forked from fixin.me/fixin.me
60 lines
3.4 KiB
Markdown
60 lines
3.4 KiB
Markdown
DESIGN
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======
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Below is a list of design decisions. The justification is to be consulted
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whenever a change is considered, to avoid regressions.
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### Data type for DB storage of numeric values (`decimal` vs `float`)
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* among database engines supported (by Rails), SQLite offers storage of
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`decimal` data type with the lowest precision, equal to the precision of
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`REAL` type (double precision float value, IEEE 754), but in a floating point
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format,
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* decimal types in other database engines offer greater precision, but store
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data in a fixed point format,
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* biology-related values differ by several orders of magnitude; storing them in
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fixed point format would only make sense if required precision would be
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greater than that offered by floating point format,
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* even then, fixed point would mean either bigger memory requirements or
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worse precision for numbers close to scale limit,
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* for a fixed point format to use the same 8 bytes of storage as IEEE
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754, precision would need to be limited to 18 digits (4 bytes/9 digits)
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and scale approximately half of that - 9,
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* double precision floating point guarantees 15 digits of precision, which
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is more than enough for all expected use cases,
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* if a decimal string with at most 15 significant digits is converted to
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the IEEE 754 double-precision format, giving a normal number, and then
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converted back to a decimal string with the same number of digits, the
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final result should match the original string,
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* if an IEEE 754 double-precision number is converted to a decimal
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string with at least 17 significant digits, and then converted back to
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double-precision representation, the final result must match the
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original number,
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* single precision floating point only guarntees 6 digits of precision,
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which is estimated to be too low for some use cases (e.g. storing
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latitude/longitude with a resolution grater than 100m),
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* double precision floating point (IEEE 754) is a standard that ensures
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compatibility with majority of database engines,
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* the same data format is used internally by Ruby as a `Float`; it
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guarantees no conversions between storage and computation,
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* as a standard with hardware implementations ensures both: computing
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efficiency and hardware/3rd party library compatibility as opposed to Ruby
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custom `BigDecimal` type,
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* at present, only normalized numbers are used and considered sufficient, in
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order to avoid potential issues with the cross-platform compatibility of
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subnormal (denormal) numbers,
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* in the future, the IEEE 754 decimal64 data type may be considered once it is
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supported by database engines.
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### Database layer vs application layer data model constraints
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* database constraints are the final guard against data integrity corruption,
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* they should safeguard against data referential integrity loss under _all_
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data (not schema) manipulation scenarios, including application level
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logic errors and direct data manipulation (e.g. through `dbconsole`),
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* application constraints can be as restrictive as database constraints or more,
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but not less, as it doesn't serve any use case,
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* proper application level constraints should prevent unhandled database
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exception occurences, e.g `ActiveRecord::InvalidForeignKey` for operations
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performed through Models (i.e. not `#delete_all` etc.)
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