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Documentation Index

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Prisma Schema Language (PSL)

The Prisma Schema Language (PSL) is the core parser, validator, and formatter for Prisma schemas. It provides the foundation for all schema-related operations across the Prisma ecosystem.

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What is PSL?

PSL is the implementation of the Prisma Schema Language - a declarative language for defining database models, relations, and configuration. The PSL crates are responsible for:
  • Parsing schema files into an Abstract Syntax Tree (AST)
  • Validating schemas against connector-specific rules and constraints
  • Analyzing schema structure and relationships
  • Formatting and reformatting schema code
  • Providing APIs for other Prisma engines to work with schemas
PSL is connector-agnostic at its core. Database-specific validations and features are handled through the connector trait system.

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Architecture Overview

The PSL implementation is organized into several focused crates with a clear dependency hierarchy:

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Crate Dependency Graph

The dependency structure is intentionally simple and linear: 
diagnostics →
schema-ast →
parser-database →
psl-core
builtin-connectors →
psl
The dml crate is a separate data structure that can optionally be produced (“lifted”) from a validated schema. It’s not part of the main dependency chain.

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Core Crates

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diagnostics

Provides error and warning reporting with source span information:
  • DatamodelError - Error types with source locations
  • DatamodelWarning - Warning types
  • Diagnostics - Collection of errors and warnings
  • Span and FileId - Source location tracking
  • Pretty-printing utilities for user-facing error messages

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schema-ast

The Abstract Syntax Tree representation:
  • Faithfully represents Prisma Schema syntax with source spans
  • Built using the Pest parser generator
  • Supports single and multi-file schemas
  • Provides AST nodes for models, fields, attributes, enums, etc.
Key exports:
  • parse_schema() - Parse string to AST
  • reformat() - Format schema code
  • SourceFile - File content wrapper

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parser-database

Connector-agnostic semantic analysis:
  • Resolves names to IDs for all schema items
  • Resolves types for fields and type aliases
  • Validates attributes on models and fields
  • Performs global validations (e.g., index name collisions)
  • Provides walker APIs for traversing validated schemas
Validation phases:
  1. Name resolution
  2. Type resolution
  3. Attribute validation
  4. Global validations
ParserDatabase never fails - it accumulates as much information as possible and returns diagnostics alongside incomplete but usable results.

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psl-core

The core implementation entry point:
  • Defines the Connector trait for database-specific behavior
  • Implements validation pipeline
  • Provides configuration parsing
  • Handles datasource and generator blocks
  • Coordinates connector-specific validations
Key traits:
  • Connector - Database connector interface
  • ExtensionTypes - Custom type system extensions

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builtin-connectors

Implements connectors for supported databases:
  • PostgreSQL - Full featured connector with extensions support
  • MySQL - Including PlanetScale compatibility
  • SQLite - Embedded database support
  • SQL Server - Microsoft SQL Server connector
  • CockroachDB - Distributed SQL database
  • MongoDB - Document database (limited QC support)
Each connector implements:
  • Native type mappings
  • Capability flags
  • Database-specific validations
  • Referential action support

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psl

The public API crate - the main entry point used by other engines: 
use psl::parse_schema;

let validated_schema = parse_schema(&schema_string)?;
Main APIs:
  • parse_schema() - Full parsing and validation
  • parse_configuration() - Parse only datasource/generator blocks
  • validate() - Validation with error tolerance
  • reformat() - Code formatting

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Usage Example

Parsing and validating a schema: 
use psl::parse_schema_without_extensions;

let schema = r#"
  datasource db {
    provider = "postgresql"
  }
  
  model User {
    id    Int    @id @default(autoincrement())
    email String @unique
    posts Post[]
  }
  
  model Post {
    id       Int  @id @default(autoincrement())
    authorId Int
    author   User @relation(fields: [authorId], references: [id])
  }
"#;

let validated = parse_schema_without_extensions(schema)?;
// Access validated schema through walker APIs

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Integration Points

PSL is used throughout the Prisma ecosystem:
  • Query Compiler - Processes schemas to produce client API and DMMF JSON
  • Schema Engine - Powers migrations and introspection
  • prisma-fmt - Language server and formatter
  • Prisma CLI - Schema validation and configuration

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Testing

Run the test suite: 
# All PSL tests
cargo test -p psl -F all

# Validation tests (declarative)
cargo test -p psl --test validation_tests

# Reformat tests
cargo test -p psl --test reformat_tests

# Update snapshots
UPDATE_EXPECT=1 cargo test -p psl
PSL uses declarative testing where .prisma files in tests/validation/ are automatically validated and their errors are compared against comment expectations.

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Key Design Principles

  1. Connector Agnostic Core - Database-specific logic lives in connectors
  2. Error Tolerance - Validation accumulates diagnostics without failing
  3. Source Fidelity - AST preserves exact source locations for errors
  4. Simple Dependencies - Linear dependency graph prevents cycles
  5. Walker Pattern - Type-safe traversal of validated schemas

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Next Steps

Parser Implementation

Deep dive into the parser and schema-ast

Validation

Learn about validation and parser-database

Connectors

Explore database connectors and capabilities

Contributing

Contributing guide for PSL development

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