Mailing-List: contact user-help@cassandra.apache.org; run by ezmlm
Precedence: bulk
Reply-To: user@cassandra.apache.org
Received-SPF: pass (nike.apache.org: domain of jbellis@gmail.com designates
 74.125.82.172 as permitted sender)
DomainKey-Signature: a=rsa-sha1; c=nofws;
        d=gmail.com; s=gamma;
        h=mime-version:in-reply-to:references:from:date:message-id:subject:to
         :content-type:content-transfer-encoding;
        b=LloSMBRE9SiNWDXkmEhLGFYgS+HlBuyrPkToPtyHRYcaiVgIKdWtS4U3VQR0yjzFDC
         4o4kCBhlOEF40rX2x4j1lbMcKr6U53Qp0BTQtPzPyKzwynxQfZ6+/uz7UrSbGaaJIHX/
         MnXrDPInUeHzoo9TeruekqUxFIkafuCtuicAE=
MIME-Version: 1.0
In-Reply-To: <x2z429591151004280002z898f8bb4ge5bb79e6b01938cd@mail.gmail.com>
References: <x2z429591151004280002z898f8bb4ge5bb79e6b01938cd@mail.gmail.com>
From: Jonathan Ellis <jbellis@gmail.com>
Date: Wed, 28 Apr 2010 09:34:08 -0500
Message-ID: <j2ve06563881004280734y2630386aja7407d3cc574c2de@mail.gmail.com>
Subject: Re: Detailed behavior of insert() operation?
To: user@cassandra.apache.org
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

2010/4/28 Roland H=E4nel <roland@haenel.me>:
> Two clients insert the same key/colum with different values at the same
> time:
>
> =A0=A0 client A does insert(keyspace,=A0key_1,
> column_name_1,=A0value_A,=A0timestamp_1,=A0consistency_level.QUORUM)
> =A0=A0 client B does insert(keyspace,=A0key_1,
> column_name_1,=A0value_B,=A0timestamp_1,=A0consistency_level.QUORUM)
>
> After that, both clients read their value:
>
> =A0=A0 client A does
> get(keyspace,=A0key_1,=A0column_name_1,=A0consistency_level.QUORUM)
> =A0=A0 client B does
> get(keyspace,=A0key_1,=A0column_name_1,=A0consistency_level.QUORUM)
>
> It is obvious that since the insert happens 'at the same time', i.e. with
> the same timestamp, we cannot say
> which value (value_A or value_B) gets written to the row. However, do we
> have a guarantee that either value_A
> or value_B is written, and that both read operations will return the same
> result?

The guarantee is that "eventually" you will get a consistent result.

Say both writes overlap such that value A is present on replicas R1
and R2, and value B is present on replica R3 (after both writes
complete).

Simultaneous read operations could then both attempt to "repair" the
other nodes, and again there could be overlap, resulting in still 2
values present, possibly on different nodes this time.

So: you can see different values on reads when there are two
"simultaneous" writes, and this can continue in the worst-case
scenario until one read's repair can finish before another begins.

--=20
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of Riptano, the source for professional Cassandra support
http://riptano.com