A Tablet represents a single replica of a Group. A tablet is served by a
single server at a time, and can move between servers due to server death or
simply load balancing.
Distances help the client pick the closest tablet out of the list of
tablets for a given request. Tablets with lower distances should generally
be preferred. Tablets with the same distance are approximately equally
close; the client can choose arbitrarily.
Distances do not correspond precisely to expected latency, geographical
distance, or anything else. Distances should be compared only between
tablets of the same group; they are not meaningful between different
groups.
A value of zero indicates that the tablet may be in the same zone as
the client, and have minimum network latency. A value less than or equal to
five indicates that the tablet is thought to be in the same region as the
client, and may have a few milliseconds of network latency. Values greater
than five are most likely in a different region, with non-trivial network
latency.
Clients should use the following algorithm:
If the request is using a directed read, eliminate any tablets that
do not match the directed read's target zone and/or replica type.
(Read-write transactions only) Choose leader tablet if it has an
distance <=5.
Group and sort tablets by distance. Choose a random
tablet with the lowest distance. If the request
is not a directed read, only consider replicas with distances <=5.
Send the request to the fallback endpoint.
The tablet picked by this algorithm may be skipped, either because it is
marked as skip by the server or because the corresponding server is
unreachable, flow controlled, etc. Skipped tablets should be added to the
skipped_tablet_uid field in RoutingHint; the algorithm above should
then be re-run without including the skipped tablet(s) to pick the next
best tablet.
incarnation indicates the freshness of the tablet information contained
in this proto. Incarnations can be compared lexicographically; if
incarnation A is greater than incarnation B, then the Tablet
corresponding to A is newer than the Tablet corresponding to B, and
should be used preferentially.
If true, the tablet should not be chosen by the client. Typically, this
signals that the tablet is unhealthy in some way. Tablets with skip
set to true should be reported back to the server in
RoutingHint.skipped_tablet_uid; this cues the server to send updated
information for this tablet should it become usable again.
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2026-01-21 UTC."],[],[]]
