The title isn't accurate because based on what I have found in my research there doesn't seem to be a way to make a function atomic in nodejs, but I will lay out my problem to see if you people can come up with something that I have not been able to think about.
I am trying to setup a scheduler where I can set my appointment time slots say 1 hr long each and when someone makes an appointment I want to make sure that the time slot is not taken before scheduling it.
So for example I decide that I will be working from 9 am to 2 pm with a time slot of one hour. Then my schedule would be 9-10, 10-11, 11-12, 12-1, 1-2.
An appointment will come in with a start time of 11 and end time of 12. I need to make sure that slot isn't already taken.
I am using mongodb with nodejs and restify.
I understand that in my appointments collection I can set an index on a combination of values like start time and end time, as discussed here Creating Multifield Indexes in Mongoose / MongoDB.
But if I decide to change my time slot from 1 hour to say 1.5 hours then I will have scheduling conflicts as the start time and end time of entries in the database will not match up with the new interval
Currently I have a function which checks to make sure that the new appointment will not conflict but I am not sure if it will work out well when I have multiple requests coming in. This is a nodejs and restify app so basically an api with a mongodb that it talks to, to handle appointments.
I am running it with multiple workers, so I am worried that at a certain point two requests will come in at the same time, handled by two different workers for the same time slot. When my conflict checking function executes it will return saying that the slot is open for both of them since no appointment has been made yet and then there will be a scheduling conflict.
Any ideas on how to combat this, or is there something in the way javascript executes so that I shouldn't have to worry about it this? All input will be appreciated
Thanks!
I ended up using https://github.com/Automattic/kue, to queue my requests and added another endpoint where you can check the status of your request. So when you want to make an appointment your request ends up in the job queue, and you can then periodically check the status of your request. This way only one appointment request gets processed at a time so no concurrency issues.
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I have stumbled upon a difficult type of problem for me. So, lets say we have an API, which creates events in the future, for example, after two weeks from this moment. During this time, we can post comments, add photos, etc. on this event. After those two week pass, I want to close this event and change it's type from 'OPEN' to 'CLOSED'. How should I achieve this?
I have tried agenda library for this task, but it seems that it is for different purpose of tasks - scheduled tasks. Are there any other options or other practices to do this?
I am using postgres database, if that's helpful.
Can the execution of an expressJS method be delayed for 30 days or more just by using setTimeout ?
Let's say I want to create an endpoint /sendMessage that send a message to my other app after a timeout of 30 days. Will my expressJS method execution will last long time enough to fire this message after this delay ?
If your server runs continuously for 30 days or more, then setTimeout() will work for that. But, it is probably not smart to rely on that fact that your server never, ever has to restart.
There are 3rd party programs/modules designed explicitly for this. If you don't want to use one of them, then what I have done in the past is I write each future firing time into a JSON file and I set a timer for it with setTimeout(). If the timer successfully fires, then I remove that time from the JSON file.
So, at any point in time, the JSON file always contains a list of times in the future that I want timers to fire for. Any timer that fires is immediately removed from the JSON file.
Anytime my server starts up, I read the times from the JSON file and reconfigure the setTimeout() for each one.
This way, even if my server restarts, I won't lose any of the timers.
In case you were wondering, the way nodejs creates timers, it does not cost you anything to have a bunch of future timers configured. Nodejs keeps the timers in a sorted linked list and the event loop just checks the time for the next timer to fire - the one at the front of the sorted list (the rest of the timers are not looked at until they get to the front of the sorted list). This means the only time it costs anything to have lots of future timers is when inserting a new timer into the sorted list and there is no regular cost in the event loop to having lots of pending timers present.
I am saving a counter number in user storage.
I want to provide some content to the user which changes daily using this counter.
So every time the counter increases by 1 the content will change.
The problem is the timezone difference.
Is there anyway to run a function, daily which will increase this counter by 1. I could use setInterval() which is a part of the NodeJs library but that won't be an accurate "daily" update for all users.
User storage is only available to you as a developer when the Action is active. This data is not available once the Action is closed, so you wouldn't be able to asynchronously update the field. If you do want asynchronous access, I'd suggest using an external database and only storing the database row key in the user's userStorage. That way you can access the data and modify it whenever you want.
The setInterval method will run a function periodically, but may not work in the way you want. It only runs the function while the runtime is active. A lot of services will shut down a runtime after a period. Cloud Functions, for example, run sometimes but then will shut down when not used. Additonally, Cloud Functions can be run several times in parallel instances, executing a setInterval function several times in parallel. That would increment the counter more times than you want.
Using a dedicated Cron service would help reduce the number of simultaneous executions while also ensuring it runs when you want.
You are unable to directly access the user's timezone within the Action, meaning you won't be able to determine the end of a day. You can get the content to change every day, but it'll have some sort of offset. To get around this, you could have several cron jobs which run for different segments of users.
Using the conv.user.locale field, you can derive their language. en-US is generally going to be for American users, which generally are going to live in the US. While this could result in an odd behavior for traveling, you can then bucket users into a particular period of execution. Running the task overnight, either 1AM or 4AM they'll probably be unaware but know that it updates overnight.
You could use the location helper to get the user's location more precisely. This may be a bit unnecessary, but you could use that value to determine their timezone and then derive that user's "midnight" to put in the correct Cron bucket.
I'm currently working on a app. This also includes some kind of group chats.
The users inside can make multiple votes, for example for kicking someone. The votes are all valid for 1 week. If all other users submit their opinion the vote gets deleted.So far so good.
I also want a logic, which deletes the vote automatically if it's expired.
So far I got the idea to store the expiration dates for the votes inside a database(MongoDB), sorted by their timestamp of expiration.
In NodeJs I'm always loading the vote with the smallest expiration date from the database.
Then I check how much time is left by subtracting the vote expiration date from the current Date
Date.now() - voteTmp;
Then I can set a timeout, which calls a function to delete the vote and automatically starts a new timeout for the next vote. Is it a problem to set a timeout with such a big number of seconds?
Do you have any better ideas?
Thank you:)
The node.js event loop is explained here:
when the event loop enters a given phase, it will perform any operations specific to that phase, then execute callbacks in that phase's queue until the queue has been exhausted or the maximum number of callbacks has executed.
On each iteration, the event loop checks for scheduled timers that satisfy the specified thresholds (delays) and executes their callbacks. Thus, the magnitude of delays for registered timers shouldn't matter.
However, in your scenario, there's a chance that you might accidentally register redundant or invalid timers (possibly after recovering from a crash). MongoDB supports (automatic) data expiration. You can instruct MongoDB to delete documents after a specified number of seconds has passed. That seems close enough to what you want to do.
I've come up with a fancy issue of synchronization in node.js, which I've not able to find an elegant solution:
I setup a express/node.js web app for retrieving statistics data from a one row database table.
If the table is empty, populate it by a long calculation task
If the record in table is older than 15 minutes from now, update it by a long calculation task
Otherwise, respond with a web page showing the record in DB.
The problem is,
when multiple users issue requests simultaneously, in case the record is old, the long calculation task would be executed once per request, instead of just once.
Is there any elegant way that only one request triggers the calculation task, and all others wait for the updated DB record?
Yes, it is called locks.
Put an additional column in your table say lock which will be of timestamp type. Once a process starts working with that record put a now+timeout time into it (by the rule of thumb I choose timeout to be 2x the average time of processing). When the process stops processing update that column with NULL value.
At the begining of processing check that column. If the value > now condition is satisfied then return some status code to client (don't force client to wait, it's a bad user experience, he doesn't know what's going on unless processing time is really short) like 409 Conflict. Otherwise start processing (also ideally processing takes place in a separate thread/process so that user won't have to wait: respond with an appropriate status code like 202 Accepted).
This now+timeout value is needed in case your processing process crashes (so we avoid deadlocks). Also remember that you have to "check and set" this lock column in transaction because of race conditions (might be quite difficult if you are working with MongoDB-like databases).