Implementing Firestore optimistic concurrency control patterns

Question

Pulumi · Accepted Answer

To implement Firestore optimistic concurrency control patterns using Pulumi, you'll typically work with transactions or use document versioning to ensure that writes to the database are not conflicting with other concurrent writes. Firestore provides transactional operations that allow you to ensure that a set of operations is atomic — that is, either all of the operations succeed, or none of them are applied.

Below, I will create a Pulumi program that demonstrates how to perform a transaction in Firestore using the `Firestore.Document` resource. This program will:

1. Set up a Firestore Document.
2. Use a transaction to update the document with the intention to prevent conflicts in a concurrent environment.

Let's walk through the Pulumi TypeScript program that defines a Firestore document and illustrates how to perform transactional updates to implement optimistic concurrency control.

```typescript
import * as pulumi from "@pulumi/pulumi";
import * as gcp from "@pulumi/gcp";

// Define a new Firestore document. This document will have a `docId`, and we'll manually manage its versioning
// field to implement optimistic concurrency control. You should initialize the version field explicitly in your
// application logic or Firestore security rules when the document is created.
const doc = new gcp.firestore.Document("myDocument", {
    collection: "myCollection",
    fields: {
        content: "Initial content",
        version: 1, // A version field used for concurrency control
    }
});

// Perform a transactional update - this is a simplistic example of implementing concurrency control by checking
// the current version of the document and incrementing it with each change. The specific fields and update logic
// may vary based on your use case.
const updatedDoc = gcp.firestore.createDocument({
    collection: "myCollection",
    documentId: "myDocument",
    fields: {
        content: "Updated content",
        version: 2, // Increment the version
    },
    // Firestore transactions or preconditions can be used here for concurrency control. In some cases, you can
    // use the Firestore REST API or client SDKs to specify preconditions like "update only if version == X".
});

// Export the document's name.
export const documentName = doc.name;
```

In this program:
- We first import the necessary Pulumi packages for executing GCP resources (`@pulumi/pulumi` and `@pulumi/gcp`).
- We create a Firestore `Document` resource representing our data, including a `version` field to track its version for concurrency control purposes.
- Next, we illustrate a transactional update where the `version` field is incremented. In practice, you would use Firestore transactions to ensure that the update is contingent on the existing `version` field value, allowing the operation to fail if there has been an intervening write by another user.

Keep in mind that this code is a simplified representation. In reality, you would use the Firestore SDK in your application to handle the get-modify-write cycle within a transaction to compare the `version` field value at read time versus at write time.

Firestore transactions involve complex operations and should be carefully designed to handle error cases and retries. For more detailed information about Firestore transactions, you can check out the Firestore documentation on transactions:
- [Firestore Document resource](https://www.pulumi.com/registry/packages/gcp/api-docs/firestore/document/)
- [Firestore transactions and optimistic concurrency](https://cloud.google.com/firestore/docs/manage-data/transactions).

Remember to install the required Pulumi packages using `npm` or `yarn` before executing the program:

```bash
npm install @pulumi/pulumi @pulumi/gcp
```

This program is a fundamental illustration of how Pulumi interacts with GCP Firestore to implement optimistic concurrency control, and actual implementations should handle various edge cases and potential failures as mentioned above.