= Repair

Cassandra is designed to remain available if one of it's nodes is down
or unreachable. However, when a node is down or unreachable, it needs to
eventually discover the writes it missed. Hints attempt to inform a node
of missed writes, but are a best effort, and aren't guaranteed to inform
a node of 100% of the writes it missed. These inconsistencies can
eventually result in data loss as nodes are replaced or tombstones
expire.

These inconsistencies are fixed with the repair process. Repair
synchronizes the data between nodes by comparing their respective
datasets for their common token ranges, and streaming the differences
for any out of sync sections between the nodes. It compares the data
with merkle trees, which are a hierarchy of hashes.

== Incremental and Full Repairs

There are 2 types of repairs: full repairs, and incremental repairs.
Full repairs operate over all of the data in the token range being
repaired. Incremental repairs only repair data that's been written since
the previous incremental repair.

Incremental repairs are the default repair type, and if run regularly,
can significantly reduce the time and io cost of performing a repair.
However, it's important to understand that once an incremental repair
marks data as repaired, it won't try to repair it again. This is fine
for syncing up missed writes, but it doesn't protect against things like
disk corruption, data loss by operator error, or bugs in Cassandra. For
this reason, full repairs should still be run occasionally.

== Usage and Best Practices

Since repair can result in a lot of disk and network io, it's not run
automatically by Cassandra. It is run by the operator via nodetool.

Incremental repair is the default and is run with the following command:

[source,none]
----
nodetool repair
----

A full repair can be run with the following command:

[source,none]
----
nodetool repair --full
----

Additionally, repair can be run on a single keyspace:

[source,none]
----
nodetool repair [options] <keyspace_name>
----

Or even on specific tables:

[source,none]
----
nodetool repair [options] <keyspace_name> <table1> <table2>
----

The repair command only repairs token ranges on the node being repaired,
it doesn't repair the whole cluster. By default, repair will operate on
all token ranges replicated by the node you're running repair on, which
will cause duplicate work if you run it on every node. The `-pr` flag
will only repair the "primary" ranges on a node, so you can repair your
entire cluster by running `nodetool repair -pr` on each node in a single
datacenter.

The specific frequency of repair that's right for your cluster, of
course, depends on several factors. However, if you're just starting out
and looking for somewhere to start, running an incremental repair every
1-3 days, and a full repair every 1-3 weeks is probably reasonable. If
you don't want to run incremental repairs, a full repair every 5 days is
a good place to start.

At a minimum, repair should be run often enough that the gc grace period
never expires on unrepaired data. Otherwise, deleted data could
reappear. With a default gc grace period of 10 days, repairing every
node in your cluster at least once every 7 days will prevent this, while
providing enough slack to allow for delays.

== Other Options

`-pr, --partitioner-range`::
  Restricts repair to the 'primary' token ranges of the node being
  repaired. A primary range is just a token range for which a node is
  the first replica in the ring.
`-prv, --preview`::
  Estimates the amount of streaming that would occur for the given
  repair command. This builds the merkle trees, and prints the expected
  streaming activity, but does not actually do any streaming. By
  default, incremental repairs are estimated, add the `--full` flag to
  estimate a full repair.
`-vd, --validate`::
  Verifies that the repaired data is the same across all nodes. Similiar
  to `--preview`, this builds and compares merkle trees of repaired
  data, but doesn't do any streaming. This is useful for
  troubleshooting. If this shows that the repaired data is out of sync,
  a full repair should be run.

`nodetool repair docs <nodetool_repair>`

== Full Repair Example

Full repair is typically needed to redistribute data after increasing
the replication factor of a keyspace or after adding a node to the
cluster. Full repair involves streaming SSTables. To demonstrate full
repair start with a three node cluster.

[source,none]
----
[ec2-user@ip-10-0-2-238 ~]$ nodetool status
Datacenter: us-east-1
=====================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address   Load        Tokens  Owns  Host ID                              Rack
UN  10.0.1.115  547 KiB     256    ?  b64cb32a-b32a-46b4-9eeb-e123fa8fc287  us-east-1b
UN  10.0.3.206  617.91 KiB  256    ?  74863177-684b-45f4-99f7-d1006625dc9e  us-east-1d
UN  10.0.2.238  670.26 KiB  256    ?  4dcdadd2-41f9-4f34-9892-1f20868b27c7  us-east-1c
----

Create a keyspace with replication factor 3:

[source,none]
----
cqlsh> DROP KEYSPACE cqlkeyspace;
cqlsh> CREATE KEYSPACE CQLKeyspace
  ... WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 3};
----

Add a table to the keyspace:

[source,none]
----
cqlsh> use cqlkeyspace;
cqlsh:cqlkeyspace> CREATE TABLE t (
           ...   id int,
           ...   k int,
           ...   v text,
           ...   PRIMARY KEY (id)
           ... );
----

Add table data:

[source,none]
----
cqlsh:cqlkeyspace> INSERT INTO t (id, k, v) VALUES (0, 0, 'val0');
cqlsh:cqlkeyspace> INSERT INTO t (id, k, v) VALUES (1, 1, 'val1');
cqlsh:cqlkeyspace> INSERT INTO t (id, k, v) VALUES (2, 2, 'val2');
----

A query lists the data added:

[source,none]
----
cqlsh:cqlkeyspace> SELECT * FROM t;

id | k | v
----+---+------
 1 | 1 | val1
 0 | 0 | val0
 2 | 2 | val2
(3 rows)
----

Make the following changes to a three node cluster:

[arabic]
. Increase the replication factor from 3 to 4.
. Add a 4th node to the cluster

When the replication factor is increased the following message gets
output indicating that a full repair is needed as per
(https://issues.apache.org/jira/browse/CASSANDRA-13079[CASSANDRA-13079]):

[source,none]
----
cqlsh:cqlkeyspace> ALTER KEYSPACE CQLKeyspace
           ... WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 4};
Warnings :
When increasing replication factor you need to run a full (-full) repair to distribute the
data.
----

Perform a full repair on the keyspace `cqlkeyspace` table `t` with
following command:

[source,none]
----
nodetool repair -full cqlkeyspace t
----

Full repair completes in about a second as indicated by the output:

[source,none]
----
[ec2-user@ip-10-0-2-238 ~]$ nodetool repair -full cqlkeyspace t
[2019-08-17 03:06:21,445] Starting repair command #1 (fd576da0-c09b-11e9-b00c-1520e8c38f00), repairing keyspace cqlkeyspace with repair options (parallelism: parallel, primary range: false, incremental: false, job threads: 1, ColumnFamilies: [t], dataCenters: [], hosts: [], previewKind: NONE, # of ranges: 1024, pull repair: false, force repair: false, optimise streams: false)
[2019-08-17 03:06:23,059] Repair session fd8e5c20-c09b-11e9-b00c-1520e8c38f00 for range [(-8792657144775336505,-8786320730900698730], (-5454146041421260303,-5439402053041523135], (4288357893651763201,4324309707046452322], ... , (4350676211955643098,4351706629422088296]] finished (progress: 0%)
[2019-08-17 03:06:23,077] Repair completed successfully
[2019-08-17 03:06:23,077] Repair command #1 finished in 1 second
[ec2-user@ip-10-0-2-238 ~]$
----

The `nodetool  tpstats` command should list a repair having been
completed as `Repair-Task` > `Completed` column value of 1:

[source,none]
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[ec2-user@ip-10-0-2-238 ~]$ nodetool tpstats
Pool Name Active   Pending Completed   Blocked  All time blocked
ReadStage  0           0           99       0              0
…
Repair-Task 0       0           1        0              0
RequestResponseStage                  0        0        2078        0               0
----