sychoi3 / SagaTracker

SagaTracker

Data structure to track a series of forward executions with its corresponding rollback executions as a saga.

Purpose

The goal of this data structure is to provide a mechanism to allow forward execution of a series of steps wrapped in a pseudo transaction that can be rolled back if any forward execution fails.

Example

Let's say there is a company that awards licenses to its users for a product. A license is awarded by performing the following actions in a single transaction (all-or-nothing):

1. (UserCatalog API) Add product to users catalog.
   • Forward: Add product to user catalog.
   • Rollback: Remove product from user catalog.
2. (DB) Decrement the number of available licenses.
   • Forward: Decrement the available license count by -1.
   • Rollback: Increment the available license count by +1.
3. (License API) Assign license to user
   • Forward: Assign license to user.
   • Rollback: no action.

For example: If the series fails at step 3, then the available license count should be incremented back by +1 and the product should be removed from the user catalog.

Limitations

This data structure is primarily useful in smaller use cases that require small bits of execution to be wrapped in a transaction and loses its usefulness at the higher feature level which may require asynchronous processing, retry policies, failure notifications, etc.

Some obvious limitations:

1. The fatter the transactions, the more brittle.
2. Higher dependency on resilient rollback execution.

Usage

SagaTracker saga = new SagaTracker();

// Add product to user's catalog.
await saga.ExecuteWithRollbackAsync(async () =>
{
    await userCatalogClient.AddProductToUserCatalogAsync("user1", "product1");

    return async () => await userCatalogClient.RemoveProductFromUserCatalogAsync("user1", "product1");
});

// Decrement license pool count.
await saga.ExecuteWithRollbackAsync(async () =>
{
    var licensePool = await DbClient.GetLicensePoolAsync("license1");
    licensePool.Count--;
    await DbClient.UpdateLicensePoolAsync(licensePool);

    return async () =>{
        licensePool.Count++;
        await DbClient.UpdateLicensePoolAsync(licensePool);
    }
});

// Assign license to user.
await saga.ExecuteWithRollbackAsync(async () =>
{
    await licenseClient.AssignLicenseAsync("user1", "product1");

    return null;
});