Never Lose a Card

What you'll learn

With versioning on, every write creates a new version and a delete adds a delete marker instead of removing data.
IfNoneMatch: "*" makes a PUT create-only; IfMatch: etag makes it update-only-if-unchanged.
A failed condition returns 412 PreconditionFailed — handle it as a normal outcome.

Section 01

Versioning

Turn on versioning with PutBucketVersioning. From then on each PUT gets a new VersionId, and older versions stay (and are billed) as noncurrent versions. A bucket can be unversioned, versioning-enabled, or versioning-suspended — once enabled, it can never go back to unversioned.

A DELETE without a version ID adds a delete marker: GET now returns 404, but every version is still there. Delete the marker and the object is back. Only a DELETE with a specific VersionId removes data permanently.

Section 02

Conditional writes

Versioning lets you recover from a lost update; a conditional write prevents it. S3 checks the condition atomically on the server, so no read-then-write race is possible.

Header	Meaning	Use it to
`IfNoneMatch: "*"`	Write only if no object has this key	Create a card exactly once
`IfMatch: "<etag>"`	Write only if the current ETag matches	Update a card you just read, without clobbering someone else's edit

Both work on PutObject and CompleteMultipartUpload. If another write to the same key is in flight, S3 may answer 409 ConditionalRequestConflict — retry.

Optimistic update with IfMatchJavaScript

const current = await client.send(new GetObjectCommand({ Bucket, Key }));
const card = JSON.parse(await current.Body.transformToString());
card.types.push("ground"); // Marshtomp evolves!

try {
  await client.send(new PutObjectCommand({
    Bucket, Key, Body: JSON.stringify(card), ContentType: "application/json",
    IfMatch: current.ETag,
  }));
} catch (err) {
  if (err.name === "PreconditionFailed") { /* someone else changed it: re-read and retry */ }
  else throw err;
}

Practice

Your turn

Two researchers upload Mudkip's card at almost the same moment. Make your upload create-only, so it fails instead of replacing a card that already exists, and handle that failure.

Add IfNoneMatch: "*" and catch the PreconditionFailed error.

// @ts-nocheck
import { S3Client, PutObjectCommand } from "@aws-sdk/client-s3";
const client = new S3Client({ region: "ap-south-1" });
const card = { number: 258, name: "Mudkip", types: ["water"], region: "Hoenn" };
async function addCard() {
  try {
    await client.send(
      new PutObjectCommand({
        Bucket: "hoenn-pokedex-media",
        Key: "cards/0258-mudkip.json",
        Body: JSON.stringify(card),
        ContentType: "application/json",
        // ⬇️ Only write if no object has this key yet.
      })
    );
    console.log("Mudkip's card created");
  } catch (err) {
    throw err;
  }
}

// @ts-nocheck
import { S3Client, PutObjectCommand } from "@aws-sdk/client-s3";

const client = new S3Client({ region: "ap-south-1" });

const card = { number: 258, name: "Mudkip", types: ["water"], region: "Hoenn" };

async function addCard() {
  try {
    await client.send(
      new PutObjectCommand({
        Bucket: "hoenn-pokedex-media",
        Key: "cards/0258-mudkip.json",
        Body: JSON.stringify(card),
        ContentType: "application/json",
        // ⬇️ Only write if no object has this key yet.

      })
    );
    console.log("Mudkip's card created");
  } catch (err) {
    throw err;
  }
}

Ctrl/⌘ Enter to run · simulated, no AWS callsSimulated — no AWS calls

Check

Check your understanding

Quick Quiz · one question, as many tries as you need.

Versioning is on. You run DeleteObject on cards/0258-mudkip.json without a version ID. What happens?

Choose one answer.