Lack of Auto-Increment Support in Cosmos DB with EF Core #1105
Description
// might want to mark this issue as bug, to be fixed in the long run
Issue
Cosmos DB does not support auto-incrementing fields like SQL's IDENTITY. When using Entity Framework Core with Cosmos DB, there is no built-in way to generate a unique, sequential global number for each entity. This makes it difficult to assign a reliable GlobalSequenceNumber across all documents.
For ex for PostgreSQL schema looks like GlobalSequenceNumber bigint GENERATED BY DEFAULT AS IDENTITY NOT NULL,
Workaround
A custom EF Core interceptor is used to set the GlobalSequenceNumber for new entities. The current workaround uses UTC ticks as a unique value, but this is not a true auto-increment and can lead to collisions under high load. The recommended robust solution is to implement a counter document with optimistic concurrency, similar to other event stores to guarantee sequential numbers in Cosmos DB.
Also not sure why for efcore GlobalSequenceNumber property is init-Only
Ugly but working fix (with reflection for init prop)
services.AddDbContext<MyOwnDbContext>((provider, options) =>
options.UseCosmos(
"...",
databaseName: "mydb-dev",
optionsx =>
{
optionsx.ConnectionMode(ConnectionMode.Gateway);
})
.AddInterceptors(provider.GetRequiredService<EntityModificationInterceptor>())
);public class EntityModificationInterceptor : SaveChangesInterceptor
{
public override InterceptionResult<int> SavingChanges(
DbContextEventData eventData,
InterceptionResult<int> result)
{
var context = eventData.Context;
if (context == null) return result;
var data = context.ChangeTracker.Entries().ToList();
foreach (var entry in data)
{
if (entry.CurrentValues.Properties.Any(p => p.Name == nameof(EventEntity.GlobalSequenceNumber)))
{
var currentValue = entry.CurrentValues[nameof(EventEntity.GlobalSequenceNumber)];
var tsValue = DateTime.UtcNow.Ticks;
long globalSequence = currentValue is long l ? l : 0;
if (globalSequence == 0)
{
if (entry.Entity is EventEntity entity)
{
var prop = entry.Entity.GetType().GetProperty(nameof(EventEntity.GlobalSequenceNumber));
if (prop != null)
{
prop.SetValue(entry.Entity, tsValue);
}
}
entry.Property(nameof(EventEntity.GlobalSequenceNumber)).CurrentValue = tsValue;
var typeName = entry.Entity.GetType().Name;
entry.Property("__id").CurrentValue = $"{typeName}|{tsValue}";
}
}
}
return result;
}
public override async ValueTask<InterceptionResult<int>> SavingChangesAsync(
DbContextEventData eventData,
InterceptionResult<int> result,
CancellationToken cancellationToken = default)
{
SavingChanges(eventData, result);
return await base.SavingChangesAsync(eventData, result, cancellationToken);
}
}