// Info represents the shard DDL information.
// This information should be persistent in etcd so can be retrieved after the DM-master leader restarted or changed.
// NOTE: `Task` and `Source` are redundant in the etcd key path for convenient.
type Info struct {
    Task   string   `json:"task"`   // data migration task name
    Source string   `json:"source"` // upstream source ID
    Schema string   `json:"schema"` // schema name of the DDL
    Table  string   `json:"table"`  // table name of the DDL
    DDLs   []string `json:"ddls"`   // DDL statements
}

// Operation represents a shard DDL coordinate operation.
// This information should be persistent in etcd so can be retrieved after the DM-master leader restarted or changed.
// NOTE: `Task` and `Source` are redundant in the etcd key path for convenient.
type Operation struct {
    ID     string   `json:"id"`     // the corresponding DDL lock ID
    Task   string   `json:"task"`   // data migration task name
    Source string   `json:"source"` // upstream source ID
    DDLs   []string `json:"ddls"`   // DDL statements
    Exec   bool     `json:"exec"`   // execute or skip the DDL statements
    Done   bool     `json:"done"`   // whether the `Exec` operation has done


    // only used to report to the caller of the watcher, do not marsh it.
    // if it's true, it means the Operation has been deleted in etcd.
    IsDeleted bool `json:"-"`
}

// Pessimist used to coordinate the shard DDL migration in pessimism mode.
type Pessimist struct {
    mu sync.RWMutex


    logger log.Logger
    cli    *clientv3.Client
    task   string
    source string


    // the shard DDL info which is pending to handle.
    pendingInfo *pessimism.Info
    // the shard DDL lock operation which is pending to handle.
    pendingOp *pessimism.Operation
}

// ShardingGroup represents a sharding DDL sync group.
type ShardingGroup struct {
    sync.RWMutex
    // remain count waiting for syncing
    // == len(sources):  DDL syncing not started or resolved
    // == 0: all DDLs synced, will be reset to len(sources) after resolved combining with other dm-workers
    // (0, len(sources)): waiting for syncing
    // NOTE: we can make remain to be configurable if needed
    remain       int
    sources      map[string]bool // source table ID -> whether source table's DDL synced
    IsSchemaOnly bool            // whether is a schema (database) only DDL TODO: zxc add schema-level syncing support later


    sourceID string                  // associate dm-worker source ID
    meta     *shardmeta.ShardingMeta // sharding sequence meta storage


    firstLocation    *binlog.Location // first DDL's binlog pos and gtid, used to restrain the global checkpoint when un-resolved
    firstEndLocation *binlog.Location // first DDL's binlog End_log_pos and gtid, used to re-direct binlog streamer after synced
    ddls             []string         // DDL which current in syncing


    flavor     string
    enableGTID bool
}

// ShardingMeta stores sharding ddl sequence
// including global sequence and each source's own sequence
// NOTE: sharding meta is not thread safe, it must be used in thread safe context.
type ShardingMeta struct {
    activeIdx int                          // the first unsynced DDL index
    global    *ShardingSequence            // merged sharding sequence of all source tables
    sources   map[string]*ShardingSequence // source table ID -> its sharding sequence
    tableName string                       // table name (with schema) used in downstream meta db


    enableGTID bool // whether enableGTID, used to compare location
}

// ShardingSequence records a list of DDLItem.
type ShardingSequence struct {
    Items []*DDLItem `json:"items"`
}

// DDLItem records ddl information used in sharding sequence organization.
type DDLItem struct {
    FirstLocation binlog.Location `json:"-"`      // first DDL's binlog Pos, not the End_log_pos of the event
    DDLs          []string        `json:"ddls"`   // DDLs, these ddls are in the same QueryEvent
    Source        string          `json:"source"` // source table ID


    // just used for json's marshal and unmarshal, because gtid.Set in FirstLocation is interface,
    // can't be marshal and unmarshal
    FirstPosition mysql.Position `json:"first-position"`
    FirstGTIDSet  string         `json:"first-gtid-set"`
}