Tablet Selection

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.

For VReplication streams a tablet also serves the role of target (vapplier). These, however, will always be the primary tablets in the target keyspace as we need to replicate the streamed writes within the target shard.

Cells and Cell Preference #

By default the TabletPicker will only look for viable (healthy and serving) source tablets of the specified tablet type(s) within the local cell (or cell alias within which the local cell belongs) of the calling process β€” the vtgate managing the VStream or the target vttablet for the VReplication stream β€” 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.

Even with the --cells flag specified, by default, the TabletPicker will give preference to a healthy and serving tablet within the local cell of the calling process. If there are multiple candidates in the local cell, it will pick one at random. If no healthy tablets exist in the local cell pool, then it will give preference to tablets within cells belonging to the same cell alias as the local cell. If none exist here, then it moves on to selecting candidates from cells provided using the --cells flag in your VReplication workflow commands.

When using the VTGate VStream API, you can override this local cell preference by specifying the CellPreference field as onlyspecified and a list of cells with Cells in the VStreamFlags request object. This will then only pick tablets from the cells provided.

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 that the tablet types should be used in the order of preference as listed in the --tablet-types flag using the --tablet-types-in-preference-order flag. For example --tablet-types "REPLICA,PRIMARY" --tablet-types-in-preference-order 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.

When using the VTGate VStream API you should instead migrate to using the new TabletOrder field in the VStreamFlags request object as usage of the "in_order" string hint will eventually be deprecated and removed.

VStream #

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