TileJSON 3.0 support with layer fields discovery and cached /capabilities responses

mapbox/tilejson/tilejson.go

@@ -2,7 +2,10 @@
 // https://github.com/mapbox/tilejson-spec
 package tilejson
 
-const Version = "2.1.0"
+const (
+	Version2 = "2.0.0"
+	Version3 = "3.0.0"
+)
 
 type GeomType string
 
@@ -135,4 +138,7 @@ type VectorLayer struct {
 	MaxZoom uint `json:"maxzoom"`
 	// Tegola supports individual layer tiles.
 	Tiles []string `json:"tiles"`
+	// REQUIRED in TileJSON 3.0.0. An object whose keys and values are the names
+	// and descriptions of attributes available in this layer.
+	Fields map[string]any `json:"fields,omitempty"`
 }

provider/postgis/postgis.go

@@ -1069,6 +1069,70 @@ func (p Provider) TileFeatures(
 	return rows.Err()
 }
 
+// LayerFields returns a map of field names to their types for a given layer.
+// It executes a sample query (LIMIT 0) to get column information without fetching data.
+func (p Provider) LayerFields(ctx context.Context, layerName string) (map[string]interface{}, error) {
+	plyr, ok := p.Layer(layerName)
+	if !ok {
+		return nil, ErrLayerNotFound{layerName}
+	}
+
+	// Use a dummy tile to replace tokens in the SQL
+	dummyTile := provider.NewTile(0, 0, 0, 256, 3857)
+	sql, err := replaceTokens(plyr.sql, &plyr, dummyTile, true)
+	if err != nil {
+		return nil, fmt.Errorf("error replacing layer tokens for layer (%s): %w", layerName, err)
+	}
+
+	// Wrap the query to get column information without fetching rows
+	sql = fmt.Sprintf("SELECT * FROM (%s) AS subquery LIMIT 0", sql)
+
+	rows, err := p.pool.Query(ctx, sql)
+	if err != nil {
+		return nil, fmt.Errorf("error querying fields for layer (%s): %w", layerName, err)
+	}
+	defer rows.Close()
+
+	fields := make(map[string]any)
+	fdescs := rows.FieldDescriptions()
+
+	for _, desc := range fdescs {
+		fieldName := desc.Name
+
+		// Skip geometry and ID fields as they're not attributes
+		if fieldName == plyr.GeomFieldName() || fieldName == plyr.IDFieldName() {
+			continue
+		}
+
+		// Map PostgreSQL OID types to simple type names
+		fieldType := postgresTypeToString(desc.DataTypeOID)
+		fields[fieldName] = fieldType
+	}
+
+	return fields, nil
+}
+
+// postgresTypeToString converts PostgreSQL OID types to simple type strings
+func postgresTypeToString(oid uint32) string {
+	// Common PostgreSQL type OIDs using pgtype constants
+	switch oid {
+	case pgtype.BoolOID:
+		return "Boolean"
+	case pgtype.Int8OID, pgtype.Int2OID, pgtype.Int4OID, pgtype.OIDOID:
+		return "Number"
+	case pgtype.Float4OID, pgtype.Float8OID, pgtype.NumericOID:
+		return "Number"
+	case pgtype.QCharOID, pgtype.NameOID, pgtype.TextOID, pgtype.BPCharOID, pgtype.VarcharOID:
+		return "String"
+	case pgtype.TimestampOID, pgtype.TimestamptzOID:
+		return "String"
+	case pgtype.JSONOID, pgtype.JSONBOID:
+		return "String"
+	default:
+		return "String"
+	}
+}
+
 func (p Provider) MVTForLayers(
 	ctx context.Context,
 	tile provider.Tile,

provider/provider.go

@@ -132,6 +132,14 @@ type Tiler interface {
 	TileFeatures(ctx context.Context, layer string, t Tile, params Params, fn func(f *Feature) error) error
 }
 
+// LayerFielder is an optional interface that providers can implement to expose
+// field information for their layers. This is used to populate the "fields"
+// property in TileJSON 3.0.0 capabilities.
+type LayerFielder interface {
+	// LayerFields returns a map of field names to their type descriptions for the given layer
+	LayerFields(ctx context.Context, layerName string) (map[string]any, error)
+}
+
 // TilerUnion represents either a Std Tiler or and MVTTiler; only one should be not nil.
 type TilerUnion struct {
 	Std Tiler