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From "Kenneth Knowles (JIRA)" <>
Subject [jira] [Commented] (BEAM-758) Per-step, per-execution nonce
Date Thu, 09 Feb 2017 04:01:41 GMT


Kenneth Knowles commented on BEAM-758:

The spirit works, as far as the Fn API will just be shipping this as "extra stuff" that the
SDK repackages and provides to its variety of DoFn. To phrase it in terms of the new {{DoFn}}
as Luke suggests, here are some Java API options:

public void processElement(ProcessContext c, @JobNonce String jobNonce, @StepId String stepId)

or with little interfaces that serve roughly the same purpose

public void processElement(ProcessContext c, JobNonce jobNonce, StepId stepId)

The latter is probably less familiar, but has the standard benefit of being less error-prone
due to isolation of the types.

> Per-step, per-execution nonce
> -----------------------------
>                 Key: BEAM-758
>                 URL:
>             Project: Beam
>          Issue Type: Improvement
>          Components: sdk-java-core
>    Affects Versions: Not applicable
>            Reporter: Daniel Halperin
>            Assignee: Sam McVeety
> In the forthcoming runner API, a user will be able to save a pipeline to JSON and then
run it repeatedly.
> Many pieces of code (e.g., BigQueryIO.Read or Write) rely on a single random value (nonce).
These values are typically generated at apply time, so that they are deterministic (don't
change across retries of DoFns) and global (are the same across all workers).
> However, once the runner API lands the existing code would result in the same nonce being
reused across jobs. Other possible solutions:
> * Generate nonce in {{Create(1) | ParDo}} then use this as a side input. Should work,
as along as side inputs are actually checkpointed. But does not work for {{BoundedSource}}.
> * If a nonce is only needed for the lifetime of one bundle, can be generated in {{startBundle}}
and used in {{finishBundle}} [or {{tearDown}}].
> * Add some context somewhere that lets user code access unique step name, and somehow
generate a nonce consistently e.g. by hashing. Will usually work, but this is similarly not
available to sources.
> Another Q: I'm not sure we have a good way to generate nonces in unbounded pipelines
-- we probably need one. This would enable us to, e.g., use {{BigQueryIO.Write}} in an unbounded
pipeline [if we had, e.g., exactly-once triggering per window]. Or generalizing to multiple

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