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Constraints

Invariant support is currently limited to referential integrity and not-null constraints. Unique and check constraints are not yet supported.

Supported​

Referential integrity​

ElectricSQL maintains referential integrity of foreign key references. So you can use foreign key relationships in your data model and rely on referential integrity:

CREATE TABLE posts (
  id UUID PRIMARY KEY
);

CREATE TABLE comments (
  id UUID PRIMARY KEY

  post_id UUID REFERENCES(posts.id) ON DELETE CASCADE
);

This works even when making writes locally in an offline database. See Introduction -> Conflict-free offline -> Preserving data integrity and the Rich-CRDT post on Compensations for more information.

Not-null constraints​

ElectricSQL supports not-null constraints as long as the constraint is defined when creating the column.

I.e.: the not-null constraint must be defined in an additive migration. So the following is supported because creating the table with new columns is additive:

CREATE TABLE items (
  -- implicit non null constraint
  id UUID PRIMARY KEY

  -- explicit non null constraint
  foo TEXT NOT NULL

  -- can be null
  bar TEXT
)

Adding a column with a not-null constraint is supported because it's additive:

ALTER TABLE items
  ADD COLUMN baz TEXT NOT NULL;

Constraining an existing column by adding a not-null constraint to it is not supported:

-- Not supported
ALTER TABLE items
  ALTER COLUMN bar TEXT NOT NULL;

This is not supported because it may invalidate concurrent, in-flight operations. Specifically, writes that were accepted locally with null values would need to be rejected, which would violate the finality of local writes.

Unsupported​

Unsupported constraints must be removed from a table before electrifying it.

Check constraints​

ElectricSQL does not currently support check constraints.

Unique constraints​

ElectricSQL does not currently support unique constraints.

Work in progress

We're working to support:

  • unique constraints using Reservations
  • single column and then multi-column check constraints using validation

See Rich-CRDTs for more information.