Class Transaction

step

• step<T>(callback: () => Promise<T>): Promise<T>

• Executes the given function locally as a single step of the transaction.

  example

  const db = new Database();
  const vertices = db.collection("vertices");
  const edges = db.collection("edges");
  const trx = await db.beginTransaction({ write: [vertices, edges] });
  
  // The following code will be part of the transaction
  const left = await trx.step(() => vertices.save({ label: "left" }));
  const right = await trx.step(() => vertices.save({ label: "right" }));
  
  // Results from preceding actions can be used normally
  await trx.step(() => edges.save({
    _from: left._id,
    _to: right._id,
    data: "potato"
  }));
  
  // Transaction must be committed for changes to take effected
  // Always call either trx.commit or trx.abort to end a transaction
  await trx.commit();

  example

  // BAD! If the callback is an async function it must only use await once!
  await trx.step(async () => {
    await collection.save(data);
    await collection.save(moreData); // WRONG
  });
  
  // BAD! Callback function must use only one arangojs call!
  await trx.step(() => {
   return collection.save(data)
     .then(() => collection.save(moreData)); // WRONG
  });
  
  // BETTER: Wrap every arangojs method call that should be part of the
  // transaction in a separate `trx.step` call
  await trx.step(() => collection.save(data));
  await trx.step(() => collection.save(moreData));

  example

  // BAD! If the callback is an async function it must not await before
  // calling an arangojs method!
  await trx.step(async () => {
    await doSomethingElse();
    return collection.save(data); // WRONG
  });
  
  // BAD! Any arangojs inside the callback must not happen inside a promise
  // method!
  await trx.step(() => {
    return doSomethingElse()
      .then(() => collection.save(data)); // WRONG
  });
  
  // BETTER: Perform any async logic needed outside the `trx.step` call
  await doSomethingElse();
  await trx.step(() => collection.save(data));
  
  // OKAY: You can perform async logic in the callback after the arangojs
  // method call as long as it does not involve additional arangojs method
  // calls, but this makes it easy to make mistakes later
  await trx.step(async () => {
    await collection.save(data);
    await doSomethingDifferent(); // no arangojs method calls allowed
  });

  example

  // BAD! The callback should not use any functions that themselves use any
  // arangojs methods!
  async function saveSomeData() {
    await collection.save(data);
    await collection.save(moreData);
  }
  await trx.step(() => saveSomeData()); // WRONG
  
  // BETTER: Pass the transaction to functions that need to call arangojs
  // methods inside a transaction
  async function saveSomeData(trx) {
    await trx.step(() => collection.save(data));
    await trx.step(() => collection.save(moreData));
  }
  await saveSomeData(); // no `trx.step` call needed

  example

  // BAD! You must wait for the promise to resolve (or await on the
  // `trx.step` call) before calling `trx.step` again!
  trx.step(() => collection.save(data)); // WRONG
  await trx.step(() => collection.save(moreData));
  
  // BAD! The trx.step callback can not make multiple calls to async arangojs
  // methods, not even using Promise.all!
  await trx.step(() => Promise.all([ // WRONG
    collection.save(data),
    collection.save(moreData),
  ]));
  
  // BAD! Multiple `trx.step` calls can not run in parallel!
  await Promise.all([ // WRONG
    trx.step(() => collection.save(data)),
    trx.step(() => collection.save(moreData)),
  ]));
  
  // BETTER: Always call `trx.step` sequentially, one after the other
  await trx.step(() => collection.save(data));
  await trx.step(() => collection.save(moreData));
  
  // OKAY: The then callback can be used if async/await is not available
  trx.step(() => collection.save(data))
    .then(() => trx.step(() => collection.save(moreData)));

  example

  // BAD! The callback will return an empty promise that resolves before
  // the inner arangojs method call has even talked to ArangoDB!
  await trx.step(async () => {
    collection.save(data); // WRONG
  });
  
  // BETTER: Use an arrow function so you don't forget to return
  await trx.step(() => collection.save(data));
  
  // OKAY: Remember to always return when using a function body
  await trx.step(() => {
    return collection.save(data); // easy to forget!
  });
  
  // OKAY: You do not have to use arrow functions but it helps
  await trx.step(function () {
    return collection.save(data);
  });

  example

  // BAD! You can not pass promises instead of a callback!
  await trx.step(collection.save(data)); // WRONG
  
  // BETTER: Wrap the code in a function and pass the function instead
  await trx.step(() => collection.save(data));

  example

  // WORSE: Calls to non-async arangojs methods don't need to be performed
  // as part of a transaction
  const collection = await trx.step(() => db.collection("my-documents"));
  
  // BETTER: If an arangojs method is not async and doesn't return promises,
  // call it without `trx.step`
  const collection = db.collection("my-documents");

  Type parameters

  • T

    Type of the callback's returned promise.

  Parameters

  • callback: () => Promise<T>

    Callback function returning a promise.

    Warning: The callback function should wrap a single call of an async arangojs method (e.g. a method on a Collection object of a collection that is involved in the transaction or the db.query method). If the callback function is async, only the first promise-returning (or async) method call will be executed as part of the transaction. See the examples below for how to avoid common mistakes when using this method.

    Note: Avoid defining the callback as an async function if possible as arangojs will throw an error if the callback did not return a promise. Async functions will return an empty promise by default, making it harder to notice if you forgot to return something from the callback.

    Note: Although almost anything can be wrapped in a callback and passed to this method, that does not guarantee ArangoDB can actually do it in a transaction. Refer to the ArangoDB documentation if you are unsure whether a given operation can be executed as part of a transaction. Generally any modification or retrieval of data is eligible but modifications of collections or databases are not.

    • • (): Promise<T>

      • Returns Promise<T>

  Returns Promise<T>