Retry transactions

[ericnormand]

Add a function + macro for doing retries when a ConcurrentModificationException happens in datomic.

This function allows us to make atomic updates to a database. Currently, we have many non-atomic updates to the database. You can identify a non-atomic update because it:

• Reads a value from the database at time t0 (does a query, gets an entity)
• Constructs a datomic transaction data structure
• Calls datomic/transact on the data structure at time tn

If anything has modified the database between t0 and tn, this transaction data may be stale, yet still be recorded. For instance:

• Server 1 wants to increment page-view. It reads the current page-view count at t0, which is 100.
• Server 2 wants to increment page-view. It reads the current page-view count at t0, which is 100.
• Server 2 is slightly faster, and transacts (inc page-view) at t1. Datomic stores page-view= 101.
• Server 1 transacts (inc page-view) at t2. In its memory, page-view is still 100. Datomic stores page-view = 101.

After two increments, page-view should be 102, but it's 101. This function, along with the database functions defined in #8, will help us make sure everything is counted.

Using this new function and the :transact database function, the corrected situation above will look like:

• Server 1 wants to increment page-view. It reads the current page-view count at t0, which is 100.
• Server 2 wants to increment page-view. It reads the current page-view count at t0, which is 100.
• Server 2 is slightly faster, and transacts [:transact userid :page-view 100 101] at t1. That means "store page-view=101 given that it is equal to 100". Datomic stores page-view= 101.
• Server 1 transacts [:transact userid :page-view 100 101] at t2. This fails with a ConcurrentModificationException.
• The retry function catches the exception which starts the Server 1 steps over.
• Server 1 reads the current page-view count at t3, which is 101.
• Server 1 transacts [:transact userid :page-view 101 102] at t4. Datomic stores page-view= 102.

Two increments, and page-view is 102, which is correct!

There is an example in election-notification-works.

[tie-rack]

Why was this closed?

[ericnormand]

I merged it together with the related branch with the transaction functions.

On Thu, Sep 10, 2015 at 8:38 AM, Christopher Shea notifications@github.com wrote:

Why was this closed?

— Reply to this email directly or view it on GitHub https://github.com/democracyworks/datomic-toolbox/pull/11#issuecomment-139236759 .