I have a scenario for my app which is similar to sending friend request in Facebook.
When user A sends friend request to user B, internally a new friend request document is created. At a later time when user B also wants to send friend request to A, system would find out that a friend request document existed and so they should be friend of each other, no new friend request document would be created.
I'm trying to figure out the case when user A and user B both simultaneously sends friend request to each other which will then create 2 friend request documents and leading to undetermined behaviour...
Thanks for your suggestions.. Really appreciated!
Edit:
A few had suggested to use a request queue to solve this; however,
I'm confused about using queue because i thought it would make my rest api endpoint process requests sequentially. Wouldn't I lose all the benefit of multi-threading by using queue? I can't help but imagine how bad it would be if my service has millions of requests queued and waiting to be executed one by one just due to this issue. Has anyone seen something along similar problems seen in production?
I had similar situation with my client which has concurrent writes in the database, What I have implemented is a Queue service.
Create a request in the queue rather than writing in the database, a separate reader will
read one message from the queue at a time and check if it is valid to write it to
database, write only if there is no previous request.
You can implement your own queue or you can use service like AWS-SQS, rabbitmq, MSMQ etc.
// Specific to your case
In mongodb write operations on a single document are atomic.
mongodb has a feature of unique index.
Hence if you insert the document with an _id(or any other unique index) with person names A and B by creating unique index for both (such as "A_B" by lexicographically sorting the names) before doing insertion. You will inherently be able to insert only one instance of that document.
// General
What essentially we would like to have are transactions but since mongodb doesn't support such, as of now. There are a few tricks to achieve this:
2 phase commits :
https://docs.mongodb.org/v3.0/tutorial/perform-two-phase-commits/
Using an external source to maintain a flag, for example using memcache which supports insertion in transactional manner/Compare and Swap.
Here if you use system calls method in frontend then you should fire one request to frontend from Database when some user like, I send you request then within a sec database send you one system call and your frontend code immediate correct the button text like
"Add a friend" to "incoming request"
or else.
if you are only setting up database then just make a system call which send it to UI when friend request arrives or as you say Document created, the further process will be handled by UI Developer.
Thank you.
if you don't like the answer then I m apologize for that but don't downvote me because I M new in Stack Overflow Community.
Related
I am developing a small REST API. As I got into analyzing all the possible failure scenarios, which I have to handle to create a reliable and stable system, I went into thinking about how to make my APIs atomic.
If we take a simple case of creating a contact through the POST API.
The server gets the POST request for the new contact.
Creates the contact in the DB.
Creates a response to send back to the client.
The server crashes before sending the response.
The client gets a timeout error (or connection refused?)
The client is bound to think that the contact creation has failed, though, in fact, the contact was in the DB.
Is this a rare case we can ignore? How do big companies deal with such an issue?
To handle this, you should make your write APIs idempotent i.e. If the same operation is executed multiple times, the result should be same as the operation was done only once.
To achieve this in your current example, you need to be able to identify a contact uniquely based on some parameter, say emailAddress. So, if the createContact is called again with the same emailAddress, check in the DB if a contact already exists with the emailAddress. If so, return the existing contact. Else, create a new contact with the emailAddress and return it.
Hope this helps.
If the request times out, the client should not make any assumption about whether it failed or succeeded.
If it is just a user making a request from a web form, then the timeout should just be exposed to the user, and they can hit the back button and check whether the operation succeeded or not, and if not they submit the request again. (This is fine as long as you always keep a consistent state. If your operation has multiple steps and fails mid way, you need to roll back.)
However if reliable messaging is important to your application you will have to use a library or build your own reliable messaging layer. This could work by having the client assign a unique ID to every request, and having another request that lets you check the result of that request ID later. Then you can do automated retries but only where necessary.
I am trying to implement simple notification system in Yii2.
I have a list of Records populated in ListView.
I would like to allow users to vote for this records. Only once per Record.
Also the Author of each record must be notified (smth like new incoming emails) about who voted for his Record.
What is the easiest way to implement this in Yii2?
Thanks for an advice!
With php only you can't do that, you need to have js at some point.
Solution One could be, from user B side you keep asking the server after certain time (using ajax), if anything(event) happened. If it does return the result with that ajax and update the page.
But then again this is not a very good practice.
If you really want a Real Time update, you might want to use socket.io. Then again using php socket is a bad idea.
So here comes the Solution Two: yii2 + redis + nodejs + socket
redis will do the communication between yii2 and nodejs. The idea is, if anything (event or update) happens yii2 will publish a event with associate data to redis channel and nodejs which is listening to that channel will catch that event and send the data (or update) to client using socket.
Here is a real time chat application tutorial using yii2+redis+nodejs+socket.
Is there a way to get list of IDs (or messages directly) which have been forwarded in some thread? I've been trying to access attachment column in FQL table but it doesn't contain anything. I've been trying to search for some kind of answer to this for a while but haven't found anything that would help me. I'm currently using this FQL query:
SELECT attachment,message_id,author_id,body,created_time FROM message WHERE thread_id = ID
Is there a way to get forwarded messages from message table?
No, the actual (and old) message table won't let you access to the forwarding information. Indeed, the forwarding system looks quite new.
However, you'll soon be able to work with it when the new unified_message table will be available. I guess these unified_ tables are still not entirely deployed. But you can already use them as development and test purposes.
Please note: We are in the process of making the new messages system
available to all users, at which point this table will replace the
existing message table. We are providing early access to this API for
registered developer accounts only until the new messaging system is
broadly available. You should use the message table for production
applications at the current time.
Actually I'm more interested in the techniques to use to achieve this task more than really building a chat system (which is an excellent concrete example). I see 2 parts:
The client needs to get registered somewhere, and we then need a unique ID per client.
The server should be able to send something to the client only from another client.
For the first part, I do not know how to get this unique id. Possibly using the new meteor auth kit ?
For the second part, I thought about building a per-client collection in which one and only one client will have access to, but it sounds heavy and In my opinion not really in the Meteor best practice. I then thought of adding a "from" and to "field" to a Message (see the regular chat example). This would do it but I'm wondering about the no privacy on them. Would a custom publish returning a filtered find do it or it is risky too i.e. would other client get the items too ? Something like:
Meteor.publish("message", function (clientID) {
return Messages.find({"dest":clientID }, {});
});
The latest Meteor todos example uses the new auth system to identify private todo entries. I would imagine that you could use the same mechanism to identify the originator and recipient of a private message in a chat like system.
Of course the filtering of which messages someone sees would need to be filtered on the server side to maintain privacy.
Does Facebook provide access to any real time APIs so that you can respond to events as soon as they happen? If not, what alternatives are there and what are their limitations? For example, if I use polling instead, will they limit my api calls? And if I try using RSS feeds, about how much delay can I expect? Or maybe it would be possible to receive and process email notifications (if I could convince a user to forward mail to another email address), as they seem to be dispatched pretty promptly.
I've never tried polling user data, but I think it will work without issues. As far as I know there are no restrictions on the number of API calls you can make on facebook.
As far as the Queries are concerned, what I have seen and I think this is how they implement it. If your query asks for too much data(takes too much time to process is how they measure this I think) - the query will just fail.
eg: I had this app that would pull all the status messages of all the friends of the user and display it in one place. I first queried for all the friends of the user - this worked okay. But at the same time if I ran a loop to get all the status messages for each friend - it would just fail.
I think you can call individual queries without issues, just be careful you query only data you need, cause, if the queries are too big or too many they will just fail. Best way to findout is running tests yourself.
The Facebook Graph API will allow you to subscribe to real time changes. You can currently only subscribe to users, permissions and errors, but they promise to allow subscribing to more objects in the future.