compared with
Current by Matt Ingenthron
on Jun 20, 2013 00:51.

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The intent of the {{REPLICA READ}} (also known as {{CMD_GET_REPLICA}} in the server) operation is to allow a client to perform retrieval operations only against one or more replicas. This would be an inconsistent read. While it could be used for nearly any purpose, the only common use case is expected to be in the event of failures when a known inconsistent read is okay.

h2. Request

h1. Client API


h2. Java


Note: Not correct, needs updating

{code}
GetFuture resf;
boolean isReplicaRead;

try {
resf = cbc.asyncGet("foo");
isReplicaRead = false;
} catch (TimeoutException ex) {
// uhoh, something went wrong, server isn't there!
resf = cbc.asyncReplicaGet("foo");
isReplicaRead = true;
} finally {

// do something useful
}

{code}

h2. .NET

h2. PHP

h2. Ruby

{code:none}
res = nil
is_replica_read = false

begin
res = cbc.get("foo")
rescue Couchbase::Error::Timeout => ex
res = cbc.get("foo", :replica => true)
is_replica_read = true
ensure
# do something useful
end
{code}

h1. Recommended Implementation
{note}These recommendations are preliminary.{note}

When a client library is processing a request on behalf of the end user, it should generally walk the list of current replicas for the vbucket to which the given key matches.

The conversation then is something along these lines.

* Application tries to retrieve item, client library cannot service this and thus replies with an error or a timeout.
* Application then requests a replica read through a similar API as listed above.
* Client library retrieves from the current configuration a map of all nodes for the item requested. Over the course of handling this function invocation by the application, it will continue to use this sequence of primary locations and replica locations even though the map may change.
** Optionally, the client may provide a function which retrieves an array/map or other structure which contains the given item and the CAS (and optionally, the expiration) from all replicas it can contact. This would be provided as a way of allowing applications to determine what the most up-to-date version of an item may be since we are not guaranteed the replication is in the order it is present in the configuration.
* The client library will then attempt to retrieve the item from each replica in the configuration, in the specified order in the configuration until the item has been retrieved or until the list of replicas has been exhausted. Upon receiving a response from a given replica, it will then return that value to the calling application.



h2. Implementation Constraints

{{REPLICA READ}} is a binary protocol only operation. It could be implemented in ASCII, but that would require changes in moxi and additional clients.

h1. Protocol level details

See authoritative protocol documentation. This section serves only as a reference.

h2. Request

{code:none}
Byte/ 0 | 1 | 2 | 3 |
/ | | | |
{code}

h2. Request Response

{code:none}

Byte/ 0 | 1 | 2 | 3 |
/ | | | |

{code}

h1. Client API


h2. Java


Note: not sure if we like this yet...

{code}

GetFuture resf;
boolean isReplicaRead;

try {
resf = cbc.asyncGet("foo");
isReplicaRead = false;
} catch (TimeoutException ex) {
// uhoh, something went wrong, server isn't there!
resf = cbc.asyncReplicaGet("foo");
isReplicaRead = true;
} finally {

// do something useful
}

{code}

h2. .NET

h2. PHP

h2. Ruby

{code:none}
res = nil
is_replica_read = false

begin
res = cbc.get("foo")
rescue Couchbase::Error::Timeout => ex
res = cbc.get("foo", :replica => true)
is_replica_read = true
ensure
# do something useful
end
{code}

h1. Recommended Implementation
{note}These recommendations are preliminary, and have not been reviewed.{note}

The replica read command is intentionally simple. It would attempt to iterate through all of the replicas as designated by the configuration supplied by the cluster in order, trying to get the specified key.


h2. Implementation Constraints

{{REPLICA READ}} is a binary protocol only operation. It could be implemented in ASCII, but that would require changes in moxi and additional clients.

h2. Implementation Questions