# Architecture Introduction Top-down overview JDBC driver Connection/session management Command execution and planning Table/index/constraints Undo log, redo log, and transactions layer B-tree engine and page-based storage allocation Filesystem abstraction ## Introduction H2 implements an embedded and standalone ANSI-SQL89 compliant SQL engine on top of a B-tree based disk store. As of October 2013, Thomas is still working on our next-generation storage engine called MVStore. This will in time replace the B-tree based storage engine. ## Top-down Overview Working from the top down, the layers look like this: JDBC driver. Connection/session management. SQL Parser. Command execution and planning. Table/Index/Constraints. Undo log, redo log, and transactions layer. B-tree engine and page-based storage allocation. Filesystem abstraction. ## JDBC Driver The JDBC driver implementation lives in `org.h2.jdbc, org.h2.jdbcx` ## Connection/session management The primary classes of interest are:
Package | Description
--- | ---
org.h2.engine.Database | the root/global class
org.h2.engine.SessionInterface | abstracts over the differences between embedded and remote sessions
org.h2.engine.Session | local/embedded session
org.h2.engine.SessionRemote | remote session
## Parser The parser lives in `org.h2.command.Parser`. It uses a straightforward recursive-descent design. See Wikipedia Recursive descent parser page. ## Command execution and planning Unlike other databases, we do not have an intermediate step where we generate some kind of IR (intermediate representation) of the query. The parser class directly generates a command execution object. Then we run some optimisation steps over the command to possibly generate a more efficient command. The primary packages of interest are:
Package | Description
--- | ---
org.h2.command.ddl | Commands that modify schema data structures
org.h2.command.dml | Commands that modify data
## Table/Index/Constraints One thing to note here is that indexes are simply stored as special kinds of tables. The primary packages of interest are:
Package | Description
--- | ---
org.h2.table | Implementations of different kinds of tables
org.h2.index | Implementations of different kinds of indices
## Undo log, redo log, and transactions layer We have a transaction log, which is shared among all sessions. See also https://en.wikipedia.org/wiki/Transaction_log http://h2database.com/html/grammar.html#set_log We also have an undo log, which is per session, to undo an operation (an update that fails for example) and to rollback a transaction. Theoretically, the transaction log could be used, but for simplicity, H2 currently uses it's own "list of operations" (usually in-memory). With the MVStore, this is no longer needed (just the transaction log). ## B-tree engine and page-based storage allocation. The primary package of interest is `org.h2.store`. This implements a storage mechanism which allocates pages of storage (typically 2k in size) and also implements a b-tree over those pages to allow fast retrieval and update. ## Filesystem abstraction. The primary class of interest is `org.h2.store.FileStore`. This implements an abstraction of a random-access file. This allows the higher layers to treat in-memory vs. on-disk vs. zip-file databases the same.