As I know REST architecture consists of "frontend" - server which receives requests from clients, and services which are actual "backend" - all business logic and data stored in services(which are exactly same application, which runs at same time on multiple servers). If every service has it's own data storage then how data synchronized?
Each service has a data set and the synchronization can be established with differents ways. Please view the example bellow.
Service A - Products - ProductDB
Service B - Users - UserDB
If a user wants to get all the products for the users with black hairs (for example) this will be a common process that will system follow:
1) Receive the request from client in Gateway
2) Go to user service and the users with black hairs.
select * from users where hair_color='black'
3) Now the user service sends the users IDs to the product service. So the product service executes the following query.
select * from product where user_id IN $(userIds)
4) Finally, the products return to the client.
Please read about gateway https://www.nginx.com/blog/building-microservices-using-an-api-gateway/
Related
I have a 2 gateways which connects to the server where user details are logged.
I can think of two ways to log users accessing the server through gateway.
First way:
The logging is done on the server side i.e
Server(port 5001) Code:
au:([user:`$()]; tim:`timestamp$()); /- Table to maintain logged users
.z.pw:{`au upsert (x;.z.n); show y; 1b}
Gateway 1:
h:hopen `::5001:a:uts1
Gateway 2:
h: hopen `::5001:b:uts2
Second way:
The logging is done from the gateway i.e
Server(port 5001) Code:
au:([user:`$()]; tim:`timestamp$()); /- Table to maintain logged users
Gateway 1:
q)h:hopen `::5001:a:uts1
q)h"`au upsert (`a;.z.p)"
Gateway 2:
q)h: hopen `::5001:b:uts2
q)h"`au upsert (`b;.z.p)"
Hence, is it good to write the user logging code on the server side(Server) or at the client side(Gateway in this case) or is there is better/standard way to do the same?
EDIT - What if we add a middleware(user manager) between multiple gateways and multiple servers, in that case will it be good to write the user logging code on the middleware(user manager) or at the client side(Gateway in this case)?
If users are connecting through a gateway to a number of servers I would implement the logging of users and authentication at the GW level. This would further abstract sensitive data away from potentially unauthorized users, and simplify keeping a master record account activity as all users need to go through the GW. Performing the account activity on the server-side would then need aggregation over multiple servers to get the full picture of system activity.
I am trying to model a Call Center with Chat communication and need your thoughts on this scenario. Real world scenario is that Customer Service Representatives[CSR] in Chat Call Center can service multiple customer chats at same time based on their capacity[integer value 1,2...]
"Chat" Agent [source]
"ChatAgent" resource unit with int parameters totalCapacity[default=3]
Using a service, incoming "Chat" from source seizes a "ChatAgent" from a resourcePool[with resourceUnit "ChatAgent"]. In this model, a "ChatAgent" accepts only 1 "Chat" inside the service block.
ResourcePool
On seize: unit.totalCapacity--;
On release: unit.totalCapacity++;
But I couldn't model a scenario where 1 "ChatAgent" can service multiple customer "Chats" at a time based on their totalCapacity like in a real chat call center.
Please advise on how I can configure this multiple agents to 1 resource seize/delay.
Updated Model
Updated ChatAgent Resource Structure
Thanks,
Shiva
Many ways of doing this, but the first thing that comes to mind is NOT to use ChatAgent as a resource (at least not the kind you use on a service block) because chats can come at any given time, and you can't have a resource taking many different agents that come at different times through the service block...
Instead you can use the following structure in the chatAgent:
The capacity of the resource will define how many agents can enter the restrictedArea block... This structure will exist inside your chatAgent resource.
Your main agent will have the following structure:
when the chat waits for an available chatAgent, if a chatAgent is available by doing:
chatAgent.beginService.entitiesInside() < chatAgent.capacity
These are the most important details to make it work... now you have to build the model properly.
Let's say there are two (or more) RESTful microservices serving JSON. Service (A) stores user information (name, login, password, etc) and service (B) stores messages to/from that user (e.g. sender_id, subject, body, rcpt_ids).
Service (A) on /profile/{user_id} may respond with:
{id: 1, name:'Bob'}
{id: 2, name:'Alice'}
{id: 3, name:'Sue'}
and so on
Service (B) responding at /user/{user_id}/messages returns a list of messages destined for that {user_id} like so:
{id: 1, subj:'Hey', body:'Lorem ipsum', sender_id: 2, rcpt_ids: [1,3]},
{id: 2, subj:'Test', body:'blah blah', sender_id: 3, rcpt_ids: [1]}
How does the client application consuming these services handle putting the message listing together such that names are shown instead of sender/rcpt ids?
Method 1: Pull the list of messages, then start pulling profile info for each id listed in sender_id and rcpt_ids? That may require 100's of requests and could take a while. Rather naive and inefficient and may not scale with complex apps???
Method 2: Pull the list of messages, extract all user ids and make bulk request for all relevant users separately... this assumes such service endpoint exists. There is still delay between getting message listing, extracting user ids, sending request for bulk user info, and then awaiting for bulk user info response.
Ideally I want to serve out a complete response set in one go (messages and user info). My research brings me to merging of responses at service layer... a.k.a. Method 3: API Gateway technique.
But how does one even implement this?
I can obtain list of messages, extract user ids, make a call behind the scenes and obtain users data, merge result sets, then serve this final result up... This works ok with 2 services behind the scenes... But what if the message listing depends on more services... What if I needed to query multiple services behind the scenes, further parse responses of these, query more services based on secondary (tertiary?) results, and then finally merge... where does this madness stop? How does this affect response times?
And I've now effectively created another "client" that combines all microservice responses into one mega-response... which is no different that Method 1 above... except at server level.
Is that how it's done in the "real world"? Any insights? Are there any open source projects that are built on such API Gateway architecture I could examine?
The solution which we used for such problem was denormalization of data and events for updating.
Basically, a microservice has a subset of data it requires from other microservices beforehand so that it doesn't have to call them at run time. This data is managed through events. Other microservices when updated, fire an event with id as a context which can be consumed by any microservice which have any interest in it. This way the data remain in sync (of course it requires some form of failure mechanism for events). This seems lots of work but helps us with any future decisions regarding consolidation of data from different microservices. Our microservice will always have all data available locally for it process any request without synchronous dependency on other services
In your case i.e. for showing names with a message, you can keep an extra property for names in Service(B). So whenever a name update in Service(A) it will fire an update event with id for the updated name. The Service(B) then gets consumes the event, fetches relevant data from Service(A) and updates its database. This way even if Service(A) is down Service(B) will function, albeit with some stale data which will eventually be consistent when Service(A) comes up and you will always have some name to be shown on UI.
https://enterprisecraftsmanship.com/2017/07/05/how-to-request-information-from-multiple-microservices/
You might want to perform response aggregation strategies on your API gateway. I've written an article on how to perform this on ASP.net Core and Ocelot, but there should be a counter-part for other API gateway technologies:
https://www.pogsdotnet.com/2018/09/api-gateway-response-aggregation-with.html
You need to write another service called Aggregator which will internally call both services and get the response and merge/filter them and return the desired result. This can be easily achieved in non-blocking using Mono/Flux in Spring Reactive.
An API Gateway often does API composition.
But this is typical engineering problem where you have microservices which is implementing databases per service pattern.
The API Composition and Command Query Responsibility Segregation (CQRS) pattern are useful ways to implement queries .
Ideally I want to serve out a complete response set in one go
(messages and user info).
The problem you've described is what Facebook realized years ago in which they decided to tackle that by creating an open source specification called GraphQL.
But how does one even implement this?
It is already implemented in various popular programming languages and maybe you can give it a try in the programming language of your choice.
Lets assume I'm building an API for a restaurant and I have the following resources:
Donut(has_chocolate,has_sprinkles)
and
Receipt(cost,donut_id)
My Web App will want to display a table of receipts for the manager to view. Unfortunately the receipt object by itself isn't useful enough, the manager needs to see whether the donut has chocolate or not.
How best do I do this - I can think of 3 implementations:
1) Do a JOIN and return the receipt resource with the has_chocolate additional field
2) Do a JOIN and return the receipt resource with a donut object containing ALL relevant donut information
3) Pull in a page of receipt objects, collect and de-duplicate the donut_ids, and use them to pull in the required donut objects - either one at a time /donut/id, or all at once /donuts?ids=id1,id2,id3
A RESTful API should have endpoints that resolve to a resource or a collection of resources. You can then do an HTTP verb on that endpoint, in this case a GET. Therefore you need to make a decision about how do you define your resources for the API user?
Are there two resources, donuts and receipts, like in your SQL?
Do you want to define a donut as a resource that has a receipt as one of the fields?
Either is fine, the problem is when you start making routes that are a resource 'with something extra on top'. That starts to become hard for the consumer to understand and not RESTful.
If it were me I would choose option 3.
Define two separate endpoints for collections of donuts and receipts: /v0/donuts/ and /v0/receipts/
Allow the consumer to join on the client side by exposing the neccessary filters /v0/donuts/?ids=1,2,3,8
You could make a /v0/events/< id>/ route but seeing as the use-case is to always get a batch of donuts then this would result in too many round-trips for the consumer.
It also sounds like you want to paginate these collections. In this case you should define a max_page_size, default_page_size and you should return your client a next_page field in the response (which is null if it's the last page). You would also have to decide what to paginate on, probably the id in this case.
This is still a theory in my mind.
I'm rebuilding my backend by splitting things into microservices. The microservices I'm imagining for starting off are:
- Order (stores order details and status of each order)
- Customer (stores customer details, addresses, orders booked)
- Service Provider (stores service provider details, status & location of each service provider, order(s) currently being processed by the service provider, etc.)
- Payment (stores payment info for each order)
- Channel (communicates with customers via email / SMS / mobile push)
I hope to be able to use PUB/SUB to create a message with corresponding data, which can be used by any other microservice subscribing to that message.
First off, I understand the concept that each microservice should have complete code & data isolation (thus, on different instances / VMs); and that all microservices should communicate strictly using HTTP REST API contracts.
My doubts are as follows:
To show a list of orders, I'll be using the Order DB to get all orders. In each Order document (I'll be using MongoDB for storage), I'll be having a customer_id Foreign Key. Now the issue of resolving customer_name by using customer_id.
If I need to show 100 orders on the page and go with the assumption that each order has a unique customer_id associated with it, then will I need to do a REST API call 100 times so as to get the names of all the 100 customer_ids?
Or, is data replication a good solution for this problem?
I am envisioning something like this w.r.t. PUB/SUB: The business center personnel mark an order as assigned & select the service provider to allot to that order. This creates a message on the cross-server PUB/SUB channel.
Then, the Channel microservice (which is on a totally different instance / VM) captures this message & sends a Push message & SMS to the service provider's device using the data within the message's contents.
Is this possible at all?
UPDATE TO QUESTION 2: I want the Order microservice to be completely independent of any other microservices that will be built upon / side-by-side it. Channel microservice is an example of a microservice that depends upon events taking place within Order microservice.
Also, please guide me as to what all technologies / libraries to use.
What I'll be developing on:
Java
MongoDB
Amazon AWS instances for each microservice.
Would appreciate anyone's help on this.
Thanks!
#1
If I need to show 100 orders and each order has a unique customer_id, will I need to do 100 REST API call?
No, just make 1 request with 100 order_id(s) and return a dictionary of order_id <=> customer_id
#2
It's a single request
POST
/orders/new
{
"selected_service_provider_id" : "123"
...
}
Which can return you order_id and you can print it locally for the customer or track progress or what have you.
On the server side, you receive an order and process it. Processing can include sending an SMS at some stage. This functionality can be implemented inside original service that received this request or as a separate call to another dedicated service.
To your first question, you don't need to do 100 queries, just one with the array of your 100 documents, like the following:
db.collection.find( { _id : { $in : [1,2,3,4] } } );
https://stackoverflow.com/a/7713461/1384539
I know this question is 1 year old, but I would like to add my answer to the first point.
One option would be to use some form of CQRS and store on the OrderDB also some of the user details when creating an order. This way when you have to show the list of orders you already have all the details you need. Also, the order document would represent a photograph of the user state at the moment of the order creation.
Of course, in case you don't have the user details when storing the order, you just need to make a GET call to the User Service, but that would be 1 call, not 100.