O'Reilly - Java Message Service

Java Message Service
A decentralized architecture may also mean that the persistence mechanism for guaranteed
messaging is pushed out to the client machines. No matter how efficient the storage
algorithm, disk I/O is always going to be the biggest bottleneck. Choosing to use such an
architecture would require that the client machines have disk storage that is both fast and
large.
There is disagreement as to whether guaranteed messaging (storing persistent messages)
benefits from a decentralized architecture. Proponents of a decentralized architecture argue
that the I/O load is distributed among the clients and is therefore faster. On the other hand,
client I/O is not nearly as reliable, nor is it as fast as a centralized server with a powerful
disk system.
7.2.4.3 Network routers and firewalls
Although technically possible, it is unlikely that a firewall administrator will allow UDP
traffic to pass through a firewall. Firewalls typically disallow all traffic, except for traffic
to or from specific hosts, using specific protocols. UDP traffic is rarely allowed through a
firewall for various reasons.
In recognition of the problems with IP multicast (lack of support, and firewall blocking),
messaging vendors that use IP multicast provide software bridge processes to carry
messaging traffic across routers and firewalls. The bridges may consist of one or more
processes connected together by HTTP, SSL, or TCP/IP.
If you're considering a vendor that supports multicasting, it is worth considering what
percentage of your message traffic is going through one of these bridges. If all of your
messages are going through the firewall over an SSL or HTTP connection, there will be
little point in using multicasting behind the firewall for performance reasons. If the routers
in your deployment environment require that a number of TCP/IP-based bridges be put in
place, the performance benefits of multicast are diminished, depending on how many of
these you have to put in place and administer. The messaging system is only as fast as its
slowest link.
If most of the message traffic is confined to your corporate LAN or a VPN and you have
full control over it, IP multicasting is a very attractive option.
7.2.4.4 Some vendors support both centralized and decentralized architectures
In recognition of these issues, the vendors who support IP multicast also provide
centralized servers using TCP/IP socket connections. This could mean you have two
different architectures to configure and support: one configuration for the nonguaranteed
one-to-many pub/sub multicast of messages within a subnet on your corporate LAN, and
another for everything else. It is important to consider what it will mean to choose one of
these architectures at deployment time, or how you will switch from one mode to the other
after your application is deployed.
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