Firebase transactions with read operations after write operations

[Veargan]

Environment details

• npm version: 6.14.6
• Node.js version: 12.8.3
• node engine: 12
• @google-cloud/firestore version: 4.5.0
• firebase-admin: 9.6.0
• firebase-functions: 3.13.2

Steps to reproduce

Docs states that you can execute reads after writes, but as it turns out you cannot. Is that a bug or misleading documentation?

1. Call a cloud function or trigger background function

const onFirestoreTestsWrite = functions.firestore
  .document('tests/{testId}')
  .onWrite(async (change, context) => {
    await admin.firestore().runTransaction(async (t) => {
      const testDoc = await admin.firestore().collection('tests').doc().get();
      t.set(testDoc.ref, {});
      t.get(testDoc.ref);
    });
  });

const tests_get = async (req, res) => {
  try {
    const test = await admin.firestore().runTransaction(async (t) => {
      const testDoc = await admin.firestore().collection('tests').doc().get();
      t.set(testDoc.ref, {});
      return t.get(testDoc.ref);
    });
    res.send({id: test.id});
  } catch (e) {
    res.status(400).send({ error: e['message'] });
  }
};

Will result in error:

Error: Firestore transactions require all reads to be executed before all writes.
    at Transaction.get (/workspace/node_modules/@google-cloud/firestore/build/src/transaction.js:76:19)
    at /workspace/lib/firestore/tests-write.js:46:11
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async Transaction.runTransaction (/workspace/node_modules/@google-cloud/firestore/build/src/transaction.js:323:26)
    at async /workspace/lib/firestore/tests-write.js:43:5 

[rafikhan]

Hi @Veargan, the documentation link you sent me says Queries and reads inside a transaction do not see the results of previous writes inside that transaction. Where did you see that reads are allowed after writes?

[Veargan]

@rafikhan yep, there's part about isolation, but also a note below:

Only the server client libraries support transactions with read operations after write operations. For transactions in the mobile/web SDKs, document reads must come before document writes.

Also, related questions so1, so2

[schmidt-sebastian]

We will fix the documentation. The SDKs do not support read after writes. The Mobile and Server SDKs only differ in how they obtain locks for documents that are read as part of a transaction.

[schmidt-sebastian]

Filed b/186414961 for documentation fix.