問題： ClickHouse 數據庫引擎，默認的到底是 Atomic 還是 Ordinary？

I found this comment in source file: src/Databases/DatabaseAtomic.h

/// All tables in DatabaseAtomic have persistent UUID and store data in /// /clickhouse_path/store/xxx/xxxyyyyy-yyyy-yyyy-yyyy-yyyyyyyyyyyy/ /// where xxxyyyyy-yyyy-yyyy-yyyy-yyyyyyyyyyyy is UUID of the table. /// RENAMEs are performed without changing UUID and moving table data. /// Tables in Atomic databases can be accessed by UUID through DatabaseCatalog. /// On DROP TABLE no data is removed, DatabaseAtomic just marks table as dropped /// by moving metadata to /clickhouse_path/metadata_dropped/ and notifies DatabaseCatalog. /// Running queries still may use dropped table. Table will be actually removed when it's not in use. /// Allows to execute RENAME and DROP without IStorage-level RWLocks class DatabaseAtomic : public DatabaseOrdinary { public:DatabaseAtomic(String name_, String metadata_path_, UUID uuid, const String & logger_name, ContextPtr context_);DatabaseAtomic(String name_, String metadata_path_, UUID uuid, ContextPtr context_);String getEngineName() const override { return "Atomic"; }UUID getUUID() const override { return db_uuid; }

ClickHouse 官網文檔說：

默認情況下，ClickHouse使用Atomic數據庫引擎。

源代碼中說：

/** Default engine of databases.* It stores tables list in filesystem using list of .sql files,* that contain declaration of table represented by SQL ATTACH TABLE query.*/ class DatabaseOrdinary : public DatabaseOnDisk { public:DatabaseOrdinary(const String & name_, const String & metadata_path_, ContextPtr context);DatabaseOrdinary(const String & name_, const String & metadata_path_, const String & data_path_,const String & logger, ContextPtr context_);String getEngineName() const override { return "Ordinary"; }

Answer

Atomic Database Engine

In version 20.5 ClickHouse first introduced database engine=Atomic.

Since version 20.10 it is a default database engine (before engine=Ordinary was used).

Those 2 database engine differs in a way how they store data on a filesystem, and engine Atomic allows to resolve some of the issues existed in engine=Ordinary.

engine=Atomic supports

non-blocking drop table / rename table
tables delete (&detach) async (wait for selects finish but invisible for new selects)
atomic drop table (all files / folders removed)
atomic table swap (table swap by “EXCHANGE TABLES t1 AND t2;”)
rename dictionary / rename database
unique automatic UUID paths in FS and ZK for Replicated

https://kb.altinity.com/engines/altinity-kb-atomic-database-engine/

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