WL#10237: X Protocol expectations for supported protobuf fields

Affects: Server-8.0   —   Status: Complete


Protobuf allows forward compatible message: If a field-id isn't known, it is ignored. A newer server that returns data to the client it doesn't know will just work. The client ignores the fields and handles what it understands.

If the behavior doesn't fit the use-case a high-level mechanism is need to announce/negotiate with the other side.


This functionality is going to help with detecting compatibility problems between client application and MySQL Server, when server receives X Protocol message containing field that it doesn't know.


The compatibility problem originates from 'protobufs' assumption that each node exchanging messages should be able to handle messages containing unknown data (different versions). In case of server application skipping unknown data can change the meaning which client "had" when he was sending the message. The assumption enforces users of 'protobuf' to design/implement their own mechanism for compatibility verification.

'protobuf' 2 stores unknown data inside "Message/MessageLite" class, which could be used to implement a runtime validation of received message. To implement proper check on using "unknown data", X Plugin would need a protobuf plugin to generate additional checks which would go deep inside all submessages and on all its fields. In protobuf 3, unknown fields are removed from the generated API/code.

Instead of doing validation when message is being received, this worklog is going to introduce new "condition key" to X Protocol expectation mechanism. The client is going to send an "Expect.Open" message containing message/fields tag chain and server is going to validate if field specified this way is present inside the definition of servers X Protocol message. This is done to ensure pipelining: message processing should be stopped when any message does not meet the expectation.

Functional requirements


  1. User MUST be able to use message with fields unsupported by the server.
  2. User MUST be able to perform a check whether a specific field is supported by X Plugin (server)
  3. User MUST be able to use check if multiple fields are supported by X Plugin (server)
  4. Querying if the server supports message fields MUST not introduce an extra round-trip when used with pipelining.
  5. A specific error message should be generated when a check fails.


  1. User MUST be able to fail whole expectation block (starting from Expect.Open) when its condition contains at last one unsupported field.

Non functional requirements

  1. There should be no degradation of X Plugin performance
  2. Code coverage shouldn't be decreased (in percent)

Interface Protocol

X Protocol messages are defined inside 'proto' files. Any change to a message defined inside 'proto' file can break compatibility between different versions of X Plugin and X Connector. This worklog is going to introduce a mechanism that allows to check that concrete field (messages field) is supported by concrete version of X Plugin. The mechanism is going to be implemented using X Protocols expectation block:

Client -> Server: Mysqlx.Expect.Open([+field_exists=12.1.1])
Client <- Server: Mysqlx.Ok
...X Connector requests, supported by X Plugin...
Client -> Server: Mysqlx.Expect.Close()
Client <- Server: Mysqlx.Ok

"Mysqlx.Expect.Open" uses conditions key to enable/disable expectations. To check if field is handled by X Plugin, following constant/key must be added:


The conditions value for the key must be non empty value, defined as chain of protobuf field-tags separated by "dots". The first element inside the chain is going to be X Protocol message id. For example to check if:

  • "StmtExecute" is supported, value should be "12" (because ID of the StmtExecute message is 12)
  • "StmtExecute.compact_metadata" is supported, value should be "12.4"
  • "StmtExecute.args.obj" is supported, value should be "12.2.3"

The dependencies between messages can be represented as a graph. Cycles inside the 'graph' are going to be overcome by allowing user to close the cycle but it must be the last field-tag inside the chain. In other words user is allowed to add relations that close the cycle but if the user would like to check a field after the cycle closure, he must check it at first occurrence of the type.

For example:

  • "StmtExecute.args.obj.fld.value.obj" - "" is not allowed because it checks the type "Any" two times. To check if "Obj" is supported by "Any" it is sufficient to execute check it as: "StmtExecute.args.obj" - "12.2.3"

Expectation open request which contains a check for a field that is not handled by X Plugin is going to be "installed" on expectations stack, still and error is send back. All clients requests which are done inside this expectation block are not executed and in return the "expectation block error" is sent for each response:

Client -> Server: Mysqlx.Expect.Open([field=unknown])
Client <- Server: Mysqlx.Error

loop all client requests inside expectation block
  Client -> Server: Any message
  Client <- Server: Mysqlx.Error

Client -> Server: Mysqlx.Expect.Close()
Client <- Server: Mysqlx.Error

Handling of unknown fields

This 'worklog' doesn't introduce any additional handling of unknown fields or values. X Plugin is going to process/execute such message thus its still possible to break the compatibility when client doesn't check if field is supported.


Specification of a feature (that introduces a field inside an existing 'protobuf' message) must say that before feature usage it is necessary to check with expectation condition "field_exists" if "tag-id-chain-to-new-field" is supported. The check must be implemented inside connector code and executed before using the feature.

Error and Warnings

The expectation message flow (behavior and its error codes) is already implemented by X Plugin , still "field-existance" expectation check must use new error code:

Property Value
Error code 5159
Error text Expectation failed: field_exists

and old code that we used with condition key "no-error" must be renamed:

Property Value
Error code 5168
Error text Expectation failed: no_error


Currently X Protocol messages are compiled in two passes: full and lite. The 'field chain' validation isn't doable on lite version of X Protocol messages. To be able to validate it with both library types, X Plugin needs to define its own "protoc plugin". The new plugin is going to generate the "field-tags-chains". To do fully autonomic chain generation (by protoc plugin), X Protocol top level messages needs to be marked with "message-ids" using protobuf custom options:

``` git diff --- a/rapid/plugin/x/protocol/mysqlx.proto +++ b/rapid/plugin/x/protocol/mysqlx.proto @@ -24,8 +24,9 @@ syntax = "proto2";

package Mysqlx; option java_package = "com.mysql.cj.mysqlx.protobuf";

+import "google/protobuf/descriptor.proto"; // comment_out_if LITE_RUNTIME

// style-guide: // // see https://developers.google.com/protocol-buffers/docs/style @@ -102,13 +103,20 @@ message ServerMessages { SQL_STMT_EXECUTE_OK = 17; RESULTSET_FETCH_DONE_MORE_OUT_PARAMS = 18; };


+// ifndef PROTOBUF_LITE +extend google.protobuf.MessageOptions { + optional ClientMessages.Type client_message_id = 100001; + optional ServerMessages.Type server_message_id = 100002; +} +// endif

// generic Ok message message Ok { optional string msg = 1; + + option (server_message_id) = OK; // comment_out_if LITE_RUNTIME }

// generic Error message @@ -133,5 +141,7 @@ message Error { enum Severity { ERROR = 0; FATAL = 1; }; + + option (server_message_id) = ERROR; // comment_out_if LITE_RUNTIME }


When "protoc plugin" gets a message which has 'client_message_id' option set, it should generate "field-tag-chains" for all its fields, recursively going through submessages. Recursion is stopped when a cycle is detected.

"protoc plugins" that use custom options need to heave already compiled protocol files. Thus the chain generation needs to be done in separate pass, where the compilation process is going to change from:

  1. compilation of full-version-proto files
  2. compilation of lite-version-proto files


  1. compilation of full-version-proto files (C++ source generation)
  2. compilation of protoc-plugin
  3. compilation of full-version-proto files with protoc-plugin (C++ header with tag-chains)
  4. compilation of lite-version-proto files (C++ source generation)

The "C++" header with tag-chains is generated using full version of 'proto' files, but still both full and lite X Plugins files can use it.

CMake generates lite version of proto files from full by uncommenting lines like:

proto // ifdef PROTOBUF_LITE: option optimize_for = LITE_RUNTIME;

The mechanism of uncommenting and commenting out needs to be improved to be able to remove custom options from lite version of proto files. It must handle:

  • commenting out single line

proto import "google/protobuf/descriptor.proto"; // comment_out_if LITE_RUNTIME

  • commenting out a block

proto // ifndef PROTOBUF_LITE extend google.protobuf.MessageOptions { optional ClientMessages.Type client_message_id = 100001; optional ServerMessages.Type server_message_id = 100002; } // endif

Expectation blocks

Condition keys are defined as integer field in "Mysqlx.Expect.Open.Condition", without any description of supported values. New enumeration was introduces to generate constants which can be used as values for "condition key" to remove magic numbers:

--- a/rapid/plugin/x/protocol/mysqlx_expect.proto +++ b/rapid/plugin/x/protocol/mysqlx_expect.proto @@ -17,8 +17,10 @@ * 02110-1301 USA */ syntax = "proto2";

+import "mysqlx.proto"; // comment_out_if PROTOBUF_LITE + // ifdef PROTOBUF_LITE: option optimize_for = LITE_RUNTIME;

// Expect operations package Mysqlx.Expect; @@ -74,8 +76,14 @@ option java_package = "com.mysql.cj.mysqlx.protobuf"; // :returns: :protobuf:msg:Mysqlx::Ok on success, :protobuf:msg:Mysqlx::Error on error // message Open { message Condition { + enum Key { + // Change error propagation behaviour + EXPECT_NO_ERROR = 1; + // Check if X Protocol field exists + EXPECT_FIELD_EXIST = 2; + };


Expectation block must be covered in MTR tests and with unit tests.