Architecture

Internal design of the lexigram-graph package.

---

Role in the System

lexigram-graph lives in the Data & Persistence tier. It implements GraphStoreProtocol and GraphProtocol from lexigram-contracts and provides pluggable backends for Neo4j and in-memory graph stores.

flowchart BT
    App[Application Code]
    Contracts[lexigram-contracts<br/>GraphStoreProtocol · GraphProtocol<br/>GraphNode · GraphEdge · GraphPath<br/>TraversalQuery · PropertyFilter]
    Graph[lexigram-graph<br/>GraphModule · GraphProvider<br/>Backends · CypherCompiler]
    Memory[In-Memory Backend<br/>Testing · Development]
    Neo4j[Neo4j Backend<br/>Production]

    App --> Contracts
    Graph --> Contracts
    Memory --> Graph
    Neo4j --> Graph

Import direction: Application code depends on GraphStoreProtocol from lexigram-contracts, never on lexigram-graph directly. The container resolves the protocol to the backend chosen at configuration time.

---

Backend Abstraction

Two protocols define the store and graph boundaries:

GraphStoreProtocol

Top-level lifecycle — connection, health, and graph database management:

class GraphStoreProtocol(Protocol):
    async def connect(self) -> None: ...
    async def disconnect(self) -> None: ...
    async def health_check(self, timeout: float = 5.0) -> HealthCheckResult: ...
    async def get_graph(self, name: str | None = None) -> GraphProtocol: ...
    async def list_graphs(self) -> list[GraphInfo]: ...
    async def create_graph(self, name: str) -> None: ...
    async def delete_graph(self, name: str) -> None: ...

GraphProtocol

All operations on a single graph database — CRUD, traversal, query, bulk, schema:

class GraphProtocol(Protocol):
    # Node operations
    async def create_node(self, labels: list[str], ...) -> NodeResult: ...
    async def get_node(self, node_id: str) -> GraphNode | None: ...
    async def find_nodes(self, ...) -> list[GraphNode]: ...
    async def update_node(self, node_id: str, ...) -> bool: ...
    async def delete_node(self, node_id: str, detach: bool = True) -> bool: ...
    async def neighbors(self, node_id: str, ...) -> list[GraphNode]: ...
    async def count_nodes(self) -> int: ...
    async def get_labels(self) -> list[str]: ...

    # Edge operations
    async def create_edge(self, source_id: str, target_id: str, ...) -> EdgeResult: ...
    async def get_edge(self, edge_id: str) -> GraphEdge | None: ...
    async def get_edges(self, node_id: str, ...) -> list[GraphEdge]: ...
    async def update_edge(self, edge_id: str, ...) -> bool: ...
    async def delete_edge(self, edge_id: str) -> bool: ...
    async def get_edge_types(self) -> list[str]: ...

    # Traversal & query
    async def traverse(self, query: TraversalQuery) -> list[GraphPath]: ...
    async def shortest_path(self, from_id: str, to_id: str, ...) -> GraphPath | None: ...
    async def query(self, query_string: str, ...) -> list[dict[str, Any]]: ...

    # Bulk & schema
    async def bulk_create_nodes(self, nodes: list[NodeSpec]) -> BulkNodeResult: ...
    async def bulk_create_edges(self, edges: list[EdgeSpec]) -> BulkEdgeResult: ...
    async def create_index(self, spec: IndexSpec) -> None: ...
    async def drop_index(self, name: str) -> None: ...
    async def create_constraint(self, spec: ConstraintSpec) -> None: ...
    async def drop_constraint(self, name: str) -> None: ...

Supported Backends

flowchart LR
    GP[GraphProvider]
    GP -->|backend=memory| IM[InMemoryGraphStore]
    GP -->|backend=neo4j| N4J[Neo4jGraphStore]
    IM --> IMG[InMemoryGraph<br/>BFS traversal·dict storage]
    N4J --> N4JG[Neo4jGraph<br/>Cypher·AsyncDriver]
    N4JG --> CC[CypherCompiler<br/>TraversalQuery → Cypher]

In-Memory (lexigram.graph.backends.memory): Pure-Python dict store. No external dependencies. BFS traversal. Default for development/testing.

Neo4j (lexigram.graph.backends.neo4j): Uses the neo4j async driver. Translates all graph operations to Cypher. Production backend.

---

Graph Model

The canonical graph data types are defined in lexigram-contracts as frozen dataclasses:

Core Types

Type	Fields	Description
GraphNode	id: str, labels: tuple[str, ...], properties: dict[str, Any]	A vertex in the graph
GraphEdge	id: str, type: str, source_id: str, target_id: str, properties: dict[str, Any]	A directed relationship
GraphPath	nodes: tuple[GraphNode, ...], edges: tuple[GraphEdge, ...]	Alternating node-edge sequence
NodeSpec	labels, properties, id?	Input spec for node creation
EdgeSpec	source_id, target_id, type, properties	Input spec for edge creation
NodeResult	id: str, created: bool	Output of node creation
EdgeResult	id: str, created: bool	Output of edge creation
BulkNodeResult	created_count: int, ids: tuple[str, ...]	Output of bulk node create
BulkEdgeResult	created_count: int, ids: tuple[str, ...]	Output of bulk edge create

Type Aliases

NodeId: TypeAlias = str | int
EdgeId: TypeAlias = str | int
Properties: TypeAlias = dict[str, Any]

Enums

Enum	Values
EdgeDirection	OUTGOING, INCOMING, BOTH
ReturnType	NODES, EDGES, PATHS, COUNT
IndexKind	BTREE, FULLTEXT, RANGE, POINT, TEXT, VECTOR
ConstraintKind	UNIQUE, EXISTS, NODE_KEY
MergeAction	CREATE, MATCH, MERGE

Property Filters

Filters compose conditions via Prop factory:

from lexigram.contracts.data.graph import Prop

filter = Prop.and_(
    Prop.eq("active", True),
    Prop.gte("age", 21),
    Prop.or_(
        Prop.eq("role", "admin"),
        Prop.eq("role", "moderator"),
    ),
)

---

Query Building

TraversalQuery

A traversal is described by StartSpec + one or more TraversalStep:

from lexigram.contracts.data.graph import TraversalQuery, StartSpec, TraversalStep
from lexigram.contracts.data.graph.enums import EdgeDirection, ReturnType

query = TraversalQuery(
    start=StartSpec(node_ids=("user-1",)),
    steps=(TraversalStep(
        edge_types=("FOLLOWS",),
        direction=EdgeDirection.OUTGOING,
        max_depth=3,
    ),),
    return_type=ReturnType.PATHS,
    limit=50,
)

Cypher Compilation

The CypherCompiler in lexigram.graph.backends.neo4j.cypher translates TraversalQuery to Cypher:

flowchart LR
    TQ[TraversalQuery] --> CC[CypherCompiler]
    CC -->|_compile_match| MATCH["MATCH path = (start_node)-[r:FOLLOWS*1..3]->(end_node)"]
    CC -->|_compile_where| WHERE["WHERE elementId(start_node) = $p1"]
    CC -->|_compile_return| RETURN["RETURN path"]
    CC -->|ORDER BY / SKIP / LIMIT| TAIL[ORDER BY end_node.name DESC<br/>SKIP 0 LIMIT 50]

Compilation produces a (cypher_string, params_dict) tuple passed directly to the Neo4j driver.

The in-memory backend uses an internal BFS instead of Cypher — _compile_traversal is not invoked.

Query Execution Flow

sequenceDiagram
    actor Client
    participant G as GraphProtocol
    participant CC as CypherCompiler
    participant Driver as Neo4j Driver
    participant Mem as InMemory Dict

    Client->>G: traverse(query)
    alt Backend is Neo4j
        G->>CC: compile_traversal(query)
        CC-->>G: (cypher, params)
        G->>Driver: session.run(cypher, params)
        Driver-->>G: records
        G->>G: map to GraphNode/Edge/Path
    else Backend is Memory
        G->>Mem: BFS over _nodes / _edges
        Mem-->>G: GraphPath list
    end
    G-->>Client: list[GraphPath]

---

Provider Lifecycle

GraphProvider

class GraphProvider(Provider):
    name = "graph"
    priority = ProviderPriority.INFRASTRUCTURE

Phase	What Happens
__init__	Accepts optional GraphConfig; falls back to environment variable defaults
register()	Binds GraphConfig as singleton. Selects and binds GraphStoreProtocol — InMemoryGraphStore or Neo4jGraphStore based on config.backend
boot()	Resolves GraphStoreProtocol and calls store.connect(). Neo4j establishes driver and creates uniqueness constraint
shutdown()	Disconnects the store. Neo4j closes the async driver; memory clears its graph dict
health_check()	Delegates to the store’s health_check(); returns DEGRADED if disabled or uninitialized

Sequence

sequenceDiagram
    actor App as Application
    participant GP as GraphProvider
    participant Container as DI Container
    participant Store as GraphStoreProtocol
    participant Backend as Backend (Neo4j/Memory)

    App->>GP: GraphModule.configure(config)
    GP->>GP: __init__(config)
    App->>Container: add_module(GraphModule)
    Container->>GP: register(registrar)
    GP->>Container: singleton(GraphConfig)
    alt backend == "neo4j"
        GP->>Container: singleton(GraphStoreProtocol, Neo4jGraphStore)
    else backend == "memory"
        GP->>Container: singleton(GraphStoreProtocol, InMemoryGraphStore)
    end
    Container->>Container: freeze()
    Container->>GP: boot(resolver)
    GP->>Container: resolve(GraphStoreProtocol)
    GP->>Store: store.connect()
    Store->>Backend: connect (Neo4j: create driver)
    App->>App: ...application runs...
    Container->>GP: shutdown()
    GP->>Store: store.disconnect()
    Store->>Backend: disconnect (Neo4j: driver.close)

---

Contracts Used

All contracts come from lexigram.contracts.data.graph:

Contract	Location	How It’s Used
GraphStoreProtocol	lexigram.contracts.data.graph.protocols	Top-level interface registered by GraphProvider
GraphProtocol	lexigram.contracts.data.graph.protocols	Returned by get_graph(); all node/edge/traversal operations
GraphNode	lexigram.contracts.data.graph.types	Return type for node CRUD
GraphEdge	lexigram.contracts.data.graph.types	Return type for edge CRUD
GraphPath	lexigram.contracts.data.graph.types	Return type for traversal and shortest-path
TraversalQuery	lexigram.contracts.data.graph.types	Input spec for traversal operations
TraversalStep	lexigram.contracts.data.graph.types	Single hop in a traversal
StartSpec	lexigram.contracts.data.graph.types	How to locate traversal start nodes
NodeSpec / EdgeSpec	lexigram.contracts.data.graph.types	Input specs for bulk operations
NodeResult / EdgeResult	lexigram.contracts.data.graph.types	Creation results
BulkNodeResult / BulkEdgeResult	lexigram.contracts.data.graph.types	Bulk operation results
GraphInfo	lexigram.contracts.data.graph.types	Graph database metadata
IndexSpec / ConstraintSpec	lexigram.contracts.data.graph.types	Schema operation specs
EdgeDirection	lexigram.contracts.data.graph.enums	Direction for edge queries
ReturnType	lexigram.contracts.data.graph.enums	What traversal returns
IndexKind	lexigram.contracts.data.graph.enums	Index type
ConstraintKind	lexigram.contracts.data.graph.enums	Constraint type
PropertyFilter	lexigram.contracts.data.graph.filters	Filter expression for nodes/edges
Prop	lexigram.contracts.data.graph.filters	Static factory for building filters

---

Source Layout

lexigram-graph/src/lexigram/graph/
├── __init__.py              # Lazy-exported public API
├── config.py                # GraphConfig, Neo4jConfig, MemoryConfig
├── constants.py             # ENV_PREFIX, BACKEND_*, defaults
├── exceptions.py            # GraphError hierarchy (12 exceptions)
├── types.py                 # NodeId, EdgeId, Properties type aliases
├── protocols.py             # Re-exports from lexigram-contracts
├── module.py                # GraphModule — configure() / stub()
├── events.py                # GraphConnectedEvent, GraphNodeCreatedEvent, etc.
├── hooks.py                 # Lifecycle hooks (future)
├── decorators.py            # Decorators (future)
├── di/
│   └── provider.py          # GraphProvider
└── backends/
    ├── base.py              # BaseGraphStore (ABC), BaseGraph (stub defaults)
    ├── memory/
    │   ├── backend.py       # InMemoryGraphStore
    │   └── graph.py         # InMemoryGraph (BFS traversal)
    └── neo4j/
        ├── backend.py       # Neo4jGraphStore
        ├── graph.py         # Neo4jGraph (Cypher-backed)
        └── cypher.py        # CypherCompiler (TraversalQuery → Cypher)

---

Exception Convention

flowchart LR
    subgraph Contracts[lexigram-contracts]
        IE[InfrastructureError]
    end
    subgraph Graph[lexigram-graph]
        GE[GraphError]
        GCE[GraphConnectionError]
        GNE[GraphNodeNotFoundError]
        GEE[GraphEdgeNotFoundError]
        TE[TraversalError]
        CCE[CypherCompilationError]
    end

    IE --> GE
    GE --> GCE
    GE --> GNE
    GE --> GEE
    GE --> TE
    TE --> CCE

12 exceptions total, all under GraphError(InfrastructureError) with unique LEX_ERR_GRAPH_0xx codes. Exception hierarchy: connection failures (GraphConnectionError), not-found errors for nodes/edges/graphs, schema errors, transaction errors, traversal errors, and Cypher compilation errors.

---

Events

The package emits lifecycle events on connection, disconnection, node creation, edge creation, and query execution:

Event	Fields
GraphConnectedEvent	backend: str
GraphDisconnectedEvent	backend: str
GraphNodeCreatedEvent	node_id: str, labels: tuple[str, ...]
GraphEdgeCreatedEvent	source_id: str, target_id: str, relationship_type: str
GraphQueryExecutedEvent	query_type: str, result_count: int

from lexigram.graph import GraphNodeCreatedEvent

@event_handler(GraphNodeCreatedEvent)
async def on_node_created(event: GraphNodeCreatedEvent) -> None:
    logger.info("node_created", node_id=event.node_id, labels=event.labels)

---

DI Registration

lexigram/graph/di/provider.py

class GraphProvider(Provider):
    async def register(self, container):
        container.singleton(GraphConfig, self._config)

        if backend == BACKEND_NEO4J:
            container.singleton(GraphStoreProtocol, factory=lambda: Neo4jGraphStore(...))
        elif backend == BACKEND_MEMORY:
            container.singleton(GraphStoreProtocol, factory=InMemoryGraphStore)

Application code configures via GraphModule:

from lexigram.graph.config import GraphConfig
from lexigram.graph.module import GraphModule

@module(imports=[GraphModule.configure(GraphConfig(backend="neo4j"))])
class AppModule(Module):
    pass

For tests:

@module(imports=[GraphModule.stub()])
class TestModule(Module):
    pass

---

Constants

constants.py defines:

Symbol	Description
BACKEND_MEMORY / BACKEND_NEO4J	Backend identifier strings
ENV_PREFIX	LEX_GRAPH__
DEFAULT_MEMORY_MAX_NODES	1,000,000
DEFAULT_MEMORY_MAX_EDGES	5,000,000
DEFAULT_NEO4J_MAX_POOL_SIZE	100
DEFAULT_CONNECT_TIMEOUT	30.0s
DEFAULT_TRAVERSAL_MAX_DEPTH	10
DEFAULT_QUERY_LIMIT	100
DEFAULT_BULK_BATCH_SIZE	1000
DEFAULT_MAX_RETRIES	3
__version__	Package version

---

Extension Points

Point	Mechanism
Custom backend	Implement GraphStoreProtocol + GraphProtocol, register via custom provider
Custom traversal strategy	Subclass CypherCompiler or implement inline (e.g., for Gremlin)
Event hooks	Subscribe to GraphNodeCreatedEvent, GraphEdgeCreatedEvent, etc.
Health check override	Override health_check() in custom backend
Config override	Pass dict or GraphConfig to GraphModule.configure()
Environment config	LEX_GRAPH__BACKEND=neo4j LEX_GRAPH__NEO4J__URI=bolt://...