional work for the garbage collector. This is somewhat of a moot point since this is usually how it goes for all objects with per-request lifetime, including the controllers.
There is a tiny fraction of time being added before you take the reading from the clock, caused by the additional work being done in loading the object and from doing lazy loading. It's negligible though - probably on the order of a few milliseconds.
Since the value is locked down, there's always the risk that you (or another developer who uses your code) might introduce a subtle bug by forgetting that the value won't change until the next request. You might consider a different naming convention. For example, instead of "now", call it "requestRecievedTime" or something like that.
Similar to the previous item, there's also the risk that your provider might be loaded with the wrong lifecycle. You might use it in a new project and forget to set the instancing, loading it up as a singleton. Then the values are locked down for all requests. There's not much you can do to enforce this, so be sure to comment it well. The <summary> tag is a good place.
You may find you need the current time in a scenario where constructor injection isn't possible - such as a static method. You'll either have to refactor to use instance methods, or will have to pass either the time or the time-provider as a parameter into the static method.
Are there better alternatives?
Yes, see Mike's answer.
You might also consider Noda Time, which has a similar concept built in, via the IClock interface, and the SystemClock and FakeClock implementations. However, both of those implementations are designed to be singletons. They help with testing, but they don't achieve your second goal of locking the time down to a single value per request. You could always write an implementation that does that though.
Code looks reasonable.
Drawback - most likely lifetime of the object will be controlled by DI container and hence user of the provider can't be sure that it always be configured correctly (per-invocation and not any longer lifetime like app/singleton).
If you have type representing "transaction" it may be better to put "Started" time there instead.
This isn't something that can be answered with a realtime clock and a query, or by testing. The developer may have figured out some obscure way of reaching the underlying library call...
So don't do that. Dependency injection also won't save you here; the issue is that you want a standard pattern for time at the start of the 'session.'
In my view, the fundamental problem is that you are expressing an idea, and looking for a mechanism for that. The right mechanism is to name it, and say what you mean in the name, and then set it only once. readonly is a good way to handle setting this only once in the constructor, and lets the compiler and runtime enforce what you mean which is that it is set only once.
// In an entity constructor:
this.CreatedAt = DateTime.UtcNow;
I am running a calculation in a PL/pgSQL function and I want to use the result of that calculation in my C++ code. What's the best way to do that?
I can insert that result into a table and use it from there but I'm not sure how well that fares with best practices. Also, I can send message to stderr with RAISE NOTICE but I don't know can I use that message in my code.
The details here are a bit thin on the ground, so it's hard to say for sure.
Strongly preferable whenever possible is to just get the function's return value directly. SELECT my_function(args) if it returns a single result, or SELECT * FROM my_function(args); if it returns a row or set of rows. Then process the result like any other query result. This is part of the basic use of simple SQL and PL/PgSQL functions.
Other options include:
Returning a refcursor. This can be useful in some circumstances where you want to return a dynamic result set or multiple result sets, though it's now mostly superseded by RETURN QUERY and RETURN QUERY EXECUTE. You then FETCH from the refcursorto get the result rows.
LISTENing for an event and having the function NOTIFY when the work is done, possibly with the result as a notify payload. This is useful when the function isn't necessarily called on the same connection as the program that wants to use its results.
Create a temporary table in the function, then SELECT from the table from the session that called the function.
Emitting log messages via RAISE and setting client_min_messages so you receive them, then processing them. This is a very ugly way to do it and should really be avoided at all costs.
INSERTing the results into an existing non-temporary table, then SELECTing them out once the transaction commits and the rows become visible to other transactions.
Which is better? It depends entirely on what you're trying to do. In almost all cases the correct thing to do is just call the function and process the return value, but there are exceptions in special cases.
Assume we have an Order class with a method called Approve. When this method is called, it checks certain conditions and either puts the Order in the state of Approved or throws an exception. In the service layer, we've got something like this:
var order = _repository.Single(o => o.ID == orderID);
order.Approve();
_context.SaveChanges(); // or _session.SaveChanges();
There are 2 ways to test this method and I'd like to hear your insight on this:
Solution 1: Stub the repository to return an Order object. Then assert the Order is in the state of "Approved".
Solution 2: Stub the repository to return a Mock Order object. Assert that Approve() method was called.
Solution 1 is easier and I personally favor state-based testing to interaction-based testing, as the latter can target implementation details and should be avoided. However, I believe testing that the given Order is in the state of Approved is not the concern of this service method. I think we need a separate test method for the Order class to test whether an exception is thrown or the Order's state is changed to Approved.
Solution 2 may sound logical as we are delegating the responsibility of Approving an Order to the Order class itself. So perhaps we need 2 tests for this service method: One to ensure it delegates the task of Approving an Order to the Order class and one to ensure it saves the changes.
What's your insight on this? Which solution do you prefer?
Cheers
Unit tests are to test whether the observed behavior is conforming to expectations/specification.
The answer to your question boils down to what you consider "expected behavior" in this case: a) if the expected behavior is that the Order is in approved state after calling the service method, then test the state; b) if the expected behavior is that the approve action is delegated, then test the method call.
You will need to test the Order object's behavior as well (so that calling Approve() changes the state to approved) in either case.
The second solution plays well as it decouples the behavior of the two objects, but if there are more than one ways that the order can be in approved state (and that's what you are testing -- case a)), then you limit the accepted behavior needlessly.
Also, I would create a separate test for testing the saving part, if that is not essential to the approval part