Table of Contents

Time to live

[TimeToLive] declares that an entity's data expires. Three stores have native expiry, with genuinely different semantics — the engine maps the declaration to each native mechanism and keeps the differences documented instead of papering over them.

[Entity("sessions")]
[TimeToLive(3600)]                    // seconds
public sealed class Session : IEntity<string>, IEntityTimeMark
{
    public string Id { get; set; } = "";
    public DateTime CreatedAt { get; set; }     // MongoDB's expiry counts from this
    public string Payload { get; set; } = "";
    public string GetKey() => Id;
    public void SetKey(string key) => Id = key;
}

What each store does with it

Store Mechanism Expiry counts from Materialized by
Cosmos DB container DefaultTimeToLive the item's last write EnsureCollection()
Cassandra table default_time_to_live the row's last write EnsureCollection()
MongoDB a TTL index (expireAfterSeconds) the date member's value, per document EnsureCollection()
Relational — no native TTL; the annotation is ignored (schedule deletes yourself)

The MongoDB nuance is the important one: Mongo expires per document, relative to a date field. The engine resolves that date member as the lifecycle CreatedAt (IEntityTimeMark) by convention, or an explicit member through the fluent model:

model.Entity<Session>(entity => entity
    .TimeToLive(x => x.LastSeenAt, TimeSpan.FromHours(1)));   // sliding expiry: refresh LastSeenAt on writes

[TimeToLive] on a MongoDB entity with no date member throws at model build with exactly that guidance — the store has the concept, so an incomplete declaration is an error, not a shrug.

Per-write TTL (Cassandra)

Cassandra can also expire individual writes, overriding the table default:

await uow.CommitAsync(o => o.WithTtl(TimeSpan.FromMinutes(5)));   // this flush's inserts expire in 5m

Checking what you declared

Explain() reports the TTL on every model, in the store's own terms — container-level, table default_time_to_live, or documents expire 3600s after CreatedAt (per-document TTL index) — so the semantic difference is always one read away. See Explain.

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