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Cassandra Documentation

Version:

5.0

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Backups

Apache Cassandra stores data in immutable SSTable files. Backups in Apache Cassandra database are backup copies of the database data that is stored as SSTable files. Backups are used for several purposes including the following:

  • To store a data copy for durability

  • To be able to restore a table if table data is lost due to node/partition/network failure

  • To be able to transfer the SSTable files to a different machine; for portability

Types of Backups

Apache Cassandra supports two kinds of backup strategies.

  • Snapshots

  • Incremental Backups

A snapshot is a copy of a table’s SSTable files at a given time, created via hard links. The DDL to create the table is stored as well. Snapshots may be created by a user or created automatically. The setting snapshot_before_compaction in the cassandra.yaml file determines if snapshots are created before each compaction. By default, snapshot_before_compaction is set to false. Snapshots may be created automatically before keyspace truncation or dropping of a table by setting auto_snapshot to true (default) in cassandra.yaml. Truncates could be delayed due to the auto snapshots and another setting in cassandra.yaml determines how long the coordinator should wait for truncates to complete. By default Cassandra waits 60 seconds for auto snapshots to complete.

An incremental backup is a copy of a table’s SSTable files created by a hard link when memtables are flushed to disk as SSTables. Typically incremental backups are paired with snapshots to reduce the backup time as well as reduce disk space. Incremental backups are not enabled by default and must be enabled explicitly in cassandra.yaml (with incremental_backups setting) or with nodetool. Once enabled, Cassandra creates a hard link to each SSTable flushed or streamed locally in a backups/ subdirectory of the keyspace data. Incremental backups of system tables are also created.

Data Directory Structure

The directory structure of Cassandra data consists of different directories for keyspaces, and tables with the data files within the table directories. Directories backups and snapshots to store backups and snapshots respectively for a particular table are also stored within the table directory. The directory structure for Cassandra is illustrated in Figure 1.

Data directory structure for backups

Figure 1. Directory Structure for Cassandra Data

Setting Up Example Tables for Backups and Snapshots

In this section we shall create some example data that could be used to demonstrate incremental backups and snapshots. We have used a three node Cassandra cluster. First, the keyspaces are created. Then tables are created within a keyspace and table data is added. We have used two keyspaces cqlkeyspace and catalogkeyspace with two tables within each.

Create the keyspace cqlkeyspace:

CREATE KEYSPACE cqlkeyspace
   WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 3};

Create two tables t and t2 in the cqlkeyspace keyspace.

USE cqlkeyspace;
CREATE TABLE t (
   id int,
   k int,
   v text,
   PRIMARY KEY (id)
);
CREATE TABLE t2 (
   id int,
   k int,
   v text,
   PRIMARY KEY (id)
);

Add data to the tables:

INSERT INTO t (id, k, v) VALUES (0, 0, 'val0');
INSERT INTO t (id, k, v) VALUES (1, 1, 'val1');

INSERT INTO t2 (id, k, v) VALUES (0, 0, 'val0');
INSERT INTO t2 (id, k, v) VALUES (1, 1, 'val1');
INSERT INTO t2 (id, k, v) VALUES (2, 2, 'val2');

Query the table to list the data:

SELECT * FROM t;
SELECT * FROM t2;

results in

id | k | v
----+---+------
 1 | 1 | val1
 0 | 0 | val0

 (2 rows)


id | k | v
----+---+------
 1 | 1 | val1
 0 | 0 | val0
 2 | 2 | val2

 (3 rows)

Create a second keyspace catalogkeyspace:

CREATE KEYSPACE catalogkeyspace
   WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 3};

Create two tables journal and magazine in catalogkeyspace:

USE catalogkeyspace;
CREATE TABLE journal (
   id int,
   name text,
   publisher text,
   PRIMARY KEY (id)
);

CREATE TABLE magazine (
   id int,
   name text,
   publisher text,
   PRIMARY KEY (id)
);

Add data to the tables:

INSERT INTO journal (id, name, publisher) VALUES (0, 'Apache Cassandra Magazine', 'Apache Cassandra');
INSERT INTO journal (id, name, publisher) VALUES (1, 'Couchbase Magazine', 'Couchbase');

INSERT INTO magazine (id, name, publisher) VALUES (0, 'Apache Cassandra Magazine', 'Apache Cassandra');
INSERT INTO magazine (id, name, publisher) VALUES (1, 'Couchbase Magazine', 'Couchbase');

Query the tables to list the data:

SELECT * FROM catalogkeyspace.journal;
SELECT * FROM catalogkeyspace.magazine;

results in

id | name                      | publisher
----+---------------------------+------------------
 1 |        Couchbase Magazine |        Couchbase
 0 | Apache Cassandra Magazine | Apache Cassandra

 (2 rows)

id | name                      | publisher
----+---------------------------+------------------
 1 |        Couchbase Magazine |        Couchbase
 0 | Apache Cassandra Magazine | Apache Cassandra

 (2 rows)

Snapshots

In this section, we demonstrate creating snapshots. The command used to create a snapshot is nodetool snapshot with the usage:

$ nodetool help snapshot

results in

NAME
       nodetool snapshot - Take a snapshot of specified keyspaces or a snapshot
       of the specified table

SYNOPSIS
       nodetool [(-h <host> | --host <host>)] [(-p <port> | --port <port>)]
               [(-pp | --print-port)] [(-pw <password> | --password <password>)]
               [(-pwf <passwordFilePath> | --password-file <passwordFilePath>)]
               [(-u <username> | --username <username>)] snapshot
               [(-cf <table> | --column-family <table> | --table <table>)]
               [(-kt <ktlist> | --kt-list <ktlist> | -kc <ktlist> | --kc.list <ktlist>)]
               [(-sf | --skip-flush)] [(-t <tag> | --tag <tag>)] [--] [<keyspaces...>]

OPTIONS
       -cf <table>, --column-family <table>, --table <table>
           The table name (you must specify one and only one keyspace for using
           this option)

       -h <host>, --host <host>
           Node hostname or ip address

       -kt <ktlist>, --kt-list <ktlist>, -kc <ktlist>, --kc.list <ktlist>
           The list of Keyspace.table to take snapshot.(you must not specify
           only keyspace)

       -p <port>, --port <port>
           Remote jmx agent port number

       -pp, --print-port
           Operate in 4.0 mode with hosts disambiguated by port number

       -pw <password>, --password <password>
           Remote jmx agent password

       -pwf <passwordFilePath>, --password-file <passwordFilePath>
           Path to the JMX password file

       -sf, --skip-flush
           Do not flush memtables before snapshotting (snapshot will not
           contain unflushed data)

       -t <tag>, --tag <tag>
           The name of the snapshot

       -u <username>, --username <username>
           Remote jmx agent username

       --
           This option can be used to separate command-line options from the
           list of argument, (useful when arguments might be mistaken for
           command-line options

       [<keyspaces...>]
           List of keyspaces. By default, all keyspaces

Configuring for Snapshots

To demonstrate creating snapshots with Nodetool on the commandline we have set auto_snapshots setting to false in the cassandra.yaml file:

auto_snapshot: false

Also set snapshot_before_compaction to false to disable creating snapshots automatically before compaction:

snapshot_before_compaction: false

Creating Snapshots

Before creating any snapshots, search for snapshots and none will be listed:

$ find -name snapshots

We shall be using the example keyspaces and tables to create snapshots.

Taking Snapshots of all Tables in a Keyspace

Using the syntax above, create a snapshot called catalog-ks for all the tables in the catalogkeyspace keyspace:

$ nodetool snapshot --tag catalog-ks catalogkeyspace

results in

Requested creating snapshot(s) for [catalogkeyspace] with snapshot name [catalog-ks] and
options {skipFlush=false}
Snapshot directory: catalog-ks

Using the find command above, the snapshots and snapshots directories are now found with listed files similar to:

./cassandra/data/data/catalogkeyspace/journal-296a2d30c22a11e9b1350d927649052c/snapshots
./cassandra/data/data/catalogkeyspace/magazine-446eae30c22a11e9b1350d927649052c/snapshots

Snapshots of all tables in multiple keyspaces may be created similarly:

$ nodetool snapshot --tag catalog-cql-ks catalogkeyspace, cqlkeyspace

Taking Snapshots of Single Table in a Keyspace

To take a snapshot of a single table the nodetool snapshot command syntax becomes as follows:

$ nodetool snapshot --tag <tag> --table <table>  --<keyspace>

Using the syntax above, create a snapshot for table magazine in keyspace catalogkeyspace:

$ nodetool snapshot --tag magazine --table magazine  catalogkeyspace

results in

Requested creating snapshot(s) for [catalogkeyspace] with snapshot name [magazine] and
options {skipFlush=false}
Snapshot directory: magazine

Taking Snapshot of Multiple Tables from same Keyspace

To take snapshots of multiple tables in a keyspace the list of Keyspace.table must be specified with option --kt-list. For example, create snapshots for tables t and t2 in the cqlkeyspace keyspace:

$ nodetool snapshot --kt-list cqlkeyspace.t,cqlkeyspace.t2 --tag multi-table

results in

Requested creating snapshot(s) for ["CQLKeyspace".t,"CQLKeyspace".t2] with snapshot name [multi-
table] and options {skipFlush=false}
Snapshot directory: multi-table

Multiple snapshots of the same set of tables may be created and tagged with a different name. As an example, create another snapshot for the same set of tables t and t2 in the cqlkeyspace keyspace and tag the snapshots differently:

$ nodetool snapshot --kt-list cqlkeyspace.t, cqlkeyspace.t2 --tag multi-table-2

results in

Requested creating snapshot(s) for ["CQLKeyspace".t,"CQLKeyspace".t2] with snapshot name [multi-
table-2] and options {skipFlush=false}
Snapshot directory: multi-table-2

Taking Snapshot of Multiple Tables from Different Keyspaces

To take snapshots of multiple tables that are in different keyspaces the command syntax is the same as when multiple tables are in the same keyspace. Each <keyspace>.<table> must be specified separately in the --kt-list option.

For example, create a snapshot for table t in the cqlkeyspace and table journal in the catalogkeyspace and tag the snapshot multi-ks.

$ nodetool snapshot --kt-list catalogkeyspace.journal,cqlkeyspace.t --tag multi-ks

results in

Requested creating snapshot(s) for [catalogkeyspace.journal,cqlkeyspace.t] with snapshot
name [multi-ks] and options {skipFlush=false}
Snapshot directory: multi-ks

Listing Snapshots

To list snapshots use the nodetool listsnapshots command. All the snapshots that we created in the preceding examples get listed:

$ nodetool listsnapshots

results in

Snapshot Details:
Snapshot name Keyspace name   Column family name True size Size on disk
multi-table   cqlkeyspace     t2                 4.86 KiB  5.67 KiB
multi-table   cqlkeyspace     t                  4.89 KiB  5.7 KiB
multi-ks      cqlkeyspace     t                  4.89 KiB  5.7 KiB
multi-ks      catalogkeyspace journal            4.9 KiB   5.73 KiB
magazine      catalogkeyspace magazine           4.9 KiB   5.73 KiB
multi-table-2 cqlkeyspace     t2                 4.86 KiB  5.67 KiB
multi-table-2 cqlkeyspace     t                  4.89 KiB  5.7 KiB
catalog-ks    catalogkeyspace journal            4.9 KiB   5.73 KiB
catalog-ks    catalogkeyspace magazine           4.9 KiB   5.73 KiB

Total TrueDiskSpaceUsed: 44.02 KiB

Finding Snapshots Directories

The snapshots directories may be listed with find –name snapshots command:

$ find -name snapshots

results in

./cassandra/data/data/cqlkeyspace/t-d132e240c21711e9bbee19821dcea330/snapshots
./cassandra/data/data/cqlkeyspace/t2-d993a390c22911e9b1350d927649052c/snapshots
./cassandra/data/data/catalogkeyspace/journal-296a2d30c22a11e9b1350d927649052c/snapshots
./cassandra/data/data/catalogkeyspace/magazine-446eae30c22a11e9b1350d927649052c/snapshots

To list the snapshots for a particular table first change to the snapshots directory for that table. For example, list the snapshots for the catalogkeyspace/journal table:

$ cd ./cassandra/data/data/catalogkeyspace/journal-296a2d30c22a11e9b1350d927649052c/snapshots && ls -l

results in

total 0
drwxrwxr-x. 2 ec2-user ec2-user 265 Aug 19 02:44 catalog-ks
drwxrwxr-x. 2 ec2-user ec2-user 265 Aug 19 02:52 multi-ks

A snapshots directory lists the SSTable files in the snapshot. A schema.cql file is also created in each snapshot that defines schema that can recreate the table with CQL when restoring from a snapshot:

$ cd catalog-ks && ls -l

results in

total 44
-rw-rw-r--. 1 ec2-user ec2-user   31 Aug 19 02:44 manifest.jsonZ
-rw-rw-r--. 4 ec2-user ec2-user   47 Aug 19 02:38 na-1-big-CompressionInfo.db
-rw-rw-r--. 4 ec2-user ec2-user   97 Aug 19 02:38 na-1-big-Data.db
-rw-rw-r--. 4 ec2-user ec2-user   10 Aug 19 02:38 na-1-big-Digest.crc32
-rw-rw-r--. 4 ec2-user ec2-user   16 Aug 19 02:38 na-1-big-Filter.db
-rw-rw-r--. 4 ec2-user ec2-user   16 Aug 19 02:38 na-1-big-Index.db
-rw-rw-r--. 4 ec2-user ec2-user 4687 Aug 19 02:38 na-1-big-Statistics.db
-rw-rw-r--. 4 ec2-user ec2-user   56 Aug 19 02:38 na-1-big-Summary.db
-rw-rw-r--. 4 ec2-user ec2-user   92 Aug 19 02:38 na-1-big-TOC.txt
-rw-rw-r--. 1 ec2-user ec2-user  814 Aug 19 02:44 schema.cql

Clearing Snapshots

Snapshots may be cleared or deleted with the nodetool clearsnapshot command. Either a specific snapshot name must be specified or the –all option must be specified.

For example, delete a snapshot called magazine from keyspace cqlkeyspace:

$ nodetool clearsnapshot -t magazine cqlkeyspace

or delete all snapshots from cqlkeyspace with the –all option:

$ nodetool clearsnapshot -all cqlkeyspace

Incremental Backups

In the following sections, we shall discuss configuring and creating incremental backups.

Configuring for Incremental Backups

To create incremental backups set incremental_backups to true in cassandra.yaml.

incremental_backups: true

This is the only setting needed to create incremental backups. By default incremental_backups setting is set to false because a new set of SSTable files is created for each data flush and if several CQL statements are to be run the backups directory could fill up quickly and use up storage that is needed to store table data. Incremental backups may also be enabled on the command line with the nodetool command nodetool enablebackup. Incremental backups may be disabled with nodetool disablebackup command. Status of incremental backups, whether they are enabled may be checked with nodetool statusbackup.

Creating Incremental Backups

After each table is created flush the table data with nodetool flush command. Incremental backups get created.

$ nodetool flush cqlkeyspace t
$ nodetool flush cqlkeyspace t2
$ nodetool flush catalogkeyspace journal magazine

Finding Incremental Backups

Incremental backups are created within the Cassandra’s data directory within a table directory. Backups may be found with following command.

$ find -name backups

results in

./cassandra/data/data/cqlkeyspace/t-d132e240c21711e9bbee19821dcea330/backups
./cassandra/data/data/cqlkeyspace/t2-d993a390c22911e9b1350d927649052c/backups
./cassandra/data/data/catalogkeyspace/journal-296a2d30c22a11e9b1350d927649052c/backups
./cassandra/data/data/catalogkeyspace/magazine-446eae30c22a11e9b1350d927649052c/backups

Creating an Incremental Backup

This section discusses how incremental backups are created in more detail using the keyspace and table previously created.

Flush the keyspace and table:

$ nodetool flush cqlkeyspace t

A search for backups and a backups directory will list a backup directory, even if we have added no table data yet.

$ find -name backups

results in

./cassandra/data/data/cqlkeyspace/t-d132e240c21711e9bbee19821dcea330/backups

Checking the backups directory will show that there are also no backup files:

$ cd ./cassandra/data/data/cqlkeyspace/t-d132e240c21711e9bbee19821dcea330/backups && ls -l

results in

total 0

If a row of data is added to the data, running the nodetool flush command will flush the table data and an incremental backup will be created:

$ nodetool flush cqlkeyspace t
$ cd ./cassandra/data/data/cqlkeyspace/t-d132e240c21711e9bbee19821dcea330/backups && ls -l

results in

total 36
-rw-rw-r--. 2 ec2-user ec2-user   47 Aug 19 00:32 na-1-big-CompressionInfo.db
-rw-rw-r--. 2 ec2-user ec2-user   43 Aug 19 00:32 na-1-big-Data.db
-rw-rw-r--. 2 ec2-user ec2-user   10 Aug 19 00:32 na-1-big-Digest.crc32
-rw-rw-r--. 2 ec2-user ec2-user   16 Aug 19 00:32 na-1-big-Filter.db
-rw-rw-r--. 2 ec2-user ec2-user    8 Aug 19 00:32 na-1-big-Index.db
-rw-rw-r--. 2 ec2-user ec2-user 4673 Aug 19 00:32 na-1-big-Statistics.db
-rw-rw-r--. 2 ec2-user ec2-user   56 Aug 19 00:32 na-1-big-Summary.db
-rw-rw-r--. 2 ec2-user ec2-user   92 Aug 19 00:32 na-1-big-TOC.txt

The backups directory for any table, such as cqlkeyspace/t is created in the data directory for that table.

Adding another row of data and flushing will result in another set of incremental backup files. The SSTable files are timestamped, which distinguishes the first incremental backup from the second:

total 72
-rw-rw-r--. 2 ec2-user ec2-user   47 Aug 19 00:32 na-1-big-CompressionInfo.db
-rw-rw-r--. 2 ec2-user ec2-user   43 Aug 19 00:32 na-1-big-Data.db
-rw-rw-r--. 2 ec2-user ec2-user   10 Aug 19 00:32 na-1-big-Digest.crc32
-rw-rw-r--. 2 ec2-user ec2-user   16 Aug 19 00:32 na-1-big-Filter.db
-rw-rw-r--. 2 ec2-user ec2-user    8 Aug 19 00:32 na-1-big-Index.db
-rw-rw-r--. 2 ec2-user ec2-user 4673 Aug 19 00:32 na-1-big-Statistics.db
-rw-rw-r--. 2 ec2-user ec2-user   56 Aug 19 00:32 na-1-big-Summary.db
-rw-rw-r--. 2 ec2-user ec2-user   92 Aug 19 00:32 na-1-big-TOC.txt
-rw-rw-r--. 2 ec2-user ec2-user   47 Aug 19 00:35 na-2-big-CompressionInfo.db
-rw-rw-r--. 2 ec2-user ec2-user   41 Aug 19 00:35 na-2-big-Data.db
-rw-rw-r--. 2 ec2-user ec2-user   10 Aug 19 00:35 na-2-big-Digest.crc32
-rw-rw-r--. 2 ec2-user ec2-user   16 Aug 19 00:35 na-2-big-Filter.db
-rw-rw-r--. 2 ec2-user ec2-user    8 Aug 19 00:35 na-2-big-Index.db
-rw-rw-r--. 2 ec2-user ec2-user 4673 Aug 19 00:35 na-2-big-Statistics.db
-rw-rw-r--. 2 ec2-user ec2-user   56 Aug 19 00:35 na-2-big-Summary.db
-rw-rw-r--. 2 ec2-user ec2-user   92 Aug 19 00:35 na-2-big-TOC.txt

Restoring from Incremental Backups and Snapshots

The two main tools/commands for restoring a table after it has been dropped are:

  • sstableloader

  • nodetool refresh

A snapshot contains essentially the same set of SSTable files as an incremental backup does with a few additional files. A snapshot includes a schema.cql file for the schema DDL to create a table in CQL. A table backup does not include DDL which must be obtained from a snapshot when restoring from an incremental backup.

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