doc: add the migration guide to v2.0.0

MIGRATION.md

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+# Migration guide
+
+## Migration from v1.x.x to v2.x.x
+
+* [Major changes](#major-changes)
+* [Main package](#main-package)
+  * [Go version](#go-version)
+  * [msgpack/v5](#msgpackv5)
+  * [Call = Call17](#call--call17)
+  * [IPROTO constants](#iproto-constants)
+  * [Request interface](#request-interface)
+  * [Request changes](#request-changes)
+  * [Response interface](#response-interface)
+  * [Response changes](#response-changes)
+  * [Future type](#future-type)
+  * [Protocol types](#protocol-types)
+  * [Connector interface](#connector-interface)
+  * [Connect function](#connect-function)
+  * [Connection schema](#connection-schema)
+  * [Schema type](#schema-type)
+* [datetime package](#datetime-package)
+* [decimal package](#decimal-package)
+* [multi package](#multi-package)
+* [pool package](#pool-package)
+* [crud package](#crud-package)
+* [test_helpers package](#test_helpers-package)
+
+### Major changes
+
+* The `go_tarantool_call_17` build tag is no longer needed, since by default
+  the `CallRequest` is `Call17Request`.
+* The `go_tarantool_msgpack_v5` build tag is no longer needed, since only the
+  `msgpack/v5` library is used.
+* The `go_tarantool_ssl_disable` build tag is no longer needed, since the
+  connector is no longer depends on `OpenSSL` by default. You could use the
+  external library [go-tlsdialer](https://github.com/tarantool/go-tlsdialer) to
+  create a connection with the `ssl` transport.
+* Required Go version is `1.20` now.
+* The `Connect` function became more flexible. It now allows to create a
+  connection with cancellation and a custom `Dialer` implementation.
+* It is required to use `Request` implementation types with the `Connection.Do`
+  method instead of `Connection.<Request>` methods.
+* The `connection_pool` package renamed to `pool`.
+
+The basic code for the `v1.12.2` release:
+```Go
+package tarantool
+
+import (
+	"fmt"
+
+	"github.com/tarantool/go-tarantool"
+)
+
+func main() {
+	opts := tarantool.Opts{User: "guest"}
+	conn, err := tarantool.Connect("127.0.0.1:3301", opts)
+	if err != nil {
+		fmt.Println("Connection refused:", err)
+		return
+	}
+
+	resp, err := conn.Insert(999, []interface{}{99999, "BB"})
+	if err != nil {
+		fmt.Println("Error:", err)
+		fmt.Println("Code:", resp.Code)
+	} else {
+		fmt.Println("Data:", resp.Data)
+	}
+}
+```
+
+At now became:
+```Go
+package tarantool
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/tarantool/go-tarantool/v2"
+)
+
+func main() {
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+	dialer := tarantool.NetDialer{
+		Address: "127.0.0.1:3301",
+		User: 	 "guest",
+	}
+	opts := tarantool.Opts{
+		Timeout: time.Second,
+	}
+
+	conn, err := tarantool.Connect(ctx, dialer, opts)
+	if err != nil {
+		fmt.Println("Connection refused:", err)
+		return
+	}
+
+	data, err := conn.Do(
+		tarantool.NewInsertRequest(999).Tuple([]interface{}{99999, "BB"})).Get()
+	if err != nil {
+		fmt.Println("Error:", err)
+	} else {
+		fmt.Println("Data:", data)
+	}
+}
+```
+
+### Main package
+
+#### Go version
+
+Required Go version is updated from `1.13` to `1.20`.
+
+#### msgpack/v5
+
+At now the `msgpack/v5` library is used for the `msgpack` encoding/decondig.
+
+Most function names and argument types in `msgpack/v5` and `msgpack.v2`
+have not changed (in our code, we noticed changes in `EncodeInt`, `EncodeUint`
+and `RegisterExt`). But there are a lot of changes in a logic of encoding and
+decoding. On the plus side the migration seems easy, but on the minus side you
+need to be very careful.
+
+First of all, `EncodeInt8`, `EncodeInt16`, `EncodeInt32`, `EncodeInt64`
+and `EncodeUint*` analogues at `msgpack/v5` encode numbers as is without loss of
+type. In `msgpack.v2` the type of a number is reduced to a value.
+
+Secondly, a base decoding function does not convert numbers to `int64` or
+`uint64`. It converts numbers to an exact type defined by MessagePack. The
+change makes manual type conversions much more difficult and can lead to
+runtime errors with an old code. We do not recommend to use type conversions
+and give preference to `*Typed` functions (besides, it's faster).
+
+There are also changes in the logic that can lead to errors in the old code,
+[as example](https://github.com/vmihailenco/msgpack/issues/327). Although in
+`msgpack/v5` some functions for the logic tuning were added (see
+[UseLooseInterfaceDecoding](https://pkg.go.dev/github.com/vmihailenco/msgpack/v5#Decoder.UseLooseInterfaceDecoding), [UseCompactInts](https://pkg.go.dev/github.com/vmihailenco/msgpack/v5#Encoder.UseCompactInts) etc), it is still impossible
+to achieve full compliance of behavior between `msgpack/v5` and `msgpack.v2`.
+So we don't go this way. We use standard settings if it possible.
+
+#### Call = Call17
+
+Call requests uses `IPROTO_CALL` instead of `IPROTO_CALL_16`.
+
+So now `Call` = `Call17` and `NewCallRequest` = `NewCall17Request`. A result
+of the requests is an array instead of array of arrays.
+
+#### IPROTO constants
+
+* IPROTO constants have been moved to a separate package [go-iproto](https://github.com/tarantool/go-iproto).
+* `PushCode` constant is removed. To check whether the current response is
+  a push response, use `IsPush()` method of the response iterator instead.
+* `ErrorNo` constant is added to indicate that no error has occurred while
+  getting the response. It should be used instead of the removed `OkCode`.
+  See `ExampleErrorNo`.
+
+#### Request interface
+
+* The method `Code() uint32` replaced by the `Type() iproto.Type`.
+* `Response` method added to the `Request` interface.
+
+#### Request changes
+
+* Requests `Update`, `UpdateAsync`, `UpdateTyped`, `Upsert`, `UpsertAsync` no
+longer accept `ops` argument (operations) as an `interface{}`. `*Operations`
+needs to be passed instead.
+* `Op` struct for update operations made private.
+* Removed `OpSplice` struct.
+* `Operations.Splice` method now accepts 5 arguments instead of 3.
+* `UpdateRequest` and `UpsertRequest` structs no longer accept `interface{}`
+for an `ops` field. `*Operations` needs to be used instead.
+
+#### Response interface
+
+* `Response` is now an interface.
+* Response header stored in a new `Header` struct. It could be accessed via
+  `Header()` method.
+
+#### Response changes
+
+* `ResponseIterator` interface now has `IsPush()` method.
+  It returns true if the current response is a push response.
+* For each request type, a different response type is created. They all
+  implement a `Response` interface. `SelectResponse`, `PrepareResponse`,
+  `ExecuteResponse`, `PushResponse` are a part of a public API.
+  `Pos()`, `MetaData()`, `SQLInfo()` methods created for them to get specific
+  info. Special types of responses are used with special requests.
+
+#### Future type
+
+* Method `Get` now returns response data instead of the actual response.
+* New method `GetResponse` added to get an actual response.
+* `Future` constructors now accept `Request` as their argument.
+* Methods `AppendPush` and `SetResponse` accepts response `Header` and data
+  as their arguments.
+* Method `Err` was removed because it was causing improper error handling.
+  You need to check an error from `Get`, `GetTyped` or `GetResponse` with
+  an addition check of a value `Response.Header().Error`, see `ExampleErrorNo`.
+
+#### Connector interface
+
+* Operations `Ping`, `Select`, `Insert`, `Replace`, `Delete`, `Update`,
+  `Upsert`, `Call`, `Call16`, `Call17`, `Eval`, `Execute` of a `Connector`
+  return response data instead of an actual responses.
+* New interface `Doer` is added as a child-interface instead of a `Do` method.
+
+#### Connect function
+
+`connection.Connect` no longer return non-working connection objects. This
+function now does not attempt to reconnect and tries to establish a connection
+only once. Function might be canceled via context. Context accepted as first
+argument, and user may cancel it in process.
+
+Now you need to pass `Dialer` as the second argument instead of URI.
+If you were using a non-SSL connection, you need to create `NetDialer`.
+For SSL-enabled connections, use `OpenSSLDialer` from the
+[go-tlsdialer](https://github.com/tarantool/go-tlsdialer) package.
+
+Please note that the options for creating a connection are now stored in
+corresponding `Dialer`, not in `Opts`.
+
+#### Connection schema
+
+* Removed `Schema` field from the `Connection` struct. Instead, new
+  `GetSchema(Doer)` function was added to get the actual connection
+  schema on demand.
+* `OverrideSchema(*Schema)` method replaced with the `SetSchema(Schema)`.
+
+#### Protocol types
+
+* `iproto.Feature` type used instead of `ProtocolFeature`.
+* `iproto.IPROTO_FEATURE_` constants used instead of local ones.
+
+#### Schema type
+
+* `ResolveSpaceIndex` function for `SchemaResolver` interface split into two:
+`ResolveSpace` and `ResolveIndex`. `NamesUseSupported` function added into the
+interface to get information if the usage of space and index names in requests
+is supported.
+* `Schema` structure no longer implements `SchemaResolver` interface.
+* `Spaces` and `SpacesById` fields of the `Schema` struct store spaces by value.
+* `Fields` and `FieldsById` fields of the `Space` struct store fields by value.
+`Index` and `IndexById` fields of the `Space` struct store indexes by value.
+* `Fields` field of the `Index` struct store `IndexField` by value.
+
+### datetime package
+
+Now you need to use objects of the Datetime type instead of pointers to it. A
+new constructor `MakeDatetime` returns an object. `NewDatetime` has been
+removed.
+
+### decimal package
+
+Now you need to use objects of the Decimal type instead of pointers to it. A
+new constructor `MakeDecimal` returns an object. `NewDecimal` has been removed.
+
+### multi package
+
+The subpackage has been deleted. You could use `pool` instead.
+
+### pool package
+
+* The `connection_pool` subpackage has been renamed to `pool`.
+* The type `PoolOpts` has been renamed to `Opts`.
+* `pool.Connect` and `pool.ConnectWithOpts`  now accept context as the first
+  argument, which user may cancel in process. If it is canceled in progress,
+  an error will be returned and all created connections will be closed.
+* `pool.Connect` and `pool.ConnectWithOpts` now accept `[]pool.Instance` as
+  the second argument instead of a list of addresses. Each instance is
+  associated with a unique string name, `Dialer` and connection options which
+  allows instances to be independently configured.
+* `pool.Connect`, `pool.ConnectWithOpts` and `pool.Add` add instances into
+  the pool even it is unable to connect to it. The pool will try to connect to
+  the instance later.
+* `pool.Add` now accepts context as the first argument, which user may cancel
+  in process.
+* `pool.Add` now accepts `pool.Instance` as the second argument instead of
+  an address, it allows to configure a new instance more flexible.
+* `pool.GetPoolInfo` has been renamed to `pool.GetInfo`. Return type has been
+  changed to `map[string]ConnectionInfo`.
+* Operations `Ping`, `Select`, `Insert`, `Replace`, `Delete`, `Update`, `Upsert`,
+  `Call`, `Call16`, `Call17`, `Eval`, `Execute` of a `Pooler` return
+  response data instead of an actual responses.
+
+### crud package
+
+* `crud` operations `Timeout` option has `crud.OptFloat64` type
+  instead of `crud.OptUint`.
+* A slice of a custom type could be used as tuples for `ReplaceManyRequest` and
+  `InsertManyRequest`, `ReplaceObjectManyRequest`.
+* A slice of a custom type could be used as objects for `ReplaceObjectManyRequest`
+  and `InsertObjectManyRequest`.
+
+### test_helpers package
+
+* Renamed `StrangerResponse` to `MockResponse`.