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Disaster Recovery

Disaster Recovery is very much mission critical for any software. Especially when it comes to data systems, the impact could be very serious leading to delay in business decisions or even wrong business decisions at times. Apache Hudi has two operations to assist you in recovering data from a previous state: "savepoint" and "restore".

Savepoint

As the name suggest, "savepoint" saves the table as of the commit time, so that it lets you restore the table to this savepoint at a later point in time if need be. Care is taken to ensure cleaner will not clean up any files that are savepointed. On similar lines, savepoint cannot be triggered on a commit that is already cleaned up. In simpler terms, this is synonymous to taking a backup, just that we don't make a new copy of the table, but just save the state of the table elegantly so that we can restore it later when in need.

Restore

This operation lets you restore your table to one of the savepoint commit. This operation cannot be undone (or reversed) and so care should be taken before doing a restore. Hudi will delete all data files and commit files (timeline files) greater than the savepoint commit to which the table is being restored. You should pause all writes to the table when performing a restore since they are likely to fail while the restore is in progress. Also, reads could also fail since snapshot queries will be hitting latest files which has high possibility of getting deleted with restore.

Runbook

Savepoint and restore can only be triggered from hudi-cli. Lets walk through an example of how one can take savepoint and later restore the state of the table.

Lets create a hudi table via spark-shell. I am going to trigger few batches of inserts.

import org.apache.hudi.QuickstartUtils._
import scala.collection.JavaConversions._
import org.apache.spark.sql.SaveMode._
import org.apache.hudi.DataSourceReadOptions._
import org.apache.hudi.DataSourceWriteOptions._
import org.apache.hudi.config.HoodieWriteConfig._

val tableName = "hudi_trips_cow"
val basePath = "file:///tmp/hudi_trips_cow"
val dataGen = new DataGenerator

// spark-shell
val inserts = convertToStringList(dataGen.generateInserts(10))
val df = spark.read.json(spark.sparkContext.parallelize(inserts, 2))
df.write.format("hudi").
options(getQuickstartWriteConfigs).
option(PRECOMBINE_FIELD_OPT_KEY, "ts").
option(RECORDKEY_FIELD_OPT_KEY, "uuid").
option(PARTITIONPATH_FIELD_OPT_KEY, "partitionpath").
option(TABLE_NAME, tableName).
mode(Overwrite).
save(basePath)

Each batch inserst 10 records. Repeating for 4 more batches.


val inserts = convertToStringList(dataGen.generateInserts(10))
val df = spark.read.json(spark.sparkContext.parallelize(inserts, 2))
df.write.format("hudi").
options(getQuickstartWriteConfigs).
option(PRECOMBINE_FIELD_OPT_KEY, "ts").
option(RECORDKEY_FIELD_OPT_KEY, "uuid").
option(PARTITIONPATH_FIELD_OPT_KEY, "partitionpath").
option(TABLE_NAME, tableName).
mode(Append).
save(basePath)

Total record count should be 50.

val tripsSnapshotDF = spark.
read.
format("hudi").
load(basePath)
tripsSnapshotDF.createOrReplaceTempView("hudi_trips_snapshot")

spark.sql("select count(partitionpath, uuid) from hudi_trips_snapshot ").show()

+--------------------------+
|count(partitionpath, uuid)|
+--------------------------+
| 50|
+--------------------------+

Let's take a look at the timeline after 5 batch of inserts.

ls -ltr /tmp/hudi_trips_cow/.hoodie 
total 128
drwxr-xr-x 2 nsb wheel 64 Jan 28 16:00 archived
-rw-r--r-- 1 nsb wheel 546 Jan 28 16:00 hoodie.properties
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:00 20220128160040171.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:00 20220128160040171.inflight
-rw-r--r-- 1 nsb wheel 4374 Jan 28 16:00 20220128160040171.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:01 20220128160124637.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:01 20220128160124637.inflight
-rw-r--r-- 1 nsb wheel 4414 Jan 28 16:01 20220128160124637.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160226172.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160226172.inflight
-rw-r--r-- 1 nsb wheel 4427 Jan 28 16:02 20220128160226172.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160229636.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160229636.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:02 20220128160229636.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160245447.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160245447.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:02 20220128160245447.commit

Let's trigger a savepoint as of the latest commit. Savepoint can only be done via hudi-cli.

./hudi-cli.sh

connect --path /tmp/hudi_trips_cow/
commits show
set --conf SPARK_HOME=<SPARK_HOME>
savepoint create --commit 20220128160245447 --sparkMaster local[2]

Let's check the timeline after savepoint.

ls -ltr /tmp/hudi_trips_cow/.hoodie
total 136
drwxr-xr-x 2 nsb wheel 64 Jan 28 16:00 archived
-rw-r--r-- 1 nsb wheel 546 Jan 28 16:00 hoodie.properties
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:00 20220128160040171.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:00 20220128160040171.inflight
-rw-r--r-- 1 nsb wheel 4374 Jan 28 16:00 20220128160040171.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:01 20220128160124637.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:01 20220128160124637.inflight
-rw-r--r-- 1 nsb wheel 4414 Jan 28 16:01 20220128160124637.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160226172.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160226172.inflight
-rw-r--r-- 1 nsb wheel 4427 Jan 28 16:02 20220128160226172.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160229636.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160229636.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:02 20220128160229636.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160245447.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160245447.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:02 20220128160245447.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:05 20220128160245447.savepoint.inflight
-rw-r--r-- 1 nsb wheel 1168 Jan 28 16:05 20220128160245447.savepoint

You could notice that savepoint meta files are added which keeps track of the files that are part of the latest table snapshot.

Now, lets continue adding few more batches of inserts. Repeat below commands for 3 times.

val inserts = convertToStringList(dataGen.generateInserts(10))
val df = spark.read.json(spark.sparkContext.parallelize(inserts, 2))
df.write.format("hudi").
options(getQuickstartWriteConfigs).
option(PRECOMBINE_FIELD_OPT_KEY, "ts").
option(RECORDKEY_FIELD_OPT_KEY, "uuid").
option(PARTITIONPATH_FIELD_OPT_KEY, "partitionpath").
option(TABLE_NAME, tableName).
mode(Append).
save(basePath)

Total record count will be 80 since we have done 8 batches in total. (5 until savepoint and 3 after savepoint)

val tripsSnapshotDF = spark.
read.
format("hudi").
load(basePath)
tripsSnapshotDF.createOrReplaceTempView("hudi_trips_snapshot")

spark.sql("select count(partitionpath, uuid) from hudi_trips_snapshot ").show()
+--------------------------+
|count(partitionpath, uuid)|
+--------------------------+
| 80|
+--------------------------+

Let's say something bad happened and you want to restore your table to a older snapshot. As we called out earlier, we can trigger restore only from hudi-cli. And do remember to bring down all of your writer processes while doing a restore.

Lets checkout timeline once, before we trigger the restore.

ls -ltr /tmp/hudi_trips_cow/.hoodie
total 208
drwxr-xr-x 2 nsb wheel 64 Jan 28 16:00 archived
-rw-r--r-- 1 nsb wheel 546 Jan 28 16:00 hoodie.properties
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:00 20220128160040171.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:00 20220128160040171.inflight
-rw-r--r-- 1 nsb wheel 4374 Jan 28 16:00 20220128160040171.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:01 20220128160124637.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:01 20220128160124637.inflight
-rw-r--r-- 1 nsb wheel 4414 Jan 28 16:01 20220128160124637.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160226172.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160226172.inflight
-rw-r--r-- 1 nsb wheel 4427 Jan 28 16:02 20220128160226172.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160229636.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160229636.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:02 20220128160229636.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160245447.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160245447.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:02 20220128160245447.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:05 20220128160245447.savepoint.inflight
-rw-r--r-- 1 nsb wheel 1168 Jan 28 16:05 20220128160245447.savepoint
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:06 20220128160620557.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:06 20220128160620557.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:06 20220128160620557.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:06 20220128160627501.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:06 20220128160627501.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:06 20220128160627501.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:06 20220128160630785.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:06 20220128160630785.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:06 20220128160630785.commit

If you are continuing in the same hudi-cli session, you can just execute "refresh" so that table state gets refreshed to its latest state. If not, connect to the table again.

./hudi-cli.sh

connect --path /tmp/hudi_trips_cow/
commits show
set --conf SPARK_HOME=<SPARK_HOME>
savepoints show
╔═══════════════════╗
║ SavepointTime ║
╠═══════════════════╣
20220128160245447
╚═══════════════════╝
savepoint rollback --savepoint 20220128160245447 --sparkMaster local[2]

Hudi table should have been restored to the savepointed commit 20220128160245447. Both data files and timeline files should have been deleted.

ls -ltr /tmp/hudi_trips_cow/.hoodie
total 152
drwxr-xr-x 2 nsb wheel 64 Jan 28 16:00 archived
-rw-r--r-- 1 nsb wheel 546 Jan 28 16:00 hoodie.properties
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:00 20220128160040171.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:00 20220128160040171.inflight
-rw-r--r-- 1 nsb wheel 4374 Jan 28 16:00 20220128160040171.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:01 20220128160124637.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:01 20220128160124637.inflight
-rw-r--r-- 1 nsb wheel 4414 Jan 28 16:01 20220128160124637.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160226172.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160226172.inflight
-rw-r--r-- 1 nsb wheel 4427 Jan 28 16:02 20220128160226172.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160229636.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160229636.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:02 20220128160229636.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:02 20220128160245447.commit.requested
-rw-r--r-- 1 nsb wheel 2594 Jan 28 16:02 20220128160245447.inflight
-rw-r--r-- 1 nsb wheel 4428 Jan 28 16:02 20220128160245447.commit
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:05 20220128160245447.savepoint.inflight
-rw-r--r-- 1 nsb wheel 1168 Jan 28 16:05 20220128160245447.savepoint
-rw-r--r-- 1 nsb wheel 0 Jan 28 16:07 20220128160732437.restore.inflight
-rw-r--r-- 1 nsb wheel 4152 Jan 28 16:07 20220128160732437.restore

Lets check the total record count in the table. Should match the records we had, just before we triggered the savepoint.

val tripsSnapshotDF = spark.
read.
format("hudi").
load(basePath)
tripsSnapshotDF.createOrReplaceTempView("hudi_trips_snapshot")

spark.sql("select count(partitionpath, uuid) from hudi_trips_snapshot ").show()
+--------------------------+
|count(partitionpath, uuid)|
+--------------------------+
| 50|
+--------------------------+

As you could see, entire table state is restored back to the commit which was savepointed. Users can choose to trigger savepoint at regular cadence and keep deleting older savepoints when new ones are created. Hudi-cli has a command "savepoint delete" to assist in deleting a savepoint. Please do remember that cleaner may not clean the files that are savepointed. And so users should ensure they delete the savepoints from time to time. If not, the storage reclamation may not happen.

Note: Savepoint and restore for MOR table is available only from 0.11.