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# Real-time relations
This document describes an intermediate mid-level data storage mechanism to support real-time queries.
The mechanism is meant to be implemented on top of a low-level data storage layer such as [LMDB] that implements all necessary storage and retrieval operations.
The mechanism is meant to be used directly or through a high-level storage/retrieval mechanism that implements more powerful operations.
This document refers to this mechanism as RTR.
## Base concepts
### Tuples
RTR centers around n-tuples of atoms of specific types.
Atom types must have a total ordering.
N-tuple types are designated with T, U, V...
Atom types are designated with t, u, v...
N-tuple type T is composed of atoms of types (t, u, v...).
N-tuple type T has a total ordering derived from the ordering of its atoms from left to right.
Values of N-tuples must have an encoding as a variable-length sequence of bytes.
### Unique relation
A unique relation is a mapping from one n-tuple to another n-tuple.
Unique relations are designated with TT, UU, VV...
Unique relation TT maps n-tuples of type T to n-tuples of type U.
TT is described as having type (T -> U).
Values of T must be unique in a unique relation. (Values of U can have duplicates.)
## Operations
All operations must have a serialized representation.
### Unique relation data definition
create_unique_relation(TT (name), T (key type), U (value type))
drop_unique_relation(TT (name))
### Unique relation data manipulation
insert_into_unique_relation(TT (name), T (key type), U (value type)) -> ok|err if duplicate
delete_from_unique_relation(TT (name), T (key type)) -> ok|err if missing
update_unique_relation(TT (name), T (key type), U (value type)) -> ok|err if missing
### One-off queries from unique relations
select_single_from_unique_relation(TT (name), T (value)) -> U (value)
select_range_from_unique_relation(TT (name), T (lower value), T (higher value) -> sorted list of (T,U) values where lower <= T < higher
### Streaming queries from unique relations
The one-off queries have a variant with the streaming_ prefix. They operate as follows:
First, they return the same result as the one-off query.
Then, until the user closes the query, any data manipulation operation is evaluated against the query condition.
If an inserted or updated tuple matches the query, then an add event with the key and value is sent to the user.
If a deleted tuple matches the query, then a remove event with the key is sent to the user.
## References
=> https://symas.com/lmdb.php [LMDB] LMDB: Lightning Memory-Mapped Database
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