Azure Cosmos DB Pre-triggers and Post-triggers
Azure Cosmos DB supports pretriggers and post-triggers.
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 it by using the Azure Cosmos DB SDKs.
Pre-triggers
Pretriggers are executed before modifying a database item. Pretriggers can't have any input parameters.
Here's an example of pre-trigger that manipulates the request message associated with the operation.
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);
}When triggers are registered, you can specify the operations that it can run with. For example, you can define that a trigger runs only before the creation of an element, and not during the update.
So, you first have to register the trigger:
await client
.GetContainer("database", "container")
.Scripts
.CreateTriggerAsync(new TriggerProperties
{
Id = "trgPreValidateToDoItemTimestamp",
Body = File.ReadAllText("@..\js\trgPreValidateToDoItemTimestamp.js"),
TriggerOperation = TriggerOperation.Create,
TriggerType = TriggerType.Pre
});Then, every time you execute an operation, you have to list the triggers to execute by specifying the ID.
dynamic newItem = new
{
category = "Personal",
name = "Groceries",
description = "Pick up strawberries",
isComplete = false
};
await client
.GetContainer("database", "container")
.CreateItemAsync(newItem, null, new ItemRequestOptions
{
PreTriggers = new List<string> { "trgPreValidateToDoItemTimestamp" }
});Post-triggers
Post-triggers are executed after modifying a database item.
The following 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;
}
}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 returned.
You have to register the trigger using the SDK:
await client
.GetContainer("database", "container")
.Scripts
.CreateTriggerAsync(new TriggerProperties
{
Id = "trgPostUpdateMetadata",
Body = File.ReadAllText(@"..\js\trgPostUpdateMetadata.js"),
TriggerOperation = TriggerOperation.Create,
TriggerType = TriggerType.Post
});Then you must specify the triggers to use when creating (or operating on) items.
var newItem = {
name: "artist_profile_1023",
artist: "The Band",
albums: ["Hellujah", "Rotators", "Spinning Top"]
};
await client
.GetContainer("database", "container")
.CreateItemAsync(newItem, null,
new ItemRequestOptions
{
PostTriggers = new List<string> { "trgPostUpdateMetadata" }
});