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An FTP publication pipeline keeps an incomplete upload off the live pathname by writing to a unique staging name, confirming the transfer, validating the staged file, and only then promoting it with the rename pair RNFR followed immediately by RNTO. The sequence gives you a clean swap when the server behaves as expected, but FTP itself does not guarantee that the rename is atomic. Whether it is atomic depends on the server and the storage behind it, and that is the boundary this article works through step by step.
What the protocol standards establish
FTP is a command and reply protocol. RFC 959 requires that every command produce at least one reply, and it describes replies as three-digit numeric codes followed by text. Replies synchronize requests with actions and tell the client what state the server is in. Client logic should branch on the numeric code and on protocol state, not on the wording of a reply, because the text varies between server implementations.
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Three base commands matter for this pipeline. STOR stores a file. RNFR names the existing object to rename, and RNTO names its new pathname. RFC 959 requires that RNTO follow RNFR immediately. The IANA FTP command registry lists STOR, RNFR, and RNTO as base commands, so the pair is part of the core protocol rather than an extension.
RFC 959 does not describe how the remote server stores files. It says nothing about transaction isolation, crash consistency, overwrite policy, or whether a reader listing the directory can see a rename as a single change. The existence of RNFR and RNTO therefore tells you that a rename can be requested over the protocol. It does not tell you that the rename is atomic on your server.
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POSIX is the source for the atomicity claim, and it applies to a local operation. The Open Group’s POSIX.1-2024 definition of rename() says that the call replaces an existing destination entry with the source entry, and that the destination name stays visible throughout, resolving to either the old file or the new one. The rationale calls the action atomic. POSIX also reports EXDEV when a rename crosses filesystems in the relevant cases. This is why the familiar temporary-file-then-rename design works well on a single POSIX filesystem. It is not evidence that a remote FTP server exposes the same semantics.
The Linux man-pages entry for rename(2) makes the same namespace point and adds a caveat for network filesystems. On NFS, if a rename returns an error, the client cannot assume the file was left where it was, because the server may have completed the rename before a crash and then handled the retry. Blind retries are therefore risky. After an uncertain result, look at the actual names and content before doing anything else.
The execution flow, line by line
The steps below are ordered. Each one is a checkpoint, and the job should not advance until the checkpoint has passed.
1. Choose the destination and the replacement policy
Decide the final pathname and whether an existing file at that path may be replaced. RFC 959 gives pathname arguments but leaves their conventions to the server site, so path syntax, permissions, case handling, and replacement behavior are properties of your deployment. Test the replacement case on your server before you write any code that depends on it. Do not assume that a rename onto an existing name will succeed, fail, or overwrite based on what the RFC says.
2. Create a unique staging pathname
The staging name must differ from the live pathname and must be unique to the job, so that two concurrent jobs cannot write to the same staging file. The naming scheme is an application choice. A leading dot and a job identifier are common conventions:
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final: /published/report.csv
staging: /published/.report.csv.7f3a9c.tmpKeep the staging file in the same directory as the destination, or at least on the same filesystem. This is what makes a local rename possible, since POSIX reports EXDEV when a rename crosses filesystems. A leading dot is a naming convention only. Anyone who lists the directory will still see the staging file, so the directory should not be a place where readers pick up files by listing alone.
3. Transfer the payload with STOR
STOR /published/.report.csv.7f3a9c.tmpSend the payload to the staging name only. The point of this step is that an interrupted or partial transfer lands in a file that is not the reader-facing name. Do not write directly to the final pathname, even as a shortcut.
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A preliminary reply is not completion. The transfer is finished only when the server sends the final reply on the control connection. RFC 959 describes two common forms: the server closes the data connection and returns a 226 reply, or the data connection stays open and the server returns 250. Read the final reply, record its numeric code, and only then continue. If the control connection drops before this reply arrives, treat the transfer as incomplete and do not promote the staged file.
5. Validate the staged payload
Run application checks against the staged file before anything is promoted. The checks depend on the data. Typical examples are an expected size, a successful parse, a schema check, a record count, a domain invariant such as no negative totals, or a comparison against a digest the producer supplies in a trusted way. FTP defines none of these checks. Keep metadata checks, which describe the file, separate from content checks, which describe what is inside it.
6. Optionally inspect server metadata
RFC 3659 defines SIZE, MDTM, MLST, and MLSD. MLST and MLSD return machine-readable facts about a path or directory listing. Support for these extensions is advertised through FEAT, so check that reply before relying on them:
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FEAT
SIZE /published/.report.csv.7f3a9c.tmp
MDTM /published/.report.csv.7f3a9c.tmpThese commands can confirm that the staged object exists, that its size matches what you sent, and that its timestamp is plausible. They cannot tell you whether the bytes are a correct report. Treat them as a cheap pre-check that sits in front of step 5, not as a substitute for it.
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7. Promote with the RNFR and RNTO pair
RNFR /published/.report.csv.7f3a9c.tmp
350 ...
RNTO /published/report.csv
250 ...Send RNFR with the staging name and confirm that the reply is the intermediate acceptance reply, 350, per RFC 959. Then send RNTO with the final name, immediately, with nothing in between on the control connection. Check the final reply of RNTO. A failed RNFR or RNTO is a failed or uncertain promotion, and the job must not be marked published. The codes you will see most often in failure cases are covered in the failure table below.
8. Reconcile an uncertain outcome
A timeout or disconnect between RNTO and its reply is the dangerous moment. The server may already have completed the rename. Before retrying, list both the staging path and the final path, and compare the file at the final name against what you expected to publish, using size, timestamp, and digest where available. If the final name holds the expected content, the promotion succeeded and the staging name should no longer exist. If the staging name still holds the payload and the final name does not match, the rename did not happen and the promotion can be retried. If neither name matches, stop and escalate. Make the retry logic depend on this reconciliation, not on the error code alone.
9. Clean up abandoned staging files
Remove a staging file only when its job state shows that no process still owns it, and when it is older than your maximum job runtime. FTP provides DELE for removing files, but the cleanup policy belongs to your application. Never use cleanup as a way to delete or overwrite the final file. A stale staging file is a leftover, not a sign that the publication failed.
Recording state and handling failures
Track the job through explicit states: created, uploading, uploaded, validated, promotion_requested, published, failed, and outcome_unknown. The job reaches published only after the final RNTO reply indicates success. Any path that ends in a timeout or an unexpected disconnect after RNTO is sent goes to outcome_unknown, not failed.
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For each attempt, log the staging path, the destination path, the server identity, the reply codes received, timestamps, and the validation result. Do not log credentials, and do not log full payloads.
The following table lists the failure cases that matter in this pipeline, what each one means, and the action to take. The reply codes are the RFC 959 meanings for these commands. Your server may add site-specific behavior, so confirm the exact codes it returns in your environment.
| Failure | What it usually indicates | Action |
|---|---|---|
Authentication rejected (530) |
The session was never established, so no file operation ran. | Fail the job. Do not retry the same credentials without checking them. |
Data connection cannot open (425) |
The transfer never started. | Safe to retry from step 3 with the same staging name, after the data connection is re-established. |
Transfer aborted (426) |
The data connection closed before the transfer finished. The staging file may be partial. | Do not validate or promote. Re-upload to a new staging name and remove the partial file once it is no longer in use. |
| Staged file missing or malformed after upload | The payload did not arrive intact, or validation found a problem with its content. | Fail validation. Do not promote. Keep the staging file for diagnosis until the retention window expires. |
RNFR rejected (for example 450, 550, or 553) |
The staging object is missing, unavailable, or its name is not allowed. | Failed promotion. Confirm the staging object’s existence with SIZE or a listing before retrying. |
RNTO rejected (for example 550 or 553) |
The final name is not permitted, the destination is protected, or the server’s replacement policy refused the rename. | Failed promotion. Do not change the final name silently. Escalate for a policy decision. |
Disconnect or timeout after RNTO was sent |
The server may have completed the rename before the reply was lost. | Mark outcome_unknown. Reconcile both names as described in step 8 before retrying. |
| Unsupported metadata extension | FEAT does not list the command, or the command returns an error. |
Skip the metadata check and rely on content validation. Do not treat the missing check as a pass. |
| Cross-filesystem staging | The staging location is not on the same filesystem as the destination, so one local rename cannot be used. | Stage in the destination directory. If that is not possible, the promotion design has to change before this pipeline is safe. |
What the swap does and does not guarantee
The swap is a useful intended outcome, and you should describe it that way. Readers of the final pathname should see either the previous file or the complete new file, and the design depends on that. Whether the server delivers it depends on what the server does with the rename, which the protocol does not specify.
Keep three boundaries separate:
- Namespace visibility. POSIX
rename()makes the directory-entry change atomic on a local filesystem. A remote FTP server may or may not expose the same behavior, so verify it on your server. - Durability. The Open Group’s rationale notes that directory operations are atomic and serializable but not necessarily durable. A rename that is visible may not survive a sudden power loss. The common pattern is to synchronize the file contents before the rename, but whether your server or storage does that is a question for the server operator.
- Consistency under failure. A failed reply does not always mean the rename did not happen, as the NFS caveat in the Linux documentation shows. The reconciliation step is what closes this gap, not the rename command.
To check your own server, create a test staging file, rename it onto an existing final name under a concurrent reader, and confirm that the reader always sees a complete file. Repeat the test with a forced disconnect after RNTO. Only claim atomic publication for a server and storage combination you have checked this way.
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The FTP promotion step and a local rename differ in ways that affect the design.
| Axis | FTP RNFR/RNTO |
Local POSIX rename() |
|---|---|---|
| Defined by | RFC 959 protocol sequence | The Open Group POSIX.1-2024 definition |
| Atomicity | Not specified by the protocol. Verify on your server. | Specified as atomic for namespace visibility in the relevant cases |
| Existing destination | Server policy. The RFC does not define overwrite. | Replaces the existing destination entry |
| Cross-filesystem | Depends on the server. Stage on the same storage where possible. | Can fail with EXDEV |
| Failure reporting | Numeric reply. An uncertain outcome is possible after timeout. | Return value and errno. Network filesystems can still produce uncertain results. |
Scope of this article
This article covers plain FTP as defined by RFC 959 and the RFC 3659 extensions. FTPS and SFTP are different. FTPS adds TLS to the FTP control and data channels, and SFTP is a separate protocol with its own file operations. Do not assume that a rename behavior verified for one applies to the other.
The atomicity claim in this pipeline is only as strong as the server and storage backend it runs on. Verify that before writing it into a runbook or a contract.
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