This article will walk you through the essentials of triggers in Azure Cosmos DB, demonstrating how to implement and call them effectively.
UNDERSTANDING TRIGGERS IN AZURE COSMOS DB
Triggers in Azure Cosmos DB are pieces of logic written in JavaScript that get executed either before (pre-triggers) or after (post-triggers) an operation within a collection. Pre-triggers are often used to validate or modify data before the data change is committed, while post-triggers allow us to retrieve information about the change or manipulate it in some way after the change is committed.
IMPLEMENTATION OF TRIGGERS
To create a trigger, you will need to specify the id, trigger type (pre-trigger or post-trigger), and operation (insert, replace, delete, or all). Below is an example of implementing a pre-trigger using JavaScript:
{
"id": "validateTodoItem",
"body": function() {
var item = getContext().getRequest().getBody();
// Validate that the item is not completed already
if(item.isComplete==true) {
throw new Error('Cannot insert a todo item that is already completed.');
}
},
"triggerOperation": "Insert",
"triggerType": "Pre"
}
In the example above, the trigger will validate every item at the time of insertion, ensuring the item isn’t marked as complete.
CALLING TRIGGERS
When you want to use a trigger, mention its id in the request options when executing an operation. It’s important to note that triggers are not automatically fired; they need to be explicitly called each time.
Below is an example of calling the pre-trigger from a .NET application:
var requestOptions = new ItemRequestOptions()
{
PreTriggers = new List
};
await container.CreateItemAsync(item, requestOptions);
In this code sample, we’re using the .NET SDK to create an item in the Cosmos DB container. We’re passing in a list of pre-triggers to fire, in this case, our `validateTodoItem` trigger will fire before the item is inserted.
It’s vital to note that Azure Cosmos DB executes triggers atomically in the scope of the transaction. If a trigger throws an exception, Azure Cosmos DB aborts the transaction.
CONCLUSION
Triggers allow you to incorporate custom logic in your applications, providing flexibility and control over various operations in Cosmos DB. It’s crucial that you consider the impact of triggers on performance and costs, as the JavaScript logic consumes request units. This knowledge of implementing and calling triggers is fundamental in effectively designing and implementing native applications utilizing Microsoft Azure Cosmos DB.
Practice Test
True or False: Triggers in Azure Cosmos DB are invoked automatically before or after the data operation.
- True
- False
Answer: True
Explanation: Triggers in Azure Cosmos DB are either pre-triggers or post-triggers and are invoked automatically before or after a specified data operation.
Which of the following cannot be used to implement triggers in Azure Cosmos DB?
- a) JavaScript
- b) Python
- c) C#
- d) Java
Answer: c) C#
Explanation: Triggers in Azure Cosmos DB are written in JavaScript.
What are the two types of triggers in Azure Cosmos DB?
- a) Pre-triggers and Post-triggers
- b) In-triggers and Out-triggers
- c) On-triggers and Off-triggers
- d) Start-triggers and End-triggers
Answer: a) Pre-triggers and Post-triggers
Explanation: The two types of triggers in Azure Cosmos DB are pre-triggers, which are invoked before a create, replace or delete operation, and post-triggers, which are invoked after a create, replace or delete operation.
True or False: Triggers must be registered in Azure Cosmos DB before they can be invoked.
- True
- False
Answer: True
Explanation: Triggers need to be registered in Azure Cosmos DB before they can be used. They are associated with a specific container in Azure Cosmos DB.
Which API is used to invoke a trigger in Azure Cosmos DB?
- a) SQL API
- b) MongoDB API
- c) Cassandra API
- d) Gremlin API
Answer: a) SQL API
Explanation: Azure Cosmos DB’s SQL API is used to invoke triggers.
True or False: A trigger can be associated with multiple containers in Azure Cosmos DB.
- True
- False
Answer: False
Explanation: Each trigger is associated with a specific container in Azure Cosmos DB.
In Azure Cosmos DB, which parameter is optional in the context object that triggers have access to?
- a) response
- b) request
- c) bindings
- d) settings
Answer: a) response
Explanation: The response object is optional because it is not accessible in pre-triggers as the operation has not yet been committed.
True or False: Post-triggers in Azure Cosmos DB can be used to validate a document after it has been written to the database.
- True
- False
Answer: True
Explanation: Post-triggers in Azure Cosmos DB are executed after an operation and can be used, for example, to validate a document after it has been written to the database.
In Azure Cosmos DB, triggers can be invoked using which of the following methods?
- a) HTTP request to a custom endpoint
- b) Directly from the Azure portal
- c) From the client SDK
- d) Using a PowerShell script
Answer: c) From the client SDK
Explanation: The Azure Cosmos DB triggers are invoked from the client SDK using the create, replace, or delete operations.
True or False: Triggers in Azure Cosmos DB can have outbound HTTP calls.
- True
- False
Answer: False
Explanation: Triggers in Azure Cosmos DB are not allowed to have outbound HTTP calls. They are intended to have minimum latency impact on the database operation calling them.
In Azure Cosmos DB, is it possible to stop a database operation from a pre-trigger by not calling the .getCurrent() method?
- Yes
- No
Answer: Yes
Explanation: In a pre-trigger, not calling the .getCurrent() method stops the following database operation.
True or False: Triggers can be idempotent in Azure Cosmos DB.
- True
- False
Answer: True
Explanation: Triggers in Azure Cosmos DB can be designed to be idempotent, meaning they can be applied more than once without changing the result beyond the initial application.
What is the key requirement while implementing triggers in Azure Cosmos DB?
- a) Use of SQL API
- b) Writing triggers in JavaScript
- c) Registering the trigger before invoking
- d) All of the above
Answer: d) All of the above
Explanation: All of these are key requirements while implementing triggers in Azure Cosmos DB.
True or False: Triggers in Azure Cosmos DB can be invoked manually.
- True
- False
Answer: False
Explanation: Triggers in Azure Cosmos DB cannot be invoked manually. They are automatically triggered before or after data operations.
In Azure Cosmos DB, can pre-triggers modify documents before they are written into the container?
- Yes
- No
Answer: Yes
Explanation: Pre-triggers are designed to operate on the documents before they are written into the container, including modifications.
Interview Questions
What is a trigger in Azure Cosmos DB?
A trigger in Azure Cosmos DB is a set of actions that are executed before or after a specific database operation, like inserting, updating, or deleting documents.
How are triggers executed in Azure Cosmos DB?
Triggers in Azure Cosmos DB are executed during any create, replace, delete, and update operation. They can be executed either before or after the operation, known as pre-triggers and post-triggers respectively.
What are the two kinds of triggers in Azure Cosmos DB?
The two kinds of triggers in Azure Cosmos DB are pre-triggers and post-triggers. Pre-triggers are executed before the operation whereas post-triggers are executed after the operation.
In what situation would a pre-trigger be used?
A pre-trigger could be used when you need to validate or modify data before it is written into the database. For example, you could use a pre-trigger to enforce business rules, or to mutate the data as it is being inserted or updated.
What is a typical use case for a post-trigger?
Post-triggers are commonly used for actions that need to happen after a database action has been performed like logging or for pushing any changes to a secondary data store.
What language is used to write triggers in Azure Cosmos DB?
JavaScript is used to write triggers in Azure Cosmos DB.
How do you implement a trigger in Azure Cosmos DB?
You implement a trigger by creating a JavaScript function and associating it with a create, replace, delete, or update operation on a specific collection in Azure Cosmos DB.
Is it mandatory to associate a trigger with every operation in Azure Cosmos DB?
No, it’s not mandatory. Triggers are optional and can be associated with operations on a case-by-case basis.
How do you call a trigger in Azure Cosmos DB?
You call a trigger in Azure Cosmos DB by specifying the trigger’s name and type when you perform the associated operation.
Can a trigger modify the output of an operation in Azure Cosmos DB?
Yes, a post-trigger can modify the output of an operation.
What happens if a trigger throws an error during execution in Azure Cosmos DB?
If a trigger throws an error, Azure Cosmos DB will not proceed with the operation, and will return an error to the client.
Can we use triggers to prevent certain operations in Azure Cosmos DB?
Yes, by throwing an exception within a trigger’s script we can prevent the associated operations from being executed in Azure Cosmos DB.
Can we use external modules in our Azure Cosmos DB triggers?
No, currently Azure Cosmos DB does not support Node.js module loader, hence you cannot use external modules in your triggers.
What is the scope of a trigger in Azure Cosmos DB?
In Azure Cosmos DB, a trigger is scoped to a single partition key value.
Are triggers always executed automatically in Azure Cosmos DB?
No, in Azure Cosmos DB, you have to explicitly request triggers to be executed by specifying them in the request options of the operation.
extutorials.com