Question

I hope my question is suitable for this site and is not too broad but I am having trouble designing the following architecture:

I have two sites. Site 1 is responsible for transferring credits between users. Site 2 is responsible for providing these users with services/products which can be paid with the credits they own in Site 1.

Lets say I have 1000 credits on Site 1. I have a service/product which costs 50 credits on Site 2 and a user wants to purchase it with his amount of credits he owns in Site 1.

Both sites communicate with REST. So for example when a user wants to purchase a service/product Site 2 prepares its request and sends it to Site 1 which makes the transaction and confirms to Site 2 that the transaction was successful (e.g. the user had enough credits for the service/product and those credits were successfully transferred to the destination)

Now here’s the tricky part. In Site 1 I have the following logic:

Begin transaction
update user set credits -= 50 where id = 1
update user set credits += 50 where id = 2
REST CALL (Site 2) Success
Site 2 response - OK, commit transaction
Commit

Since REST is a call to a different site the transaction might take some time to complete. In the mean time is the whole table locked for any transactions or are the rows for user 1 and user 2 locked? Is this the proper way to implement my logic? Am I missing something?

Thank you in advance.

Answer 1

This is in response to your question on Casey’s answer:

Yes, as long as you do it like this:

• Site 2:

  1. Customer logs in.
  2. Ask Site 1 for credit total & transaction history (GET request) for this user (user 1).
  3. (Any awaiting transactions which receive ‘transaction succeeded’ responses are made available for download/dispatch)
  4. Use credit total to enable “Buy” buttons for things that can be afforded.
  5. Customer clicks a Buy button
  6. generate a transaction ID unique to site 2, store in database along with details of who bought what, when they did so, with state = pending. tell user transaction has been received and they should be notified soon whether it was successful (HTTP 202 response)
  7. POST purchase request to Site 1, including authentication (don’t want forged requests to cause people to spend money they don’t want to spend) and the transaction ID.

• Site 1

  1. validate authentication
  2. verify that Site 2’s transactionID has not been used before by site 2. if it has, return an error, if not:
  3. Begin transaction
  4. update user set credits -= 50 where id = 1
  5. update user set credits += 50 where id = 2
  6. insert into transactions remoteSiteID = ‘Site2’, remoteTransactionID = tID, user = 1
     You would not need the remoteSiteID field if site2 is the only site using credits from site1
  7. Commit
  8. REST CALL (Site 2) Success

• Site 2:

EITHER:
1. Receive REST success call, make purchase available for download/dispatch, display some message to user saying purchase processing complete. Update local transaction record, state=succeeded.

OR
2. Site 2 is down. Transaction success will be noted next time background polling process runs (which checks status of purchase requests awaiting responses) or next time customer logs in (in which case poll is initiated too–step 3 in first list)

If you have not received a response to a transaction, perform a GET using the transaction ID. If the response is an error, Site 1 did not receive the original request, Site 2 is free to repeat the transaction (POST) request. If the response is ‘transaction failed’ then the user didn’t have enough credits, update transaction record on site 2 accordingly. if result is ‘transaction succeeded’ record that too.

if a transaction fails N number of times, or a certain period elapses since the user clicked the button (say 5 minutes) then Site 2 stops retrying the purchase.