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https://github.com/matrix-construct/construct
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104 lines
5 KiB
Markdown
104 lines
5 KiB
Markdown
## Database
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### Organization
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The database subsystem is split into three interfaces.
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- IRCd developer interface. These classes are for developers to simply use the
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database. An overview of them is given below. They do not require RocksDB headers
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and expose no RocksDB internals. These interfaces are included in the standard
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include stack. The headers are readily found listed in `<ircd/db.h>`. The
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definitions are in `ircd/db.cc`.
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- Database developer interface. These classes implement various RocksDB virtual
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interfaces. They are all declared as nested classes of`ircd::db::database` Most of
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these are not used directly by general users of the database; though they are
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technically frontend interfaces to RocksDB. They help us create the IRCd
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developer interface in `ircd/db.cc`. Headers are in `<ircd/db/database/*>`
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but most are not included in the standard include stack.
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- Database environment interface. These classes implement additional RocksDB
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virtual interfaces for backend callbacks operations. We connect RocksDB to `ircd::fs`
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and `ircd::ctx` et al on this surface. Headers are in `<ircd/db/database/env/*>`
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and are never included in the standard include stack. Definitions are in a
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separate unit `db_env.cc`.
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- Database port interface (bonus). This hack helps us trick RocksDB into using
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`ircd::ctx` rather than posix threads. We achieve this by conjuring magics using
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shared library PLT indirection while praying that the class layout and semantics
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RocksDB's compiler has assumed does not thwart our plans. These definitions are
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in `ircd/db_port.cc`.
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### Interface
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The database here is a strictly schematized key-value grid built from the primitives of
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`column`, `cell` and `row`. We use the database as both a flat-object store (matrix
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event db) using cells, columns and rows; as well as binary data block store (matrix
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media db) using just a single column.
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#### Columns
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A column is a key-value store. We specify columns in the database descriptors when
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opening the database and (though technically somewhat possible) don't change them
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during runtime. The database must be opened with the same descriptors in properly
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aligned positions every single time. All keys and values in a column can be iterated.
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Columns can also be split into "domains" of keys (see: `db/index.h`) based on the
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key's prefix, allowing a seek and iteration isolated to just one domain.
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In practice we create a column to present a single property in a JSON object. There
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is no recursion and we also must know the name of the property in advance to specify
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a descriptor for it. Applied, we create a column for each property in the Matrix
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event object† and then optimize that column specifically for that property.
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```
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{
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"origin_server_ts": 15373977384823,
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"content": {"msgtype": "m.text", "body": "hello"}
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}
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```
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For the above example consider two columns: first `origin_server_ts` is optimized for
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timestamps by having values which are fixed 8 byte signed integers; next the content
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object is stored in whole as JSON text in the content column. Recursion is not yet
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supported but theoretically we can create more columns to hold nested properties
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if we want further optimizations.
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† Note that the application in the example has been further optimized; not
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every property in the Matrix event has a dedicated column, and an additional
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meta-column is used to store full JSON events (albeit compressed) which is
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may be selected to improve performance by making a single larger request
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rather than several smaller requests to compose the same data.
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#### Rows
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Since `columns` are technically independent key-value stores (they have their own
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index), when an index key is the same between columns we call this a `row`. In
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the Matrix event example, each property of the same event is sought together in a
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`row`. A row seek is optimized and the individual cells are queried concurrently and
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iterated in lock-step together.
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There is a caveat to rows even though the queries to individual cells are
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made concurrently: each cell still requires an individual request to the
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storage device. The device has a fixed number of total requests it can service
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concurrently.
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#### Cells
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A `cell` is a gratuitious interface representing of a single value in a `column` with
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a common key that should be able to form a `row` between columns. A `row` is comprised
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of cells.
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### Important notes
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!!!
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The database system is plugged into the userspace context system to facilitate IO. This means
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that an expensive database call (mostly on the read side) that has to do disk IO will suspend
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your userspace context. Remember that when your userspace context resumes on the other side
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of the call, the state of IRCd and even the database itself may have changed. We have a suite
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of tools to mitigate this.
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!!!
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* While the database schema is modifiable at runtime (we can add and remove columns on
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the fly) the database is very picky about opening the exact same way it last closed.
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This means, for now, we have the full object schema explicitly specified when the DB
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is first opened. All columns exist for the lifetime of the DB, whether or not you have
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a handle to them.
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