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website: Document the consistency modes

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Armon Dadgar 2014-04-21 13:46:57 -07:00
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@ -131,6 +131,39 @@ only for data in their datacenter. When a request is received for a remote datac
the request is forwarded to the correct leader. This design allows for lower latency
transactions and higher availability without sacrificing consistency.
## Consistency Modes
Although all writes to the replicated log go through Raft, reads are more
flexible. To support various tradeoffs that developers may want, Consul
supports 3 different consistency modes for reads.
The three read modes are:
* default - Raft makes use of leader leasing, providing a time window
in which the leader assumes it's role is stable. However, if a leader
is partitioned from the remaining peers, they may elect a new leader
while the leader is still holding the lease. This means there are 2 leader
nodes. There is no risk of a split-brain since the old leader will be
unable to commit new logs. However, if the old leader services any reads
the values are potentially stale. The default consistency mode relies only
on leader leasing, exposing clients to potentially stale values. We make
this trade off because reads are fast, usually strongly consistent, and
only stale in a hard to trigger situation. The time window of stale reads
is also bounded, since the leader will step down due to the partition.
* consistent - This mode is strongly consistent without caveats. It requires
that a leader verify with a quorum of peers that it is still leader. This
introduces an additional round-trip to all server nodes. The trade off is
always consistent reads, but increased latency due to an extra round trip.
* stale - This mode allows any server to service the read, regardless of if
it is the leader. This means reads can be arbitrarily stale, but are generally
within 50 milliseconds of the leader. The trade off is very fast and scalable
reads but values will be stale. This mode allows reads without a leader, meaning
a cluster that is unavailable will still be able to respond.
For more documentation about using these various modes, see the [HTTP API](/docs/agent/http.html).
## Deployment Table
Below is a table that shows for the number of servers how large the