Client: what it does in a system design

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Client

Generates open-loop or closed-loop load with read/write mix.

Settings

Mode: Open loop sends requests at the scenario's rate whether or not earlier ones finished, so a slow system piles up work. Closed loop uses a fixed number of virtual users who each wait for a reply, pause, then ask again.
Key distribution: Which keys reads ask for. Uniform picks every key equally. Zipfian makes a few keys very popular, like real traffic, which helps caches but concentrates load on hot keys.
Write-key strategy: Which keys writes touch. By default writes hit the same keys as reads, overwriting existing values. Create new makes each write a brand new key, like inserting new records.
Think time (closed-loop): In closed-loop mode, how long each virtual user pauses after a reply before sending the next request. Longer pauses mean less load. Shapes: constant, lognormal (mostly near the mean), exponential, uniform. Unused in open loop.
Timeout (ms): How long the client waits for an answer before giving up on that attempt, and retrying if it is allowed to.
Answers lost on the way back: Chance that a reply is lost on its way back even though the server did the work. The client sees a failure and may retry, so a write can run twice unless an idempotency key is used. 0 is never, 1 is always.
Retries: Whether the client tries again after a failure. Waiting between tries keeps retries from piling onto a struggling database.
Object size (KB): Size of each stored object in KB, fixed per key for the whole run. Bigger objects take longer to send and can fill a component's network bandwidth. Leave unset to ignore bandwidth. Shapes: constant, lognormal, exponential, uniform.
Share of the traffic: The fraction of the scenario's total traffic this client generates. 1 is all of it, 0.5 is half. Use it to split load between several clients.
Reads: The fraction of this client's requests that are reads, the rest are writes. 0.9 means 90% reads. Overrides the scenario's own read/write mix. More writes load the database and invalidate caches more.