In micrometer, we can create a new gauge doing something like
myMeterRegistry.gauge("my_metric", 69);
See the code here https://github.com/micrometer-metrics/micrometer/blob/master/micrometer-core/src/main/java/io/micrometer/core/instrument/MeterRegistry.java#L468
Would be possible to include a "prefix" name by default for my myMeterRegistry object?
Manually, it should like:
myeterRegistry.gauge("myprefix_my_metric", 69);
My goal is that every developer that creates a gauge metric in my application does not have to take care of adding the "myprefix_" at the beginning of the metric name
A MeterFilter would let you do that (but don't!):
new MeterFilter() {
#Override
public Meter.Id map(Meter.Id id) {
return id.withName("myprefix." + id.getName());
}
}
However a common prefix is typically a smell of an incorrect dimensionality. Usually users try to add a region, host, or the application's name as a prefix. Those are better provided as tags since then you can aggregate across systems and use common dashboards.
The commonTags approach is recommended:
registry.config().commonTags("team", "myteam", "region", "us-east-1");
For hierarchical meter registries, tags will be included in the name as a prefix.
Related
I have a user resource in a module that gets used by several different nodes. Now I want to add this user to a group but only in one specific node. Is there a good solution for this?
Module looks something like this:
class testmodule::basics {
user { 'testuser':
ensure => present,
home => '/home/testuser',
managehome => true,
}
}
Node manifest:
node 'testnode' {
include testmodule::basics
# here I would like to add the user to a group
# something like this (obviously does not work because of duplicate resource)
user { 'testuser':
groups => 'testgroup',
membership => 'minimum',
}
}
You have several alternatives, split among several general categories.
Category 1 - use external data to communicate which secondary groups the user should have. The particular datum might be a flag to indicate whether the user should be in the secondary group, or it might be an actual array of the appropriate secondary groups. You might then obtain it either by directly calling the lookup() or hiera() function, depending on which version of Puppet you are using, or by creating a class parameter for it, and using automatic data binding.
Example:
modules/testmodule/manifests/basics.pp:
class testmodule::basics($secondary_groups = []) {
user { 'testuser':
ensure => present,
home => '/home/testuser',
managehome => true,
groups => $secondary_groups
}
}
data/nodes/special.my.com.yaml:
---
testmodule::basics::secondary_groups:
- testgroup
Category 2 - Set up a class parameter to receive the distinguishing data, just as in one of the category 1 options, and feed the data in via an external node classifier (ENC), instead of external data. Setting up and enabling an ENC has much broader implications than feeding data to a single class, however, so I don't really recommend this unless you are already using or planning to use an ENC.
Category 3 - Perform a resource parameter override where needed. This could be almost a drop-in change to your example manifest, though it would be better to put the override in a separate class than to perform it directly in the node block. In a class that inherits from testmodule::basics, you can use resource parameter override syntax, like so:
modules/testmodule/manifests/basics/special.pp:
class testmodule::basics::special inherits testmodule::basics {
User['testuser'] {
groups => 'testgroup'
}
}
If you want to perform such an override in a node block or in an unrelated class, however, then you need to do it via a collector:
node 'testnode' {
include testmodule::basics
User<title == 'testuser'> {
groups => 'testgroup'
}
}
To two varieties of overrides have some subtle differences beyond the scopes in which they may be used, so do read the docs for more information.
So I'm trying to refactor rewrite my app in a DDD way. This is a simple app with 3 classes :
Configuration(name)
Environment(name)
Property(key)
I use it to view and edit configuration files per environment. One Configuration can be viewed as a table with Property as row and Environment as column.
At this time, the Configuration is an entity and Environment and Property are value objects. But now I jave to implement the us ecase to set a Value to a Property for a given Environment. My first idea was this one :
class Configuration(name) {
environments = SetOf[Environment]
properties = SetOf[Property]
setValue(property, environment, value) {
knowEnv = environments.get(environment)
knowEnv.setValue(property, value)
}
}
class Environment(name) {
properties = MapOf[Property, Value]
setValue(property, value) {
properties[property] = value
}
}
But doing so will change my Environment from a value object to an entity. So I started to think (too much) and have trouble to find the "best" solution. That's why I came here to ask you, experts, how would you implement this use case.
Thanks
From what you've posted it does sound as though each Environment is a unique thing with an identity. I'm guessing your Environments are probably platforms or development environments? So it probably should be an entity.
It does sound as though your Environment could be edited, used, created, etc. independently of anything else. In this case it probably shouldn't exist as part of another aggregate, so it should be it's own aggregate root (even if it's just a single entity). Therefore it would have it's own repository. This is a point that isn't blatantly obvious in the Evans DDD book, but an entity on it's own, is considered an aggregate root (made up of just one object).
If you wish to reference an Environment from another aggregate root, you would reference it by its unique id (not as a object reference). You would then need another technique/method to retrieve these Environments.
This might seem to fly in the face of the old data-centric dogma, but you can do all sorts of things, like data caching your Environments (as there's probably a limited amount and they probably change infrequently) or employ CQRS.
Given the discussion and comments received on this questions I decided to keep the Environment immutable as value object. Setting a property value will then produce a new Environment :
class Configuration(name) {
environments = SetOf[Environment]
properties = SetOf[Property]
setValue(property, environment, value) {
knowEnv = environments.get(environment)
updatedEnv = knowEnv.setValue(property, value)
environments.replace(knowEnv, updatedEnv)
}
}
class Environment(name) {
properties = MapOf[Property, Value]
setValue(property, value) {
copy = new Environment(name)
copy.properties = properties
copy.properties[property] = value
return copy
}
}
It is simple to use and acceptable for our use cases.
I've taken a read to the Advantages of message based web services article and am wondering if there is there a recommended style/practice to versioning Restful resources in ServiceStack? The different versions could render different responses or have different input parameters in the Request DTO.
I'm leaning toward a URL type versioning (i.e /v1/movies/{Id}), but I have seen other practices that set the version in the HTTP headers (i.e Content-Type: application/vnd.company.myapp-v2).
I'm hoping a way that works with the metadata page but not so much a requirement as I've noticed simply using folder structure/ namespacing works fine when rendering routes.
For example (this doesn't render right in the metadata page but performs properly if you know the direct route/url)
/v1/movies/{id}
/v1.1/movies/{id}
Code
namespace Samples.Movies.Operations.v1_1
{
[Route("/v1.1/Movies", "GET")]
public class Movies
{
...
}
}
namespace Samples.Movies.Operations.v1
{
[Route("/v1/Movies", "GET")]
public class Movies
{
...
}
}
and corresponding services...
public class MovieService: ServiceBase<Samples.Movies.Operations.v1.Movies>
{
protected override object Run(Samples.Movies.Operations.v1.Movies request)
{
...
}
}
public class MovieService: ServiceBase<Samples.Movies.Operations.v1_1.Movies>
{
protected override object Run(Samples.Movies.Operations.v1_1.Movies request)
{
...
}
}
Try to evolve (not re-implement) existing services
For versioning, you are going to be in for a world of hurt if you try to maintain different static types for different version endpoints. We initially started down this route but as soon as you start to support your first version the development effort to maintain multiple versions of the same service explodes as you will need to either maintain manual mapping of different types which easily leaks out into having to maintain multiple parallel implementations, each coupled to a different versions type - a massive violation of DRY. This is less of an issue for dynamic languages where the same models can easily be re-used by different versions.
Take advantage of built-in versioning in serializers
My recommendation is not to explicitly version but take advantage of the versioning capabilities inside the serialization formats.
E.g: you generally don't need to worry about versioning with JSON clients as the versioning capabilities of the JSON and JSV Serializers are much more resilient.
Enhance your existing services defensively
With XML and DataContract's you can freely add and remove fields without making a breaking change. If you add IExtensibleDataObject to your response DTO's you also have a potential to access data that's not defined on the DTO. My approach to versioning is to program defensively so not to introduce a breaking change, you can verify this is the case with Integration tests using old DTOs. Here are some tips I follow:
Never change the type of an existing property - If you need it to be a different type add another property and use the old/existing one to determine the version
Program defensively realize what properties don't exist with older clients so don't make them mandatory.
Keep a single global namespace (only relevant for XML/SOAP endpoints)
I do this by using the [assembly] attribute in the AssemblyInfo.cs of each of your DTO projects:
[assembly: ContractNamespace("http://schemas.servicestack.net/types",
ClrNamespace = "MyServiceModel.DtoTypes")]
The assembly attribute saves you from manually specifying explicit namespaces on each DTO, i.e:
namespace MyServiceModel.DtoTypes {
[DataContract(Namespace="http://schemas.servicestack.net/types")]
public class Foo { .. }
}
If you want to use a different XML namespace than the default above you need to register it with:
SetConfig(new EndpointHostConfig {
WsdlServiceNamespace = "http://schemas.my.org/types"
});
Embedding Versioning in DTOs
Most of the time, if you program defensively and evolve your services gracefully you wont need to know exactly what version a specific client is using as you can infer it from the data that is populated. But in the rare cases your services needs to tweak the behavior based on the specific version of the client, you can embed version information in your DTOs.
With the first release of your DTOs you publish, you can happily create them without any thought of versioning.
class Foo {
string Name;
}
But maybe for some reason the Form/UI was changed and you no longer wanted the Client to use the ambiguous Name variable and you also wanted to track the specific version the client was using:
class Foo {
Foo() {
Version = 1;
}
int Version;
string Name;
string DisplayName;
int Age;
}
Later it was discussed in a Team meeting, DisplayName wasn't good enough and you should split them out into different fields:
class Foo {
Foo() {
Version = 2;
}
int Version;
string Name;
string DisplayName;
string FirstName;
string LastName;
DateTime? DateOfBirth;
}
So the current state is that you have 3 different client versions out, with existing calls that look like:
v1 Release:
client.Post(new Foo { Name = "Foo Bar" });
v2 Release:
client.Post(new Foo { Name="Bar", DisplayName="Foo Bar", Age=18 });
v3 Release:
client.Post(new Foo { FirstName = "Foo", LastName = "Bar",
DateOfBirth = new DateTime(1994, 01, 01) });
You can continue to handle these different versions in the same implementation (which will be using the latest v3 version of the DTOs) e.g:
class FooService : Service {
public object Post(Foo request) {
//v1:
request.Version == 0
request.Name == "Foo"
request.DisplayName == null
request.Age = 0
request.DateOfBirth = null
//v2:
request.Version == 2
request.Name == null
request.DisplayName == "Foo Bar"
request.Age = 18
request.DateOfBirth = null
//v3:
request.Version == 3
request.Name == null
request.DisplayName == null
request.FirstName == "Foo"
request.LastName == "Bar"
request.Age = 0
request.DateOfBirth = new DateTime(1994, 01, 01)
}
}
Framing the Problem
The API is the part of your system that exposes its expression. It defines the concepts and the semantics of communicating in your domain. The problem comes when you want to change what can be expressed or how it can be expressed.
There can be differences in both the method of expression and what is being expressed. The first problem tends to be differences in tokens (first and last name instead of name). The second problem is expressing different things (the ability to rename oneself).
A long-term versioning solution will need to solve both of these challenges.
Evolving an API
Evolving a service by changing the resource types is a type of implicit versioning. It uses the construction of the object to determine behavior. Its works best when there are only minor changes to the method of expression (like the names). It does not work well for more complex changes to the method of expression or changes to the change of expressiveness. Code tends to be scatter throughout.
Specific Versioning
When changes become more complex it is important to keep the logic for each version separate. Even in mythz example, he segregated the code for each version. However, the code is still mixed together in the same methods. It is very easy for code for the different versions to start collapsing on each other and it is likely to spread out. Getting rid of support for a previous version can be difficult.
Additionally, you will need to keep your old code in sync to any changes in its dependencies. If a database changes, the code supporting the old model will also need to change.
A Better Way
The best way I've found is to tackle the expression problem directly. Each time a new version of the API is released, it will be implemented on top of the new layer. This is generally easy because changes are small.
It really shines in two ways: first all the code to handle the mapping is in one spot so it is easy to understand or remove later and second it doesn't require maintenance as new APIs are developed (the Russian doll model).
The problem is when the new API is less expressive than the old API. This is a problem that will need to be solved no matter what the solution is for keeping the old version around. It just becomes clear that there is a problem and what the solution for that problem is.
The example from mythz's example in this style is:
namespace APIv3 {
class FooService : RestServiceBase<Foo> {
public object OnPost(Foo request) {
var data = repository.getData()
request.FirstName == data.firstName
request.LastName == data.lastName
request.DateOfBirth = data.dateOfBirth
}
}
}
namespace APIv2 {
class FooService : RestServiceBase<Foo> {
public object OnPost(Foo request) {
var v3Request = APIv3.FooService.OnPost(request)
request.DisplayName == v3Request.FirstName + " " + v3Request.LastName
request.Age = (new DateTime() - v3Request.DateOfBirth).years
}
}
}
namespace APIv1 {
class FooService : RestServiceBase<Foo> {
public object OnPost(Foo request) {
var v2Request = APIv2.FooService.OnPost(request)
request.Name == v2Request.DisplayName
}
}
}
Each exposed object is clear. The same mapping code still needs to be written in both styles, but in the separated style, only the mapping relevant to a type needs to be written. There is no need to explicitly map code that doesn't apply (which is just another potential source of error). The dependency of previous APIs is static when you add future APIs or change the dependency of the API layer. For example, if the data source changes then only the most recent API (version 3) needs to change in this style. In the combined style, you would need to code the changes for each of the APIs supported.
One concern in the comments was the addition of types to the code base. This is not a problem because these types are exposed externally. Providing the types explicitly in the code base makes them easy to discover and isolate in testing. It is much better for maintainability to be clear. Another benefit is that this method does not produce additional logic, but only adds additional types.
I am also trying to come with a solution for this and was thinking of doing something like the below. (Based on a lot of Googlling and StackOverflow querying so this is built on the shoulders of many others.)
First up, I don’t want to debate if the version should be in the URI or Request Header. There are pros/cons for both approaches so I think each of us need to use what meets our requirements best.
This is about how to design/architecture the Java Message Objects and the Resource Implementation classes.
So let’s get to it.
I would approach this in two steps. Minor Changes (e.g. 1.0 to 1.1) and Major Changes (e.g 1.1 to 2.0)
Approach for minor changes
So let’s say we go by the same example classes used by #mythz
Initially we have
class Foo { string Name; }
We provide access to this resource as /V1.0/fooresource/{id}
In my use case, I use JAX-RS,
#Path("/{versionid}/fooresource")
public class FooResource {
#GET
#Path( "/{id}" )
public Foo getFoo (#PathParam("versionid") String versionid, (#PathParam("id") String fooId)
{
Foo foo = new Foo();
//setters, load data from persistence, handle business logic etc
Return foo;
}
}
Now let’s say we add 2 additional properties to Foo.
class Foo {
string Name;
string DisplayName;
int Age;
}
What I do at this point is annotate the properties with a #Version annotation
class Foo {
#Version(“V1.0")string Name;
#Version(“V1.1")string DisplayName;
#Version(“V1.1")int Age;
}
Then I have a response filter that will based on the requested version, return back to the user only the properties that match that version. Note that for convenience, if there are properties that should be returned for all versions, then you just don’t annotate it and the filter will return it irrespective of the requested version
This is sort of like a mediation layer. What I have explained is a simplistic version and it can get very complicated but hope you get the idea.
Approach for Major Version
Now this can get quite complicated when there is a lot of changes been done from one version to another. That is when we need to move to 2nd option.
Option 2 is essentially to branch off the codebase and then do the changes on that code base and host both versions on different contexts. At this point we might have to refactor the code base a bit to remove version mediation complexity introduced in Approach one (i.e. make the code cleaner) This might mainly be in the filters.
Note that this is just want I am thinking and haven’t implemented it as yet and wonder if this is a good idea.
Also I was wondering if there are good mediation engines/ESB’s that could do this type of transformation without having to use filters but haven’t seen any that is as simple as using a filter. Maybe I haven’t searched enough.
Interested in knowing thoughts of others and if this solution will address the original question.
I'm refactoring a project using DDD, but am concerned about not making too many Entities their own Aggregate Root.
I have a Store, which has a list of ProductOptions and a list of Products. A ProductOption can be used by several Products. These entities seem to fit the Store aggregate pretty well.
Then I have an Order, which transiently uses a Product to build its OrderLines:
class Order {
// ...
public function addOrderLine(Product $product, $quantity) {
$orderLine = new OrderLine($product, $quantity);
$this->orderLines->add($orderLine);
}
}
class OrderLine {
// ...
public function __construct(Product $product, $quantity) {
$this->productName = $product->getName();
$this->basePrice = $product->getPrice();
$this->quantity = $quantity;
}
}
Looks like for now, DDD rules as respected. But I'd like to add a requirement, that might break the rules of the aggregate: the Store owner will sometimes need to check statistics about the Orders which included a particular Product.
That means that basically, we would need to keep a reference to the Product in the OrderLine, but this would never be used by any method inside the entity. We would only use this information for reporting purposes, when querying the database; thus it would not be possible to "break" anything inside the Store aggregate because of this internal reference:
class OrderLine {
// ...
public function __construct(Product $product, $quantity) {
$this->productName = $product->getName();
$this->basePrice = $product->getPrice();
$this->quantity = $quantity;
// store this information, but don't use it in any method
$this->product = $product;
}
}
Does this simple requirement dictates that Product becomes an aggregate root? That would also cascade to the ProductOption becoming an aggregate root, as Product has a reference to it, thus resulting in two aggregates which have no meaning outside a Store, and will not need any Repository; looks weird to me.
Any comment is welcome!
Even though it is for 'reporting only' there is still a business / domain meaning there. I think that your design is good. Although I would not handle the new requirement by storing OrderLine -> Product reference. I would do something similar to what you already doing with product name and price. You just need to store some sort of product identifier (SKU?) in the order line. This identifier/SKU can later be used in a query. SKU can be a combination of Store and Product natural keys:
class Sku {
private String _storeNumber;
private String _someProductIdUniqueWithinStore;
}
class OrderLine {
private Money _price;
private int _quantity;
private String _productName;
private Sku _productSku;
}
This way you don't violate any aggregate rules and the product and stores can be safely deleted without affecting existing or archived orders. And you can still have your 'Orders with ProductX from StoreY'.
Update: Regarding your concern about foreign key. In my opinion foreign key is just a mechanism that enforces long-living Domain relationships at the database level. Since you don't have a domain relationship you don't need the enforcement mechanism as well.
In this case you need the information for reporting which has nothing to do with the aggregate root.
So the most suitable place for it would be a service (could be a domain service if it is related to business or better to application service like querying service which query the required data and return them as DTOs customizable for presentation or consumer.
I suggest you create a statistics services which query the required data using read only repositories (or preferable Finders) which returns DTOs instead of corrupting the domain with query models.
Check this
Just wanted to get the groups thoughts on how to handle configuration details of entities.
What I'm thinking of specifically is high level settings which might be admin-changed. the sort of thing that you might store in the app or web.config ultimately, but from teh DDD perspective should be set somewhere in the objects explicitly.
For sake of argument, let's take as an example a web-based CMS or blog app.
A given blog Entry entity has any number of instance settings like Author, Content, etc.
But you also might want to set (for example) default Description or Keywords that all entries in the site should start with if they're not changed by the author. Sure, you could just make those constants in the class, but then the site owner couldn't change the defaults.
So my thoughts are as follows:
1) use class-level (static) properties to represent those settings, and then set them when the app starts up, either setting them from the DB or from the web.config.
or
2) use a separate entity for holding the settings, possibly a dictionary, either use it directly or have it be a member of the Entry class
What strikes you all as the most easy / flexible? My concerns abou the first one is that it doesn't strike me as very pluggable (if I end up wanting to add more features) as changing an entity's class methods would make me change the app itself as well (which feels like an OCP violation). The second one feels like it's more heavy, though, especially if I then have to cast or parse values out of a dictionary.
I would say that that whether a value is configurable or not is irrelevant from the Domain Model's perspective - what matters is that is is externally defined.
Let's say that you have a class that must have a Name. If the Name is always required, it must be encapsulated as an invariant irrespective of the source of the value. Here's a C# example:
public class MyClass
{
private string name;
public MyClass(string name)
{
if(name == null)
{
throw new ArgumentNullException("name");
}
this.name = name;
}
public string Name
{
get { return this.name; }
set
{
if(value == null)
{
throw new ArgumentNullException("name");
}
this.name = value;
}
}
}
A class like this effectively protects the invariant: Name must not be null. Domain Models must encapsulate invariants like this without any regard to which consumer will be using them - otherwise, they would not meet the goal of Supple Design.
But you asked about default values. If you have a good default value for Name, then how do you communicate that default value to MyClass.
This is where Factories come in handy. You simply separate the construction of your objects from their implementation. This is often a good idea in any case. Whether you choose an Abstract Factory or Builder implementation is less important, but Abstract Factory is a good default choice.
In the case of MyClass, we could define the IMyClassFactory interface:
public interface IMyClassFactory
{
MyClass Create();
}
Now you can define an implementation that pulls the name from a config file:
public ConfigurationBasedMyClassFactory : IMyClassFactory
{
public MyClass Create()
{
var name = ConfigurationManager.AppSettings["MyName"];
return new MyClass(name);
}
}
Make sure that code that needs instances of MyClass use IMyClassFactory to create it instead of new'ing it up manually.