What is the standard pattern of orchestrating microservices?
If a microservice only knows about its own domain, but there is a flow of data that requires that multiple services interact in some manner, what's the way to go about it?
Let's say we have something like this:
Invoicing
Shipment
And for the sake of the argument, let's say that once an order has been shipped, the invoice should be created.
Somewhere, someone presses a button in a GUI, "I'm done, let's do this!"
In a classic monolith service architecture, I'd say that there is either an ESB handling this, or the Shipment service has knowledge of the invoice service and just calls that.
But what is the way people deal with this in this brave new world of microservices?
I do get that this could be considered highly opinion-based. but there is a concrete side to it, as microservices are not supposed to do the above.
So there has to be a "what should it by definition do instead", which is not opinion-based.
Shoot.
The Book Building Microservices describes in detail the styles mentioned by #RogerAlsing in his answer.
On page 43 under Orchestration vs Choreography the book says:
As we start to model more and more complex logic, we have to deal with
the problem of managing business processes that stretch across the
boundary of individual services. And with microservices, we’ll hit
this limit sooner than usual. [...] When it comes to actually
implementing this flow, there are two styles of architecture we could
follow. With orchestration, we rely on a central brain to guide and
drive the process, much like the conductor in an orchestra. With
choreography, we inform each part of the system of its job and let it
work out the details, like dancers all find‐ ing their way and
reacting to others around them in a ballet.
The book then proceeds to explain the two styles. The orchestration style corresponds more to the SOA idea of orchestration/task services, whereas the choreography style corresponds to the dumb pipes and smart endpoints mentioned in Martin Fowler's article.
Orchestration Style
Under this style, the book above mentions:
Let’s think about what an orchestration solution would look like for
this flow. Here, probably the simplest thing to do would be to have
our customer service act as the central brain. On creation, it talks
to the loyalty points bank, email service, and postal service [...],
through a series of request/response calls. The
customer service itself can then track where a customer is in this
process. It can check to see if the customer’s account has been set
up, or the email sent, or the post delivered. We get to take the
flowchart [...] and model it directly into code. We could even use
tooling that implements this for us, perhaps using an appropriate
rules engine. Commercial tools exist for this very purpose in the form
of business process modeling software. Assuming we use synchronous
request/response, we could even know if each stage has worked [...]
The downside to this orchestration approach is that the customer
service can become too much of a central governing authority. It can
become the hub in the middle of a web and a central point where logic
starts to live. I have seen this approach result in a small number of
smart “god” services telling anemic CRUD-based services what to do.
Note: I suppose that when the author mentions tooling he's referring to something like BPM (e.g. Activity, Apache ODE, Camunda). As a matter of fact, the Workflow Patterns Website has an awesome set of patterns to do this kind of orchestration and it also offers evaluation details of different vendor tools that help to implement it this way. I don't think the author implies one is required to use one of these tools to implement this style of integration though, other lightweight orchestration frameworks could be used e.g. Spring Integration, Apache Camel or Mule ESB
However, other books I've read on the topic of Microservices and in general the majority of articles I've found in the web seem to disfavor this approach of orchestration and instead suggest using the next one.
Choreography Style
Under choreography style the author says:
With a choreographed approach, we could instead just have the customer
service emit an event in an asynchronous manner, saying Customer
created. The email service, postal service, and loyalty points bank
then just subscribe to these events and react accordingly [...]
This approach is significantly more decoupled. If some
other service needed to reach to the creation of a customer, it just
needs to subscribe to the events and do its job when needed. The
downside is that the explicit view of the business process we see in
[the workflow] is now only implicitly reflected in our system [...]
This means additional work is needed to ensure that you can monitor
and track that the right things have happened. For example, would you
know if the loyalty points bank had a bug and for some reason didn’t
set up the correct account? One approach I like for dealing with this
is to build a monitoring system that explicitly matches the view of
the business process in [the workflow], but then tracks what each of
the services do as independent entities, letting you see odd
exceptions mapped onto the more explicit process flow. The [flowchart]
[...] isn’t the driving force, but just one lens through
which we can see how the system is behaving. In general, I have found
that systems that tend more toward the choreographed approach are more
loosely coupled, and are more flexible and amenable to change. You do
need to do extra work to monitor and track the processes across system
boundaries, however. I have found most heavily orchestrated
implementations to be extremely brittle, with a higher cost of change.
With that in mind, I strongly prefer aiming for a choreographed
system, where each service is smart enough to understand its role in
the whole dance.
Note: To this day I'm still not sure if choreography is just another name for event-driven architecture (EDA), but if EDA is just one way to do it, what are the other ways? (Also see What do you mean by "Event-Driven"? and The Meanings of Event-Driven Architecture). Also, it seems that things like CQRS and EventSourcing resonate a lot with this architectural style, right?
Now, after this comes the fun. The Microservices book does not assume microservices are going to be implemented with REST. As a matter of fact in the next section in the book, they proceed to consider RPC and SOA-based solutions and finally REST. An important point here is that Microservices does not imply REST.
So, What About HATEOAS? (Hypermedia as the Engine of Application State)
Now, if we want to follow the RESTful approach we cannot ignore HATEOAS or Roy Fielding will be very much pleased to say in his blog that our solution is not truly REST. See his blog post on REST API Must be Hypertext Driven:
I am getting frustrated by the number of people calling any HTTP-based
interface a REST API. What needs to be done to make the REST
architectural style clear on the notion that hypertext is a
constraint? In other words, if the engine of application state (and
hence the API) is not being driven by hypertext, then it cannot be
RESTful and cannot be a REST API. Period. Is there some broken manual
somewhere that needs to be fixed?
So, as you can see, Fielding thinks that without HATEOAS you are not truly building RESTful applications. For Fielding, HATEOAS is the way to go when it comes to orchestrating services. I am just learning all this, but to me, HATEOAS does not clearly define who or what is the driving force behind actually following the links. In a UI that could be the user, but in computer-to-computer interactions, I suppose that needs to be done by a higher level service.
According to HATEOAS, the only link the API consumer truly needs to know is the one that initiates the communication with the server (e.g. POST /order). From this point on, REST is going to conduct the flow, because, in the response of this endpoint, the resource returned will contain the links to the next possible states. The API consumer then decides what link to follow and move the application to the next state.
Despite how cool that sounds, the client still needs to know if the link must be POSTed, PUTed, GETed, PATCHed, etc. And the client still needs to decide what payload to pass. The client still needs to be aware of what to do if that fails (retry, compensate, cancel, etc.).
I am fairly new to all this, but for me, from HATEOAs perspective, this client, or API consumer is a high order service. If we think it from the perspective of a human, you can imagine an end-user on a web page, deciding what links to follow, but still, the programmer of the web page had to decide what method to use to invoke the links, and what payload to pass. So, to my point, in a computer-to-computer interaction, the computer takes the role of the end-user. Once more this is what we call an orchestrations service.
I suppose we can use HATEOAS with either orchestration or choreography.
The API Gateway Pattern
Another interesting pattern is suggested by Chris Richardson who also proposed what he called an API Gateway Pattern.
In a monolithic architecture, clients of the application, such as web
browsers and native applications, make HTTP requests via a load
balancer to one of N identical instances of the application. But in a
microservice architecture, the monolith has been replaced by a
collection of services. Consequently, a key question we need to answer
is what do the clients interact with?
An application client, such as a native mobile application, could make
RESTful HTTP requests to the individual services [...] On the surface
this might seem attractive. However, there is likely to be a
significant mismatch in granularity between the APIs of the individual
services and data required by the clients. For example, displaying one
web page could potentially require calls to large numbers of services.
Amazon.com, for example,
describes how some
pages require calls to 100+ services. Making that many requests, even
over a high-speed internet connection, let alone a lower-bandwidth,
higher-latency mobile network, would be very inefficient and result in
a poor user experience.
A much better approach is for clients to make a small number of
requests per-page, perhaps as few as one, over the Internet to a
front-end server known as an API gateway.
The API gateway sits between the application’s clients and the
microservices. It provides APIs that are tailored to the client. The
API gateway provides a coarse-grained API to mobile clients and a
finer-grained API to desktop clients that use a high-performance
network. In this example, the desktop clients make multiple requests
to retrieve information about a product, whereas a mobile client
makes a single request.
The API gateway handles incoming requests by making requests to some
number of microservices over the high-performance LAN. Netflix, for
example,
describes
how each request fans out to on average six backend services. In this
example, fine-grained requests from a desktop client are simply
proxied to the corresponding service, whereas each coarse-grained
request from a mobile client is handled by aggregating the results of
calling multiple services.
Not only does the API gateway optimize communication between clients
and the application, but it also encapsulates the details of the
microservices. This enables the microservices to evolve without
impacting the clients. For example, two microservices might be
merged. Another microservice might be partitioned into two or more
services. Only the API gateway needs to be updated to reflect these
changes. The clients are unaffected.
Now that we have looked at how the API gateway mediates between the
application and its clients, let’s now look at how to implement
communication between microservices.
This sounds pretty similar to the orchestration style mentioned above, just with a slightly different intent, in this case, it seems to be all about performance and simplification of interactions.
Trying to aggregate the different approaches here.
Domain Events
The dominant approach for this seems to be using domain events, where each service publish events regarding what have happened and other services can subscribe to those events.
This seems to go hand in hand with the concept of smart endpoints, dumb pipes that is described by Martin Fowler here: http://martinfowler.com/articles/microservices.html#SmartEndpointsAndDumbPipes
Proxy
Another apporach that seems common is to wrap the business flow in its own service.
Where the proxy orchestrates the interaction between the microservices like shown in the below picture:
.
Other patterns of the composition
This page contains various composition patterns.
So, how is orchestration of microservices different from orchestration of old SOA services that are not “micro”? Not much at all.
Microservices usually communicate using http (REST) or messaging/events. Orchestration is often associated with orchestration platforms that allow you to create a scripted interaction among services to automate workflows. In the old SOA days, these platforms used WS-BPEL. Today's tools don't use BPEL. Examples of modern orchestration products: Netflix Conductor, Camunda, Zeebe, Azure Logic Apps, Baker.
Keep in mind that orchestration is a compound pattern that offers several capabilities to create complex compositions of services. Microservices are more often seen as services that should not participate in complex compositions and rather be more autonomous.
I can see a microservice being invoked in an orchestrated workflow to do some simple processing, but I don’t see a microservice being the orchestrator service, which often uses mechanisms such as compensating transactions and state repository (dehydration).
So you're having two services:
Invoice micro service
Shipment micro service
In real life, you would have something where you hold the order state. Let's call it order service. Next you have order processing use cases, which know what to do when the order transitions from one state to another. All these services contain a certain set of data, and now you need something else, that does all the coordination. This might be:
A simple GUI knowing all your services and implementing the use cases ("I'm done" calls the shipment service)
A business process engine, which waits for an "I'm done" event. This engine implements the use cases and the flow.
An orchestration micro service, let's say the order processing service itself that knows the flow/use cases of your domain
Anything else I did not think about yet
The main point with this is that the control is external. This is because all your application components are individual building blocks, loosely coupled. If your use cases change, you have to alter one component in one place, which is the orchestration component. If you add a different order flow, you can easily add another orchestrator that does not interfere with the first one. The micro service thinking is not only about scalability and doing fancy REST API's but also about a clear structure, reduced dependencies between components and reuse of common data and functionality that are shared throughout your business.
HTH, Mark
If the State needs to be managed then the Event Sourcing with CQRS is the ideal way of communication. Else, an Asynchronous messaging system (AMQP) can be used for inter microservice communication.
From your question, it is clear that the ES with CQRS should be the right mix. If using java, take a look at Axon framework. Or build a custom solution using Kafka or RabbitMQ.
You can implement orchestration by using spring State machine model.
Steps
Add below dependency to your project ( if you are using Maven)
<dependency>
<groupId>org.springframework.statemachine</groupId>
<artifactId>spring-statemachine-core</artifactId>
<version>2.2.0.RELEASE</version>
</dependency>
Define states and events e.g. State 1, State 2 and Event 1 and Event 2
Provide state machine implementation in buildMachine() method.
configureStates
configureTransitions
Send events to state machine
Refer to documentation page for complete code
i have written few posts on this topic:
Maybe these posts can also help:
API Gateway pattern - Course-grained api vs fine-grained apis
https://www.linkedin.com/pulse/api-gateway-pattern-ronen-hamias/
https://www.linkedin.com/pulse/successfulapi-ronen-hamias/
Coarse-grained vs Fine-grained service API
By definition a coarse-grained service operation has broader scope than a fine-grained service, although the terms are relative. coarse-grained increased design complexity but can reduce the number of calls required to complete a task. at micro-services architecture coarse-grained may reside at the API Gateway layer and orchestrate several micro-services to complete specific business operation. coarse-grained APIs needs to be carefully designed as involving several micro-services that managing different domain of expertise has a risk to mix-concerns in single API and breaking the rules described above. coarse-grained APIs may suggest new level of granularity for business functions that where not exist otherwise. for example hire employee may involve two microservices calls to HR system to create employee ID and another call to LDAP system to create a user account. alternatively client may have performed two fine-grained API calls to achieve the same task. while coarse-grained represents business use-case create user account, fine-grained API represent the capabilities involved in such task. further more fine-grained API may involve different technologies and communication protocols while coarse-grained abstract them into unified flow. when designing a system consider both as again there is no golden approach that solve everything and there is trad-off for each. Coarse-grained are particularly suited as services to be consumed in other Business contexts, such as other applications, line of business or even by other organizations across the own Enterprise boundaries (typical B2B scenarios).
the answer to the original question is SAGA pattern.
I have a scenario where I have multiple websites using a commnon dll for authentication and general user detail fetching.
I now need to update the common dll with a slightly different login logic and it means I'll need to push this new dll into every website and do a release process for each.
I'm wondering whether it's better to host the common authentication methods in a webservice of some sort then have the websites call that internally. Would it be an internal web service? ajax callbacks from an server side only website? Or stick with the dll method to ensure code changes doesn't break the sites?
Are there any security concerns when not using a dll for this kind of task?
Using a webservice seems a good way to do that. I will cause less memory usage and can be updated independantly from the wesites (if ever needed).
You could go for a WCF services (with dual tcp?) maybe.
Both approaches work in my opinion, but there are significant differences between them that we should keep in mind.
First of all, all of this depends also on the language you are using, because sometimes the best theoretical answer is not always easily implemented in each and every language, making it practical unusable.
So, with this in mind, the best way for me is to have an internal web service, who deals with all requests regarding this "authentication and general user detail module", assuming that all websites use the same DB (or data layer) (otherwise, you will need to create a web service for each one and it's another completely different story). This approach will give you flexibility and maintainability. You could use direct ajax requests to this web service, or make the calls from you website server, and them reply to the browser already with that information. (this second option is more time consuming but much more secure, and if it is a real internal web service (ie hosted on the same machine, the lag will not be noticeable)).
The dll approach should if you need to apply the same business logic to different services. In practical terms: you have two completely separated web sites, that use the same kind of authentication logic. Keep in mind that using this approach to websites that use the same data layer, will force you most of the times to have "deprecated ways" working together with the new implementations, while you are updating the dll on all websites.
Regards,
I am planning to write a .net web application using SOA, which means data operations are made using web methods. There will be many, many methods so I got the next questions:
how should i handle security?
should i split them into more services?
call them using reflection?
Any tips will help because i am new to SOA..
I would suggest you use WCF instead of .Net web-services. WCF gives you a lot of flexibility regarding security and many more aspects. Especially: SOA does not equal web-services. With WCF you can configure the channel your data is sent over (i.e. HTTP, TCP, MSMQ, etc.).
Regarding Reflection, I see no reason to use it. Reflection is slow, hard to debug and not really related to SOA at all. Debugging SOA's is challenging enough, so use reflection sparingly.
As you can imagine, that's not a simple subject. So I would partition it this way: minimally, your question comprises two aspects of security:
Authentication: knowing who your calling party is
Authorization: knowing what that calling part is allowed to do
You have different options for both. For ex. you can handle authentication through multiple standards like WS-{Security|Trust|etc} and, in the other end, authorization through AzMan roles (which BTW doesn't scale very well).
With respect to technology, I agree with other posts, you should opt for WCF. That allows you to leverage those standards and present you more options for the different aspects of security, including auditing.
I'm going to build an API for a web app and I'm interested in what people can suggest as good practices.
I'm already planning to make it versioned (version 1 can only control certain aspects of the system, version 2 could control more, but this may need a change in the way authentication is performed that would be incompatible with version 1), and the authentication will be distinct from the standard username/password people use to log in (if someone does use a malicious tool it won't open them up to full impersonation, just whatever the api allows).
Does anyone have further ideas, or examples of sites with particularly good APIs you have used?
Read the RESTful Web Services book, which give you a good overview of how to use REST in practice, and get to up to speed quickly enough to get started now, with some confidence. This is more useful than just looking at an existing API, because it also discusses design choices and trade-offs.
1) Bake the version number directly into the URL rather than passing it as a parameter, since that gives you complete freedom to change the organization of your API namespace with each version bump.
2) Keep your URL rewriting rules (if any) as simple/lean as possible (but no simpler), while making your URLs as beautiful as possible (but no more).
3) Always look for the best HTTP status code you can find for each response (and don't forget about 202 and 207, for example).
4) Implement fascist parameter validation logic, and informative error messages.
5) Use HTTP request headers where appropriate instead of parameters (like Accept, for example, to allow clients to specify the desired data format of the response).
6) Organize your "nouns" in such a way that the URLs used by different client audiences are separated near the "root" of your URL tree (this makes it easier to enforce different authentication mechanisms for those different audiences if needed, or even map different portions of your URL tree to different servers).
7) If you're serving regular web pages off the same domain as your APIs and use the same authentication credentials, require an X-Requested-With header in your API requests so as to avoid XSRF vulnerabiities.
I would take a look at proven APIs:
YouTube API
Twitter API
There's a lot of argument about whether these APIs are "good" but I think their success is demonstrated, and they're all easy to use.
Use REST.
RESTful web services architecture is easy to implement and uses the strengths and semantics of HTTP for what they were intended. It's resource-oriented, just like the web itself.
Amazon Web Services, Google and many others offer REST APIs to interact with their products.
Use REST.
Read up on standards for APIs, or copy the ideas from one of the popular ones.
Be careful when authenticating users.
Start very very simple.
Build a site that uses your API (even if it's not useful) to check things work. Perhaps you could build a mobile version of the site or something that forces you to use the API in a lot of depth.