Add different data providers and improve docs

This adds support for switching between in-memory, local filesystem, s3
or sqlite.

stores/sqlite/documents.go

@@ -0,0 +1,60 @@
+package sqlite
+
+import (
+	"bytes"
+	"context"
+	"excalidraw-complete/core"
+	"fmt"
+
+	"database/sql"
+	"log"
+
+	_ "github.com/mattn/go-sqlite3"
+	"github.com/oklog/ulid/v2"
+)
+
+var savedDocuments = make(map[string]core.Document)
+
+type documentStore struct {
+	db *sql.DB
+}
+
+func NewDocumentStore(dataSourceName string) core.DocumentStore {
+	// db, err := sql.Open("sqlite3", ":memory:")
+	db, err := sql.Open("sqlite3", dataSourceName)
+
+	if err != nil {
+		log.Fatal(err)
+	}
+	sts := `CREATE TABLE IF NOT EXISTS documents (id TEXT PRIMARY KEY, data BLOB);`
+	_, err = db.Exec(sts)
+	if err != nil {
+		log.Fatal(err)
+	}
+	return &documentStore{db}
+}
+
+func (s *documentStore) FindID(ctx context.Context, id string) (*core.Document, error) {
+	var data []byte
+	err := s.db.QueryRowContext(ctx, "SELECT data FROM documents WHERE id = ?", id).Scan(&data)
+	if err != nil {
+		if err == sql.ErrNoRows {
+			return nil, fmt.Errorf("document with id %s not found", id)
+		}
+		return nil, err
+	}
+	document := core.Document{
+		Data: *bytes.NewBuffer(data),
+	}
+	return &document, nil
+}
+
+func (s *documentStore) Create(ctx context.Context, document *core.Document) (string, error) {
+	id := ulid.Make().String()
+	data := document.Data.Bytes()
+	_, err := s.db.ExecContext(ctx, "INSERT INTO documents (id, data) VALUES (?, ?)", id, data)
+	if err != nil {
+		return "", err
+	}
+	return id, nil
+}