Introduction #

For both VTGate VStreams and VTTablet VReplication streams we must choose a tablet to serve the role of source (vstreamer). This tablet selection is performed by the internal TabletPicker component.

Cells #

By default the TabletPicker will only look for viable (healthy and serving) source tablets of the specified tablet type(s) within the local cell of the calling process — the vtgate managing the VStream or the target vttablet for the VReplication stream — and it will select a random one from the candidate list. If you want to support cross-cell streams then you will need to specify the list of cells or any CellAlias that contain a list of cells using the --cells flag in your VReplication workflow commands like MoveTables or the VStreamFlags.Cells field in a VStreamRequest.

Tablet Types #

VReplication #

For VReplication, the server side default which determines the candidate types made available for potential selection as the source for a stream is set using the vttablet --vreplication_tablet_type flag (default value is in_order:REPLICA,PRIMARY). The target tablet will provide this value to the TabletPicker to determine the viable source tablet candidates. You can override this default on the client side using your workflow command's --tablet_types flag.

You can also specify an order of preference for the tablet types using the in_order: prefix in both the server and client flags. For example, --tablet_types "in_order:REPLICA,PRIMARY" would cause a replica source tablet to be used whenever possible and a primary tablet would only be used as a fallback in the event that there are no viable replica tablets available at the time.

VStream #

For a VStream there is no default tablet type. You must specify an individual tablet type using the VStreamRequest.TabletType field.