This small library allows you to incrementally save the transaction log of a Datomic cloud database (may also work with on-prem, though there is an official story there).
This library is not maintained, and was not usable in the real production scenarios we needed, though the approach is technically sound. Use at your own risk. We do not accept issues or pull requests on this repository. We recommend you contact the Datomic team for an officially-supported backup/restore solution.
This library supports creating a clone of a Datomic database by reading the transaction log and saving that to a durable storage (incrementally). Restoration of the database involves playing back this transaction log into a new database. The one major complication is that the new database will assign different IDs to the new entities as they are transacted, thus, we must keep track of what these new IDs are in the new database so we can remap them in later incremental steps.
The storage for the backups is represented with the protocol:
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BackupStore- A place to store the actual transactions
This library comes with a sample BackupStore based on Filesystems and AWS S3.
If you want to use the supplied S3 file store then you MUST add the following to your dependencies (they are not included as transitive dependencies in case you want to use something else):
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com.datomic/dev-local- if you want to play with it in dev mode -
com.amazonaws/aws-java-sdk-s3- if you want to use the S3BackupStore
You will then need to add the backup library to your application that you are deploying to the cloud, and run (on a periodic basis) an incremental backup.
Basically create a BackupStore (db-store in the example) that your restoration process will be able to see,
and then periodically run a function like this:
(defn backup-step!
"Backs up a maximum of txns-per-backup-segment, starting where we last left off. Returns the actual number
of transactions written."
[dbname conn db-store txns-per-backup-segment]
(let [{:keys [end-t]} (dcbp/last-segment-info db-store dbname)]
(if (and end-t (pos? end-t))
(let [next-start (inc end-t)
max-t (min (+ next-start txns-per-backup-segment) (:t (d/db conn)))]
(if (pos? (- max-t next-start))
(try
(let [{:keys [start-t end-t]} (cloning/backup-segment! dbname conn db-store next-start max-t)]
(inc (- end-t start-t)))
(catch Exception e
(log/error e "Incremental backup step failed!")
0))
(do
(log/trace "There was nothing new to back up on" dbname)
0)))
(do
(log/warn "Not doing incremental backup step for" dbname "because there is no base backup in the store.")
0))))Basically you just call backup-segment! on a periodic basis and it makes a new chunk.
You can run a fast initial backup with backup!, which is capable of creating a large number of chunks (in parallel)
for your initial step. Then you can start running the above loop without it taking as long to catch up.
On a different machine that uses a different Datomic deployment (WARNING: restore uses the db names you use when backing up), you will need to make a simple application that uses this library to read the backup segments and apply them to a new (empty) database. The restore restores timestamps of transactions, so you MUST NOT transact anything against the target databases, or restore will not be able to proceed.
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ℹ️
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We could relax this and lose the transaction times…perhaps as an option. Not implemented. |
The restore-segment! function automatically figures out the next segment to restore, and applies it. It returns
:nothing-new-available if there is nothing else to do, and :restored-segment it restored a segment. The
simplest restore process would be:
(while (= :restored-segment (cloning/restore-segment! dbname conn dbstore {})))Of course you’ll want to do something more complex for streaming replication, since new segments will arrive after this loop finishes. You’ll probably want to log progress, sleep when there is nothing available, etc.
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Two schema items are added to the TARGET database ::cloning/last-source-transaction and ::cloning/original-id.
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last-source-transactionis the last t that has been restored. It is an ident, and there is exactly ONE datom in the database (with no history) that has this as both that ID and attribute:[::cloning/last-source-transaction ::cloning/last-source-transaction 2] -
original-idis stored on every restored entity. It is the ID that the entity has in the SOURCE database, and is used for remapping incoming IDs, and as audit/source information.
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You cannot write transactions to the target database during a restore and expect the restore to be able to continue. The restore tries to include the reified transactions to maintain any auditing data you may have added to the database. As such it is also restoring the original tx time, which it cannot do if you transact something that has a time close to "now".
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Using an elision predicate on the restore can cause transactions to become empty. Those transactions will be still be added to the database so that
last-source-transactionis updated. This ensures that interrupted restores can be properly restarted, and that the basis of every transaction will match the original. -
If you’ve been running Datomic Cloud for a while, you probably have upgraded storage and base schema at some point. The restore code should handle upgrades fine, but of course if you were to try to restore to an older version of a Datomic cloud template from a newer source it will almost certainly fail.
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