Do we need complex sending strategies for GCM / FCM? - algorithm

Currently I'm working on a SaaS with support for multiple tenants that can enable push notifications for their user-bases.
I'm thinking of using a message queue to store all pushes and send them with a separate service. That new service would need to read from the queue and send the push notifications.
My question now is: Do I need to come up with a complex sending strategy? I know that with GCM has a limit of 1000 devices per request, so this needs to be considered. I also can't wait for x pushes to fly in as this might delay a previous push from being sent. My next thought was to create a global array and fill it with pushes from the queue. A loop would then fetch that array every say 1 second and send pushes. This way pushes would get sent for sure and I wouldn't exceed the 1000 devices limit.
So ... although this might work I'm not sure if an infinite loop is the best way to go. I'm wondering if GCM / FCM even has a request limit? If not, I wouldn't need to aggregate the pushes in the first place and I could ditch the loop. I could simply fire a request for each push that gets pulled from the queue.
Any enlightenment on this topic or improvement of my prototypical algorithm would be great!

Do I need to come up with a complex sending strategy?
Not really. GCM/FCM is pretty simple enough. Just send the message towards the GCM/FCM server and it would queue it on it's own, then (as per it's behavior) send it as soon as possible.
I know that with GCM has a limit of 1000 devices per request, so this needs to be considered.
I think you're confusing the 1000 devices per request limit. The 1000 devices limit refers to the number of registration tokens you add in the list when using the registration_ids parameter:
This parameter specifies a list of devices (registration tokens, or IDs) receiving a multicast message. It must contain at least 1 and at most 1000 registration tokens.
This means you can only send to 1000 devices with the same message payload in a single request (you can then do a batch request (1000/each request) if you need to).
I'm wondering if GCM / FCM even has a request limit?
AFAIK, there is no such limit. Ditch the loop. Whenever you successfully send a message towards the GCM/FCM server, it will enqueue and keep the message until such time that it is available to send.

Related

ZeroMQ: Can I use kvmsg class to add sequence numbers to messages? If yes, how?

I'm implementing a mechanism to detect packet loss in ZeroMQ PUSH/PULL socket type.
1) I was wondering if kvmsg can be used for the same?
2) I would like the client to detect gaps in sequence numbers if there are any loss of packets and implement a resend mechanism accordingly.
Assuming that kvmsg can cope with arbitrary message structures, then yes. Alternatives include Google Protocol Buffers, XML, etc.
In general one does this by adding a field to the messages that you send, perhaps called "sequence". The software you've written for the PUSH end will set this to zero for the very first message, 1 for the next, incrementing by 1 for each message. The PULL end then simply checks the sequence.
However, the real question is, why is this required by your application? ZMQ guarantees ( in normal circumstances ) delivery of messages. That's kinda the whole point of it. PUSH/PULL means that exactly one PULLer will receive a PUSHed message. If you have one PUSH and one PULL, every PUSHed message will be delivered in the correct order with no loss to the PULLer, barring catastrophic network failures. AFAIK it will even deal with temporary network problems for you, managing reconnection, etc, and still deliver messages in the correct order.
Messages that cannot be sent because the outgoing queue on the PUSH end is full will result in the zmq_send() returning an error, so the PUSH end already knows that a message wasn't sent.
Is there something else more complex about the application?

Mass Text to Speech using Plivo

I am looking at integrating Plivo with our platform to make outgoing text to speech calls. All of our calls made, will be a customized message of about 20 words, or less than a 30 second call.
Daily, we'll batch about 10,000 calls at the same time. It appears I would have to make 10,000 rest API calls vs being able to send a batch at one time, each one with it's own answer_url. Does anyone have experience with this, seems like a ton of overhead.
Another option may be to use parameters in the answer_url, so I can send a list of all phone numbers at once and then based on a parameterized answer_url, tell Plivo what to do next.
With Plivo, you can make bulk outbound calls where you can specify multiple numbers and a single answer_url. See https://www.plivo.com/docs/getting-started/make-bulk-calls/ for a getting started doc.
For each call made, Plivo makes a request to that answer url with the to/from numbers (see this link for more details). Then, based on the to/from numbers, your answer_url can respond with the TTS message to be played for that particular number. You would just need to have a database where you can lookup the number to get the message to play for each request to your answer_url.

Pricing: Are push notifications really free?

According to the parse.com pricing page, push notifications are free up to 1 million unique recipients.
API calls are free up to 30 requests / second.
I want to make sure there is no catch here.
An example will clarify: I have 100K subscribed users. I will send weekly push notifications to them. In a month, that will be 4 push "blasts" with 100K recipients each. Is this covered by the free tier? Would this count as 4 API calls, 400K API calls, or some other amount?
100k users is 1/10 the advertised unique recipient limit, so that should be okay.
Remember that there's a 10sec timeout, too. So the only way to blast 100k pushes within the free-tier resource limits is to create a scheduled job that spends about 2 hours (that's a safe rate of 15 req/sec) doing pushes and writing state so you can pick up later where you left off.
Assuming there's no hidden gotcha (you'll probably need to discover those empirically), I think the only gotcha in plain sight is the fact that the free tier allows only one (1) scheduled job. Any other long-running processing -- and there are bound to be some on 100k users -- are going to have to share the job, making the what-should-this-single-job-work-on-now logic pretty complex.
You should take a look at the FAQ for Parse.com:
https://www.parse.com/plans/faq
What is considered an API request?
Anytime you make a network call to
Parse on behalf of your app using one of the Parse SDKs or REST API,
it counts as an API request. This does include things like queries,
saves, logins, amongst other kinds of requests. It also includes
requests to send push notifications, although this is seen as a single
request regardless of how many recipients are targeted. Serving Parse
files counts as an API request, including static assets served from
Parse Hosting. Analytics requests do have a special exemption. You can
send us your analytics events any time without being limited by your
app's request limit.

ZeroMQ XREP -- Endpoint disappearing

I am using a standard LRU queue as defined by the ZeroMQ guide figure 41, and I am wondering how to add in protection so that I don't send messages to end points that have disappeared (server crash, OOM killer, anything along those lines).
From the documentation I read that XREP will just drop the message if it is going to a non-existant end-point, and there is no way I get notified about that. Is there a way to get such a notification? Should I just send out a "ping" first and if I don't get a response then that "worker" is dead meat to me? How will I know that it is the same client that I just sent the ping to that I am getting the message back from?
Or is my use case not a good one for ZeroMQ? I just want to make sure that a message has been received, I don't want it being dropped on the floor without my knowledge...
Pinging a worker to know if it is alive will cause a race condition: the worker might well answer the ping just before it dies.
However, if you assume that a worker will not die during a request processing (you can do little in this case), you can reverse the flow of communication between the workers and the central queue. Let the worker fetch a request from the queue (using a REQ/REP connection) and have it send the answer along with the original envelope when the processing is done (using the same socket as above, or even better through a separate PUSH/PULL connection).
With this scenario, you know that a dead worker will not be sent requests, as it will be unable to fetch them (being dead…). Moreover, your central queue can even ensure that it receives an answer to every request in a given time. If it does not, it can put the request back in the queue so that a new worker will fetch it shortly after. This way, even if a worker dies while processing a request, the request will eventually be served.
(as a side note: be careful if the worker crashes because of a particular request - you do not want to kill your workers one by one, and might want to put a maximum number of tries for a request)
Edit: I wrote some code implementing the other direction to explain what I mean.

Async Request-Response Algorithm with response time limit

I am writing a Message Handler for an ebXML message passing application. The message follow the Request-Response Pattern. The process is straightforward: The Sender sends a message, the Receiver receives the message and sends back a response. So far so good.
On receipt of a message, the Receiver has a set Time To Respond (TTR) to the message. This could be anywhere from seconds to hours/days.
My question is this: How should the Sender deal with the TTR? I need this to be an async process, as the TTR could be quite long (several days). How can I somehow count down the timer, but not tie up system resources for large periods of time. There could be large volumes of messages.
My initial idea is to have a "Waiting" Collection, to which the message Id is added, along with its TTR expiry time. I would then poll the collection on a regular basis. When the timer expires, the message Id would be moved to an "Expired" Collection and the message transaction would be terminated.
When the Sender receives a response, it can check the "Waiting" collection for its matching sent message, and confirm the response was received in time. The message would then be removed from the collection for the next stage of processing.
Does this sound like a robust solution. I am sure this is a solved problem, but there is precious little information about this type of algorithm. I plan to implement it in C#, but the implementation language is kind of irrelevant at this stage I think.
Thanks for your input
Depending on number of clients you can use persistent JMS queues. One queue per client ID. The message will stay in the queue until a client connects to it to retrieve it.
I'm not understanding the purpose of the TTR. Is it more of a client side measure to mean that if the response cannot be returned within certain time then just don't bother sending it? Or is it to be used on the server to schedule the work and do what's required now and push the requests with later response time to be done later?
It's a broad question...

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