How to wait for a specific Job to be processed inside a Controller

[amitaibu]

I'm working on a Bid system (many items, and each one can have Bids offered for it). When a Bid is submitted, I'd like to process it inside a Job to guarantee that no two Bids are processed at the same time (so if two bidders offer the same price, the first bid is guaranteed to win).

My questions is now around the need of the controller to wait for that Job, before returning the answer from the Controller.

Maybe I could use Postgress Triggers, and watch for changes similar to how it's done on Runner.hs?- but I would also need to verify that the Job that triggered the change, is that Job the Controller inserted.

So my Controller would maybe look something like this:

action CreateBidAction = do

    -- Create a "Mailbox", so once our Job is processed, we could return a response from the Controller.
    mailbox <- newEmptyMVar

    -- Create the Job
    job <- newRecord @BidJob |> create

    -- This non existing function, should wait for the trigger and compare it with 
    -- the created Job.
    -- if Job is ours it will `putMvar` so Controller can proceed.
    watchForJob job


    -- Waiting for our Job..
    receivedJob <- takeMvar mailbox

So my question is how to approach the watchForJob job from the above example (unless someone has a better approach!)

[mpscholten]

You could try the following:

• redirect to a view after scheduling the job
• use auto refresh to have the view update whenever the job is completed

action CreateBidAction = do
    job <- newRecord @BidJob |> create
    redirectTo ShowBidJobAction { bidJobId = get #id bidJob }

action ShowBidJobAction { bidJobId } = autoRefresh do -- <---- we use auto refresh here
    bidJob <- fetch bidJobId
    bid <- fetch (get #bidId bidJob)
    render ShowBidJobView { .. }

with the view:

render ShowBidJobView { .. } = [hsx|
    {if status == JobStatusSucceeded then renderBid bid else "waiting for bid to process"}
|]
    where
        status = get #status bidJob

[amitaibu]

Thanks @mpscholten!

I guess with the new DataSync I could also achieve something similar if my frontend was in Elm.