Add changeable_without_restart column to system.server_settings

[skyoct]

Fixes #57902

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Replaced undocumented (boolean) column is_hot_reloadable in system table system.server_settings by (Enum8) column changeable_without_restart with possible values No, Yes, IncreaseOnly and DecreaseOnly. Also documented the column.

[serxa]

Looks like too many letters, Can we leave just M and change a few other macros?

[skyoct]

@serxa Hi, I'm sorry, I don't quite understand what you mean. Perhaps you are referring to the macro's name being too long?

[serxa]

I was thinking it should be possible to use M(TYPE, NAME, DEFAULT, DESCRIPTION, FLAGS, RUNTIME_RELOAD) directly, but now I see that SERVER_SETTINGS is passed directly to DECLARE_SETTINGS_TRAITS which is used in a lot of places, so changing it is not a good idea.

But, yes macro's name still feels too long to me, just because almost everywhere we use M(X, Y, Z) do declare such a list. Let's ask @rschu1ze for his opinion. I was just glad to see this PR :-)

[skyoct]

@serxa Ok, The new macro name does sound awkward, and I will make sure to modify it later.

[robot-clickhouse]

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[rschu1ze]

There is no restriction that new columns must be appended, we can insert them at arbitrary positions and even permute the order of existing columns.

Thoughts:

• is_obsolete should be the last column
• is_hot_reloadable and runtime_reload are related columns and should therefore be adjacent to each other
• let's move the two former columns prior to is_obsolete.

[rschu1ze]

runtime_reload isn't a great name (is_hot_reloadable isn't either but it is too late to change it, I guess).

So if I understand correctly, 'is_hot_reloadableshows *if* a server setting can be changed andruntime_reload` shows how it can be changed. There is some duplication here. I think the latter is conceptually a full replacement of the former, right?

[skyoct]

@rschu1ze I think perhaps the value of "is_hot_reloadable" is problematic. It seems that only those that undergo changes (where the set value is inconsistent with the displayed value) will be shown as true.

[skyoct]

ClickHouse/src/Storages/System/StorageSystemServerSettings.cpp

Lines 38 to 61 in 20c0347

	std::unordered_map<std::string, std::string> updated = {
	{"max_server_memory_usage", std::to_string(total_memory_tracker.getHardLimit())},
	{"allow_use_jemalloc_memory", std::to_string(total_memory_tracker.getAllowUseJemallocMmemory())},
	{"max_table_size_to_drop", std::to_string(context->getMaxTableSizeToDrop())},
	{"max_partition_size_to_drop", std::to_string(context->getMaxPartitionSizeToDrop())},
	{"max_concurrent_queries", std::to_string(context->getProcessList().getMaxSize())},
	{"max_concurrent_insert_queries", std::to_string(context->getProcessList().getMaxInsertQueriesAmount())},
	{"max_concurrent_select_queries", std::to_string(context->getProcessList().getMaxSelectQueriesAmount())},
	{"background_buffer_flush_schedule_pool_size", std::to_string(CurrentMetrics::get(CurrentMetrics::BackgroundBufferFlushSchedulePoolSize))},
	{"background_schedule_pool_size", std::to_string(CurrentMetrics::get(CurrentMetrics::BackgroundSchedulePoolSize))},
	{"background_message_broker_schedule_pool_size", std::to_string(CurrentMetrics::get(CurrentMetrics::BackgroundMessageBrokerSchedulePoolSize))},
	{"background_distributed_schedule_pool_size", std::to_string(CurrentMetrics::get(CurrentMetrics::BackgroundDistributedSchedulePoolSize))}
	};
	if (context->areBackgroundExecutorsInitialized())
	{
	updated.insert({"background_pool_size", std::to_string(context->getMergeMutateExecutor()->getMaxThreads())});
	updated.insert({"background_move_pool_size", std::to_string(context->getMovesExecutor()->getMaxThreads())});
	updated.insert({"background_fetches_pool_size", std::to_string(context->getFetchesExecutor()->getMaxThreads())});
	updated.insert({"background_common_pool_size", std::to_string(context->getCommonExecutor()->getMaxThreads())});
	}

ClickHouse/src/Storages/System/StorageSystemServerSettings.cpp

Line 79 in 20c0347

res_columns[7]->insert((it != updated.end()) ? true : false);

[skyoct]

@rschu1ze Hi, Can I delete "is_hot_reloadable" and change "runtime_reload" to "hot_reload"?

[rschu1ze]

@skyoct I just came back to your PR (after holidays) and checked deeper. Someone added a boolean column is_hot_reloadable already and it overlaps with what this PR tries to achieve. Luckily, is_hot_reloadable was never documented, so we might get away with hijacking it for our purposes, i.e. change its data type from Bool to Enum8 with values Yes, No, IncreaseOnly and DecreaseOnly. Also, the existing map updated could be extended with an enum to indicate how the value is reloadable. With that, ServerSettings.h/cpp can remain as is. I made these changes and pushed them.

The advantage of the new approach is that ServerSettings.h/cpp and StorageSystemServerSettings.cpp cannot easily go out-of-sync (imagine someone adds ChangeableAtRuntime::Yes in ServerSettings but forgets to modify the updated map in StorageSystemServerSettings.cpp)...

But to be fair, all of that is based on the fact that the periodic re-loads of the server config only update the server components (e.g. memory tracking) with the new setting values but do not store the settings themselves (so we can't get them directly). If that was the case, your approach would be better. I actually like it conceptually better as we could then use the ChangeableWithoutRestart property to accept/reject changes of server settings at runtime (right now, it is only used by the system view). Anyways, that would be a bigger rewrite.

Besides that, I named the new column changeable_without_restart (as mentioned already) and marked various caches as such. I guess there are a lot more server settings which can be changed at runtime but that can be fixed in separate PR(s).

[azat]

The advantage of the new approach is that ServerSettings.h/cpp and StorageSystemServerSettings.cpp cannot easily go out-of-sync (imagine someone adds ChangeableAtRuntime::Yes in ServerSettings but forgets to modify the updated map in StorageSystemServerSettings.cpp)...

This may also force developers to write the reloading handler for a newly added settings.