diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000000000000000000000000000000000000..99bee6fa15b4aea5582461d665c3dfe109a0e210
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,5 @@
+ALL:
+	@echo "THIS IS ONLY FOR DEVELOPMENT. DON'T USE IT"
+
+formatting:
+	clang-format -i $$(find proto/ -type f -iname "*.proto")
diff --git a/proto/caosdb/entity/v1alpha1/main.proto b/proto/caosdb/entity/v1alpha1/main.proto
index f920154ec81f126bdff8407c93741b78a25051e5..91450968c89fbc800c3ed6fb90208e6f0f32baab 100644
--- a/proto/caosdb/entity/v1alpha1/main.proto
+++ b/proto/caosdb/entity/v1alpha1/main.proto
@@ -22,9 +22,11 @@
 syntax                      = "proto3";
 option  java_multiple_files = true;
 option  java_package        = "org.caosdb.api.entity.v1alpha1";
+option  cc_enable_arenas    = true;
 package caosdb.entity.v1alpha1;
 
-// Entity messages codes.
+// Messages codes. Entities and transactions contain errors, warnings and info
+// messages which use these codes.
 enum MessageCode {
   // Unspecific code. Use this for anything where a client is not expected to
   // handle this message in a specific way.
@@ -35,6 +37,251 @@ enum MessageCode {
   MESSAGE_CODE_UNKNOWN = 1;
   // The requested entity (retrieve, delete, or update) does not exist.
   MESSAGE_CODE_ENTITY_DOES_NOT_EXIST = 2;
+  // The entity has no properties.
+  MESSAGE_CODE_ENTITY_HAS_NO_PROPERTIES = 3;
+
+  // TODO(tf) This entity has been deleted successfully.
+  MESSAGE_CODE_ENTITY_HAS_BEEN_DELETED_SUCCESSFULLY = 5;
+
+  // TODO(tf) Entity has unqualified properties.
+  MESSAGE_CODE_ENTITY_HAS_UNQUALIFIED_PROPERTIES = 6;
+
+  // TODO(tf) Entity has unqualified parents.
+  MESSAGE_CODE_ENTITY_HAS_UNQUALIFIED_PARENTS = 7;
+
+  // TODO(tf) DESCRIPTION!!!
+  MESSAGE_CODE_PARSING_FAILED = 8;
+
+  // TODO(tf) Unknown datatype.
+  MESSAGE_CODE_UNKNOWN_DATATYPE = 9;
+
+  // TODO(tf) Unknown importance.
+  MESSAGE_CODE_UNKNOWN_IMPORTANCE = 10;
+
+  // TODO(tf) Entity has no ID.
+  MESSAGE_CODE_ENTITY_HAS_NO_ID = 11;
+
+  // TODO(tf) Entity is required by other entities which are not to be deleted.
+  MESSAGE_CODE_REQUIRED_BY_PERSISTENT_ENTITY = 12;
+
+  // TODO(tf) Property has no datatype.
+  MESSAGE_CODE_NO_DATATYPE = 13;
+
+  // TODO(tf) Entity has no description.
+  MESSAGE_CODE_ENTITY_HAS_NO_DESCRIPTION = 14;
+
+  // TODO(tf) Entity has no name.
+  MESSAGE_CODE_ENTITY_HAS_NO_NAME = 15;
+
+  // TODO(tf) An obligatory property is missing.
+  MESSAGE_CODE_OBLIGATORY_PROPERTY_MISSING = 16;
+
+  // TODO(tf) Entity has no parents.
+  MESSAGE_CODE_ENTITY_HAS_NO_PARENTS = 17;
+
+  // TODO(tf) Entity can not be identified due to name duplicates.
+  MESSAGE_CODE_NAME_DUPLICATES = 18;
+
+  // TODO(tf) Entity has no name and no ID.
+  MESSAGE_CODE_ENTITY_HAS_NO_NAME_AND_NO_ID = 19;
+
+  // TODO(tf) Reference property has no refid.
+  MESSAGE_CODE_REFERENCE_HAS_NO_REFID = 20;
+
+  // TODO(tf) No target path specified.
+  MESSAGE_CODE_NO_TARGET_PATH = 21;
+
+  // TODO(tf) This target path is not allowed.
+  MESSAGE_CODE_TARGET_PATH_NOT_ALLOWED = 22;
+
+  // TODO(tf) This target path does already exist.
+  MESSAGE_CODE_TARGET_PATH_EXISTS = 23;
+
+  // TODO(tf) Entity has no unit.
+  MESSAGE_CODE_ENTITY_HAS_NO_UNIT = 24;
+
+  // TODO(tf) Cannot parse value.
+  MESSAGE_CODE_CANNOT_PARSE_VALUE = 25;
+
+  // TODO(tf) Checksum test failed. File is corrupted.
+  MESSAGE_CODE_CHECKSUM_TEST_FAILED = 26;
+
+  // TODO(tf) Size test failed. File is corrupted.
+  MESSAGE_CODE_SIZE_TEST_FAILED = 27;
+
+  // TODO(tf) Could not create parent folder in the file system.
+  MESSAGE_CODE_CANNOT_CREATE_PARENT_FOLDER = 28;
+
+  // TODO(tf) File has not been uploaded.
+  MESSAGE_CODE_FILE_HAS_NOT_BEEN_UPLOAED = 29;
+
+  // TODO(tf) Thumbnail has not been uploaded.
+  MESSAGE_CODE_THUMBNAIL_HAS_NOT_BEEN_UPLOAED = 30;
+
+  // TODO(tf) Could not move file to it's target folder.
+  MESSAGE_CODE_CANNOT_MOVE_FILE_TO_TARGET_PATH = 31;
+
+  // TODO(tf) Cannot parse value to datetime format
+  // (yyyy-mm-dd'T'hh:mm:ss[.fffffffff][TimeZone]).
+  MESSAGE_CODE_CANNOT_PARSE_DATETIME_VALUE = 32;
+
+  // TODO(tf) Cannot parse value to double.
+  MESSAGE_CODE_CANNOT_PARSE_DOUBLE_VALUE = 33;
+
+  // TODO(tf) Cannot parse value to integer.
+  MESSAGE_CODE_CANNOT_PARSE_INT_VALUE = 34;
+
+  // TODO(tf) Cannot parse value to boolean (either 'true' or 'false', ignoring
+  // case).
+  MESSAGE_CODE_CANNOT_PARSE_BOOL_VALUE = 35;
+
+  // TODO(tf) Could not connect to MySQL server.
+  MESSAGE_CODE_CANNOT_CONNECT_TO_DATABASE = 36;
+
+  // TODO(tf) Please check if your MySQL has all required procedures installed.
+  MESSAGE_CODE_MYSQL_PROCEDURE_EXCEPTION = 37;
+
+  // TODO(tf) This error occurred while parsing the xml. Probably it has a wrong
+  // encoding.
+  MESSAGE_CODE_REQUEST_HAS_WRONG_ENCODING = 38;
+
+  // TODO(tf) Request body didn't contain the expected Elements.
+  MESSAGE_CODE_REQUEST_DOESNT_CONTAIN_EXPECTED_ELEMENTS = 39;
+
+  // TODO(tf) File is not in drop-off box.
+  MESSAGE_CODE_FILE_NOT_IN_DROPOFFBOX = 40;
+
+  // TODO(tf) File could not be found.
+  MESSAGE_CODE_FILE_NOT_FOUND = 41;
+
+  // TODO(tf) Could not move file to tmp folder.
+  MESSAGE_CODE_CANNOT_MOVE_FILE_TO_TMP = 42;
+
+  // TODO(tf) Insufficient read permission for this file. Please make it
+  // readable.
+  MESSAGE_CODE_CANNOT_READ_FILE = 43;
+
+  // TODO(tf) Insufficient read permission for this file's thumbnail. Please
+  // make it readable.
+  MESSAGE_CODE_CANNOT_READ_THUMBNAIL = 44;
+
+  // TODO(tf) No file representation submitted.
+  MESSAGE_CODE_NO_FILE_REPRESENTATION_SUBMITTED = 45;
+
+  // TODO(tf) DESCRIPTION?
+  MESSAGE_CODE_FILE_IS_EMPTY = 46;
+
+  // TODO(tf) A warning occured while processing an entity with the strict flag.
+  MESSAGE_CODE_WARNING_OCCURED = 47;
+
+  // TODO(tf) DESCRIPTION?
+  MESSAGE_CODE_UNKNOWN_JOB = 48;
+
+  // TODO(tf) Name is already in use. Choose a different name or reuse an
+  // existing entity.
+  MESSAGE_CODE_NAME_IS_NOT_UNIQUE = 49;
+
+  // TODO(tf) This entity cannot be identified uniquely due to name dublicates
+  MESSAGE_CODE_CANNOT_IDENTIFY_ENTITY_UNIQUELY = 50;
+
+  // TODO(tf) This query finished with errors.
+  MESSAGE_CODE_QUERY_EXCEPTION = 51;
+
+  // TODO(tf) This entity is empty.
+  MESSAGE_CODE_ENTITY_IS_EMPTY = 52;
+
+  // TODO(tf) An unknown error occured during the transaction and it was rolled
+  // back.
+  MESSAGE_CODE_TRANSACTION_ROLL_BACK = 53;
+
+  // TODO(tf) The file upload failed for an unknown reason.
+  MESSAGE_CODE_FILE_UPLOAD_FAILED = 54;
+
+  // TODO(tf) Unknown unit. Values with this unit cannot be converted to other
+  // units when used in search queries.
+  MESSAGE_CODE_UNKNOWN_UNIT = 55;
+
+  // TODO(tf) You are not allowed to do this.
+  MESSAGE_CODE_AUTHORIZATION_ERROR = 56;
+
+  // TODO(tf) Reference not qualified. The value of this Reference Property is
+  // to be a child of its data type.
+  MESSAGE_CODE_REFERENCE_IS_NOT_ALLOWED_BY_DATATYPE = 57;
+
+  // TODO(tf) Cannot parse EntityACL.
+  MESSAGE_CODE_CANNOT_PARSE_ENTITY_ACL = 58;
+
+  // TODO(tf) This entity cannot be identified due to name duplicates.
+  MESSAGE_CODE_ENTITY_NAME_DUPLICATES = 59;
+
+  // TODO(tf) This data type cannot be identified due to name duplicates.
+  MESSAGE_CODE_DATA_TYPE_NAME_DUPLICATES = 60;
+
+  // TODO(tf) This entity cannot be identified as it didn't come with a name or
+  // id.
+  MESSAGE_CODE_ENTITY_HAS_NO_NAME_OR_ID = 61;
+
+  // TODO(tf) Affiliation is not defined for this child-parent constellation.
+  MESSAGE_CODE_AFFILIATION_ERROR = 62;
+
+  // TODO(tf) An error occured during the parsing of this query. Maybe you use a
+  // wrong syntax?
+  MESSAGE_CODE_QUERY_PARSING_ERROR = 63;
+
+  // TODO(tf) A property which has 'name' as its parent must have a TEXT data
+  // type.
+  MESSAGE_CODE_NAME_PROPERTIES_MUST_BE_TEXT = 64;
+
+  // TODO(tf) This entity had parent duplicates. That is meaningless and only
+  // one parent had been inserted.
+  MESSAGE_CODE_PARENT_DUPLICATES_WARNING = 65;
+
+  // TODO(tf) This entity had parent duplicates. Parent duplicates are
+  // meaningless and would be ignored (and inserted only once). But these
+  // parents had diverging inheritance instructions which cannot be processed.
+  MESSAGE_CODE_PARENT_DUPLICATES_ERROR = 66;
+
+  // TODO(tf) One or more entities are not qualified. None of them have been
+  // inserted/updated/deleted.
+  MESSAGE_CODE_ATOMICITY_ERROR = 67;
+
+  // TODO(tf) There is no such role '" + role + "'.
+  MESSAGE_CODE_NO_SUCH_ENTITY_ROLE = 68;
+
+  // TODO(tf) This entity cannot be deleted due to dependency problems
+  MESSAGE_CODE_REQUIRED_BY_UNQUALIFIED = 69;
+
+  // TODO(tf) This entity has an invalid reference.
+  MESSAGE_CODE_ENTITY_HAS_INVALID_REFERENCE = 70;
+
+  // TODO(tf) Referenced entity does not exist.
+  MESSAGE_CODE_REFERENCED_ENTITY_DOES_NOT_EXIST = 71;
+
+  // TODO(tf) This reference cannot be identified due to name duplicates.
+  MESSAGE_CODE_REFERENCE_NAME_DUPLICATES = 72;
+
+  // TODO(tf) The datatype which is to be inherited could not be detected due to
+  // divergent datatypes of at least two parents.
+  MESSAGE_CODE_DATATYPE_INHERITANCE_AMBIGUOUS = 73;
+
+  // TODO(tf) This datatype does not accept collections of values (e.g. Lists).
+  MESSAGE_CODE_DATA_TYPE_DOES_NOT_ACCEPT_COLLECTION_VALUES = 74;
+
+  // TODO(tf) This unit cannot be parsed.
+  MESSAGE_CODE_CANNOT_PARSE_UNIT = 75;
+
+  // TODO(tf) This property is an additional property which has no corresponding
+  // property among the properties of the parents.
+  MESSAGE_CODE_ADDITIONAL_PROPERTY = 76;
+
+  // TODO(tf) This property overrides the datatype.
+  MESSAGE_CODE_PROPERTY_WITH_DATATYPE_OVERRIDE = 77;
+
+  // TODO(tf) This property overrides the description.
+  MESSAGE_CODE_PROPERTY_WITH_DESC_OVERRIDE = 78;
+
+  // TODO(tf) This property overrides the name.
+  MESSAGE_CODE_PROPERTY_WITH_NAME_OVERRIDE = 79;
 }
 
 // Messages are used by server's or client's to transport *transient*
@@ -115,6 +362,12 @@ message Property {
   string unit = 6;
   // Property datatype
   string datatype = 7;
+  // Errors of this property
+  repeated Message errors = 8;
+  // Warnings of this property
+  repeated Message warnings = 9;
+  // Info messages of this property
+  repeated Message infos = 10;
 }
 
 // Parent of an entity.
@@ -125,40 +378,127 @@ message Parent {
   string name = 2;
   // Description of the parent entity.
   string description = 3;
-}
-
-// Request a transaction of an entity by its ID. This can be used to implement
-// a retrieval or deletion.
-message ByIdRequest {
-  // ID of requested entity
-  string id = 1;
+  // Errors of this parent
+  repeated Message errors = 4;
+  // Warnings of this parent
+  repeated Message warnings = 5;
+  // Info messages of this parent
+  repeated Message infos = 6;
 }
 
 // Response to a retrieve request
 message RetrieveResponse {
   // Payload of the retrieval
-  Entity entity = 1;
+  oneof query_response {
+    // Result of a FIND query or a request by id.
+    Entity entity = 1;
+    // Result of a SELECT query
+    SelectQueryResult select_result = 3;
+    // Result of a COUNT query
+    int64 count_result = 4;
+  }
+}
+
+// CaosDB Query
+message Query {
+  // The string representation of the query, e.g. "FIND Person WITH name =
+  // Aisha"
+  string query = 1;
+}
+
+// A single result row of a SELECT query
+message SelectQueryRows {
+  // The cells of the table row.
+  repeated string cells = 1;
+}
+
+// A table which is a result of a SELECT query
+message SelectQueryResult {
+  // A single row which contains the SELECT'ed fields
+  SelectQueryRows header = 1;
+  // The actual results
+  repeated SelectQueryRows data_rows = 2;
+}
+
+// Request which specifies one or multiple entities by query or by id.
+message QueryOrIdRequest {
+  // We wrap these two together because this makes it easier to add further
+  // requests (like "name", or "path") in the future.
+  oneof wrapped_request {
+    // A single id
+    string id = 1;
+    // A query
+    Query query = 2;
+  }
+}
+
+// Single response containing either an id, a COUNT query result, a SELECT-table
+// or complete entities.
+message TransactionResponse {
+  // We wrap these together in order to make this a repeatable field in the
+  // MultiTransactionResponse.
+  oneof wrapped_response {
+    // Single response to a delete transaction.
+    IdResponse delete_response = 1;
+    // Single response to an update transaction.
+    IdResponse update_response = 2;
+    // Single response to a retrieve transaction.
+    RetrieveResponse retrieve_response = 3;
+    // Single response to an insert transaction.
+    IdResponse insert_response = 4;
+  }
+}
+
+// Single request containing either a query, an id or a complete entity.
+message TransactionRequest {
+  // We wrap these together in order to make this a repeatable field in the
+  // MultiTransactionRequest.
+  oneof wrapped_requests {
+    // Single request for a retrieve transaction.
+    QueryOrIdRequest retrieve_request = 1;
+    // Single request for an update transaction.
+    Entity update_request = 2;
+    // Single request for an insert transaction.
+    Entity insert_request = 3;
+    // Single request for a delete transaction.
+    QueryOrIdRequest delete_request = 4;
+  }
+}
+
+// Wraps the reponses to multiple sub-transactions of mixed types.
+message MultiTransactionResponse {
+  // The actual payload.
+  repeated TransactionResponse responses = 1;
   // Transaction errors.
-  repeated Message errors = 2;
+  repeated Message transaction_errors = 2;
   // Transaction warnings.
-  repeated Message warnings = 3;
+  repeated Message transaction_warnings = 3;
   // Transaction info messages.
-  repeated Message infos = 4;
+  repeated Message transaction_infos = 4;
 }
 
-// Wrapper for different kinds of retrievals (currently, only by ID). We need
-// this in order to make a repeated field of these different kinds of
-// retrievals.
-message RetrieveRequest {
-  // Wrapped special kind of request.
-  oneof wrapped_request {
-    // A single retrieve
-    ByIdRequest by_id = 1;
-  }
+// Wraps multiple sub-transaction requests of mixed types.
+message MultiTransactionRequest {
+  // The actual payload.
+  repeated TransactionRequest requests = 1;
+}
+
+// A response containing only the id (of a deleted, inserted, or updated
+// entity) and the usual trias of entity messages.
+message IdResponse {
+  // The entity id.
+  string id = 1;
+  // Entity errors.
+  repeated Message entity_errors = 2;
+  // Entity warnings.
+  repeated Message entity_warnings = 3;
+  // Entity info messages.
+  repeated Message entity_infos = 4;
 }
 
 // Entity Transaction Service
 service EntityTransactionService {
-  // Issue a single retrieve request
-  rpc Retrieve(RetrieveRequest) returns (RetrieveResponse);
+  // Issue multiple mixed requests
+  rpc MultiTransaction(MultiTransactionRequest)
+      returns (MultiTransactionResponse);
 }