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An “unknown result” is not the same as a confirmed failure. In FoundationDB, commit_unknown_result means the client cannot tell whether a transaction committed; retrying may therefore run its effects twice. That is one specific kind of uncertainty, not a universal diagnosis for every bug or error described as “we don’t know what happened.”
What commit_unknown_result actually tells you
FoundationDB’s Developer Guide describes “Transactions with unknown results” as a situation in which the client did not receive enough information to know whether a commit succeeded. The transaction may have committed, or it may not have. The error is not a definitive report that it failed.
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The guide gives two possible causes: the client may lose its connection to the commit proxy after sending the commit, or a FoundationDB failure may occur during commit. In either case, the client may not receive a response that settles the outcome. The FoundationDB 7.4.7 Error Codes page defines error 1021 in the same terms: the transaction may or may not have committed. FoundationDB Developer Guide; FoundationDB 7.4.7 Error Codes.
Why an ordinary retry can duplicate work
FoundationDB’s on_error() treats commit_unknown_result as retryable. But retryable does not mean that replaying arbitrary application logic is harmless. If the first attempt committed and only its response was lost, running the transaction again can apply its effects a second time.
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FoundationDB’s guide puts the key design question plainly: “In these cases, you must consider the idempotence of the transaction.” An operation is idempotent when committing it twice has the same effect as committing it once. For example, setting a value to a known state can be idempotent; adding an amount to a balance generally is not, unless the application guards against a duplicate.
Make retries safe with an application-level identity
For a deposit-like operation, the guide’s pattern is to create a stable identifier before entering the retry loop, then use a unique transaction record to detect whether that deposit has already been applied. The transaction checks for the record before changing the balance and records the identifier as part of the operation.
- Create the operation’s stable identifier once, outside the retry loop. Do not generate a new identifier for each attempt.
- In each transaction attempt, check whether a record with that identifier already exists.
- If it exists, avoid applying the effect again. If it does not, apply the effect and write the uniquely identified record atomically with it.
- Retry only with the same identifier, and ensure the database’s uniqueness and transaction rules make the check-and-write safe under concurrent attempts.
This is a design pattern, not a drop-in fix for every application. The identifier’s scope, uniqueness constraints, retention, and relationship to the side effect must match the data model. A duplicate guard that is not atomic with the effect can still leave a race or inconsistency.
The guarantee is specific to this error
The Developer Guide states that once commit_unknown_result is received, the transaction is no longer in flight: it either committed or did not, and if it did not commit, it will not commit later. That bounded guarantee means an idempotent transaction can be retried under this particular error without a delayed first commit racing the retry.
Do not apply that guarantee to every error that sounds uncertain. FoundationDB’s documentation distinguishes transaction_timed_out and operation_cancelled as errors without the same assurance. For those cases, the appropriate recovery depends on the specific error and what evidence the application can inspect; the commit_unknown_result rule alone does not resolve them. FoundationDB Developer Guide.
Why “unknown” is not a complete bug taxonomy
The phrase “we don’t know what happened” can describe different knowledge states. A caller may not know whether an operation took effect; the operation may or may not still take effect later; a retry may be safe or duplicative; and the available evidence may differ. Those are separate questions, and answering one does not answer the others.
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FoundationDB’s documented case establishes a useful distinction for its own commit error, but it does not classify unrelated application bugs, network failures, or database errors. For any other error, identify its documented semantics independently: what is known about the effect, whether a delayed effect remains possible, what a retry would do, and what state or records can verify the outcome.
Automatic idempotency is not a universal answer
The FoundationDB 7.4.8 documentation labels automatic idempotency experimental and not recommended for production. It also says that transaction_timed_out and cluster_version_changed can still indicate unknown status. These details are version-sensitive, so check the documentation for the exact FoundationDB release in use before relying on the feature. FoundationDB 7.4.8 Automatic Idempotency.
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