Processing Transactions

After a transaction data structure is built, you must submit it to the transactor for processing. The transactor queues transactions and processes them serially.

Submitting Transactions

Once a transaction data structure is built, you submit it to the transactor by calling transact or transact-async.

In the Peer API, both functions return a Clojure future that will eventually contain the transaction's result.

In the Client API, datomic.client.api/transact will block until a transaction result is returned, while datomic.client.api.async/transact will return a core.async channel that will eventually contain the transaction's result.

Transaction Timeouts

When a transaction times out, the peer does not know whether the transaction succeeded, and will need to query a recent value of the database to discover what happened.

Monitoring Transactions

Peers can monitor all transactions being processed by the system's transactor. In the Peer API, tx-report-queue returns a queue of transaction notifications.

The queue delivers a report for every transaction submitted while a peer is connected to the database, even those submitted by other peers.

Reports are records with the following keys:

key	usage
:db-before	database value before the transaction
:db-after	database value after the transaction
:tx-data	datoms produced by the transaction
:tempids	argument to resolve-tempid

It is the responsibility of your application to empty the queue. When you are done monitoring the queue, remove it by calling remove-tx-report-queue otherwise it will continue to accumulate transaction reports and consume memory.

This example shows how to connect to the notification queue, and retrieve a transaction report from it:

(let [tx-report-q (d/tx-report-queue conn)
      tx          (.poll tx-report-q)]
  (d/remove-tx-report-queue conn)
  tx)

The tx-data of a transaction report contains the set of datoms created by a transaction (both assertions and retractions). The tx-data can be used as an input source for a query. The query below uses the tx-data and the database value after the transaction was applied to show each datom of the transaction.

[:find ?e ?aname ?v ?added
 :in $ [[?e ?a ?v _ ?added]]
 :where
 [?e ?a ?v _ ?added]
 [?a :db/ident ?aname]]

The query expects the db-after and tx-data values of a transaction report as its two input sources, in that order.

Check Query for more information on using sets of tuples as input sources in queries.