asynchronous data update

rubyonrails-talk
1

Hi, We are developing an e-commerce site in ROR. We have a table which stores the prices of our products. To save on repeated database queries, we read the table and store it in memory.

As the admin can change the prices through the management console we are facing the problem of updating the in-memory data. We are running the application on mutiple servers,with multiple instances of mongrels serving the requests. Thus all the mongrels have their own copy of the in-memory data.

Now we think there are few issues

1. We would need to update it on every mongrel. 2. We don't want the scene where there are different servers/mongrels have different version of the pricing. 3. We are hoping there is some way where we won't need to shut down the whole site !

We tried to use the SEE-SAW. but i think it doesn't take the business of mongrels into account. Thus if one of the mongrels is serving a long request while it receives the restart request, things go wrong and the mongrel dies in the end.

We have been discussing this issue and come up with the following solution.

1. Copy the prices to the user's session (which is anyway read every time) and use that to create the shopping cart etc. 2. Update the each of the mongrels through an http request/ web service.

This way the user will see only one version of the prices throughout his transaction. But this again has some problem. What if a user somehow keeps his session to be alive for a very long duration ( if he knows the pending price update.) Even though we don't keep persistent shopping cart, a malicious user can, though a script, keep his session alive for days/ weeks or even months and thus use the old prices. We did use this trick for our online submission of assignments in college :wink: As a last resort we can check the staleness of session prices in the last checkout step.

We think this is a common problem and every e-commerce site should be facing this but can't seem to find any published solution. We are hoping some can put some light or point us in the right direction. Any help would be appreciated

A.K

2

a.k wrote:

We are developing an e-commerce site in ROR. We have a table which stores the prices of our products. To save on repeated database queries, we read the table and store it in memory.

As the admin can change the prices through the management console we are facing the problem of updating the in-memory data. We are running the application on mutiple servers,with multiple instances of mongrels serving the requests. Thus all the mongrels have their own copy of the in-memory data. ... 2. Update the each of the mongrels through an http request/ web service.

The above is one of the methods I've used. In your case the mongrel getting the price change update would do a GET of some 'reload_prices' action on each handler. You can limit the IP addresses able to call this action.

Another method I've used is Ruby mmap. This allows no-overhead sharing of a Ruby string between a set of mongrels on a single server. This only really comes into its own when the changes you want to propagate are small but frequent.