Hi, thanks for your answers! > Are you asking if writes are atomic at the partition level? If so yes. If you have N columns in a simple k/v schema, and you send a write with X/N of those columns set, all X will be updated at the same time wherever that writes goes.
Even for cross dc replication? (And yes, it has nothing about CL at all). I'm not familiar with Cassandra internals and implementation details, but if I understand correctly there are at least 3 possible implementations for replication: 1. Row level, each row is eventually consistent as a bunch of cells. User cannot see the mix of two concurrent updates, so he can only see row as before or after write but not in intermediate state (write are linearised). Looks like this case is not about Cassandra because it should require read before write operation to achieve this and such thing has huge impact on Cassandra basic properties as it is an AP system in general. 2. Write level, each write is applied / replicated atomically and this guarantees isolation across ONLY updated columns during this write. Enough for my case. 3. Cell level, each cell "lives" its' own life without any sync with other cells in row (excluding PK of course). Worst case scenario. To clarify a bit my case. I just insert rows with unique PK (no overwrites happen) into CF and hope that row will be atomically replicated (eventually) with isolation guarantees (whole row or nothing) to another datacenter. ср, 8 мая 2019 г. в 01:15, Avinash Mandava <avin...@vorstella.com>: > Are you asking if writes are atomic at the partition level? If so yes. If > you have N columns in a simple k/v schema, and you send a write with X/N of > those columns set, all X will be updated at the same time wherever that > writes goes. > > The CL thing is more about how tolerant you are to stale data, i.e. if you > write in one DC and you absolutely can't tolerate reads from a remote DC > showing stale data, you would have to write at EACH_QUORUM and read at > LOCAL_QUORUM. While I'm not one for blanket advice, and certainly you can > make the decision on this tradeoff, this is a last resort situation, one of > those "supported features" that you ought to be wary of, as it's a bit off > from the intended design/usage of the system. > > On Tue, May 7, 2019 at 2:58 PM Rahul Singh <rahul.xavier.si...@gmail.com> > wrote: > >> Depends on the consistency level you are setting on write and read. >> >> What CL are you writing at and what CL are you reading at? >> >> The consistency level tells the coordinator when to send acknowledgement >> of a write and whether to cross DCs to confirm a write. It also tells the >> coordinator how many replicas to read and whether or not to cross DCs to >> get consensus. >> >> Eg. Local_quorum is different from Quorum. >> Local_quorum guarantees Data was saved to a quorum of nodes on the DC on >> which the Coordinator accepted the write. Similarly it would only check >> nodes in that DC. Quorum would check across DCs in the whole cluster. >> On May 7, 2019, 12:11 PM -0500, Alexey Knyshev <alexey.knys...@gmail.com>, >> wrote: >> >> Hi there! >> >> Could someone please explain how Column Family would be replicated and >> "visible / readable" in the following scenario? Having multiple >> geo-distributed datacenters with significant latency (up to 100ms RTT). >> Let's name two of them A and B and consider the following 2 cases: >> >> 1. Cassandra client X inserts row into Column Family (CF) with >> Primary Key = PK (all cells are set - no nulls possible). Write >> coordinator >> is in dc A. All cells in this write should have the same writetime. For >> simplicity let's assume that Cassandra coordinator node sets writetime. >> After some amount of time (< RTT) client Y reads whole row (select * ...) >> from the same CF with same PK talking to coordinator node from, another dc >> (B). Is it possible that client Y will get some cells as NULLs, I mean, is >> it possible to read some already replicated cells and for others get >> NULLs, >> or does Cassandra guarantee row-level isolation / atomic write for that >> insert? Assume that row (all cells for same PK will never be updated / >> deleted afterwards. >> 2. Same as in p.1 but after first write at PK same client (X) updates >> some columns for the same PK. Will be this update isolated / atomically >> written and eventually visible in another dc. Will client see isolated >> state as it was before write or after it? >> >> Thanks in advance! >> >> >> -- >> linkedin.com/profile >> <https://www.linkedin.com/profile/view?id=AAMAABn6oKQBDhBteiQnWsYm-S9yxT7wQkfWhSw> >> >> github.com/alexeyknyshev >> bitbucket.org/alexeyknyshev >> >> > > -- > www.vorstella.com > 408 691 8402 > -- linkedin.com/profile <https://www.linkedin.com/profile/view?id=AAMAABn6oKQBDhBteiQnWsYm-S9yxT7wQkfWhSw> github.com/alexeyknyshev bitbucket.org/alexeyknyshev
