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[[known_issues_and_limitations]]
= Known Issues and Limitations

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== Schema

=== Primary keys

* The primary key may not be changed after the table is created.
  You must drop and recreate a table to select a new primary key.

* The columns which make up the primary key must be listed first in the schema.

* The primary key of a row may not be modified using the `UPDATE` functionality.
  To modify a row's primary key, the row must be deleted and re-inserted with
  the modified key. Such a modification is non-atomic.

* Columns with `DOUBLE`, `FLOAT`, or `BOOL` types are not allowed as part of a
  primary key definition. Additionally, all columns that are part of a primary
  key definition must be `NOT NULL`.

* Auto-generated primary keys are not supported.

* Cells making up a composite primary key are limited to a total of 16KiB after the internal
  composite-key encoding done by Kudu.

=== Columns

* `CHAR` and complex types like `ARRAY`, `MAP`, and `STRUCT` are not yet
  supported.

* Type, nullability and type attributes (i.e. precision and scale of `DECIMAL`,
  length of `VARCHAR`) of existing columns cannot be changed by altering the
  table.

* Tables can have a maximum of 300 columns by default.

=== Tables

* Tables must have an odd number of replicas, with a maximum of 7.

* The replication factor is set at table creation time and cannot be changed
  later. As a workaround, create a new table with the same schema and
  the desired replication factor and insert the contents of the old table
  into the new one.

=== Cells (individual values)

* By default, cells cannot be larger than 64KiB before encoding or compression.

=== Other usage limitations

* Secondary indexes are not supported.

* Multi-row transactions are not yet supported.

* Relational features, like foreign keys, are not supported.

* Identifiers such as column and table names are restricted to be valid UTF-8 strings.
  Additionally, a maximum length of 256 characters is enforced.

* Dropping a column does not immediately reclaim space. Compaction must run first.

* There is no way to run compaction manually, but dropping the table will reclaim the
  space immediately.

== Partitioning Limitations

* Tables must be manually pre-split into tablets using simple or compound primary
  keys. Automatic splitting is not yet possible. Range partitions may be added
  or dropped after a table has been created. See
  <<schema_design.adoc#schema_design,Schema Design>> for more information.

* Data in existing tables cannot currently be automatically repartitioned. As a workaround,
  create a new table with the new partitioning and insert the contents of the old
  table.

* Tablets that lose a majority of replicas (such as 1 left out of 3) require manual
  intervention to be repaired.

== Cluster management

* Recommended maximum point-to-point latency within a Kudu cluster is 20 milliseconds.

* Recommended minimum point-to-point bandwidth within a Kudu cluster is 10 Gbps.

* If you intend to use the location awareness feature to place tablet servers in
  different locations, it is recommended to measure the bandwidth and latency between servers
  to ensure they fit within the above guidelines.

* All masters must be started at the same time when the cluster is started for the very first time.

== Server management

* Production deployments should configure a least 4 GiB of memory for tablet servers,
  and ideally more than 16 GiB when approaching the data and tablet <<Scale>> limits.

* Write ahead logs (WAL) can only be stored on one disk.

* Tablet servers can’t change address/port.

* Kudu has a requirement on having the server-side clock synchronized with NTP.
  Kudu masters and tablet servers crash if detecting that the clock is out of
  sync. For the clock synchronization, either synchronize the local clock
  using an NTP server such as `ntpd` or `chronyd`, or use the built-in NTP
  client for Kudu masters and tablet servers.

== Scale

Kudu is known to run seamlessly across a wide array of environments and workloads
with minimal expertise and configuration at the following scale:

* 3 master servers

* 100 tablet servers

* 8 TiB of stored data per tablet server, post-replication and post-compression.

* 1000 tablets per tablet server, post-replication.

* 60 tablets per table, per tablet server, at table-creation time.

* 10 GiB of stored data per tablet.

Staying within these limits will provide the most predictable and straightforward
Kudu experience.

However, experienced users who run on modern hardware, use the latest
versions of Kudu, test and tune Kudu for their use case, and work closely with
the community, can achieve much higher scales comfortably. Below are some
anecdotal values that have been seen in real world production clusters:

* 3 master servers

* 300+ tablet servers

* 10+ TiB of stored data per tablet server, post-replication and post-compression.

* 4000+ tablets per tablet server, post-replication.

* 50 GiB of stored data per tablet. Going beyond this can cause issues such a
  reduced performance, compaction issues, and slow tablet startup times.

== Security Limitations

* Row-level authorization is not available.

* Data encryption at rest is not directly built into Kudu. Encryption of
  Kudu data at rest can be achieved through the use of local block device
  encryption software such as `dmcrypt`.

* Server certificates generated by Kudu IPKI are incompatible with
  link:https://www.bouncycastle.org/[bouncycastle] version 1.52 and earlier. See
  link:https://issues.apache.org/jira/browse/KUDU-2145[KUDU-2145] for details.

== Other Known Issues

The following are known bugs and issues with the current release of Kudu. They will
be addressed in later releases. Note that this list is not exhaustive, and is meant
to communicate only the most important known issues.

* If a tablet server has a very large number of tablets, it may take several minutes
  to start up. It is recommended to limit the number of tablets per server to 1000
  or fewer. Consider this limitation when pre-splitting your tables. If you notice slow
  start-up times, you can monitor the number of tablets per server in the web UI.

* NVM-based cache doesn't work reliably on RH6/CentOS6
  (see link:https://issues.apache.org/jira/browse/KUDU-2978[KUDU-2978]).

* When upgrading a Kudu cluster to 1.11.0 version with existing pre-1.11.0
  tables, the `live_row_count` and `on_disk_size` metrics might produce
  inconsistent readings in some scenarios
  (see link:https://issues.apache.org/jira/browse/KUDU-2986[KUDU-2986]).

* In Kudu 1.10.0 and Kudu 1.11.0, the kudu-binary JAR (targeted for
  containerized Kudu deployments using mini-cluster) contains libnuma dynamic
  library. Also, if building Kudu binaries in release mode with default cmake
  settings, the libnuma library is linked statically with the Kudu binaries
  (add `-DKUDU_LINK=dynamic` when running cmake to avoid that). The library is
  licensed under LGPL v.2.1, however the ASF thirdparty license policy
  explicitly prohibits including such contents into releases: see
  link:https://www.apache.org/legal/resolved.html#category-x[Category X]. This
  issue has been addressed in 1.10.1 and 1.11.1 patch releases correspondingly
  (see link:https://issues.apache.org/jira/browse/KUDU-2990[KUDU-2990]).

* Due to a bug in SSSD PAC plugin of version prior to 1.16, a KRPC connection
  negotiation may stuck and the whole process wouldn't be able to negotiate
  any new connection for about 5 minutes in a secure Kudu cluster using SSSD.
  If using SSSD in secure Kudu cluster deployments, make sure SSSD packages are
  of version 1.16 or newer
  (see link:https://issues.apache.org/jira/browse/KUDU-3217[KUDU-3217]).