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How to write stored procedures, triggers, and user-defined functions in Azure Cosmos DB

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APPLIES TO: NoSQL

Azure Cosmos DB provides language-integrated, transactional execution of JavaScript that lets you write stored procedures, triggers, and user-defined functions (UDFs). When you use the API for NoSQL in Azure Cosmos DB, you can define the stored procedures, triggers, and UDFs using JavaScript. You can write your logic in JavaScript and execute it inside the database engine. You can create and execute triggers, stored procedures, and UDFs by using the Azure portal, the JavaScript query API in Azure Cosmos DB, and the Azure Cosmos DB for NoSQL SDKs.

To call a stored procedure, trigger, or UDF, you need to register it. For more information, see How to work with stored procedures, triggers, and user-defined functions in Azure Cosmos DB.

Note

For partitioned containers, when executing a stored procedure, a partition key value must be provided in the request options. Stored procedures are always scoped to a partition key. Items that have a different partition key value aren't visible to the stored procedure. This also applies to triggers.

Note

Server-side JavaScript features, including stored procedures, triggers, and UDFs, don't support importing modules.

Tip

Azure Cosmos DB supports deploying containers with stored procedures, triggers, and UDFs. For more information, see Create an Azure Cosmos DB container with server-side functionality.

How to write stored procedures

Stored procedures are written using JavaScript, and they can create, update, read, query, and delete items inside an Azure Cosmos DB container. Stored procedures are registered per collection, and can operate on any document or an attachment present in that collection.

Note

Azure Cosmos DB has a different charging policy for stored procedures. Because stored procedures can execute code and consume any number of request units (RUs), each execution requires an upfront charge. This ensures that stored procedure scripts don't impact backend services. The amount charged upfront equals the average charge consumed by the script in previous invocations. The average RUs per operation is reserved before execution. If the invocations have a lot of variance in RUs, your budget utilization might be affected. As an alternative, you should use batch or bulk requests instead of stored procedures to avoid variance around RU charges.

Here's a simple stored procedure that returns a "Hello World" response.

var helloWorldStoredProc = {
    id: "helloWorld",
    serverScript: function () {
        var context = getContext();
        var response = context.getResponse();

        response.setBody("Hello, World");
    }
}

The context object provides access to all operations that can be performed in Azure Cosmos DB, as well as access to the request and response objects. In this case, you use the response object to set the body of the response to be sent back to the client.

Once written, the stored procedure must be registered with a collection. To learn more, see How to use stored procedures in Azure Cosmos DB.

Create items using stored procedures

When you create an item using a stored procedure, the item is inserted into the Azure Cosmos DB container and an ID for the newly created item is returned. Creating an item is an asynchronous operation and depends on the JavaScript callback functions. The callback function has two parameters: one for the error object in case the operation fails, and another for a return value, in this case, the created object. Inside the callback, you can either handle the exception or throw an error. If a callback isn't provided and there's an error, the Azure Cosmos DB runtime throws an error.

The stored procedure also includes a parameter to set the description as a boolean value. When the parameter is set to true and the description is missing, the stored procedure throws an exception. Otherwise, the rest of the stored procedure continues to run.

The following example of a stored procedure takes an array of new Azure Cosmos DB items as input, inserts it into the Azure Cosmos DB container, and returns the count of the items inserted. In this example, we're using the ToDoList sample from the Quickstart .NET API for NoSQL.

function createToDoItems(items) {
    var collection = getContext().getCollection();
    var collectionLink = collection.getSelfLink();
    var count = 0;

    if (!items) throw new Error("The array is undefined or null.");

    var numItems = items.length;

    if (numItems == 0) {
        getContext().getResponse().setBody(0);
        return;
    }

    tryCreate(items[count], callback);

    function tryCreate(item, callback) {
        var options = { disableAutomaticIdGeneration: false };

        var isAccepted = collection.createDocument(collectionLink, item, options, callback);

        if (!isAccepted) getContext().getResponse().setBody(count);
    }

    function callback(err, item, options) {
        if (err) throw err;
        count++;
        if (count >= numItems) {
            getContext().getResponse().setBody(count);
        } else {
            tryCreate(items[count], callback);
        }
    }
}

Arrays as input parameters for stored procedures

When you define a stored procedure in the Azure portal, input parameters are always sent as a string to the stored procedure. Even if you pass an array of strings as an input, the array is converted to a string and sent to the stored procedure. To work around this, you can define a function within your stored procedure to parse the string as an array. The following code shows how to parse a string input parameter as an array:

function sample(arr) {
    if (typeof arr === "string") arr = JSON.parse(arr);

    arr.forEach(function(a) {
        // do something here
        console.log(a);
    });
}

Transactions within stored procedures

You can implement transactions on items within a container by using a stored procedure. The following example uses transactions within a fantasy football gaming app to trade players between two teams in a single operation. The stored procedure attempts to read the two Azure Cosmos DB items, each corresponding to the player IDs passed in as an argument. If both players are found, then the stored procedure updates the items by swapping their teams. If any errors are encountered along the way, the stored procedure throws a JavaScript exception that implicitly aborts the transaction.

function tradePlayers(playerId1, playerId2) {
    var context = getContext();
    var container = context.getCollection();
    var response = context.getResponse();

    var player1Item, player2Item;

    // query for players
    var filterQuery =
    {
        'query' : 'SELECT * FROM Players p where p.id = @playerId1',
        'parameters' : [{'name':'@playerId1', 'value':playerId1}] 
    };

    var accept = container.queryDocuments(container.getSelfLink(), filterQuery, {},
        function (err, items, responseOptions) {
            if (err) throw new Error("Error" + err.message);

            if (items.length != 1) throw "Unable to find player 1";
            player1Item = items[0];

            var filterQuery2 =
            {
                'query' : 'SELECT * FROM Players p where p.id = @playerId2',
                'parameters' : [{'name':'@playerId2', 'value':playerId2}]
            };
            var accept2 = container.queryDocuments(container.getSelfLink(), filterQuery2, {},
                function (err2, items2, responseOptions2) {
                    if (err2) throw new Error("Error " + err2.message);
                    if (items2.length != 1) throw "Unable to find player 2";
                    player2Item = items2[0];
                    swapTeams(player1Item, player2Item);
                    return;
                });
            if (!accept2) throw "Unable to read player details, abort ";
        });

    if (!accept) throw "Unable to read player details, abort ";

    // swap the two players’ teams
    function swapTeams(player1, player2) {
        var player2NewTeam = player1.team;
        player1.team = player2.team;
        player2.team = player2NewTeam;

        var accept = container.replaceDocument(player1._self, player1,
            function (err, itemReplaced) {
                if (err) throw "Unable to update player 1, abort ";

                var accept2 = container.replaceDocument(player2._self, player2,
                    function (err2, itemReplaced2) {
                        if (err) throw "Unable to update player 2, abort"
                    });

                if (!accept2) throw "Unable to update player 2, abort";
            });

        if (!accept) throw "Unable to update player 1, abort";
    }
}

Bounded execution within stored procedures

The following is an example of a stored procedure that bulk-imports items into an Azure Cosmos DB container. The stored procedure handles bounded execution by checking the boolean return value from createDocument, and then uses the count of items inserted in each invocation of the stored procedure to track and resume progress across batches.

function bulkImport(items) {
    var container = getContext().getCollection();
    var containerLink = container.getSelfLink();

    // The count of imported items, also used as the current item index.
    var count = 0;

    // Validate input.
    if (!items) throw new Error("The array is undefined or null.");

    var itemsLength = items.length;
    if (itemsLength == 0) {
        getContext().getResponse().setBody(0);
    }

    // Call the create API to create an item.
    tryCreate(items[count], callback);

    // Note that there are 2 exit conditions:
    // 1) The createDocument request was not accepted.
    //    In this case the callback will not be called, we just call setBody and we are done.
    // 2) The callback was called items.length times.
    //    In this case all items were created and we don’t need to call tryCreate anymore. Just call setBody and we are done.
    function tryCreate(item, callback) {
        var isAccepted = container.createDocument(containerLink, item, callback);

        // If the request was accepted, the callback will be called.
        // Otherwise report the current count back to the client,
        // which will call the script again with the remaining set of items.
        if (!isAccepted) getContext().getResponse().setBody(count);
    }

    // This is called when container.createDocument is done in order to process the result.
    function callback(err, item, options) {
        if (err) throw err;

        // One more item has been inserted, increment the count.
        count++;

        if (count >= itemsLength) {
            // If we created all items, we are done. Just set the response.
            getContext().getResponse().setBody(count);
        } else {
            // Create the next document.
            tryCreate(items[count], callback);
        }
    }
}

Async/await with stored procedures

The following stored procedure example uses async/await with Promises using a helper function. The stored procedure queries for an item and replaces it.

function async_sample() {
    const ERROR_CODE = {
        NotAccepted: 429
    };

    const asyncHelper = {
        queryDocuments(sqlQuery, options) {
            return new Promise((resolve, reject) => {
                const isAccepted = __.queryDocuments(__.getSelfLink(), sqlQuery, options, (err, feed, options) => {
                    if (err) reject(err);
                    resolve({ feed, options });
                });
                if (!isAccepted) reject(new Error(ERROR_CODE.NotAccepted, "queryDocuments was not accepted."));
            });
        },

        replaceDocument(doc) {
            return new Promise((resolve, reject) => {
                const isAccepted = __.replaceDocument(doc._self, doc, (err, result, options) => {
                    if (err) reject(err);
                    resolve({ result, options });
                });
                if (!isAccepted) reject(new Error(ERROR_CODE.NotAccepted, "replaceDocument was not accepted."));
            });
        }
    };

    async function main() {
        let continuation;
        do {
            let { feed, options } = await asyncHelper.queryDocuments("SELECT * from c", { continuation });

            for (let doc of feed) {
                doc.newProp = 1;
                await asyncHelper.replaceDocument(doc);
            }

            continuation = options.continuation;
        } while (continuation);
    }

    main().catch(err => getContext().abort(err));
}

How to write triggers

Azure Cosmos DB supports pre-triggers and post-triggers. Pre-triggers are executed before modifying a database item, and post-triggers are executed after modifying a database item. Triggers aren't automatically executed. They must be specified for each database operation where you want them to execute. After you define a trigger, you should register and call a pre-trigger by using the Azure Cosmos DB SDKs.

Pre-triggers

The following example shows how a pre-trigger is used to validate the properties of an Azure Cosmos DB item that's being created. This example uses the ToDoList sample from the Quickstart .NET API for NoSQL to add a timestamp property to a newly added item if it doesn't contain one.

function validateToDoItemTimestamp() {
    var context = getContext();
    var request = context.getRequest();

    // item to be created in the current operation
    var itemToCreate = request.getBody();

    // validate properties
    if (!("timestamp" in itemToCreate)) {
        var ts = new Date();
        itemToCreate["timestamp"] = ts.getTime();
    }

    // update the item that will be created
    request.setBody(itemToCreate);
}

Pre-triggers can't have any input parameters. The request object in the trigger is used to manipulate the request message associated with the operation. In the previous example, the pre-trigger is run when creating an Azure Cosmos DB item, and the request message body contains the item to be created in JSON format.

When triggers are registered, you can specify the operations that it can run with. This trigger should be created with a TriggerOperation value of TriggerOperation.Create, which means that using the trigger in a replace operation isn't permitted.

For examples of how to register and call a pre-trigger, see pre-triggers and post-triggers.

Post-triggers

The following example shows a post-trigger. This trigger queries for the metadata item and updates it with details about the newly created item.

function updateMetadata() {
    var context = getContext();
    var container = context.getCollection();
    var response = context.getResponse();

    // item that was created
    var createdItem = response.getBody();

    // query for metadata document
    var filterQuery = 'SELECT * FROM root r WHERE r.id = "_metadata"';
    var accept = container.queryDocuments(container.getSelfLink(), filterQuery,
        updateMetadataCallback);
    if(!accept) throw "Unable to update metadata, abort";

    function updateMetadataCallback(err, items, responseOptions) {
        if(err) throw new Error("Error" + err.message);

        if(items.length != 1) throw 'Unable to find metadata document';

        var metadataItem = items[0];

        // update metadata
        metadataItem.createdItems += 1;
        metadataItem.createdNames += " " + createdItem.id;
        var accept = container.replaceDocument(metadataItem._self,
            metadataItem, function(err, itemReplaced) {
                    if(err) throw "Unable to update metadata, abort";
            });

        if(!accept) throw "Unable to update metadata, abort";
        return;
    }
}

One thing that's important to note is the transactional execution of triggers in Azure Cosmos DB. The post-trigger runs as part of the same transaction for the underlying item itself. An exception during the post-trigger execution fails the whole transaction. Anything committed is rolled back and an exception is returned.

For examples of how to register and call a pre-trigger, see pre-triggers and post-triggers.

How to write user-defined functions

The following sample creates a UDF to calculate income tax for various income brackets. This user-defined function would then be used inside a query. For the purposes of this example, assume there's a container called Incomes with properties as follows:

{
   "name": "Daniel Elfyn",
   "country": "USA",
   "income": 70000
}

The following function definition calculates income tax for various income brackets:

function tax(income) {
    if (income == undefined)
        throw 'no input';

    if (income < 1000)
        return income * 0.1;
    else if (income < 10000)
        return income * 0.2;
    else
        return income * 0.4;
}

For examples of how to register and use a UDF, see How to work with user-defined functions in Azure Cosmos DB.

Logging

When using stored procedures, triggers, or UDFs, you can log the steps by enabling script logging. A string for debugging is generated when EnableScriptLogging is set to true, as shown in the following examples:

let requestOptions = { enableScriptLogging: true };
const { resource: result, headers: responseHeaders} = await container.scripts
      .storedProcedure(Sproc.id)
      .execute(undefined, [], requestOptions);
console.log(responseHeaders[Constants.HttpHeaders.ScriptLogResults]);

Next steps

Learn more concepts and how to write or use stored procedures, triggers, and UDFs in Azure Cosmos DB: