JavaScript Transactions

JavaScript transactions are different from transactions in SQL.

In SQL, transactions are started with explicit BEGIN or START TRANSACTION command. Following any series of data retrieval or modification operations, an SQL transaction is finished with a COMMIT command, or rolled back with a ROLLBACK command. There may be client/server communication between the start and the commit/rollback of an SQL transaction.

In ArangoDB, a transaction is always a server-side operation, and is executed on the server in one go, without any client interaction. All operations to be executed inside a transaction need to be known by the server when the transaction is started.

There are no individual BEGIN, COMMIT or ROLLBACK transaction commands in ArangoDB. Instead, a transaction in ArangoDB is started by providing a description of the transaction to the db._executeTransaction() JavaScript function:

db._executeTransaction(options);

This function automatically starts a transaction, executes all required data retrieval and/or modification operations, and automatically commit the transaction at the end. If an error occurs during transaction execution, the transaction is automatically aborted, and all changes are rolled back.

JavaScript API

Execute Transaction

db._executeTransaction(options)

Executes a server-side JavaScript transaction.

options must be an object and have the following attributes:

• collections: A sub-object that defines which collections you want to use in the transaction. It can have the following sub-attributes:
  • read: A single collection or a list of collections to use in the transaction in read-only mode.
  • write: A single collection or a list of collections to use in the transaction in write or read mode.
  • exclusive: A single collection or a list of collections to acquire exclusive write access for.
• action: A JavaScript function or a string with JavaScript code containing all the instructions to be executed inside the transaction. If the code runs through successfully, the transaction is committed at the end. If the code throws an exception, the transaction is aborted and all database operations are be rolled back.

Additionally, options can have the following optional attributes:

• params: Optional arguments passed to the function specified in action.
• allowImplicit: Allow reading from undeclared collections.
• waitForSync: An optional boolean flag that, if set, forces the transaction to write all data to disk before returning.
• lockTimeout: A numeric value that can be used to set a timeout in seconds for waiting on collection locks. This option is only meaningful when using exclusive locks. If not specified, a default value of 900 seconds is used. Setting lockTimeout to 0 makes ArangoDB not time out waiting for a lock.
• maxTransactionSize: Transaction size limit in bytes.

Declaration of Collections

All collections participating in a transaction need to be declared beforehand. This is necessary to ensure proper locking and isolation.

Collections can be used in a transaction in write mode or in read-only mode.

If any data modification operations are to be executed, the collection must be declared for use in write mode. The write mode allows modifying and reading data from the collection during the transaction (i.e. the write mode includes the read mode).

Contrary, using a collection in read-only mode will only allow performing read operations on a collection. Any attempt to write into a collection used in read-only mode will make the transaction fail.

Collections for a transaction are declared by providing them in the collections attribute of the object passed to the _executeTransaction() function. The collections attribute can have the sub-attributes read, write, and exclusive:

db._executeTransaction({
  collections: {
    write: [ "users", "logins" ],
    read: [ "recommendations" ]
  }
});

read, write, and exclusive are optional attributes, and only need to be specified if the operations inside the transactions demand for it.

The attribute values can each be lists of collection names or a single collection name (as a string):

db._executeTransaction({
  collections: {
    write: "users",
    read: "recommendations"
  }
});

Note: It is optional to specify collections for read-only access by default. Even without specifying them, it is still possible to read from such collections from within a transaction, but with relaxed isolation. Please refer to Transactions Locking for more details.

In order to make a transaction fail when a non-declared collection is used inside for reading, the optional allowImplicit sub-attribute of collections can be set to false:

db._executeTransaction({
  collections: {
    read: "recommendations",
    allowImplicit: false  /* this disallows read access to other collections
                             than specified */
  },
  action: function () {
    var db = require("@arangodb").db;
    return db.foobar.toArray(); /* will fail because db.foobar must not be accessed
                                   for reading inside this transaction */
  }
});

The default value for allowImplicit is true. Write-accessing collections that have not been declared in the collections array is never possible, regardless of the value of allowImplicit.

Declaration of Data Modification and Retrieval Operations

All data modification and retrieval operations that are to be executed inside the transaction need to be specified in a JavaScript function, using the action attribute:

db._executeTransaction({
  collections: {
    write: "users"
  },
  action: function () {
    // all operations go here
  }
});

Any valid JavaScript code is allowed inside action but the code may only access the collections declared in collections. action may be a JavaScript function as shown above, or a string representation of a JavaScript function:

db._executeTransaction({
  collections: {
    write: "users"
  },
  action: "function () { doSomething(); }"
});

Note that any operations specified in action will be executed on the server, in a separate scope. Variables will be bound late. Accessing any JavaScript variables defined on the client-side or in some other server context from inside a transaction may not work. Instead, any variables used inside action should be defined inside action itself:

db._executeTransaction({
  collections: {
    write: "users"
  },
  action: function () {
    var db = require(...).db;
    db.users.save({ ... });
  }
});

When the code inside the action attribute is executed, the transaction is already started and all required locks have been acquired. When the code inside the action attribute finishes, the transaction will automatically commit. There is no explicit commit command.

To make a transaction abort and roll back all changes, an exception needs to be thrown and not caught inside the transaction:

db._executeTransaction({
  collections: {
    write: "users"
  },
  action: function () {
    var db = require("@arangodb").db;
    db.users.save({ _key: "hello" });
    // will abort and roll back the transaction
    throw "doh!";
  }
});

There is no explicit abort or roll back command.

As mentioned earlier, a transaction commits automatically when the end of the action function is reached and no exception were thrown. In this case, the user can return any legal JavaScript value from the function:

db._executeTransaction({
  collections: {
    write: "users"
  },
  action: function () {
    var db = require("@arangodb").db;
    db.users.save({ _key: "hello" });
    // will commit the transaction and return the value "hello"
    return "hello";
  }
});

Passing parameters to transactions

Arbitrary parameters can be passed to transactions by setting the params attribute when declaring the transaction. This feature is handy to re-use the same transaction code for multiple calls but with different parameters.

A basic example:

db._executeTransaction({
  collections: { },
  action: function (params) {
    return params[1];
  },
  params: [ 1, 2, 3 ]
});

The above example will return 2.

Some example that uses collections:

db._executeTransaction({
  collections: { 
    write: "users",
    read: [ "c1", "c2" ]
  },
  action: function (params) {
    var db = require('@arangodb').db;
    var doc = db.c1.document(params['c1Key']);
    db.users.save(doc);
    doc = db.c2.document(params['c2Key']);
    db.users.save(doc);
  },
  params: { 
    c1Key: "foo", 
    c2Key: "bar" 
  }
});

Throwing Exceptions

If you catch errors in your transaction, try to get them solved. If you can’t, you may want to mimic original ArangoDB error messages to ease the control flow of your invoking environment. This can be done like this:

db._executeTransaction({
  collections: {},
  action: function () {
    const arangodb = require('@arangodb');
    var err = new arangodb.ArangoError();
    err.errorNum = arangodb.ERROR_BAD_PARAMETER;
    err.errorMessage = "who's bad?";
    throw err;
  }
});

For a complete list of possible ArangoDB errors, see Error codes and meanings.

Custom Exceptions

You may want to define custom exceptions inside of a transaction. To have the exception propagate upwards properly, please throw an an instance of base JavaScript Error class or a derivative. To specify an error number, include it as the errorNumber field. As an example:

db._executeTransaction({
  collections: {},
  action: function () {
    var err = new Error('My error context');
    err.errorNum = 1234;
    throw err;
  }
});

Note: In previous versions, custom exceptions which did not have an Error-like form were simply converted to strings and exposed in the exception field of the returned error. This is no longer the case, as it had the potential to leak unwanted information if improperly used.

Note: In some versions the above example wouldn’t propagate the errorNum to the invoking party, you may need to upgrade your ArangoDB.

Examples

Single Collection Example

The first example writes 3 documents into a collection named c1. The c1 collection needs to be declared in the write attribute of the collections attribute passed to the _executeTransaction() function.

The action attribute contains the actual transaction code to be executed. This code contains all data modification operations (3 in this example).

// setup
db._create("c1");

db._executeTransaction({
  collections: {
    write: [ "c1" ]
  },
  action: function () {
    var db = require("@arangodb").db;
    db.c1.save({ _key: "key1" });
    db.c1.save({ _key: "key2" });
    db.c1.save({ _key: "key3" });
  }
});
    db.c1.count(); // 3

Aborting the transaction by throwing an exception in the action function will revert all changes, so as if the transaction never happened:

// setup
db._create("c1");

db._executeTransaction({
  collections: {
    write: [ "c1" ]
  },
  action: function () {
    var db = require("@arangodb").db;
    db.c1.save({ _key: "key1" });
    db.c1.count(); // 1
    db.c1.save({ _key: "key2" });
    db.c1.count(); // 2
    throw "doh!";
  }
});

db.c1.count(); // 0

The automatic rollback is also executed when an internal exception is thrown at some point during transaction execution:

// setup
db._create("c1");

db._executeTransaction({
  collections: {
    write: [ "c1" ]
  },
  action: function () {
    var db = require("@arangodb").db;
    db.c1.save({ _key: "key1" });
    // will throw duplicate a key error, not explicitly requested by the user
    db.c1.save({ _key: "key1" });  
    // we'll never get here...
  }
});

db.c1.count(); // 0

As required by the consistency principle, aborting or rolling back a transaction will also restore secondary indexes to the state at transaction start.

Cross-collection Transactions

There’s also the possibility to run a transaction across multiple collections. In this case, multiple collections need to be declared in the collections attribute, e.g.:

// setup
db._create("c1");
db._create("c2");

db._executeTransaction({
  collections: {
    write: [ "c1", "c2" ]
  },
  action: function () {
    var db = require("@arangodb").db;
    db.c1.save({ _key: "key1" });
    db.c2.save({ _key: "key2" });
  }
});

db.c1.count(); // 1
db.c2.count(); // 1

Again, throwing an exception from inside the action function will make the transaction abort and roll back all changes in all collections:

// setup
db._create("c1");
db._create("c2");

db._executeTransaction({
  collections: {
    write: [ "c1", "c2" ]
  },
  action: function () {
    var db = require("@arangodb").db;
    for (var i = 0; i < 100; ++i) {
      db.c1.save({ _key: "key" + i });
      db.c2.save({ _key: "key" + i });
    }
    db.c1.count(); // 100
    db.c2.count(); // 100
    // abort
    throw "doh!"
  }
});

db.c1.count(); // 0
db.c2.count(); // 0

HTTP API

See the HTTP Interface for JavaScript Transactions documentation.