diff --git a/pkg/__test__/adapter_test.go b/pkg/__test__/adapter_test.go
new file mode 100644
index 0000000..039540c
--- /dev/null
+++ b/pkg/__test__/adapter_test.go
@@ -0,0 +1,44 @@
+package tests
+
+import (
+	"fmt"
+	"testing"
+
+	_ "github.com/mattn/go-sqlite3"
+	"l12.xyz/dal/adapter"
+)
+
+func TestAdapterBasic(t *testing.T) {
+	a := adapter.DBAdapter{Type: "sqlite3"}
+	db, err := a.Open("file::memory:?cache=shared")
+	if err != nil {
+		t.Fatalf("failed to open db: %v", err)
+	}
+	defer db.Close()
+	_, err = db.Exec("CREATE TABLE test (id INTEGER PRIMARY KEY, name TEXT)")
+	if err != nil {
+		t.Fatalf("failed to create table: %v", err)
+	}
+	_, err = db.Exec("INSERT INTO test (name) VALUES (?)", "test")
+	if err != nil {
+		t.Fatalf("failed to insert data: %v", err)
+	}
+	rows, err := a.Query(adapter.Query{
+		Db:         "file::memory:?cache=shared",
+		Expression: "SELECT * FROM test",
+		Data:       []interface{}{},
+	})
+	if err != nil {
+		t.Fatalf("failed to query data: %v", err)
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var id int
+		var name string
+		err = rows.Scan(&id, &name)
+		if err != nil {
+			t.Fatalf("failed to scan row: %v", err)
+		}
+		fmt.Printf("id: %d, name: %s\n", id, name)
+	}
+}
diff --git a/pkg/__test__/go.mod b/pkg/__test__/go.mod
new file mode 100644
index 0000000..4921031
--- /dev/null
+++ b/pkg/__test__/go.mod
@@ -0,0 +1,17 @@
+module l12.xyz/dal/tests
+
+go 1.22.6
+
+replace l12.xyz/dal/builder v0.0.0 => ../builder
+
+replace l12.xyz/dal/utils v0.0.0 => ../utils
+
+require l12.xyz/dal/adapter v0.0.0
+
+replace l12.xyz/dal/adapter v0.0.0 => ../adapter
+
+require (
+	github.com/mattn/go-sqlite3 v1.14.22
+	github.com/pkg/errors v0.9.1 // indirect
+	l12.xyz/dal/utils v0.0.0 // indirect
+)
diff --git a/pkg/__test__/go.sum b/pkg/__test__/go.sum
new file mode 100644
index 0000000..acfeee2
--- /dev/null
+++ b/pkg/__test__/go.sum
@@ -0,0 +1,4 @@
+github.com/mattn/go-sqlite3 v1.14.22 h1:2gZY6PC6kBnID23Tichd1K+Z0oS6nE/XwU+Vz/5o4kU=
+github.com/mattn/go-sqlite3 v1.14.22/go.mod h1:Uh1q+B4BYcTPb+yiD3kU8Ct7aC0hY9fxUwlHK0RXw+Y=
+github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
+github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
diff --git a/pkg/adapter/DBAdapter.go b/pkg/adapter/DBAdapter.go
new file mode 100644
index 0000000..b9563bb
--- /dev/null
+++ b/pkg/adapter/DBAdapter.go
@@ -0,0 +1,92 @@
+package adapter
+
+import (
+	"database/sql"
+	"fmt"
+	"time"
+)
+
+/**
+ * DBAdapter
+ * Automatically creates connections for each database URL.
+ * Executes queries on the specified database.
+ * TODO: Closes connections older than 2 minutes.
+**/
+type DBAdapter struct {
+	Type              string
+	MaxAttempts       int
+	_db               map[string]*sql.DB
+	_connect_attempts map[string]int
+	_connect_time     map[string]int64
+}
+
+func (a *DBAdapter) Open(url string) (*sql.DB, error) {
+	if a._db == nil {
+		a._db = make(map[string]*sql.DB)
+	}
+	if a._connect_attempts == nil {
+		a._connect_attempts = make(map[string]int)
+	}
+	if a._connect_time == nil {
+		a._connect_time = make(map[string]int64)
+	}
+	if a.MaxAttempts == 0 {
+		a.MaxAttempts = 6
+	}
+	if _, ok := a._db[url]; !ok {
+		a._db[url], _ = sql.Open(a.Type, url)
+	} else {
+		err := a._db[url].Ping()
+		if err != nil {
+			a._db[url] = nil
+			a._connect_attempts[url]++
+			time.Sleep(time.Duration(5) * time.Second)
+			if a._connect_attempts[url] > a.MaxAttempts {
+				return nil, fmt.Errorf(`failed to connect to "%s"`, url)
+			}
+			return a.Open(url)
+		}
+	}
+	a._connect_attempts[url] = 0
+	a._connect_time[url] = time.Now().Unix()
+	return a._db[url], nil
+}
+
+func (a *DBAdapter) GetDB(url string) *sql.DB {
+	if a._db == nil {
+		return nil
+	}
+	return a._db[url]
+}
+
+func (a *DBAdapter) Close() {
+	for _, db := range a._db {
+		db.Close()
+	}
+	a._db = nil
+	a._connect_attempts = nil
+}
+
+func (a *DBAdapter) Query(req Query) (*sql.Rows, error) {
+	db, err := a.Open(req.Db)
+	if err != nil {
+		return nil, err
+	}
+	sfmt, err := db.Prepare(req.Expression)
+	if err != nil {
+		return nil, err
+	}
+	return sfmt.Query(req.Data...)
+}
+
+func (a *DBAdapter) Exec(req Query) (sql.Result, error) {
+	db, err := a.Open(req.Db)
+	if err != nil {
+		return nil, err
+	}
+	sfmt, err := db.Prepare(req.Expression)
+	if err != nil {
+		return nil, err
+	}
+	return sfmt.Exec(req.Data...)
+}
diff --git a/pkg/adapter/types.go b/pkg/adapter/types.go
index a0a4b71..ecbd57c 100644
--- a/pkg/adapter/types.go
+++ b/pkg/adapter/types.go
@@ -1,5 +1,11 @@
 package adapter
 
+type Query struct {
+	Db         string        `json:"db"`
+	Expression string        `json:"expr"`
+	Data       []interface{} `json:"data"`
+}
+
 type CtxOpts map[string]string
 
 type Context interface {
diff --git a/pkg/builder/Builder.go b/pkg/builder/Builder.go
index e0223ea..a8d0b64 100644
--- a/pkg/builder/Builder.go
+++ b/pkg/builder/Builder.go
@@ -5,6 +5,13 @@ import (
 	"strings"
 )
 
+type Builder struct {
+	TableName  string
+	TableAlias string
+	Parts      SQLParts
+	Ctx        Context
+}
+
 type SQLParts struct {
 	Operation string
 	From      string
@@ -21,13 +28,6 @@ type SQLParts struct {
 	Update    UpdateData
 }
 
-type Builder struct {
-	Parts      SQLParts
-	TableName  string
-	TableAlias string
-	Ctx        Context
-}
-
 func New(ctx Context) *Builder {
 	return &Builder{
 		Parts: SQLParts{
@@ -135,6 +135,8 @@ func (b *Builder) OnConflict(fields ...string) *Builder {
 	if b.Parts.Operation == "UPDATE" {
 		b.Parts.Update.Upsert = convertConflict(b.Ctx, fields...)
 		b.Parts.Update.UpsertExp = "DO NOTHING"
+	} else {
+		panic("OnConflict is only available for UPDATE operation")
 	}
 	return b
 }
@@ -142,6 +144,8 @@ func (b *Builder) OnConflict(fields ...string) *Builder {
 func (b *Builder) DoUpdate(fields ...string) *Builder {
 	if b.Parts.Operation == "UPDATE" {
 		b.Parts.Update.UpsertExp = convertUpsert(fields)
+	} else {
+		panic("DoUpdate is only available for UPDATE operation")
 	}
 	return b
 }
@@ -149,6 +153,8 @@ func (b *Builder) DoUpdate(fields ...string) *Builder {
 func (b *Builder) DoNothing() *Builder {
 	if b.Parts.Operation == "UPDATE" {
 		b.Parts.Update.UpsertExp = "DO NOTHING"
+	} else {
+		panic("DoNothing is only available for UPDATE operation")
 	}
 	return b
 }
diff --git a/pkg/builder/go.mod b/pkg/builder/go.mod
index 25a38e6..e6579ae 100644
--- a/pkg/builder/go.mod
+++ b/pkg/builder/go.mod
@@ -2,7 +2,7 @@ module l12.xyz/dal/builder
 
 go 1.22.6
 
-require l12.xyz/dal/utils v0.0.0 // indirect
+require l12.xyz/dal/utils v0.0.0
 
 replace l12.xyz/dal/utils v0.0.0 => ../utils
 
diff --git a/pkg/server/go.mod b/pkg/server/go.mod
new file mode 100644
index 0000000..1d2f90d
--- /dev/null
+++ b/pkg/server/go.mod
@@ -0,0 +1,3 @@
+module l12.xyz/dal/server
+
+go 1.22.6