1. Drivers
Piotr Sarna
piotr@sarna.dev sarna@turso.tech

2. Piotr Sarna
● Wrote a few patches for the Linux kernel
● Ex-maintainer of ScyllaDB and Seastar
● Staff @Turso
● Maintainer of libSQL

3. Drivers
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Communication tools for database users
● binary protocols
● HTTP
● gRPC

4. Clients vs Servers
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5. Clients vs Servers
Clients:
● want stuff done as fast as possible
● don’t usually care much about other clients
Servers:
● want to have things to do all the time
● but not too many things
● cooperate with other fellow servers
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6. Workload types
a.k.a. types of annoying clients
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interactive:
“webpage users”
batch:
“Spark job”

7. Throughput vs Goodput
Throughput:
measure of how much data you can squeeze through
Goodput:
measure of much useful data you can squeeze through
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8. Throughput vs Goodput
Throughput:
1MiB of data, retried 10 times, 10MiB worth of error logs => 20 MiB/s!
Goodput:
1MiB of data, retried 10 times, 10MiB worth of error logs => 1 MiB/s
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(nobody cares)

9. Timeouts
User-side timeouts: “whew, they finally gave up trying”
Server-side timeouts: “I’m tired, boss”
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10. User-side timeouts
Tricky.
A server doesn’t necessarily know the user gave up.
May trigger a retry, which just makes the load worse.
Drivers can’t assume the servers know.
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11. Server-side timeouts
Not tricky.
A nice backpressure mechanism.
Server should clearly inform why the request was refused.
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12. Context
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13. Topology
Some databases expose their topology, some don’t.
For those who do, drivers should track topology changes.
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14. Topology
Ways of learning topology
● polling servers for topology information
● subscribing to topology change events
● taking part in the topology change process
○ gossiping
○ consensus algorithms
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15. Metadata
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● load statistics
○ current load
○ historical load
● priority
○ prefer servers with faster disks
○ prefer servers with more memory
● physical distance

16. Caching
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17. Caching
■ prepared statements
■ query results
■ topology
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18. Caching prepared statements
Example algorithms:
■ LRU
○ with multiple pools to avoid thrashing
■ LFU
Do not prepare oneshot statements.
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19. Caching query results
Local cache -> relaxed consistency guarantees
Usually better to be done in the app, not in the driver
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20. Caching topology & schema
Clients cannot assume they are up-to-date with topology or schema.
On failed assumption:
● client tries to contact a dead node
● client contacts a node that doesn’t have certain data anymore
● client contacts a node that is not yet up-to-date with all data
Server reaction:
● refuse to execute, ask to refresh topology and/or schema
● refuse to execute, reroute to the right node
● refuse to execute, send updated topology/schema info
● act as proxy
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21. Retries
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22. Retries
a.k.a. clients being even more annoying than usual
To retry, or not to retry?
● timeouts
○ read timeouts
○ write timeouts
● temporary errors
○ overload
○ dead node
○ schema mismatch
● permanent errors
○ bad syntax
○ authentication error
○ authorization error
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23. Idempotence
Can a query be safely executed multiple times?
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Query OK to retry?
SELECT * FROM t; yep
CREATE TABLE IF NOT EXISTS t(v int PRIMARY KEY); sure
INSERT INTO T(v) VALUES (42) USING TIMESTAMP 123; ok*
UPDATE t SET v = v + 42; nnnope*

24. Retry policies
● do not retry
● retry on the same server
● retry on another server
● retry after some delay
● retry speculatively
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25. Paging
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26. Paging
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Serving results in smaller bits.
● protects server memory
● protects clients' memory
● improves overall latency
● can be hidden behind driver API: e.g. result streams
● users can tweak parameters: page size

27. Concurrency
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28. Concurrency
Doing multiple things at a time.
High == good, but to a degree.
Modern hardware loves concurrency.
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29. Keep in touch!
Piotr Sarna
Staff @Turso
piotr@sarna.dev
sarna@turso.tech
@sarna_dev