Cosmos DB consistency levels — Strong, Bounded, Session, Consistent Prefix, Eventual

Five consistency levels, from strongest (and slowest, most expensive) to weakest.

Level	Read sees writes from	Latency	RU cost	Default?
Strong	All committed writes globally	Highest	2x	no
Bounded Staleness	Writes within K operations or T time	Medium-high	2x	no
Session	Your own writes immediately; others eventually	Low	1x	yes
Consistent Prefix	Writes in order, but possibly stale	Low	1x	no
Eventual	Writes in any order, eventually	Lowest	1x	no

Strong

Linearizable reads — every read sees the most recent committed write across all regions. Only available with single write-region accounts.

Use when: financial balance, inventory count where over-sell is unacceptable.

Bounded Staleness

Reads lag the writes by at most K versions or T seconds. Useful when you need predictable staleness without the cost of Strong.

Use when: dashboards that tolerate up to N seconds of lag but need an upper bound.

Session (default)

The most useful default. A client sees its own writes immediately; sees other clients' writes eventually. Backed by a session token returned with every write.

var resp = await container.CreateItemAsync(item);
SessionToken token = resp.Headers.Session;       // capture
// Later, on the same logical user session
var read = await container.ReadItemAsync<Item>(id, pk,
    new ItemRequestOptions { SessionToken = token });

Use when: per-user UX (user reads their own data, others' data slightly stale is fine).

Consistent Prefix

Reads may be stale, but never see writes out of order. Good for "live feed" UX where order matters more than freshness.

Use when: notifications, log streams.

Eventual

No ordering, no staleness bound. Cheapest reads.

Use when: precomputed metrics, last-known-value caches where wrong-order does not matter.

Cost — the surprise

Strong and Bounded Staleness cost 2x RUs per read compared with Session/Eventual. On a busy collection this doubles your bill.

Region setup tradeoff

Setup	Strongest level available
Single write region	Strong
Multi-region writes (active-active)	Session is the strongest you can get

If you want Strong, you give up active-active. There is no free lunch on global distribution.

How to choose

• Start with Session. It is the default for a reason.
• Move stronger only when you have a concrete correctness story (Bounded Staleness for dashboards, Strong for balance/inventory).
• Move weaker (Eventual) only when reads are cheap-but-cold metrics and the cost saving is measurable.