This section contains descriptions of server settings that cannot be changed at the session or query level.
These settings are stored in the `config.xml` file on the ClickHouse server.
Other settings are described in the “[Settings](../../operations/settings/index.md#session-settings-intro)” section.
Before studying the settings, read the [Configuration files](../../operations/configuration-files.md#configuration_files) section and note the use of substitutions (the `incl` and `optional` attributes).
This setting should be used with extra caution since forwarded address can be easily spoofed - server accepting such authentication should not be accessed directly but rather exclusively through a trusted proxy.
The maximum number of threads that will be used for performing flush operations for Buffer-engine tables in the background.
Type: UInt64
Default: 16
## background_common_pool_size
The maximum number of threads that will be used for performing a variety of operations (mostly garbage collection) for \*MergeTree-engine tables in a background.
Type: UInt64
Default: 8
## background_distributed_schedule_pool_size
The maximum number of threads that will be used for executing distributed sends.
Type: UInt64
Default: 16
## background_fetches_pool_size
The maximum number of threads that will be used for fetching data parts from another replica for \*MergeTree-engine tables in a background.
Sets a ratio between the number of threads and the number of background merges and mutations that can be executed concurrently. For example, if the ratio equals to 2 and
`background_pool_size` is set to 16 then ClickHouse can execute 32 background merges concurrently. This is possible, because background operations could be suspended and postponed. This is needed to give small merges more execution priority. You can only increase this ratio at runtime. To lower it you have to restart the server.
The same as for `background_pool_size` setting `background_merges_mutations_concurrency_ratio` could be applied from the `default` profile for backward compatibility.
Algorithm used to select next merge or mutation to be executed by background thread pool. Policy may be changed at runtime without server restart.
Could be applied from the `default` profile for backward compatibility.
Possible values:
- "round_robin" — Every concurrent merge and mutation is executed in round-robin order to ensure starvation-free operation. Smaller merges are completed faster than bigger ones just because they have fewer blocks to merge.
- "shortest_task_first" — Always execute smaller merge or mutation. Merges and mutations are assigned priorities based on their resulting size. Merges with smaller sizes are strictly preferred over bigger ones. This policy ensures the fastest possible merge of small parts but can lead to indefinite starvation of big merges in partitions heavily overloaded by INSERTs.
Type: String
Default: round_robin
## background_message_broker_schedule_pool_size
The maximum number of threads that will be used for executing background operations for message streaming.
Type: UInt64
Default: 16
## background_move_pool_size
The maximum number of threads that will be used for moving data parts to another disk or volume for *MergeTree-engine tables in a background.
Type: UInt64
Default: 8
## background_pool_size
Sets the number of threads performing background merges and mutations for tables with MergeTree engines. You can only increase the number of threads at runtime. To lower the number of threads you have to restart the server. By adjusting this setting, you manage CPU and disk load. Smaller pool size utilizes less CPU and disk resources, but background processes advance slower which might eventually impact query performance.
Before changing it, please also take a look at related MergeTree settings, such as `number_of_free_entries_in_pool_to_lower_max_size_of_merge` and `number_of_free_entries_in_pool_to_execute_mutation`.
Type: UInt64
Default: 16
## background_schedule_pool_size
The maximum number of threads that will be used for constantly executing some lightweight periodic operations for replicated tables, Kafka streaming, and DNS cache updates.
The maximum number of threads to execute BACKUP requests.
Type: UInt64
Default: 16
## backups_io_thread_pool_queue_size
The maximum number of jobs that can be scheduled on the Backups IO Thread pool. It is recommended to keep this queue unlimited (0) due to the current S3 backup logic.
Type: UInt64
Default: 0
## cache_size_to_ram_max_ratio
Set cache size to RAM max ratio. Allows lowering the cache size on low-memory systems.
Type: Double
Default: 0.5
## concurrent_threads_soft_limit_num
The maximum number of query processing threads, excluding threads for retrieving data from remote servers, allowed to run all queries. This is not a hard limit. In case if the limit is reached the query will still get at least one thread to run. Query can upscale to desired number of threads during execution if more threads become available.
Zero means Unlimited.
Type: UInt64
Default: 0
## concurrent_threads_soft_limit_ratio_to_cores
Same as concurrent_threads_soft_limit_num, but with ratio to cores.
Type: UInt64
Default: 0
## default_database
Default database name.
Type: String
Default: default
## disable_internal_dns_cache
Disables the internal DNS cache. Recommended for operating ClickHouse in systems
with frequently changing infrastructure such as Kubernetes.
The maximum read speed in bytes per second for all backups on server. Zero means unlimited.
Type: UInt64
Default: 0
## max_backups_io_thread_pool_free_size
If the number of **idle** threads in the Backups IO Thread pool exceeds `max_backup_io_thread_pool_free_size`, ClickHouse will release resources occupied by idling threads and decrease the pool size. Threads can be created again if necessary.
Type: UInt64
Default: 0
## max_backups_io_thread_pool_size
The maximum number of threads that would be used for IO operations for BACKUP queries
Limit on total number of concurrently executed queries. Zero means Unlimited. Note that limits on insert and select queries, and on the maximum number of queries for users must also be considered. See also max_concurrent_insert_queries, max_concurrent_select_queries, max_concurrent_queries_for_all_users. Zero means unlimited.
Limit on total number of concurrently waiting queries. Execution of a waiting query is blocked while required tables are loading asynchronously (see `async_load_databases`). Note that waiting queries are not counted when `max_concurrent_queries`, `max_concurrent_insert_queries`, `max_concurrent_select_queries`, `max_concurrent_queries_for_user` and `max_concurrent_queries_for_all_users` limits are checked. This correction is done to avoid hitting these limits just after server startup. Zero means unlimited.
:::note
This setting can be modified at runtime and will take effect immediately. Queries that are already running will remain unchanged.
The maximum number of threads that would be used for IO operations
Type: UInt64
Default: 100
## max_local_read_bandwidth_for_server
The maximum speed of local reads in bytes per second. Zero means unlimited.
Type: UInt64
Default: 0
## max_local_write_bandwidth_for_server
The maximum speed of local writes in bytes per second. Zero means unlimited.
Type: UInt64
Default: 0
## max_partition_size_to_drop
Restriction on dropping partitions.
If the size of a [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table exceeds `max_partition_size_to_drop` (in bytes), you can’t drop a partition using a [DROP PARTITION](../../sql-reference/statements/alter/partition.md#drop-partitionpart) query.
This setting does not require a restart of the ClickHouse server to apply. Another way to disable the restriction is to create the `<clickhouse-path>/flags/force_drop_table` file.
If the size of a [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table exceeds `max_table_size_to_drop` (in bytes), you can’t delete it using a [DROP](../../sql-reference/statements/drop.md) query or [TRUNCATE](../../sql-reference/statements/truncate.md) query.
This setting does not require a restart of the ClickHouse server to apply. Another way to disable the restriction is to create the `<clickhouse-path>/flags/force_drop_table` file.
If number of tables is greater than this value, server will throw an exception. 0 means no limitation. View, remote tables, dictionary, system tables are not counted. Only count table in Atomic/Ordinary/Replicated/Lazy database engine.Default value: 0
The maximum amount of storage that could be used for external aggregation, joins or sorting.
Queries that exceed this limit will fail with an exception. Zero means unlimited.
Also see `max_temporary_data_on_disk_size_for_user` and `max_temporary_data_on_disk_size_for_query`.
Type: UInt64
Default: 0
## max_thread_pool_free_size
If the number of **idle** threads in the Global Thread pool is greater than `max_thread_pool_free_size`, then ClickHouse releases resources occupied by some threads and the pool size is decreased. Threads can be created again if necessary.
Type: UInt64
Default: 1000
## max_thread_pool_size
The maximum number of threads that could be allocated from the OS and used for query execution and background operations.
Type: UInt64
Default: 10000
## mmap_cache_size
Sets the cache size (in bytes) for mapped files. This setting allows avoiding frequent open/close calls (which are very expensive due to consequent page faults), and to reuse mappings from several threads and queries. The setting value is the number of mapped regions (usually equal to the number of mapped files). The amount of data in mapped files can be monitored in the tables system.metrics and system.metric_log with the `MMappedFiles` and `MMappedFileBytes` metrics. Also, in system.asynchronous_metrics and system.asynchronous_metrics_log by the `MMapCacheCells` metric, and in system.events, system.processes, system.query_log, system.query_thread_log, system.query_views_log by the `CreatedReadBufferMMap`, `CreatedReadBufferMMapFailed`, `MMappedFileCacheHits`, `MMappedFileCacheMisses` events.
Note that the amount of data in mapped files does not consume memory directly and is not accounted for in query or server memory usage — because this memory can be discarded similar to the OS page cache. The cache is dropped (the files are closed) automatically on the removal of old parts in tables of the MergeTree family, also it can be dropped manually by the `SYSTEM DROP MMAP CACHE` query.
TThe maximum number of jobs that can be scheduled on the Global Thread pool. Increasing queue size leads to larger memory usage. It is recommended to keep this value equal to `max_thread_pool_size`. Zero means unlimited.
Type: UInt64
Default: 10000
## tmp_policy
Policy for storage with temporary data. Also see the MergeTree Table Engine documentation.
:::note
- Only one option can be used to configure temporary data storage: `tmp_path` ,`tmp_policy`, `temporary_data_in_cache`.
-`move_factor`, `keep_free_space_bytes`,`max_data_part_size_bytes` and are ignored.
- Policy should have exactly *one volume* with *local* disks.
:::
**Example**
When `/disk1` is full, temporary data will be stored on `/disk2`.
Configures a command to obtain a key to be used by [encryption codecs](../../sql-reference/statements/create/table.md#create-query-encryption-codecs). Key (or keys) should be written in environment variables or set in the configuration file.
Storing keys in the configuration file is not recommended. It isn't secure. You can move the keys into a separate config file on a secure disk and put a symlink to that config file to `config.d/` folder.
To manually turn on error history collection [`system.error_log`](../../operations/system-tables/error_log.md), create `/etc/clickhouse-server/config.d/error_log.xml` with the following content:
To disable `error_log` setting, you should create the following file `/etc/clickhouse-server/config.d/disable_error_log.xml` with the following content:
The delay during which a dropped table can be restored using the [UNDROP](/docs/en/sql-reference/statements/undrop.md) statement. If `DROP TABLE` ran with a `SYNC` modifier, the setting is ignored.
(see [database_catalog_unused_dir_hide_timeout_sec](../../operations/server-configuration-parameters/settings.md#database_catalog_unused_dir_hide_timeout_sec))
- interval – The interval for sending, in seconds.
- timeout – The timeout for sending data, in seconds.
- root_path – Prefix for keys.
- metrics – Sending data from the [system.metrics](../../operations/system-tables/metrics.md#system_tables-metrics) table.
- events – Sending deltas data accumulated for the time period from the [system.events](../../operations/system-tables/events.md#system_tables-events) table.
- events_cumulative – Sending cumulative data from the [system.events](../../operations/system-tables/events.md#system_tables-events) table.
- asynchronous_metrics – Sending data from the [system.asynchronous_metrics](../../operations/system-tables/asynchronous_metrics.md#system_tables-asynchronous_metrics) table.
Expired time for HSTS in seconds. The default value is 0 means clickhouse disabled HSTS. If you set a positive number, the HSTS will be enabled and the max-age is the number you set.
Perform mlockall after startup to lower first queries latency and to prevent clickhouse executable from being paged out under high IO load. Enabling this option is recommended but will lead to increased startup time for up to a few seconds.
Keep in mind that this parameter would not work without "CAP_IPC_LOCK" capability.
A username and a password used to connect to other servers during [replication](../../engines/table-engines/mergetree-family/replication.md). Also the server authenticates other replicas using these credentials. So, `interserver_http_credentials` must be the same for all replicas in a cluster.
-`allow_empty` — If `true`, then other replicas are allowed to connect without authentication even if credentials are set. If `false`, then connections without authentication are refused. Default value: `false`.
-`old` — Contains old `user` and `password` used during credential rotation. Several `old` sections can be specified.
ClickHouse supports dynamic interserver credentials rotation without stopping all replicas at the same time to update their configuration. Credentials can be changed in several steps.
To enable authentication, set `interserver_http_credentials.allow_empty` to `true` and add credentials. This allows connections with authentication and without it.
To change existing credentials, move the username and the password to `interserver_http_credentials.old` section and update `user` and `password` with new values. At this point the server uses new credentials to connect to other replicas and accepts connections with either new or old credentials.
Allow multiple servers to listen on the same address:port. Requests will be routed to a random server by the operating system. Enabling this setting is not recommended.
Backlog (queue size of pending connections) of the listen socket.
Default value: `4096` (as in linux [5.4+](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=19f92a030ca6d772ab44b22ee6a01378a8cb32d4)).
So even if you have `TcpExtListenOverflows` (from `nstat`) non zero and this counter grows for ClickHouse server it does not mean that this value need to be increased, since:
-`size`– Rotation policy: Maximum size of the log files in bytes. Once the log file size exceeds this threshold, it is renamed and archived, and a new log file is created.
-`count`– Rotation policy: How many historical log files Clickhouse are kept at most.
-`stream_compress`– Compress log messages using LZ4. Set to `1` or `true` to enable.
-`console`– Do not write log messages to log files, instead print them in the console. Set to `1` or `true` to enable. Default is
`1` if Clickhouse does not run in daemon mode, `0` otherwise.
-`console_log_level`– Log level for console output. Defaults to `level`.
-`formatting`– Log format for console output. Currently, only `json` is supported).
-`use_syslog` - Also forward log output to syslog.
-`syslog_level` - Log level for logging to syslog.
| %Y | Year as a decimal number, e.g. 2017 | 2023 |
| %y | Last 2 digits of year as a decimal number (range [00,99]) | 23 |
| %C | First 2 digits of year as a decimal number (range [00,99]) | 20 |
| %G | Four-digit [ISO 8601 week-based year](https://en.wikipedia.org/wiki/ISO_8601#Week_dates), i.e. the year that contains the specified week. Normally useful only with %V | 2023 |
| %g | Last 2 digits of [ISO 8601 week-based year](https://en.wikipedia.org/wiki/ISO_8601#Week_dates), i.e. the year that contains the specified week. | 23 |
| %b | Abbreviated month name, e.g. Oct (locale dependent) | Jul |
| %h | Synonym of %b | Jul |
| %B | Full month name, e.g. October (locale dependent) | July |
| %m | Month as a decimal number (range [01,12]) | 07 |
| %U | Week of the year as a decimal number (Sunday is the first day of the week) (range [00,53]) | 27 |
| %W | Week of the year as a decimal number (Monday is the first day of the week) (range [00,53]) | 27 |
| %V | ISO 8601 week number (range [01,53]) | 27 |
| %j | Day of the year as a decimal number (range [001,366]) | 187 |
| %d | Day of the month as a zero-padded decimal number (range [01,31]). Single digit is preceded by zero. | 06 |
| %e | Day of the month as a space-padded decimal number (range [1,31]). Single digit is preceded by a space. | 6 |
-`address` — The address of syslog in format `host\[:port\]`. If omitted, the local daemon is used.
-`hostname` — The name of the host from which logs are send. Optional.
-`facility` — The syslog [facility keyword](https://en.wikipedia.org/wiki/Syslog#Facility). Must be specified uppercase with a “LOG_” prefix, e.g. `LOG_USER`, `LOG_DAEMON`, `LOG_LOCAL3`, etc. Default value: `LOG_USER` if `address` is specified, `LOG_DAEMON` otherwise.
-`format`– Log message format. Possible values: `bsd` and `syslog.`
To enable JSON logging support, use the following snippet:
```xml
<logger>
<formatting>
<type>json</type>
<names>
<date_time>date_time</date_time>
<thread_name>thread_name</thread_name>
<thread_id>thread_id</thread_id>
<level>level</level>
<query_id>query_id</query_id>
<logger_name>logger_name</logger_name>
<message>message</message>
<source_file>source_file</source_file>
<source_line>source_line</source_line>
</names>
</formatting>
</logger>
```
**Renaming keys for JSON logs**
Key names can be modified by changing tag values inside the `<names>` tag. For example, to change `DATE_TIME` to `MY_DATE_TIME`, you can use `<date_time>MY_DATE_TIME</date_time>`.
**Omitting keys for JSON logs**
Log properties can be omitted by commenting out the property. For example, if you do not want your log to print `query_id`, you can comment out the `<query_id>` tag.
The server will need access to the public Internet via IPv4 (at the time of writing IPv6 is not supported by Sentry) for this feature to be functioning properly.
-`send_logical_errors`–`LOGICAL_ERROR` is like an `assert`, it is a bug in ClickHouse. This boolean flag enables sending this exceptions to sentry (default: `false`).
-`endpoint`– You can override the Sentry endpoint URL for sending crash reports. It can be either a separate Sentry account or your self-hosted Sentry instance. Use the [Sentry DSN](https://docs.sentry.io/error-reporting/quickstart/?platform=native#configure-the-sdk) syntax.
-`anonymize` - Avoid attaching the server hostname to the crash report.
-`http_proxy` - Configure HTTP proxy for sending crash reports.
-`debug` - Sets the Sentry client into debug mode.
-`tmp_path` - Filesystem path for temporary crash report state.
-`environment` - An arbitrary name of an environment in which the ClickHouse server is running. It will be mentioned in each crash report. The default value is `test` or `prod` depending on the version of ClickHouse.
For more information, see the section [Creating replicated tables](../../engines/table-engines/mergetree-family/replication.md#creating-replicated-tables).
If the size of a [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table exceeds `max_table_size_to_drop` (in bytes), you can’t delete it using a [DROP](../../sql-reference/statements/drop.md) query or [TRUNCATE](../../sql-reference/statements/truncate.md) query.
This setting does not require a restart of the ClickHouse server to apply. Another way to disable the restriction is to create the `<clickhouse-path>/flags/force_drop_table` file.
If the size of a [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table exceeds `max_partition_size_to_drop` (in bytes), you can’t drop a partition using a [DROP PARTITION](../../sql-reference/statements/alter/partition.md#drop-partitionpart) query.
This setting does not require a restart of the ClickHouse server to apply. Another way to disable the restriction is to create the `<clickhouse-path>/flags/force_drop_table` file.
ClickHouse uses threads from the Global Thread pool to process queries. If there is no idle thread to process a query, then a new thread is created in the pool. `max_thread_pool_size` limits the maximum number of threads in the pool.
If the number of **idle** threads in the Global Thread pool is greater than `max_thread_pool_free_size`, then ClickHouse releases resources occupied by some threads and the pool size is decreased. Threads can be created again if necessary.
The maximum number of jobs that can be scheduled on the Global Thread pool. Increasing queue size leads to larger memory usage. It is recommended to keep this value equal to [max_thread_pool_size](#max-thread-pool-size).
ClickHouse uses threads from the IO Thread pool to do some IO operations (e.g. to interact with S3). `max_io_thread_pool_size` limits the maximum number of threads in the pool.
If the number of **idle** threads in the IO Thread pool exceeds `max_io_thread_pool_free_size`, ClickHouse will release resources occupied by idling threads and decrease the pool size. Threads can be created again if necessary.
ClickHouse uses threads from the Backups IO Thread pool to do S3 backup IO operations. `max_backups_io_thread_pool_size` limits the maximum number of threads in the pool.
If the number of **idle** threads in the Backups IO Thread pool exceeds `max_backup_io_thread_pool_free_size`, ClickHouse will release resources occupied by idling threads and decrease the pool size. Threads can be created again if necessary.
The maximum number of jobs that can be scheduled on the Backups IO Thread pool. It is recommended to keep this queue unlimited due to the current S3 backup logic.
Sets the number of threads performing background merges and mutations for tables with MergeTree engines. This setting is also could be applied at server startup from the `default` profile configuration for backward compatibility at the ClickHouse server start. You can only increase the number of threads at runtime. To lower the number of threads you have to restart the server. By adjusting this setting, you manage CPU and disk load. Smaller pool size utilizes less CPU and disk resources, but background processes advance slower which might eventually impact query performance.
Before changing it, please also take a look at related MergeTree settings, such as [number_of_free_entries_in_pool_to_lower_max_size_of_merge](../../operations/settings/merge-tree-settings.md#number-of-free-entries-in-pool-to-lower-max-size-of-merge) and [number_of_free_entries_in_pool_to_execute_mutation](../../operations/settings/merge-tree-settings.md#number-of-free-entries-in-pool-to-execute-mutation).
Sets a ratio between the number of threads and the number of background merges and mutations that can be executed concurrently. For example, if the ratio equals to 2 and
`background_pool_size` is set to 16 then ClickHouse can execute 32 background merges concurrently. This is possible, because background operations could be suspended and postponed. This is needed to give small merges more execution priority. You can only increase this ratio at runtime. To lower it you have to restart the server.
The same as for `background_pool_size` setting `background_merges_mutations_concurrency_ratio` could be applied from the `default` profile for backward compatibility.
Sets the limit on how much RAM is allowed to use for performing merge and mutation operations.
Zero means unlimited.
If ClickHouse reaches this limit, it won't schedule any new background merge or mutation operations but will continue to execute already scheduled tasks.
If `true` all non-system databases with `Ordinary`, `Atomic` and `Replicated` engine will be loaded asynchronously after the ClickHouse server start up. See `system.asynchronous_loader` table, `tables_loader_background_pool_size` and `tables_loader_foreground_pool_size` server settings. Any query that tries to access a table, that is not yet loaded, will wait for exactly this table to be started up. If load job fails, query will rethrow an error (instead of shutting down the whole server in case of `async_load_databases = false`). The table that is waited for by at least one query will be loaded with higher priority. DDL queries on a database will wait for exactly that database to be started up. Also consider setting a limit `max_waiting_queries` for the total number of waiting queries.
Sets the number of threads performing load jobs in foreground pool. The foreground pool is used for loading table synchronously before server start listening on a port and for loading tables that are waited for. Foreground pool has higher priority than background pool. It means that no job starts in background pool while there are jobs running in foreground pool.
Sets the number of threads performing asynchronous load jobs in background pool. The background pool is used for loading tables asynchronously after server start in case there are no queries waiting for the table. It could be beneficial to keep low number of threads in background pool if there are a lot of tables. It will reserve CPU resources for concurrent query execution.
To manually turn on metrics history collection [`system.metric_log`](../../operations/system-tables/metric_log.md), create `/etc/clickhouse-server/config.d/metric_log.xml` with the following content:
To disable `metric_log` setting, you should create the following file `/etc/clickhouse-server/config.d/disable_metric_log.xml` with the following content:
Support for SSL is provided by the `libpoco` library. The available configuration options are explained in [SSLManager.h](https://github.com/ClickHouse-Extras/poco/blob/master/NetSSL_OpenSSL/include/Poco/Net/SSLManager.h). Default values can be found in [SSLManager.cpp](https://github.com/ClickHouse-Extras/poco/blob/master/NetSSL_OpenSSL/src/SSLManager.cpp).
- privateKeyFile – The path to the file with the secret key of the PEM certificate. The file may contain a key and certificate at the same time.
- certificateFile – The path to the client/server certificate file in PEM format. You can omit it if `privateKeyFile` contains the certificate.
- caConfig (default: none) – The path to the file or directory that contains trusted CA certificates. If this points to a file, it must be in PEM format and can contain several CA certificates. If this points to a directory, it must contain one .pem file per CA certificate. The filenames are looked up by the CA subject name hash value. Details can be found in the man page of [SSL_CTX_load_verify_locations](https://www.openssl.org/docs/man3.0/man3/SSL_CTX_load_verify_locations.html).
- verificationMode (default: relaxed) – The method for checking the node’s certificates. Details are in the description of the [Context](https://github.com/ClickHouse-Extras/poco/blob/master/NetSSL_OpenSSL/include/Poco/Net/Context.h) class. Possible values: `none`, `relaxed`, `strict`, `once`.
- verificationDepth (default: 9) – The maximum length of the verification chain. Verification will fail if the certificate chain length exceeds the set value.
- loadDefaultCAFile (default: true) – Wether built-in CA certificates for OpenSSL will be used. ClickHouse assumes that builtin CA certificates are in the file `/etc/ssl/cert.pem` (resp. the directory `/etc/ssl/certs`) or in file (resp. directory) specified by the environment variable `SSL_CERT_FILE` (resp. `SSL_CERT_DIR`).
- cacheSessions (default: false) – Enables or disables caching sessions. Must be used in combination with `sessionIdContext`. Acceptable values: `true`, `false`.
- sessionIdContext (default: `${application.name}`) – A unique set of random characters that the server appends to each generated identifier. The length of the string must not exceed `SSL_MAX_SSL_SESSION_ID_LENGTH`. This parameter is always recommended since it helps avoid problems both if the server caches the session and if the client requested caching. Default value: `${application.name}`.
- sessionCacheSize (default: [1024\*20](https://github.com/ClickHouse/boringssl/blob/master/include/openssl/ssl.h#L1978)) – The maximum number of sessions that the server caches. A value of 0 means unlimited sessions.
- sessionTimeout (default: [2h](https://github.com/ClickHouse/boringssl/blob/master/include/openssl/ssl.h#L1926)) – Time for caching the session on the server.
- extendedVerification (default: false) – If enabled, verify that the certificate CN or SAN matches the peer hostname.
- fips (default: false) – Activates OpenSSL FIPS mode. Supported if the library’s OpenSSL version supports FIPS.
- privateKeyPassphraseHandler (default: `KeyConsoleHandler`)– Class (PrivateKeyPassphraseHandler subclass) that requests the passphrase for accessing the private key. For example: `<privateKeyPassphraseHandler>`, `<name>KeyFileHandler</name>`, `<options><password>test</password></options>`, `</privateKeyPassphraseHandler>`.
- invalidCertificateHandler (default: `RejectCertificateHandler`) – Class (a subclass of CertificateHandler) for verifying invalid certificates. For example: `<invalidCertificateHandler> <name>RejectCertificateHandler</name> </invalidCertificateHandler>` .
Logging events that are associated with [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md). For instance, adding or merging data. You can use the log to simulate merge algorithms and compare their characteristics. You can visualize the merge process.
Queries are logged in the [system.part_log](../../operations/system-tables/part_log.md#system_tables-part-log) table, not in a separate file. You can configure the name of this table in the `table` parameter (see below).
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
Queries are logged in the [system.query_log](../../operations/system-tables/query_log.md#system_tables-query_log) table, not in a separate file. You can change the name of the table in the `table` parameter (see below).
-`table`– Name of the system table the queries will be logged in.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
If the table does not exist, ClickHouse will create it. If the structure of the query log changed when the ClickHouse server was updated, the table with the old structure is renamed, and a new table is created automatically.
Data for the query cache is allocated in DRAM. If memory is scarce, make sure to set a small value for `max_size_in_bytes` or disable the query cache altogether.
Queries are logged in the [system.query_thread_log](../../operations/system-tables/query_thread_log.md#system_tables-query_thread_log) table, not in a separate file. You can change the name of the table in the `table` parameter (see below).
-`table`– Name of the system table the queries will be logged in.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
If the table does not exist, ClickHouse will create it. If the structure of the query thread log changed when the ClickHouse server was updated, the table with the old structure is renamed, and a new table is created automatically.
Setting for logging views (live, materialized etc) dependant of queries received with the [log_query_views=1](../../operations/settings/settings.md#log-query-views) setting.
Queries are logged in the [system.query_views_log](../../operations/system-tables/query_views_log.md#system_tables-query_views_log) table, not in a separate file. You can change the name of the table in the `table` parameter (see below).
-`table`– Name of the system table the queries will be logged in.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
If the table does not exist, ClickHouse will create it. If the structure of the query views log changed when the ClickHouse server was updated, the table with the old structure is renamed, and a new table is created automatically.
-`level` — Maximum Message Level (by default `Trace`) which will be stored in a table.
-`database` — Database name.
-`table` — Table name.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/index.md) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
Settings for the [asynchronous_insert_log](../../operations/system-tables/asynchronous_insert_log.md#system_tables-asynchronous_insert_log) system table for logging async inserts.
Parameters:
-`database` — Database name.
-`table` — Table name.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` defined.
-`flush_interval_milliseconds` — Interval for flushing data from the buffer in memory to the table.
Settings for the [crash_log](../../operations/system-tables/crash-log.md) system table operation.
Parameters:
-`database` — Database for storing a table.
-`table` — Table name.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/index.md) for a system table. Can't be used if `partition_by` or `order_by` defined.
-`flush_interval_milliseconds` — Interval for flushing data from the buffer in memory to the table.
-`max_size_rows`– Maximal size in lines for the logs. When non-flushed logs amount reaches max_size, logs dumped to the disk.
Default: 1048576.
-`reserved_size_rows`– Pre-allocated memory size in lines for the logs.
Default: 8192.
-`buffer_size_rows_flush_threshold`– Lines amount threshold, reaching it launches flushing logs to the disk in background.
Default: `max_size_rows / 2`.
-`flush_on_crash` - Indication whether logs should be dumped to the disk in case of a crash.
Default: false.
-`storage_policy`– Name of storage policy to use for the table (optional)
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md/#settings) that control the behavior of the MergeTree (optional).
The default server configuration file `config.xml` contains the following settings section:
Settings for the [backup_log](../../operations/system-tables/backup_log.md) system table for logging `BACKUP` and `RESTORE` operations.
Parameters:
-`database` — Database name.
-`table` — Table name.
-`partition_by` — [Custom partitioning key](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) for a system table. Can't be used if `engine` is defined.
-`order_by` - [Custom sorting key](../../engines/table-engines/mergetree-family/mergetree.md#order_by) for a system table. Can't be used if `engine` is defined.
-`engine` - [MergeTree Engine Definition](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) for a system table. Can't be used if `partition_by` or `order_by` is defined.
-`flush_interval_milliseconds` — Interval for flushing data from the buffer in memory to the table.
-`max_size_rows`– Maximal size in lines for the logs. When non-flushed logs amount reaches max_size, logs dumped to the disk.
Default: 1048576.
-`reserved_size_rows`– Pre-allocated memory size in lines for the logs.
Default: 8192.
-`buffer_size_rows_flush_threshold`– Lines amount threshold, reaching it launches flushing logs to the disk in background.
Default: `max_size_rows / 2`.
-`flush_on_crash` - Indication whether logs should be dumped to the disk in case of a crash.
Default: false.
-`storage_policy`– Name of storage policy to use for the table (optional).
-`settings` - [Additional parameters](../../engines/table-engines/mergetree-family/mergetree.md#settings) that control the behavior of the MergeTree (optional).
Configuration of clusters used by the [Distributed](../../engines/table-engines/special/distributed.md) table engine and by the `cluster` table function.
The time zone is necessary for conversions between String and DateTime formats when DateTime fields are output to text format (printed on the screen or in a file), and when getting DateTime from a string. Besides, the time zone is used in functions that work with the time and date if they didn’t receive the time zone in the input parameters.
The directory with user files. Used in the table function [file()](../../sql-reference/table-functions/file.md), [fileCluster()](../../sql-reference/table-functions/fileCluster.md).
The directory with user scripts files. Used for Executable user defined functions [Executable User Defined Functions](../../sql-reference/functions/index.md#executable-user-defined-functions).
The directory with user defined files. Used for SQL user defined functions [SQL User Defined Functions](../../sql-reference/functions/index.md#user-defined-functions).
ClickHouse uses ZooKeeper for storing metadata of replicas when using replicated tables. If replicated tables are not used, this section of parameters can be omitted.
-`session_timeout_ms` — Maximum timeout for the client session in milliseconds.
-`operation_timeout_ms` — Maximum timeout for one operation in milliseconds.
-`root` — The [znode](http://zookeeper.apache.org/doc/r3.5.5/zookeeperOver.html#Nodes+and+ephemeral+nodes) that is used as the root for znodes used by the ClickHouse server. Optional.
-`fallback_session_lifetime.min` - If the first zookeeper host resolved by zookeeper_load_balancing strategy is unavailable, limit the lifetime of a zookeeper session to the fallback node. This is done for load-balancing purposes to avoid excessive load on one of zookeeper hosts. This setting sets the minimal duration of the fallback session. Set in seconds. Optional. Default is 3 hours.
-`fallback_session_lifetime.max` - If the first zookeeper host resolved by zookeeper_load_balancing strategy is unavailable, limit the lifetime of a zookeeper session to the fallback node. This is done for load-balancing purposes to avoid excessive load on one of zookeeper hosts. This setting sets the maximum duration of the fallback session. Set in seconds. Optional. Default is 6 hours.
ClickHouse uses the setting for all the tables on the server. You can change the setting at any time. Existing tables change their behaviour when the setting changes.
When creating a table, specify the corresponding [engine setting](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table). The behaviour of an existing table with this setting does not change, even if the global setting changes.
If `use_minimalistic_part_header_in_zookeeper = 1`, then [replicated](../../engines/table-engines/mergetree-family/replication.md) tables store the headers of the data parts compactly using a single `znode`. If the table contains many columns, this storage method significantly reduces the volume of the data stored in Zookeeper.
After applying `use_minimalistic_part_header_in_zookeeper = 1`, you can’t downgrade the ClickHouse server to a version that does not support this setting. Be careful when upgrading ClickHouse on servers in a cluster. Don’t upgrade all the servers at once. It is safer to test new versions of ClickHouse in a test environment, or on just a few servers of a cluster.
The configurable settings within `<distributed_ddl>` include:
- **path**: the path in Keeper for the `task_queue` for DDL queries
- **profile**: the profile used to execute the DDL queries
- **pool_size**: how many `ON CLUSTER` queries can be run simultaneously
- **max_tasks_in_queue**: the maximum number of tasks that can be in the queue. Default is 1,000
- **task_max_lifetime**: delete node if its age is greater than this value. Default is `7 * 24 * 60 * 60` (a week in seconds)
- **cleanup_delay_period**: cleaning starts after new node event is received if the last cleaning wasn't made sooner than `cleanup_delay_period` seconds ago. Default is 60 seconds
If this section is specified, the path from [users_config](../../operations/server-configuration-parameters/settings.md#users-config) and [access_control_path](../../operations/server-configuration-parameters/settings.md#access_control_path) won't be used.
The `user_directories` section can contain any number of items, the order of the items means their precedence (the higher the item the higher the precedence).
You can also define sections `memory` — means storing information only in memory, without writing to disk, and `ldap` — means storing information on an LDAP server.
-`server` — one of LDAP server names defined in `ldap_servers` config section. This parameter is mandatory and cannot be empty.
-`roles` — section with a list of locally defined roles that will be assigned to each user retrieved from the LDAP server. If no roles are specified, user will not be able to perform any actions after authentication. If any of the listed roles is not defined locally at the time of authentication, the authentication attempt will fail as if the provided password was incorrect.
Sets the memory size (in bytes) for a stack trace at every peak allocation step. The data is stored in the [system.trace_log](../../operations/system-tables/trace_log.md) system table with `query_id` equal to an empty string.
Allows to collect random allocations and deallocations and writes them in the [system.trace_log](../../operations/system-tables/trace_log.md) system table with `trace_type` equal to a `MemorySample` with the specified probability. The probability is for every allocation or deallocations, regardless of the size of the allocation. Note that sampling happens only when the amount of untracked memory exceeds the untracked memory limit (default value is `4` MiB). It can be lowered if [total_memory_profiler_step](#total-memory-profiler-step) is lowered. You can set `total_memory_profiler_step` equal to `1` for extra fine-grained sampling.
If set to true, then alter operations will be surrounded by parentheses in formatted queries. This makes the parsing of formatted alter queries less ambiguous.
Used to regulate how resources are utilized and shared between merges and other workloads. Specified value is used as `workload` setting value for all background merges. Can be overridden by a merge tree setting.
Used to regulate how resources are utilized and shared between mutations and other workloads. Specified value is used as `workload` setting value for all background mutations. Can be overridden by a merge tree setting.
