Each of these attributes are described in the following
sections.
Queue TTL
The TTL for a queue defines how long (in milliseconds)
each event published to the queue will remain available
to consumers. Specifying a TTL of 0 will mean that events
will remain on the queue indefinitely. If you specify
a TTL of 10000, then after each event has been on the
queue for 10000 milliseconds, it will be automatically
removed by the server.
Queue Capacity
The capacity of a queue defines the maximum number
of events may remain on a queue once published. Specifying
a capacity of 0 will mean that there is no limit to
the number of events on a queue. If you specify a capacity
of 10000, then if there are 10000 events on a queue,
and another event is published to the queue, the 1st
event will be automatically removed by the server.
Queue Type
Nirvana queues can be of 4 different types, simple,
reliable, persistent and mixed. The difference lies
in the type of physical storage used for each type and
the performance overhead associated with each type.
Simple Queues
Simple queues have their messages stored in the Nirvana
Realm’s own memory space supplying a high-speed
queue type. The difference between a Simple and Reliable
is the fact that the event ids are reset to 0 in a Simple
queue whenever the Nirvana Server is restarted.
Reliable Queues
Reliable queues have their messages stored in the Nirvana
Realm’s own memory space. The first fact that
is implied is that the maximum number of bytes that
that all messages across all reliable queues within
a Nirvana Realm is limited by the maximum heap size
available to the Java Virtual Machine hosting that Realm.
The second fact implied is that if the Nirvana Realm
is restarted for any reason, all messages stored on
reliable queues will be removed from the queue as a
matter of policy. However, as Nirvana guarantees not
to ever reuse event ids within a queue, new messages
published in those queues will get assigned event ids
incremented from the event id of the last message prior
to the previous instance stopping.
Persistent Queues
Persistent queues have their messages stored in the
Nirvana Realm’s persistent queue disk based store.
The persistent queue store is a high performance file
based store that uses a separate file for each queue
on that Realm facilitating migrating whole queues to
different Realms. All messages published to a persistent
queue will be stored to disk, hence it is guaranteed
that they will be kept and delivered to subscribers
until it is purged or removed as a result of a TTL or
capacity policy. Messages purged from a persistent queue
are marked as deleted however the store size will not
be reduced until maintenance is performed on the queue
using the Nirvana AdminTool. This augments the high
performance of the Nirvana Realm.
Mixed Queues
Mixed queues allow the users to specify whether the
event is stored persistently or in memory as well as
that the Time To Live (TTL) of the individual event.
On construction of a Mixed queue the TTL and Capacity
can be set, if the user supplies a TTL for an event
this is used instead of the queue
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