# Apache Cassandra to Scylla Migration Process¶

Migrating data from Apache Cassandra to an eventually consistent data store such as Scylla for a high volume, low latency application and verifying its consistency, is a multi-step process. It involves the following high level steps:

1. Creating the same schema from Apache Cassandra in Scylla, though there can be some variation
2. Configuring your application/s to perform dual writes (still reading only from Apache Cassandra)
3. Taking a snapshot of all to-be-migrated data from Apache Cassandra
5. Verification period: dual writes and reads, Scylla serves reads. Logging mismatches, until minimal data mismatch threshold is reached
6. Apache Cassandra End Of Life: Scylla only for reads and writes

Note

steps 2 and 5 are required for Live migration only (meaning with on-going traffic and no downtime).

Dual Writes: Application logic is updated to write to both DBs

Forklifting: Migrate historical data from Apache Cassandra SSTables to Scylla

Dual Reads: Ongoing validation of data sync between the two DBs

Live Migration: Migrating from DB-OLD to DB-NEW timeline

## Procedure¶

1. Create manually / Migrate your schema (keyspaces, tables and user defined type, if used) on / to your Scylla cluster. When migrating from Apache Cassandra 3.x some schema updates are required (see limitations and known issues section).
• Export schema from Apache Cassandra: cqlsh [IP] "-e DESC SCHEMA" > orig_schema.cql
• Import schema to Scylla: cqlsh [IP] --file 'adjusted_schema.cql'

Note

It is recommended to alter the schema of tables you plan to migrate as following:

• set compaction min_threshold to 2, allowing compactions to get rid of duplication faster. Migrating using sstable loader may create a lot of temporary duplication of the disk.
• Increase gc_grace_seconds (default 10 days) to a higher value, making sure you will not lose tombstones during the migration process. Recommended value is 315360000 (10 years).

Make sure to change both parameters back to the original value once the migration is done (using ALTER TABLE)

Note

Scylla Open Source 3.0 and later and Scylla Enterprise 2019.1 and later support Materialized View(MV) and Secondary Index(SI).

When migrating data from Apache Cassandra with MV or SI you can either:

• Create the MV and SI as part of the schema, so each new insert will be indexed.
• Upload all the data with sstableloader first, and only then create the secondary indexes and MVs

In either case, only use the sstableloader to load the base table sstable. Do not load the index and view data - let Scylla index for you.

1. If you wish to perform the migration process without any downtime, please configure your application/s to perform dual writes, to both data stores, Apache Cassandra and Scylla (see below code snippet for dual writes). Before doing that, and as a general guidance make sure to use client generated timestamp (writetime). If you do not, the data on Scylla and Apache Cassandra can be considered different, while it is the same.

Note: your application/s should continue reading and writing from Apache Cassandra until the entire migration process is completed, data integrity validated and dual writes and reads verification period performed to your satisfaction.

Dual writes and client generated timestamp Python code snippet

# put both writes (cluster 1 and cluster 2) into a list
writes = []
#insert 1st statement into db1 session, table 1
writes.append(db1.execute_async(insert_statement_prepared[0], values))
#insert 2nd statement into db2 session, table 2
writes.append(db2.execute_async(insert_statement_prepared[1], values))

# loop over futures and output success/fail
results = []
for i in range(0,len(writes)):
try:
row = writes[i].result()
results.append(1)
except Exception:
results.append(0)
#log exception if you like
#logging.exception('Failed write: %s', ('Cluster 1' if (i==0) else 'Cluster 2'))

results.append(values)
log(results)

#did we have failures?
if (results[0]==0):
#do something, like re-write to cluster 1
log('Write to cluster 1 failed')
if (results[1]==0):
#do something, like re-write to cluster 2
log('Write to cluster 2 failed')

for x in range(0,RANDOM_WRITES):
#explicitly set a writetime in microseconds
values = [ random.randrange(0,1000) , str(uuid.uuid4()) , int(time.time()*1000000) ]

execute( values )


See the full code example here

1. On each Apache Cassandra node, take a snapshot for every keyspace using nodetool snapshot command. This will flush all sstables to disk and generate a snapshots folder with an epoch timestamp for each underlying table in that keyspace.
Folder path post snapshot: /var/lib/cassandra/data/keyspace/table-[uuid]/snapshots/[epoch_timestamp]/
1. We strongly advise against running the sstableloader tool directly on the Scylla cluster, as it will consume resources from Scylla, instead you should run the sstableloader from intermediate node/s. To do that, you need to install scylla-tools-core package (it includes the sstableloader tool).

You need to make sure you have connectivity to both the Apache Cassandra and Scylla clusters. There are two ways to do that, both require having a file system in place (RAID is optional):

• Option 1 (recommended): copy the sstable files from the Apache Cassandra cluster to a local folder on the intermediate node.
• Option 2: NFS mount point on the intermediate node to the sstable files located in the Apache Cassandra nodes.
1. After installing the relevant pkgs (detailed in the links), edit /etc/exports file on each Apache Cassandra node and add the following in a single line:

[Full path to snapshot ‘epoch’ folder] [Scylla_IP](rw,sync,no_root_squash,no_subtree_check)

2. Restart NFS server sudo systemctl restart nfs-kernel-server

3. Create a new folder on one of the Scylla nodes and use it as a mount point to the Apache Cassandra node

Example:

sudo mount [Cassandra_IP]:[Full path to snapshots ‘epoch’ folder] /[ks]/[table]

Note: both the local folder, or the NFS mount point paths must end with /[ks]/[table] format, used by the sstableloader for parsing purposes (see sstableloader help for more details).

2. If you cannot use intermediate node/s (see the previous step), then you have two options:

• Option 1: Copy the sstable files to a local folder on one of your Scylla cluster nodes. Preferably on a disk or disk-array which is not part of the Scylla cluster RAID, yet still accessible for the sstableloader tool.

Note: copying it to the Scylla RAID, will require sufficient disk space (Apache Cassandra sstable snapshots size x2 < 50% of Scylla node capacity), in order to contain both the copied sstables files and the entire data migrated to Scylla (keyspace RF should also be taken into account).

• Option 2: NFS mount point on Scylla nodes to the sstable files located in the Apache Cassandra nodes (see NFS mount instructions in the previous step). This saves the additional disk space needed for the 1st option.

Note: both the local folder or the NFS mount point paths must end with /[ks]/[table] format, used by the sstableloader for parsing purposes (see sstableloader help for more details).

3. Use Scylla sstableloader tool (NOT the Apache Cassandra one which has the same name) to load the sstables. Running without any parameters will present the list of options and usage. Most important are the sstables directory and the Scylla node IP.

Examples:

• sstableloader -d [Scylla IP] .../[ks]/[table]
• sstableloader -d [scylla IP] .../[mount point] (in /[ks]/[table] format)
1. We recommend running several sstableloaders in parallel and utilizing all Scylla nodes as targets for sstable loading. Start with one keyspace and its underlying sstable files from all Apache Cassandra nodes. After completion, continue to the next keyspace and so on.

Note: limit the sstableloader speed by using the throttling -t parameter, considering your physical HW, live traffic load and network utilization (see sstableloader help for more details).

2. Once you completed loading the sstable files from all keyspaces, you can use cqlsh or any other tool to validate the data migrated successfully. We strongly recommend configuring your application to perform both writes and reads to/from both data stores. Apache Cassandra (as is, up to this point) and Scylla (now as primary) for a verification period. Keep track of the number of requests for which the data in both these data stores mismatched.

3. Apache Cassandra end of life: once you are confident in your Scylla cluster, you can flip the flag in your application/s, stop writes and reads against the Cassandra cluster, and make Scylla your sole target/source.

## Failure Handling¶

What should I do if sstableloader fails?

Each loading job is per keyspace/table_name, that means in any case of failure, you need to repeat the loading job. As you are loading the same data (partially loaded before the failure), compactions will take care of any duplication.

What should I do if an Apache Cassandra node fails?

If the node that failed was a node you were loading sstables from, then the sstableloader will also fail. If you were using RF>1 then the data exists on other node/s. Hence you can continue with the sstable loading from all the other Cassandra nodes. Once completed, all your data should be on Scylla.

What should I do if a Scylla node fails?

How to rollback and start from scratch?

1. Stop the dual writes to Scylla
2. Stop Scylla service sudo systemctl stop scylla-server
3. Use cqlsh to perform truncate on all data already loaded to Scylla
4. Start the dual writes again to Scylla
5. Take a new snapshot of all Cassandra nodes
6. Start loading sstables again to Scylla from the NEW snapshot folder

## Notes, Limitations and Known Issues¶

1. Duration data type is only supported in Scylla 2.1 and later (issue-2240). This is relevant only when migrating from Apache Cassandra 3.X.
2. Changes in table schema from Apache Cassandra 3.0 that requires adjustments for Scylla 2.x, 1.x table schema:
• Changes in create table (issue-8384)
• crc_check_chance out of compression options (issue-9839)
3. Scylla 2.x CQL client cqlsh does not display the millisecond values of a timestamp data type. (scylla-tools-java/issues #36)
4. Nodetool tablestats partition keys (estimated) number in Scylla, post migration from Apache Cassandra, differs by 20% less up to 120% more than the original amount in Cassandra (issue-2545)
5. Scylla 2.x is using Apache Cassandra 2.x file format. These means that migrating from Apache Cassandra 3.x to Scylla 2.x will result in a different storage space of the same data on the Scylla cluster. Scylla 3.x is using the same foramt as Cassandra 3.x

### Counters¶

In version 2.1, Apache Cassandra changed how the counters work. The previous design had some hard to fix issues which meant that there is no safe and general way of converting counter data from the old format to the new one. As a result, counter cells created before version 2.1 may contain old-format information even after migration to the latest Cassandra version. As Scylla implements only the new counter design, this imposes restrictions on how counters can be migrated from Cassandra.

Copying counter SSTables over to Scylla is unsafe and, by default, disallowed. Even if you use SSTableloader which is a safe way to copy the tables, it will refuse to load data in the legacy format.

Note

If you used the same Counter SSTables with Apache Cassandra from before version 2.1, the migration to Scylla will not work

Schema differences between Apache Cassandra 3.x and Scylla 2.x and 1.x

The following table illustrates the default schema differences between Apache Cassandra 3.x and Scylla 2.x, 1.x

Notable differences:

• ‘caching’ section is supported in Scylla, yet requires adjustments to the schema (see below).
• ‘crc_check_chance’ (marked in bold) is NOT supported in Scylla, make sure you remove it from the schema.
Apache Cassandra 3.10 (uses 3.x Schema) Scylla 2.x 1.x (uses Apache Cassandra 2.1 Schema)
CREATE KEYSPACE mykeyspace WITH replication =
{'class': 'SimpleStrategy', 'replication_factor': '1'}
AND durable_writes = true;

CREATE TYPE mykeyspace.udt_info (
birthday date,
nationality text,
height int);

CREATE TABLE
mykeyspace.all_types_no_counter_no_duration (
aascii ascii,
abigint bigint,
ablob blob,
aboolean boolean,
afloat float,
afrozen_udt frozen<udt_info>,
ainet inet,
aint int,
alist list<int>,
amap map<int, int>,
aset set<int>,
asmallint smallint,
atext text,
atime time,
atimestamp timestamp,
atimeuuid timeuuid,
atinyint tinyint,
atuple frozen<tuple<int, text>>,
auuid uuid,
avarchar text,
avarint varint,
PRIMARY KEY (aascii, abigint))
WITH CLUSTERING ORDER BY (abigint ASC)
AND bloom_filter_fp_chance = 0.01
AND comment = ''

CREATE KEYSPACE mykeyspace WITH replication =
{'class': 'SimpleStrategy', 'replication_factor': '1'}
AND durable_writes = true;

CREATE TYPE mykeyspace.udt_info (
birthday date,
nationality text,
height int);

CREATE TABLE
mykeyspace.all_types_no_counter_no_duration (
aascii ascii,
abigint bigint,
ablob blob,
aboolean boolean,
afloat float,
afrozen_udt frozen<udt_info>,
ainet inet,
aint int,
alist list<int>,
amap map<int, int>,
aset set<int>,
asmallint smallint,
atext text,
atime time,
atimestamp timestamp,
atimeuuid timeuuid,
atinyint tinyint,
atuple frozen<tuple<int, text>>,
auuid uuid,
avarchar text,
avarint varint,
PRIMARY KEY (aascii, abigint))
WITH CLUSTERING ORDER BY (abigint ASC)
AND bloom_filter_fp_chance = 0.01
AND comment = ''

AND caching = {'keys': 'ALL','rows_per_partition': 'NONE'}
AND compaction = {'class': 'org.apache.cassandra.db.
compaction.SizeTieredCompactionStrategy',
'max_threshold': '32', 'min_threshold': '4'}
AND compression = {'chunk_length_in_kb': '64','class':
'org.apache.cassandra.io.compress.LZ4Compressor'}
AND speculative_retry = '99PERCENTILE'


(Adjust for Apache Cassandra 2.1 schema)

AND caching = '{"keys":"ALL","rows_per_partition":"ALL"}'
AND compaction = {'class': 'SizeTieredCompactionStrategy'}
AND compression = {'sstable_compression':
'org.apache.cassandra.io.compress.LZ4Compressor'}
AND speculative_retry = '99.0PERCENTILE'

AND crc_check_chance = 1.0 (Remove from Apache Cassandra 2.1 schema)
AND dclocal_read_repair_chance = 0.1
AND default_time_to_live = 0
AND gc_grace_seconds = 864000
AND max_index_interval = 2048
AND memtable_flush_period_in_ms = 0
AND min_index_interval = 128

AND dclocal_read_repair_chance = 0.1